Geophysical Journal International

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1 Detailed source analysis of 0 Gyeongju earthquake sequences: aftershock interaction within a complex fault system Abbreviated Title: Detailed source analysis of 0 Gyeongju earthquake sequences Journal: Geophysical Journal International Manuscript ID GJI-S--0 Manuscript Type: Research Paper Date Submitted by the Author: -Jun-0 Complete List of Authors: Woo, Jeong-Ung; Seoul National University, School of Earth and Environmental Sciences Rhie, Junkee; Seoul National University, School of Earth and Environmental Sciences; Kim, Seongryong; Seoul National University, School of Earth and Environmental Sciences Kang, Tae-Seob; Pukyong National University, Department of Earth Environmental Sciences Kim, Kwang-Hee; Pusan National University, Geological Sciences Kim, YoungHee; Seoul National University, School of Earth and Environmental Sciences Keywords: Earthquake source observations < SEISMOLOGY, Dynamics and mechanics of faulting < TECTONOPHYSICS, Dynamics: seismotectonics < TECTONOPHYSICS, Continental neotectonics < TECTONOPHYSICS, Earthquake interaction, forecasting, and prediction < SEISMOLOGY

2 Page of Geophysical Journal International Title: Detailed source analysis of 0 Gyeongju earthquake sequences: aftershock interaction within a complex fault system Abbreviated Title: Detailed source analysis of 0 Gyeongju earthquake sequences Jeong-Ung Woo, Junkee Rhie *, Seongryong Kim, Tae-Seob Kang, Kwang-Hee Kim, YoungHee Kim School of Earth and Environmental Sciences, Seoul National University, Seoul 0, Korea Department of Earth and Environmental Sciences, Pukyong National University / Busan, Korea Department of Geological Sciences, Pusan National University, Busan, Korea * Corresponding author rhie@snu.ac.kr Phone: Fax: +--- Key words: Earthquake source observation; Dynamics and mechanics of faulting; Seismotectonics; Continental neotectonics; Earthquake interaction, forecasting, and prediction

3 Page of SUMMARY The seismicity of the Korean Peninsula is low because of its location away from the tectonic plate boundary. Due to its low seismicity, the characteristics and detailed mechanisms of earthquake occurrences in this region have not been well studied. On September 0, a moderate earthquake (ML.) occurred in the Gyeongju area in the southeast part of the Korean Peninsula 0 minutes after the onset of a ML. event. Including a ML. event occurring a week after these two events, an unprecedented large number of aftershocks were reported in the area. A detailed analysis of the source parameters of the 0 Gyeongju earthquake sequences needed to be undertaken to understand the reactivation process of the multi-fault system in the Gyeongsang Basin, where the earthquake sequences are located. As a case study, it will contribute to our understanding of the characteristics of intraplate earthquakes with complex fault systems. In this study, we determined the precise locations of the earthquake sequences using the data from a temporary seismic network, as well as the regional permanent seismic networks, to depict the detailed fault structures. From the spatiotemporal distribution of the hypocentres, we observed that the ML. event and the ML. event lay on two sub-parallel fault planes striking to the NNE-SSW at a depth range of km and that the ML. event is seated on a conjugate fault plane to the parallel faults. As one of the more interesting features, it is confirmed that the seismic activities on the fault where the ML. event occurred temporarily halted as soon as the ML. event occurred. For the events located on the two parallel faults, the Gutenberg-Richter law b-value and Omori s law p-value are estimated at ~ with some variations of the p-values. The stress inversion results based on the inverted focal mechanisms suggest that heterogeneity between the intermediate and minimum principal stresses exists along the NNE-SSW

4 Page of Geophysical Journal International direction and the vertical direction. The Coulomb stress changes due to the finite-fault slip models of the ML. event and the ML. event correlate well with the temporal changes in the off-fault seismicity. The possibility that the Coulomb stress changes caused by the ML. event advanced the occurrence of the ML. event was tested. Multifaceted observations, such as the parallel or conjugate fault planes related to the three largest events with ML > and the turn-off of seismicity on the fault attributed to the ML. event immediately after onset of the ML. event, may reflect the reactivation processes of a complex fault system. This will offer a distinctive case study to understand the general characteristics of intraplate earthquakes in multi-fault systems.

5 Page of INTRODUCTION Intraplate earthquakes occur in tectonic plates with low strain rates, away from plate boundaries (Johnston ). Although they host only % of the world s seismic moment release, some of these events are potentially responsible for a disproportionate amount of destruction and death (Gangopadhyay & Talwani 00; Talwani 0) in some of the more densely populated areas, such as Charleston earthquake (Durá-Gómez & Talwani 00), Tangshan earthquake (Chen et al. ), and 00 Bhuj earthquake (Rastogi, 00). In spite of the importance in terms of seismic hazard analysis, the general characteristics of intraplate earthquakes are not well understood. It is widely accepted that intraplate earthquakes are caused by the tectonic forces transmitted from plate boundaries and located in preexisting zones of weakness such as failed rifts (Bodin & Horton 00; Sykes ; Talwani & Rajendran ; Tavakoli et al. 00; Zoback ). However, the clear causality between the preexisting weak zones and the location of the intraplate earthquakes has not been verified, in that there are a few reported cases where intraplate earthquakes occurred regardless of old geological surface faults (Assumpção et al. 00; Ferreira et al. ). Furthermore, it is difficult to identify the correlation because it is rare for a fault rupture to extend to the surface (Adams ). Although our understanding of the characteristics of intraplate earthquakes has advanced slowly due to their rarity, the increasing number of installations of seismographs in intraplate regions and the diverse approaches appropriate for array networks have expanded our knowledge about these earthquakes (Bianchi et al. 0; Matos et al. 0; Ross et al. 0; Soto-Cordero et al. 0; Talwani 0). The southern part of the Korean Peninsula lies within the eastern margin of the Eurasian Plate and the nearest tectonic boundary from the peninsula is ~ 00 km to the

6 Page of Geophysical Journal International southeast at the Ryukyu and Nankai Trough (So et al. 0), and thus it is classified as a intraplate region where the frequency of earthquakes is relatively low compared to that on plate boundaries. Based on an earthquake catalogue by the Korea Meteorological Administration (KMA), only 0 earthquakes with ML were instrumentally recorded in and around the Korean peninsula since. The laregest amongst them, a ML. earthquake in Gyeongju, South Korea, on September 0, produced strong coseismic ground shaking, which was sufficient to be felt throughout South Korea (Hong et al. 0). Including the ML. event, followed 0 minutes later by the occurrence of the ML. event, and the largest aftershock with ML. recorded after a week, hundreds of events were reported near the epicentre of the ML. earthquake (Fig., Fig. a, Kim et al. 0b). Hereafter, for convenience, we refer to the three events of ML., ML., and ML. as E, E (main shock), and E, respectively. The epicentres of the three events are located in the Gyeongsang Basin where several systematic fault sets, such as the Yangsan Fault System and the Gaum Fault System, are developed (Fig., Fig. a, Chang ; Hwang et al. 00) and microseismic activities occur along the surface geological lineaments or faults (Han et al. 0). Many studies on the Gyeongju earthquake sequences have analyzed their hypocentres (Hong et al. 0; Kim et al. 0b; Lee et al. 0; Son et al. 0) using regional permanent seismic networks. They have found that a deep seated fault system at a depth range of 0 km is responsible for the sequences. Some of the studies have estimated that the recovered fault plane of E is striking to the NNE-SSW direction and steeply dipping to the SE direction with a right-lateral strike-slip motion based on the regional moment tensor inversion and the hypocentre distribution (Hong et al. 0b; Kim et al. 0a; Son et al. 0). Since the Yangsan Fault is located close to the

7 Page of epicentres and the total amount of dextral displacement on the fault is nearly 0 km (Kyung 00), the possible correlation between the Yangsan Fault System and the Gyeongju earthquake sequences have been addressed (Kim et al. 0a; Kim et al. 0c; Uchide & Song, 0). However, it is difficult to prove the direct causality between the deep seated fault plane and the surface faults because the focal depth exceeds 0 km and no surface deformation due to E has been reported (Park et al. 0). A large portion of the earthquake sequences correspond to microseismic events with magnitudes less than, and many studies concerning the microearthquakes have limited the research topic to hypocentres or magnitudes based on the data of permanent seismic stations. In particular, since only a few stations exist near the epicentre of E (i.e., only stations are located within 0 km of the epicentre of E), the source parameters such as focal hypocentres, mechanisms and magnitudes cannot be highly resolved due to the poor station coverage. Therefore, to conduct further analysis on the detailed source parameters, a denser seismic network is favoured to secure a wide range of azimuths and take-off angles. For this, a temporary aftershock monitoring network was deployed and operated in and around the Gyeongju area within a few days (Kim et al. 0a). Early research using the data from the temporary seismographs found that the detected number of events almost doubled the catalogue reported by the KMA for the first 0 days of continuous data (Kim et al. 0a). In this study, we investigated the detailed source parameters related to the 0 Gyeongju earthquake sequences using a larger data set obtained from both permanent and temporary seismic recording stations. The basic goal is to create the most complete catalogue of the earthquakes that occurred in the Gyeongsang Basin from 0 to 0. To do this, we first determined the location of the earthquakes that were detected by an

8 Page of Geophysical Journal International automatic algorithm and resolved the detailed fault structures from their spatial distribution. The magnitudes of the events were calculated using the KMA catalogue events. The moment tensor solutions of selected events with ML were inverted using long period waveform data, and the focal mechanisms of fairly well recorded events were determined from the first motions of P-wave polarities. Based on our catalogue, we conducted further analysis including statistical parameters, the tectonic stress field, and the role of stress transfer induced by E and E. The multifaceted approaches of the source parameter analysis described above will contribute to a better understanding of the reactivation process of interacting complex faults in the Gyeongsang Basin and the generalized characteristics of intraplate earthquakes that occur in complex fault systems.

9 Page of DATA ACQUISITION The data set is divided into two parts: the continuous waveforms recorded at the permanent seismic stations and those of the temporary seismic stations that were deployed in the aftershock period (Fig. b). The permanent seismic networks are operated by Korean Meteorological Administration (KMA) and Korea Institute of Geoscience and Mineral Resources (KIGAM). We selected permanent stations near the epicentre of E to detect and locate the earthquake sequences and two of them were used to analyze the moment tensors of 0 selected events together with three other permanent stations located in the Gyeongsang Basin (Fig. a). The temporary seismic network was installed to monitor the aftershock sequences and consists of broadband seismometers and covers an area of ~ km in and around the epicentre of E. Teams from Seoul National University, Pukyong National University and Pusan National University have been collaborating on the installation of the temporary seismometers. The first operation of a seismometer successfully began within two hours of E and all temporary seismometers were fully established days later. To detect not only the earthquake sequences but also background seismicity near E, we used the continuous waveform data of the permanent seismic stations from January 0 to December 0, if available. In the operating period of the temporary seismic network, the data from the temporary seismic stations were also used. For the epicentre of E, the data coverage of the permanent stations yields the azimuthal gap of 0 and the minimum epicentral distance of km. However, by adding data from the temporary instruments, the two parameters could be reduced to and 0. km, respectively. Therefore, it is expected that not only additional events with small magnitudes can be detected, but also the source parameters of the earthquake sequences can be analyzed at high

10 Page of Geophysical Journal International resolution scales using both the permanent stations and the temporary stations (See the details about the temporary seismic network in Kim et al. 0a). HYPOCENTER DETERMINATION. Absolute location determination To observe the distribution of the earthquake sequences within a few kilometres from the epicentre of E and the seismicity changes throughout the Gyeongsang Basin associated with the earthquake sequences, we detected events from continuous seismic waveform data and determined their absolute locations using the single difference method (HYPOELLIPSE, Lahr ). Phase arrival times for the determination of each event were automatically measured from the recursive short-term average to long-term average ratio (STA/LTA) method (Grigoli et al. 0; Withers et al. ; Woo et al. 0). Then, incorrectly measured phase arrival times were manually inspected to discriminate overlapping events and remove false detections. A regional -dimensional velocity model for the Gyeongsang Basin suggested by Kim et al. (0) was applied for all the other procedures as well as the absolute hypocentre inversion. The mean standard deviations of the determined hypocentres correspond to the 0. km horizontally and. km vertically, while the average of the root mean square (RMS) of the travel times is 0.0 s (Fig. e). During the period from days before, to 0 days after, the onset of E, earthquakes were automatically detected and a total of earthquakes within 0 km of the epicentre of E were selected through manual inspection (Fig. ). Among the confirmed events, only events ( %) were detected before the onset of E. These

11 Page 0 of events are mainly observed in offshore regions or along several local fault structures (Fig. a). However, only two events were located within 0 km of the epicentre of E during this period, and thus it is considered that the background seismicity near the aftershock region is insignificant. Since E ruptured near the Gyeongju area, a total of events (0 %) occurred within 0 km from the epicentre of E and the distribution of the hypocentres showed a trend along the NNE-SSW direction (Fig. b). Although their epicentres are mainly located between the two surface fault traces of the Yangsan Fault and Deokcheon Fault striking to NNE-SSW or N-S direction (Kim et al. 0c), the spatial distribution of the earthquake sequences does not exactly coincide with the surface geological features (Fig. b). In cross section parallel to the dip direction, the earthquake sequences deepen to the SE direction at a depth range of 0 km, which is consistent with the results of most previous studies (Fig. c and d; Kim et al. 0a; Kim et al. 0b; Lee et al. 0). After the onset of E, the number of earthquakes located outside 0 km of the epicentre of E is ( %). Similarly, the background seismic activities were observed in offshore regions or along surface faults as well. However, parts of the region have experienced temporal changes in seismic activities. For example, the area where subsidiary local faults, such as the Ulsan Fault System and the Dongrae Fault System intersect each other is known as one of the high seismic activity zones (Han et al. 0), and the seismicity near the region has shifted km to the south. As another example, the earthquakes that occurred near the Miryang Fault System, the strike of which is similar to the Yangsan Fault System, have migrated to the north by a few kilometres. This result indicates that the pattern of the seismicity has changed due to the occurrence of the Gyeongju earthquake sequences, although it is hard to validate how long these changes might be maintained.

12 Page of Geophysical Journal International Double-difference relocation The spatial distribution of the aftershocks determined from measured phase arrival times is rather scattered and the uncertainties in the locations are not suitable for the delineation of the detailed geometries of the fault system. In order to highly resolve their spatial pattern, the earthquake sequences near the epicentre of E were relocated using the Double-Difference algorithm (HypoDD, Waldhauser & Ellsworth 000). The input data for the inversion consists of the relative arrival times measured from,, waveform cross-correlation pairs and those calculated from,0 picked arrival time pairs. Similar to the waveform cross-correlation technique applied in Hauksson & Shearer (00), time windows centered at measured arrival times were interpolated at a sampling rate of 000 and the maximum cross-correlation coefficient (maxcc) of s time windows were measured by allowing up to ±. s time shifts. Here, all the seismograms were bandpass-filtered from to 0 Hz before the cross-correlation. To sort out the appropriate data for the Double-Difference method, we only used the waveform pairs with maxcc 0.. Since the waveforms of the three largest events (E, E, and E) are not suitable for measuring differential travel times using the cross-correlation, due to the different frequency contents compared with those of other events, the differential travel times calculated from the phase arrival times were included to determine the relative location of the three events. To check whether the data obtained from the phase picking distort the relocation results or not, the inversion using only the cross-correlation data was performed, while we confirmed that there were no significant changes in the results. For the bootstrapping to test the relocation uncertainties, the events were relocated by resampling the differential travel

13 Page of times for each event pair. The mean of standard deviations from these 0 relocations corresponds to m horizontally and m vertically, respectively. Including the three largest events, a total of events are relocated within 0 km from the epicentre of E (Fig. a). This number is ~ times greater than the relocated events of previous studies, which only used data of permanent station networks for a similar period of time. This result illustrates the importance of aftershock monitoring networks for observations of microseismic events (Kim et al. 0b; Son et al. 0). In the map view, the aftershocks are primarily aligned along the N E and extend km in the strike direction and km in the perpendicular direction. The two depth profile views show that the focal depths of the earthquakes collapse to a depth range of km, compared with their initial location (Fig. c-d and Fig. c-d). Comparing the spatial pattern of the relocated hypocentres with that obtained from the single difference method, we can clearly observe the distinctive fault geometries that cannot be resolved in the initial distribution. In particular, it seems that the largest events (E and E) occur independently on two distinct sub-parallel faults with an offset of m, while E event occurred on a conjugate fault plane with a strike of ESE-WNW (Fig. d). For convenience, hereafter, we refer the three fault planes attributed with E, E, and E as the F, F, and F, respectively. The size and shape of each fault plane are estimated from the hypocentre distributions. The F strikes at NE and deepens towards the southeast direction with a dip angle of in the depth range of km (Fig. a). The F is km long and. km wide, and makes-up a square-like fault plane (Fig. b). Located at the east of the F, the F has the same strike with F, but the dip angle of the fault plane varies with the depth

14 Page of Geophysical Journal International range, and can be separated as: () the Fa with a dip of and a depth range of.. km; and () Fb with a dip of and a depth range of. km (Fig. a, c-d). The E is located on the Fb whose dip angle is well matched with that of one nodal plane of the calculated moment tensor solution of E. The widths of the Fa and Fb correspond to. km and. km, respectively, while the lengths of the Fa and Fb are the same at. km. The strike of the F is set to N E, since the earthquakes near E are aligned perpendicular to the strike of F and F. However, the number of earthquakes distributed over the streak is not enough to infer the dip angle. Therefore, we set the F to a vertical fault plane based on the moment tensor inversion of E (Fig. a). Using the spatial distribution of the aftershocks for an hour from the onset of E, the length and width of the F were set to km and 0. km, respectively (Fig. ). The parameters of each fault plane are summarized in Table and the schematic diagram is illustrated in Fig.. Comparing the locations of the three major events (E, E, and E) with the spatial distribution of the aftershocks on each fault, we observed that E started to rupture at the northeastern part on the F and it propagated to the southwest (Fig. b). In contrast, the rupture of E seems to have propagated to the northeast (Fig. c and d). This result is consistent with the finite fault model inverted from empirical Green s functions of Uchide & Song (0). Although E lay on the south west tip of the F, the earthquake are spatially separated from the F in a map view, indicating that the F and F form a conjugate fault system (Fig. a). AFTERSHOCK STATISTICS

15 Page of Magnitude estimation As one of the basic seismic source parameters, such as hypocentre, magnitude is a physical parameter that quantifies the size of an earthquake and is required in order to study the characteristics of seismic activities such as earthquake-size distribution, the seismicity rate or seismic hazard. Among the various methods to estimate the magnitude of an event, we use the scaling relationship between magnitude differences and amplitude ratios for estimation of the magnitude of the earthquake sequences. Assuming that two events have similar ray paths, focal mechanisms and source time histories, the magnitude difference dm between them is expressed as the product of the amplitude ratio and a scaling parameter c as: dm=clog0α () where α is amplitude ratio of one event to the other event, and c is a linear scaling parameter between the amplitude ratio and the magnitude difference (Shelly et al. 0). In this study, the linear relationship was demonstrated using the KMA catalogue and then the magnitudes of each relocated events was determined from the estimated scaling parameter c and the KMA catalogue events. To retain the appropriate event pairs that can be assumed to have sufficiently similar source parameters, a waveform cross-correlation technique was applied. For two waveforms with a maxcc greater than 0. on a particular channel at a station, the amplitude ratios were measured as the slope of the principal vector extracted from the principal component analysis (PCA). This method is known to give reliable amplitude ratios because it applies full waveforms unlike the method that uses only the differences in peak amplitude values (Shelly et al. 0). Fig. illustrates that for the events of

16 Page of Geophysical Journal International the KMA catalogue, common logarithms of amplitude ratios are approximately linearly proportional to the magnitude differences. The c values estimated by the least squares method and the PCA algorithm are not significantly different at 0. and 0.. Of the two methods, the PCA algorithm is the more appropriate approach for extracting patterns between measured variables containing uncertainties. Therefore, the c values are taken from the PCA algorithm, since both amplitude ratio and local magnitude are measurements. The waveforms of our relocated earthquakes and KMA catalogue events were cross-correlated in order to relatively determine their magnitudes. In the case of maxcc values above 0., the amplitude ratio of the two waveforms was measured using the slope of the principal vector and the magnitudes of the relocated events were calculated from the selected c values and the local magnitude of the KMA catalogue events. To increase the reliability of the results, the magnitude of a relocated earthquake was estimated as the arithmetic mean of the calculated magnitudes, but only if at least 0 amplitude ratios were measured. Here, we define the estimated magnitudes as Mrel since Eq. () can be valid for a limited range of magnitudes. By this process, the magnitudes of earthquakes were relatively determined. The minimum value of the estimated magnitude is 0., the mean of the standard deviations of the calculated magnitudes from amplitude ratios is 0., and the number of average amplitude ratios used for the scale measurements is.. Earthquake size distribution and aftershock decay rate As factors representing the characteristics of aftershock occurrences, the frequency-size distribution and the decay rate of the seismicity have been analyzed and

17 Page of discussed in many cases (Abdelfattah et al. 0; Aktar et al. 00; Ansari 0; Enescu et al. 0; De Gori et al. 0; Wiemer & Katsumata ; Zhao et al. 00). The former characteristics can be expressed in the following way: Log0N = a - bm () where N is the cumulative number of earthquakes with magnitude equal to or greater than M. The a-value and b-value, which are generally treated as constants, reflect the characteristics of earthquakes in a particular region and period. However, they can change with time or location. In particular, the b-values are linked to various physical properties such as asperities, effective stress, material heterogeneity, crack density, thermal gradient, and tectonic regimes (Goebel et al. 0; Raub et al. 0; Schorlemmer et al. 00; Tormann et al. 0; Westerhaus et al. 00; Wiemer & Katsumata ). Therefore, analyzing the temporal and spatial changes of b-values is very important for identifying the properties of earthquakes and the faults that caused them. The frequency of aftershocks generally decreases in inverse proportion to time and the tendency can be expressed using Omori's formula: R(t)=K(t+c) -p () where K, c, and p are constants to be determined. In particular, the p-value that represents a measure of the decay rate of the seismic activities is known to vary from 0. to. (Wiemer & Katsumata ), and there are many factors that are attributed with the variation in the p-value, including structural heterogeneity, stress and temperature in the crust (Utsu & Ogata ).

18 Page of Geophysical Journal International In this study, it can be predicted that the minimum magnitude of completeness (Mc) that can hold Eq. () will change over time due to the expansion of the temporary observation network, and that this change will affect estimates of b-values and p-values. Therefore, we initially investigated the temporal variations in b-values for every set of 00 events in chronological order by allowing duplication of 0 events, considering the time-variant Mc (Fig. a). We then estimated the b-values and p-values for the two event sets located on the two faults (F and F) for the maximum value of Mc (Fig. ). The b- value for each event set is evaluated from the maximum likelihood method (MLM) by Aki () with a magnitude bin size of 0., while the uncertainty of each estimated b-value is calculated using the method by Shi & Bolt (). The Mc in Eq. is calculated using a modified version of the goodness-of-fit method by Wiemer & Wyss (000). After the calculation of the goodness-of-fit value for every Mc with a 0. interval, we find the minimum Mc (Mcmin) of which the goodness-of-fit value is greater than 0. and then take the Mc that yields the maximum goodness-of-fit value in the range of [Mcmin, Mcmin+ΔM]. The constants and their standard deviations in Eq. () for each event data with the magnitude greater than Mc are determined using the maximum likelihood procedure by Ogata (). The results of the time-variant Mc and b-values of the earthquake sequences are presented in Fig.. For the first days, the Mc decreases from. to ~ and remains at about for the subsequent period (Fig. b). The b-value, on the other hand, is 0. ± 0.0 at the initial stage and increases to. ± 0.0 during the days after the onset of E (Fig. a). The b-values of the total data set, the events on the F, and the events on the F are approximately the same at 0. ± 0.0, 0. ± 0.0, and 0. ± 0.0, respectively, but the p-values of the three event sets are.0 ± 0.0, 0. ± 0.0, and

19 Page of ± 0.0, respectively, which are slightly different from each other (Fig. ). At this time, the Mc for calculating b-values and p-values is fixed at., which is the maximum value over time. The initial three days of decreasing Mc values correspond exactly to the period when the temporary observation network is being expanded. The Mc-value may be temporarily overestimated during the early earthquake sequences due to the high level of background noises. However, we checked that the temporal change in the Mc over time does not appear with the earthquakes detected using the permanent stations. Therefore, we have demonstrated that the expansion of the temporary observation network has quantitatively decreased the magnitude of completely detected events. Simply assuming a b-value of, the fact that the Mc for the extended period of the temporary observation network was lowered by 0. indicates that the number of completely detected events increases by a factor of approximately. This agrees well with the number of earthquakes detected only by the permanent stations, which corresponds to % of all detected events. The b-value temporarily increased from 0. to. during the initial three day period in which the seismic observation network was being expanded, although it is unknown whether the increasing pattern of the b-values is a continuation from the period before the onset of E. This increasing tendency over the three days cannot be explained by the decrease in the Mc over the same period. Because the slope of the frequency-magnitude distribution increases with respect to the Mc, using a constant Mc makes the increasing tendency of the b-values stand out. For all time periods, indeed, it is observed that the b-values estimated using the highest Mc (=.), do not vary

20 Page of Geophysical Journal International significantly. The two event segments marked as solid red dots have days offset and their b-value difference is estimated as 0.. To determine whether the difference between the two b-values is statistically significant or not, we applied the Utsu s test (Utsu ) and the probability that there would be no difference in the b-value between the two sets of populations are estimated as 0 - %, confirming that the difference in the b-values of the two sets of data would not be statistically insignificant. The tendency for b-values to increase after the onset of an earthquake has been observed in other cases: 00 ms.0 Parkfield earthquake (Shcherbakov et al. 00) and th September Mw. event between Izmit epicentre and Lake Sapanca (Raub et al. 0). Raub et al. (0) estimated that the increase in b-values is related not only to the release of stress, but also to the unclamping of faults. Hosono & Yoshida (00) reported that the expected number of relatively large earthquakes is lacking in later earthquake sequences compared with the expected number from the modified Omori formula for an early time period, and this effect may result in the relatively small b- values at the initial stages. Another reason that can cause the small b-values is that microseismic events may not be detected due to the low signal to noise ratios (SNRs) of seismic coda waves. The estimated b-values for all data sets, E, and E are all close to and consistent to the estimation of.0 for the KMA catalogue events (Hong et al. 0). This b-value of also corresponds to the commonly observed value in other earthquake sequences (Wu et al. 0; Ansari 0). The p-value of.0 for the entire data set is considerably lower than the. of Hong et al. (0), which uses the KMA catalogue events. The disparity can be attributed to the use of different fitting ranges in Omori's Law, or the additional

21 Page 0 of consideration of the Mc for determination of the p-values.

22 Page of Geophysical Journal International MOMENT TENSOR SOLUTIONS AND FOCAL MECHANISMS From the moment tensor and focal mechanism of an individual earthquake, the fault plane and the slip motion of that earthquake can be estimated with the hypocentre distribution. Moreover, these can be applicable to the stress inversion, which shows the local stress heterogeneity. If the seismic wave propagation characteristics in a region have been well studied and the observed waveforms have high SNRs, the moment tensor calculated using long period waveform data is generally the best representative of the seismic source mechanism. Conversely, the first motions of P waves recorded at each station can be used to determine the focal mechanisms of small earthquakes where the SNRs of the observed waveforms are low, and thus the long period waveform inversion cannot be applied. In this study, the deviatoric moment tensor solutions were determined using the ISOLA software of Sokos & Zahradnik (00) for earthquakes with ML. For earthquakes clearly recorded at many stations, the first P-wave polarities at each station are manually measured and then applied to the FOCMEC software of Snoke (00) to calculate their focal mechanisms. After several trials have been attempted for the appropriate stations and filtering ranges, the moment tensor solutions of 0 selected earthquakes were determined using the waveforms recorded at five stations, and two different ranges of cosine filters were applied with respect to their magnitudes. The optimal focal depths were determined from the moment tensor inversion by testing different depth ranges with a km bin. To obtain reliable results from the measured first P-wave polarities, the focal mechanism analysis was performed only in those cases where the earthquake had more than 0 polarity data with various ranges of azimuth and take-off angles. Up to

23 Page of three measurement errors were allowed for the first P-wave polarity and all the solutions were obtained with minimal polarity error for each earthquake. Among the solutions, one optimal solution is selected that minimizes the sum of the distances from P, T, and B axes of each solution to that of others on the focal sphere. All possible solutions, including the selected optimal solution are visually inspected to confim that their mechanisms are similar to each other, and we retained focal mechanisms. Fig. illustrates two examples of moment tensor inversion results and deviatoric moment tensors, while the detailed fault parameters of the 0 selected events are given in Fig. 0a-c and Table. Although some earthquakes have considerable compensated linear vector dipole components of ~ 0 %, double couple components are generally dominant. The inverted moment tensor solutions are commonly classified as strike-slip events, and one nodal plane is parallel to the fault planes of F and F obtained from the earthquake distribution. Assuming that the fault rupture of each earthquake propagates on the F or F, the true fault plane will have a NNE-SSW strike direction with a right-lateral movement. Fig. represents the density distribution for the inverted focal mechanisms onto the triangular diagram of Frohlich (). The number of earthquakes that correspond to strike-slip events and thrust events were ( %) and ( %), respectively. The remaining events were classified as odd faulting types, while no normal faulting events were observed. By looking at the spatial distribution of the strike-slip events and thrust events, which account for % of the calculated focal mechanisms, we can observe that both types of focal mechanisms occur on the F and F (Fig. a and c, and Fig. a and c).

24 Page of Geophysical Journal International The trend and plunge of the P axis of the strike-slip events are ± and 0 ±, respectively, and this is very similar to those of the thrust events which are ± 0 and ± 0, respectively, indicating that the trend of the maximum principal stress axis is homogeneous at 0 0 (Fig. b and Fig. b). However, despite co-existence of the two faulting types on the F and F, the detailed spatial distributions of the strike-slip and thrust events are quite different from each other, implying that the other principal stresses are locally heterogeneous. Due to the similarity of the P-axes between the strike-slip events and thrust events, a slight polarity measurement error can change the faulting type of a focal mechanism (See the polarity differences of Figs. d and d). Therefore, the first motion of the P-wave recorded at the stations near the epicentre was tested to confirm the existence of the two faulting types. In general, if a seismological observatory is located near the epicentre and a strike-slip fault is assumed, the first motion of the P-wave has a very weak signal, as the ray path will pass approximately through the B-axis on the focal sphere, whereas the first motion of the P wave of a thrust event is clearly observed as positive because the ray path will sample the point near the T axis on the focal sphere. From the vertical components of the waveforms recorded at the station installed near the epicentre, the first motions of the P-waves of the strike-slip events are recorded as weak emergent arrivals, whereas those of the thrust events are identified clearly positive arrivals, and some of them have a similar amplitude to those of the S waves recorded following the P-waves. In opposing cases, we confirm that the waveforms recorded from stations at which the corresponding ray paths passed through the B-axis in focal sphere have the opposite tendency.

25 Page of In order to cross-check the reliability of the inverted fault parameters calculated by the methods implemented in the FOCMEC and ISOLA software, the focal mechanisms of the four earthquakes inverted from both methods were compared (Fig. 0d). All four events can be found to have similar fault parameters as strike-slip faults, which proves that the inverted parameters are quite reliable.

26 Page of Geophysical Journal International STRESS INVERSION As mentioned in the former chapter, inverted focal mechanisms of the events are mainly divided into the strike-slip and thrust events under the homogeneous P axis and their spatial distributions are different from each other. One of the main observations is that the thrust type of earthquakes rarely occurs deeper than about km, while strike-slip earthquakes do (e.g., the sub-region of C in Fig. c and Fig. c). Another important observation is that the thrust type of events are dominantly scattered in the area with depths shallower than.km and horizontal distances greater than km on the A-A profile (e.g., the sub-regions of C in Fig. c and Fig. c). Since the fault motion is affected by the local stress field, this spatial heterogeneity of the fault types can be interpreted to reflect the spatial changes in the tectonic stress field. To examine the heterogeneity in the stress field beneath the epicentres of Gyeongju earthquake sequences, the study area is divided into five sub-regions at the point at which the spatial distribution of the two fault types varies and the stress inversion is performed for each sub-region (Figs. c, c and ). The inversion process was applied via the MSATSI software, which was redesigned from the conventional SATSI algorithm for use in the MATLAB environment (Hardebeck & Michael 00; Martínez- Garzón et al. 0). As the real fault plane cannot be exactly distinguished from the two nodal planes of each focal mechanism, the fault plane is randomly selected from the two options. No regularization for the spatial difference of the stress field is applied since sub-regions are manually selected from the distribution of the fault plane. Detailed inversion parameters, such as the number of the fault planes used and the direction of each principal axis are presented in Fig.. The stress inversion results indicate that the

27 Page of strikes and dip angles of the maximum stress components (σ ) for all sub-regions are well constrained as ~ 0 N and ~ 0, respectively. This result is well matched with the uniform P-axis directions, implying that the maximum horizontal stress (σ Hmax ) is homogeneous in the study area. However, the intermediate stress field (σ ) and the minimum stress field (σ ) of the subregions have locally different regimes, which may have resulted from spatial differences in the strike-slip events and thrust events. In the shallow part (focal depths less than. km), the directions of σ in the C and C correspond to the minimum horizontal stress (σ hmin ) and the lithostatic stress (σ v ), events and thrust events, which are consistent with the inversion results. However, the estimation of σ contains quite large uncertainties, such that the interpretations should be treated with caution. In the intermediate depth range of. km, the amplitudes of σ and σ are very close to each other, such that the stress field for these in C and C cannot be well constrained, although the number of strike slip events is even larger than that of thrust events in both cases (Fig. ). The portion of the odd faulting events, which plotted in the undesignated area in the ternary diagram of Fig., may be attributed to the results. For block C, located below a depth of km, all the principal stress orientations are well constrained because 0 % of the focal mechanisms in this block are classified as typical strike-slip events, which can be observed in the moment tensor solutions. In the stress-field inversion process, the stress ratios (R) for the blocks are calculated. The R value is defined as (σ -σ )/(σ -σ ) which varies from 0 to. The tectonic regime is transpressional (or transtensional) if the R value is above (or below) 0. (Bohnhoff et al. 00). As shown in Fig., all the R values are greater than or equal to 0.. The R values of the C and C blocks are nearly one, implying that the magnitudes

28 Page of Geophysical Journal International of the two principal stresses σ and σ are very close to each other. In the same vein, we can observe that the uncertainties in σ and σ get smaller as an R value departs from one. In block C, where the uncertainty of the stress inversion results is relatively low and the σ and σ can be considered as the vertical stress and minimum horizontal stresses, the principal stresses normalized by vertical stress can be derived from the Coulomb Friction Law and the R-value, with the assumption that excessive differential stresses would be released by slip along optimally oriented fault planes (Jaeger et al. 00; Soh et al. 0). Based on the frictional coefficient (μ) of 0. and the hydrostatic pore pressure, the relative minimum horizontal stresses (σ hmin /σ v ) and the relative maximum horizontal stresses (σ Hmax /σ v ) correspond to 0.0 and., respectively, which are consistent to the results of Soh et al. (0). If μ varies from 0. to following the Byerlee s Law (Byerlee ), σ hmin /σ v and σ Hmax /σ v have a range of and..0, which do not significantly change the results when μ is 0.. Therefore, at a depth of km, the amplitude of σ Hmax, σ v, and σ hmin are,, and MPa, respectively, if we assume the unit weight of granitic rocks for the continental crusts (. knm - km -, Soh et al. 0).

29 Page of STRESS TRANSFER ANALYSIS The fifty minute gap in the origin times of the E and E events, and the m offset of the two faults (F and F) may be related to the static stress interaction in a multi-fault system. The occurrence of E and E closely related in space and time can cause the static stress changes due to Coulomb stress transfer to the other part of the fault planes, and affect the rupture process on each fault plane to be advanced or delayed. In other words, the perturbed stress field caused by the rupture of E may have changed the onset of E forward and vice versa. One of the favorable approaches in regards to the triggering of earthquake aftershocks is the Coulomb hypothesis, which explains the increased (or decreased) seismicity with the regional static stress changes. For specific receiver fault geometry, the relationships amongst the coulomb stress change (Δσ f ), the shear stress change (Δτ), the normal stress change (Δσ), and the pore pressure change (ΔP) on this fault are given as: Δσ f =Δτ+μ(Δσ+ΔP) () where μ is the coefficient of internal friction and unclamping is defined as positive for the normal stress (Lay and Wallace, ). If we use the apparent frictional coefficient μ instead of μ, which involves both the effects of the pore pressure changes and normal stress changes, then Eq. is rewritten as (King et al. ; Reasenberg & Simpson ): Δσ f =Δτ+μ Δσ () We applied two approaches for the Coulomb stress calculations via the Coulomb. software (Lin & Stein 00; Toda et al. 00). Firstly, the Coulomb stress changes imparted by both E and E were analyzed for optimally oriented strike-slip faults to

30 Page of Geophysical Journal International resolve the temporal variation of the off-fault seismicity. For this, we used the hypocentres determined by the single difference method in Fig., while the receiver fault planes were assumed to be optimally oriented to the maximum stress field of the N E direction with a strength of bars (Hong et al. 0). We then calculated the Coulomb stress change on F (and F) under the E (and E) slip model to check whether the static stress changes on each fault plane correlated with its seismicity. Following the scheme of Hong et al. (0), the apparent frictional coefficient μ, Young s modulus and Poisson s ratio were set to 0., 0 GPa, and 0., respectively. The finite rupture models of E and E of Uchide & Song (0) were used to configure the slip amounts on the faults. Fig. shows the Coulomb stress changes generated by both E and E for optimally oriented faults at a depth of km with the seismicity in and around the Gyeongsang Basin during 0 0, under the maximum stress field of the direction N E with the strength of bars (Hong et al. 0). It is observed that the seismicity distal from the earthquake sequences (i.e., the area outside of the black box in Fig. ) has increased within the area of positive Coulomb stress changes, whereas the rate of the earthquake occurrence diminishes within the negative lobes; a result that is consistent with that of Hong et al. (0). More specifically, the effects of the coulomb stress changes were inversely proportional to the epicentral distances. Within the km from the epicentre of E, the ratio difference of the earthquake occurrence density between lobes of positive and negative Coulomb stress changes steeply increased after E. The increasing tendency of the ratio difference gets weaker the greater the epicentral distances (see the upper right panels of Fig. ). Note that this interpretation

31 Page 0 of should be treated with careful consideration due to the possible discrepancy between the receiver faults and optimally oriented faults. The calculated coulomb stress change on F for the fault model of E represents a negative value at the point where E occurs. However, an elliptical area with extremely positive Coulomb stress changes was observed near the loci of E; not more than 00 m away (Fig. a and b). Therefore, if the uncertainties of the finite slip model and relocated hypocentre are taken into account, the Coulomb static stress changes by E may have advanced the occurrence of E, although many other factors besides the static stress change could be related to the rupture process of E, such as the dynamic stress change, fault geometries, and the regional stress state (Gomberg et al. 0). Based on the calculation of the variation in the Coulomb stress on F, assuming the finite fault model in E, the thin band area with a km width and the area beneath the depth of km show increased Coulomb stress (Fig. c). Comparing this area with the location of the seismic activity for hours after E, they seem to be similar to each other, and the observation implies that the Coulomb stress change caused by the occurrence of E may have affected the seismicity on F.

32 Page of Geophysical Journal International DISCUSSION. Fault system complexity The detailed spatial distribution of the Gyeongju earthquake sequences represents that of the E and E events occurring sequentially on the two independent fault planes of the F and F separated by 0 minutes. These two faults are sub-parallel to each other with an offset of 00 00m. As such, the complex fault system, in which two or more fault planes are developed parallel in the intraplate region, can be found in other cases (Durá-Gómez & Talwani 00; Rabak et al. 00; Yano & Matsubara 0). Yano & Matsubara (0) reported that a part of the 0 Kumamoto earthquake sequences are aligned with two vertical fault planes in the northeastern area of Mt. Aso. In particular, two moderate-sized aftershocks (M ) were located on either side of the fault plane. This is consistent with our results, implying that each visually identified fault from the hypocentre distribution can be attributed to major seismic events. Rabak et al. (00) estimated that there are two fault planes with the NE-SW direction in the depth range of km using the hypocentre distribution of the 0 Enola earthquake sequences, and they are thought to be a system of conduits possibly filled by fluid. Although the focal depths of the Gyeongju earthquake sequences are deeper at a range of km, the possibility that fluid migration is related to the complex seismicity cannot be ruled out. The tendency for the dip of F to change at a depth of ~. km can be interpreted as an artifact due to the uncertainty in the seismic wave velocity model assumed for the hypocentre determination. However, the result does not significantly change even if a velocity model with a fast velocity anomaly of kms - in the depth range of the Fa or other models with no velocity discontinuities at around km are

33 Page of assumed in the relocation procedures. Therefore, it is likely that the fault plane is bent at a depth of ~. km at which the dip of the F changes, or that two distinct faults with different dips are crossing. Near the epicentre of the F, we observed that the earthquake sequences are distributed as a streak along the direction perpendicular to F and F. Therefore, the fault plane of E is considered to be another fault segment that constitutes part of the Gyeongju earthquake sequences. As an example of how conjugate fault systems are developed, conjugate faults can sequentially propagate (0 Sumatra earthquake, Meng et al. 0; 000 Wharton Basin earthquake, Robinson et al. 00), and the fault planes activated by the main shock can be perpendicular to each other (000 Western Tottori earthquake, Fukuyama et al. 00). In particular, despite its larger magnitude of Mw., the 000 Western Tottori earthquake has similar characteristics to the Gyeongju earthquake sequences in that one side of the conjugate fault system is relatively small compared to the other side, and it is located at the tip of the fault system.. Interactions between complex faults The hypocentre distributions for an hour after each of the three major earthquakes (E, E, and E) and the one nodal plane of each of their moment tensor solutions are clearly matched with their rupture plane, such that each assumed fault plane is considered to have been determined reasonably accurately. However, seismic activities on F abruptly diminished immediately after E, which occurred about 0 minutes later than E (Fig. b and d). Moreover, after the occurrence of E, most of the seismicity is confined to F at a depth range shallower than km, with deeper seismic

34 Page of Geophysical Journal International activity beginning not until one hour after E (e.g., orange dots in Fig. ). This is in contrast to the seismic activity distributed over all areas of F immediately after E. Since it is possible to use the static stress change to account for the increase or decrease of seismic activity in certain areas after an earthquake occurred, the static stress change triggered by the occurrence of E may temporally impede earthquakes on F. In spite of that, the Coulomb stress changes on F calculated by the finite slip model of E are not negative in all areas (Fig. c). Therefore, in addition to the coulomb stress changes of E, other factors such as elastic modulus and initial stress states are required to explain the seismic quiescence on F after the onset of E.. A possible microscale heterogeneity in the complex fault system The b values for F and F are almost identical at approximately, but the p- value for F is estimated to be significantly less than that for F. As mentioned earlier, this difference may be related to the seismic quiescence on F for about an hour starting with the onset of E. To check for the effect, the p-value for F was re-estimated after removing the one hour gap, and it remained at 0. ± 0.0, which is still less than those for the all data set and the events on F. Therefore, it is reasonable that the decay rate of the seismicity on F is generally slower than that on F. Still, many other factors describe the spatial changes in the p-values in aftershock areas: stress, fault heterogeneity, and crustal heat flow (Enescu et al. 00; Wiemer & Katsumata ), requiring that further analysis on the parameters should be conducted. The focal mechanisms consisting of the Gyeongju earthquake sequences are divided into the strike-slip events ( %), thrust events ( %), and the odd faulting

35 Page of events ( %), which are characterized as being intermediate between those faulting types. The Nobi fault area in the central Japan has a quite similar composition of focal mechanisms in that both strike-slip and reverse fault earthquakes are observed (Katsumata et al. 0). However, unlike the Nobi fault area, Gyeongju earthquake sequences do not have local perturbations in the maximum stress field. Instead, we observed the spatial heterogeneity of σ and σ throughout the study area. Where the depth is shallower than. km, the σ corresponds to the minimum horizontal stress in the northeastern part of the A-A profile, with an horizontal distance of > km, whereas it corresponds to the lithostatic stress in the southwest part. This difference may result from the relatively weak minimum stress in the northern region. Strike-slip fault events are dominantly observed in areas where the depth is deeper than km, which means that the overburden pressure increases with depth, and is always greater than the minimum horizontal stress below a certain depth. The maximum Mrel of the reverse fault earthquakes determined by the focal mechanism inversion corresponds to., and none of the fault types calculated from the moment tensor inversion has a thrust regime. Indeed, from the hypocentre distribution, no characteristic structures are identified along the direction of NNE-SSW, the strike direction of the reverse faults. Therefore, the reverse fault events occur on smaller fault planes than those of the strike-slip events, and microcracks that cannot be identified by the epicentre distribution may be developed throughout the study area, generating the thrust events that occur in response to the stress field at each location.. Model Implication

36 Page of Geophysical Journal International From before the beginning of the Gyeongju earthquake sequences, many moderate earthquakes have occurred along surface faults located in and around Gyeongsang Basin (Fig. a; Lee & Jin ; Han et al. 0; Kim et al. 0b). Therefore, the faults identified from the hypocentre distribution of the sequences were more likely to be preexisting fault systems where local stress had accumulated rather than newly formed fault planes. According to Hwang et al. (00), a block rotation of the Gyeongsang Basin, generated by the subduction of plates or ridges, or by a collision of plates, can be responsible for the current fault system observed at the surface. In particular, two surface faults can be identified near the epicentres of the earthquake sequences. The Yangsan Fault at the east side of the epicentres is extended throughout the Gyeongsang Basin with a length of 0 00 km and a horizontal offset of. km (Fig. a). Therefore, the possibility that the Yangsan Fault System and the 0 Gyeongju earthquake would be related is taken for granted. However, in the detailed distribution of the earthquake sequences, it is clearly observed that there is an angular difference of 0 0 between them. Kim et al. (0a) proposed in relation to the issue, that it may present as a strike-slip duplex consisting of two parallel master faults with interconnecting Riedel shear structures. However, according to the theory of strike-slip faulting systems, the sense and stepping direction of en echelon fault segments control the deformation regime at offsets or step-overs separating the en echelon segments, and thus the Riedel shears connecting the two dextral strike-slip faults in the study area corresponds to extensional oversteps. However, normal faulting events that can be observed in extensional tectonic regimes are not found in our inverted focal mechanisms (Fig. ). Therefore, based on the spatial distribution of relocated earthquake sequences and their focal mechanism solutions, we suggest that the strain accumulation on distinct

37 Page of veiled pre-existing faults beneath the aftershock region is responsible for the reactivation process of the earthquake sequences, rather than directly related structures with the two surface faults. The discrepancy between local seismicity and geological lineaments is also observed in the seismicity of the northeastern Brazil craton. It is reported that the occurrence of events in the northeastern Brazil craton is sometimes not directly related to the major lineament, although the areas close to neotectonic faults are statistically likely to have higher seismic activity (Assumpção et al. 0; Moura et al., 0). CONCLUSIONS In this study, the source parameters of Gyeongju earthquake sequences and years background seismicity were analyzed in detail using continuous waveforms recorded from two permanent seismic networks and a temporary seismic network. From the spatial distribution of about,00 earthquake sequences, the fault planes of each of the three major earthquakes with ML > were identified at a depth range of km. In particular, the two events with ML > occurred on two sub-parallel fault planes (F and F), which have the same strike of º, while the dip to the southeast direction occurs at angles in the range of º. The third largest event (E) seems to have occurred in a vertical fault plane with a strike perpendicular to those of F and F. The strike direction of the three fault planes constructed from the hypocentre distribution does not coincide with the strike of the two surface faults located near the epicentres, which implies that deep seated fault systems, which are not related to the surface faults were reactivated in response to the current stress field.

38 Page of Geophysical Journal International From the focal mechanism solutions obtained from the first motions of the P- waves, strike-slip events and thrust events were most commonly found in this region. Of these, the strike-slip events are clearly matched with the moment-tensor solutions calculated by the long period waveform inversion. The difference in the spatial distribution between the strike-slip events and thrust events can be considered to reflect the local heterogeneity of the minimum stress field and lithostatic stress field since their P axes are very similar to each other. In particular, reverse fault events are rarely observed at depths of km or more, implying that the minimum horizontal stress is no longer greater than the lithostatic stress in the areas deeper than km. From the magnitudes of the earthquake sequences estimated by scaling the amplitude ratios and the magnitude differences, the temporal variations of the b-values and Mc were analyzed over time. The b-values increased over the initial three days, and their variation is not significant for the subsequent period. If the b-values are assumed to be a stress-meter indicator, the increase of b-values over time seems to reflect the stress releasing process. The measured b-values for both fault planes of F and F were close to, which are similar to those for other earthquake sequences. Unlike the b-values, the p-values for both fault planes were different from each other, which may require more complex parameters such as the stress transfer between the fault planes. The Coulomb stress changes caused by E and E clearly correlated with the spatial change of the seismic activities, which is observed along geological structures in the Gyeongsang Basin. The occurrence of E may have been facilitated by the Coulomb stress changes as well as the dynamic stress changes due to the E. On the other hand, the Coulomb stress changes caused by E appears to have been affected by the

39 Page of seismicity on F. However, a sudden reduction in the seismic activity on F for approximately an hour following the onset of E is difficult to explain only by Coulomb stress change, and it may require complex fault properties to resolve it. We suggest that our enigmatic observations, such as the complicated spatio-temporal distribution of seismicity, heterogeneous stress field, and the p-value difference between the two parallel faults, could be closely related to the complex fault properties, and our results can be the basic data to resolve them. Acknowledgements We appreciate Hobin Lim and one/two anonymous reviewers for careful comments. We thank Korea Meteorological Administration (KMA) and Korea Institute of Geosciences and Mineral Resources (KIGAM) for sharing continuous waveform data for this study. We are grateful to not only all landowners and organizations who allowed us to install and operate temporary seismic stations on their properties but also graduate students in the Gyeongju earthquake research group for maintaining the temporary seismic network. This work was supported by the Nuclear Safety Research Program through the Korea Foundation Of Nuclear Safety(KoFONS) using the financial resource granted by the Nuclear Safety and Security Commission(NSSC) of the Republic of Korea. (No. 000 ) References

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51 Page 0 of TABLE Table. Fault parameters for the three major earthquakes. Fault ID Width (km) Length (km) Max Strike / Dip (º, Strike / Dip (º, Min Depth Number of Depth Area (km ) aftershock Moment tensor (km) Earthquake (km) distribution) inversion) F.. / / 0 Fa..... / / 0 Fb / F / NA 0 /

52 Page of Geophysical Journal International Table. Source parameters of the ten earthquakes used in the moment tensor analysis. Event Origin Event ID (E) (E) (E) Time (UTC) Longitude Depth (DD; Latitude (ºN) (yy/mm/dd, (ºE) MTI, km) Magnitude(Mw) Strike (º) Dip (º) Rake (º) hh:mm:ss.ss) /0/,.;.0. 0:::0 ;. / / - / - /0/,.;.. ::.. / 0 / / /0/,.;.0. ::..0 / 0 / 0-0 / - /0/,.;.0. ::.0. 0 / / - / -0 /0/,.00;.0.0 ::.. / 0 / / /0/,.00; / 0 ; / ; - / - ;..0. ::. 0 / 0 0 / 0 0 / 0 /0/,.; / ; / ; - / -0 ;..0. ::. 0 / 0 / / /0/0,.; / 0 ; / ; / ;... ::0. 0 / 0 / / /0/0,.; / ; / ; / ;..0.0 ::0. / / /

53 Page of //,.0;... / / - / - 0::. The depth of each event is calculated from both Double-Difference (DD) relocation and the moment tensor inversion (MTI). If we obtain the focal mechanism of an event using first P-wave motions, we ordinarily describe the fault parameters obtained from the waveform inversion and first P-wave motions.

54 Page of Geophysical Journal International FIGURES Figure. (a) Regional tectonic setting of the 0 Gyeongju earthquake. Major geologic units are separated with black lines: PB (Pyungnam Basin), OB (Ongjin Basin), IB (Imjingang Belt), GM (Gyeonggi Massif), OFB (Okcheon Fold Belt), YM (Youngnam Massif), GB (Gyeongsang Basin), and YB (Yeonil Basin). Inset illustrates the location of Korean Peninsula. The box with red lines indicates the extent of (b). The hypocentre of E is shown with its moment tensor. The triangles represent the stations which recorded the waveforms used to invert the moment tensor inversion of the ten selected events. (b) The distribution of the stations used to locate the aftershocks. Data period is from st January 0 to th December 0. The three major events are shown with their moment tensors, which were calculated via the low frequency waveform inversion. The two fault lines located near the main shock are denoted with green lines. The other

55 Page of lineaments are shown as dark grey lines. The rex box corresponds to the extent of Fig. b and Fig. a.

56 Page of Geophysical Journal International Figure. Distribution of the hypocentres calculated from the visually inspected P-phase and S-phase arrival times, and the histogram of the root mean squares (RMS) of the location uncertainties. (a) The observed seismicity in and around Gyeongsang Basin for 0-0, and (b) a higher magnification view. The seismicity before and after the onset

57 Page of of E are shown with red and black dots, respectively. Geological structures, such as faults and lineaments, are denoted as dark grey lines and two surface faults near the epicentre of the Gyeongju earthquake sequences are shown with green lines. The name of several major surface ruptures are presented with abbreviations: GUF (Gaum Fault); MiRF (Miryang Fault); MoRF (Moryang Fault); YSF (Yangsan Fault); DRF (Dongrae Fault); USF (Ulsan Fault). The red box in (a) corresponds to the extent of (b). (c-d) Two crosssections along profiles A-A (oriented NºE) and B-B (oriented NºE). (e) The probability function of the RMS histogram for the determined hypocentres. The red and black lines are the probability functions for the seismicity before and after the occurrence of E. The green, red, and blue stars in (b), (c), and (d) are the location of E, E, and E, respectively.

58 Page of Geophysical Journal International Figure. Distribution of the relocated hypocentres for the 0 aftershocks located in the mainshock area. (a) Distribution of the epicentres of the Gyeongju earthquake sequences. Tectonic structures, such as faults and lineaments, are denoted as dark grey lines. Two surface faults near the epicentre of the Gyeongju earthquake sequences are shown with green lines. (b) Six hour seismicity at the early stage of aftershocks as a function of focal depth. (c-d) The depth distributions along A-A (oriented NºE) and B-B (oriented NºE). The three moment tensor solutions of E, E, and E are distinguished as three different colors of compressional quadrants: green (E), red (E), and blue (E). The seismic activity during three selected periods with different

59 Page of background colors in (b) are highlighted in (a), (c), and (d) by matching their colors. The blue dots in (a), (c), and (d) represent the seismic activities for an hour starting from the onset of E.

60 Page of Geophysical Journal International Figure. Configuration of the detailed fault structure based on the hypocentre relocation shown in Fig.. (a) The depth distribution along profile B-B (oriented NºE). The estimated fault planes of F, Fa, and Fb are denoted as green, orange, and red lines. The events on each fault plane are projected within the plane and have offsets less than 0. km. (b-d) Distribution of the events on F (b), Fa (c), and Fb (d) at the crosssection along profile A-A (oriented NºE) with the corresponding fault contours. The green, blue, and red stars in (a), (b), and (d) are the locations of E, E, and E, respectively, and the rupture directivity of E and E is described in (b-d).

61 Page 0 of Figure. A three-dimensional schematic diagram to illustrate the spatial distribution of the aftershocks, the geometry of the four faults (F, Fa, Fb, and F) and the rupture directivity of E and E. Detailed information on each fault is summarized in Table.

62 Page of Geophysical Journal International Figure. Determination of the c value which scales the amplitude ratio and magnitude difference. Two methods were tested for estimation of the c value: the least squares method (red line; c = 0.), and the principal component analysis method (blue line; c = 0.).

63 Page of Figure. (a) The b-value as a function of time. The horizontal and vertical error bars indicate the standard deviation of event origin times and b-values, respectively. The b- value difference between two event sets denoted as solid red dots was tested by the Utsu s test. (b) The magnitude of completeness (blue dots) and the goodness of fit (green dots) as a function of time.

64 Page of Geophysical Journal International Figure. (Top) The frequency-magnitude distribution of the three event clusters: all events (left), F (middle), and F (right). The grey lines in the three figures represent the value of the minimum magnitude of completeness. (Bottom) the occurrence rate of the three event clusters: all events (left), F (middle), and F (right). The black dots and red lines represent the occurrence rate of observed and predicted events by fitting Omori s law with a bin size of day, respectively. The black lines and the blue dashed lines are the cumulative number of observed and predicted events by fitting Omori s law with a bin size of day, respectively.

65 Page of Figure. Two moment tensor inversion results for (a) E, and (b) another event with Mw.. The black lines and red lines indicate the observed waveforms and synthetic waveforms for each station. The epicentre and the stations used in the inversion are shown as a yellow circle and grey triangles, respectively. Different frequency limits of the

66 Page of Geophysical Journal International cosine filtering were applied to the observed waveforms and synthetic waveforms (0.0, 0.0, 0.0, and 0. for the three largest events, and 0.0, 0.0, 0., and 0. for the others).

67 Page of Figure 0. (a-c) The spatial distribution of the 0 selected events with their deviatoric moment tensors. Each number above the moment tensor in (a) represents the moment magnitude of the event. The compressive quadrants of the three largest events in (b) and (c) are colored in green (E), red (E), and blue (E). (d) Comparison of focal mechanism solutions obtained from first P-wave motions to the corresponding moment tensor results. The nodal planes, P-axis and T-axis of each focal mechanism solution is

68 Page of Geophysical Journal International illustrated with red lines, a red circle, and a red triangle, respectively. Those from the double couple component of each moment tensor are illustrated with thick black lines, a black circle, and a black triangle, respectively. The compressive quadrants of each moment tensor are colored in grey. The other symbols are matched with those in Fig. (a, c, d).

69 Page of Figure. The probability density distribution of the focal mechanism solutions projected onto the triangle diagram of Frohlich (). Typical examples of focal mechanisms are illustrated on the three edges of the projection to aid understanding.

70 Page of Geophysical Journal International Figure. (a-c) The hypocentre distribution of the strike-slip events classified by the method of Frohlich () and the histograms of the trends and plunges of P-axes. All the relocated hypocentres are denoted as grey dots on the map (a) and cross-sections (c). The strike-slip events, for which sine-squared values of the plunges of the B axes are above or equal to 0., are represented with their focal mechanisms. The green, red, and

71 Page 0 of blue stars in (a), (c) are the locations of E, E, and E, respectively. For the two crosssections (c), the probability histogram for the strike-slip events is also shown. Note that E, E, and E are strike-slip events and their compressional quadrants of its focal mechanism are marked by difference colours. Each subregion (C-) in (c) indicates the extent of the stress inversion in Fig.. (d) Three examples of strike-slip events. The open and closed circles are the compressional and dilatational P-wave polarities, respectively. One optimal solution and all possible solutions with the minimum number of the polarity errors are denoted as thick black lines and grey lines. All the focal mechanisms in (a-c and d) are represented with the conventional lower hemisphere projection.

72 Page of Geophysical Journal International Figure. (a-c) The hypocentre distribution of the thrust events classified by the method of Frohlich () and the histograms of the trends and plunges of P-axes. All the relocated hypocentres are shown as grey dots on the map (a) and cross-sections (c). The thrust events, for which sine-squared values of the plunges of the T axes are above or equal to 0., are represented with their focal mechanisms. For the two cross-sections

73 Page of (c), the probability histogram for the strike-slip events is also shown. (d) Three examples of the thrust events. All the symbols of (a-c and d) are matched with those in Fig..

74 Page of Geophysical Journal International Figure. Stress inversion results for the events in the different blocks (C-C). Firstly, the study area is divided into the left section (C, C, and C) and the right section (C and C) at the km point on the A-A profile. Then, each section is split into two or

75 Page of three subsection on the baiss of several depth ranges. For each subplot, the depth ranges, the relative stress magnitude (R), and the number of focal mechanisms used (N) are shown below the inversion results, while each subplot is illustrated with a pie-chart representing the relative portions of strike-slip events (red), thrust events (blue), and other type of events (green). The determined three principal stress components (σ > σ > σ ) are represented by 000 bootstrapping results. For all the subsections, the maximum principal stress (σ ) are well resolved with low uncertainty, whereas the other principal stress components are not constrained due to the mixed population of strikeslip events and thrust events.

76 Page of Geophysical Journal International Figure. Coulomb stress change on uniformly oriented receiver faults at a depth of km. Each sub fault element is assumed to be an area of km. The red dots and green dots in the map correspond to the red dots and black dots in Fig. a, respectively. Fault lines described in Fig. a are represented as grey lines and green lines, as well. The off-fault seismic activities are regarded as the earthquakes that occur outside the black box. For each epicentral distance bin, the percentage difference between the off-fault

77 Page of seismicity densities of the areas with increased Coulomb stress changes and those with decreased Coulomb stress changes is illustrated in the inset histogram.

78 Page of Geophysical Journal International Figure. Coulomb stress changes imparted by finite slip models on the subfault elements of other faults. (a-b) Coulomb stress changes on (a) Fa, and (b) Fb caused by the finite slip model for E. The seismic activities on F between the onsets of E and E are represented with yellow dots. (c) Coulomb stress changes on F caused by the