The source process of the 2001 July 26 Skyros Island (Greece) earthquake

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1 Geophys. J. Int. (2004) 156, doi: /j X x The source process of the 2001 July 26 Skyros Island (Greece) earthquake Zafeiria Roumelioti, 1 Anastasia Kiratzi 1 and Douglas Dreger 2 1 Department of Geophysics, Aristotle University of Thessaloniki, PO Box 352-1, Thessaloniki, Greece. kiratzi@geo.auth.gr 2 Berkeley Seismological Laboratory, University of California, 281 McCone Hall, Berkeley, CA 94720, USA Accepted 2003 September 10. Received 2003 September 1; in original form 2002 November 4 1 INTRODUCTION On 2001 July 26 at 00:21:39 UTM, a strong earthquake (M 6.5) occurred in the North Aegean Sea, around 130 km NE from the metropolitan city of Athens. The earthquake was relocated at N and E, at a depth of 13 km (Roumelioti et al. 2003), a few kilometers NW of the island of Skyros (Fig. 1). Even though the earthquake was felt in a wide region, it did not cause human losses or extended damage mainly because its epicentre was located at sea, far enough from densely populated areas. Some damage was only observed on the island of Skyros, concerning mainly old residential buildings and a historical monastery located inside the island s castle. A landslide was also formed, causing damage to cars that were parked beneath the castle s hill. The focal mechanism of the Skyros earthquake (Fig. 1), as derived from teleseismic waveform modeling (Benetatos et al. 2002), yields almost pure strike slip faulting (plane 1: strike = 145, dip = 80, rake = 8 ; plane 2: strike = 54, dip = 82, rake = 170 ). Past studies (e.g. Papazachos et al. 1984; Rocca et al. 1985; Kiratzi et al. 1991, among many others) have shown that the North Aegean area, where the earthquake occurred, is tectonically controlled by the continuation of the North Anatolian Fault zone in the Aegean Sea, which is well known for its dextral strike-slip character. However, aftershock epicentres of the 2001 July sequence (Melis et al. SUMMARY The spatial and temporal distribution of slip during the 2001 July 26 Skyros (Greece) earthquake (Moment magnitude M 6.5) is investigated using broadband data recorded at regional distances. The applied method involves estimation of the source time functions of the examined event through an empirical Green s function approach and inversion of their shapes to estimate kinematic source parameters. Our test inversions to statistically identify the fault plane, together with the distribution of aftershocks clearly indicate sinistral strike-slip faulting. In view of the fact that the Skyros epicentre lies near the western termination of the dextral strike-slip North Anatolian Fault (NAF) into the Aegean Sea, this sinistral strike-slip motion, for the first time instrumentally identified, has great tectonic significance. The best values searched through the inversion are 0.7 s for the rise time, and 2.4 km s 1 for the rupture velocity. Most of the slip appears to be concentrated in a relatively small area around the hypocentre, while a smaller slip patch was found at relatively large depth (18 24 km). At least two of the large aftershocks following the main event also occurred at the deeper part of the fault. Smaller amounts of slip are distributed in a wider area with dimensions similar to those inferred from the aftershock distribution studies and the empirical relations applicable to Greece. Key words: Aegean, Skyros, slip distribution, source time function. 2001; Drakatos et al. 2002; Roumelioti et al. 2003) indicate that the Skyros event occurred along the NW SE oriented plane of the focal mechanism, thus implying sinistral strike-slip faulting. This fact lends great scientific interest to the examined earthquake. Kiratzi (2002) studies the North Anatolian Fault continental Greece interaction and in that work a model is proposed for the Skyros event. Thus, in the present paper we will focus on the study of the source process of this event. The source process of the Skyros earthquake is studied through an inversion of source time functions (STF) estimated from broadband regional data (Mori & Hartzell 1990; Dreger 1994). The applied method involves estimation of the required STFs through the use of empirical Green s functions (EGF). This approach is particularly attractive to study earthquakes in seismotectonically complex environments, such as the Aegean Sea area, where computation of theoretical Green s functions can be a troublesome task. We also focus on the discrimination of the fault plane between the two nodal planes of the focal mechanism, by examining their comparative performance in reproducing the observed STFs. 2 DATA The 2001 July 26 Skyros earthquake was the first large earthquake to be very well recorded by the Greek broadband network, which was GJI Seismology C 2004 RAS 541

2 542 Z. Roumelioti, A. Kiratzi and D. Dreger Figure 1. Regional map with the locations of the broadband stations used in this study (triangles) with respect to the earthquake s hypocentre (star). recently established by the National Observatory of Athens (NOA). The stations of the network are equipped with Lennartz LE-3D/20s, three-component seismometers and provide an excellent coverage of the examined area. We used all three components from a total of 12 stations that provided good signal-to-noise ratios for both the mainshock and large aftershocks records. The original velocity data were corrected for the instrument response and integrated to displacement. Subsequently they were band-passed filtered from 0.05 to 1.0 Hz, which is the frequency range used in the inversions. The azimuthal distribution of the employed stations, with regard to the epicentre of the examined earthquake, is shown in Fig METHOD APPLIED The applied method for earthquake source inversion was initially proposed by Mori & Hartzell (1990) in the study of local earthquakes and later extended to regional distances by Dreger (1994). Under this method, source time functions of the examined event are estimated through the use of a nearby smaller event, with similar focal mechanism, as an empirical Green s function (Hartzell 1978). The method can be described based on the mathematical representation of the far-field wave displacement of a point source, U(ω) recorded at distance r and azimuth φ: U(ω) = M(ω) G(ω,r,φ) R I (ω) (1) where ω is the angular frequency, M is the moment rate function (source time function) of the event, G is the Green s function response of the medium along the wave path, which includes the effects of attenuation and geometrical spreading, R is the radiation pattern factor and I is the response of the recording instrument. In order to obtain the required moment rate function, M, the effects of geometrical spreading and attenuation, as well as those of the recording instrument, have to be removed. The instrument responses of modern seismometers are known with great precision and therefore can be easily removed. Nevertheless, the theoretical computation of the effect of the propagation path involves numerical and analytical difficulties and reflects the uncertainty in the knowledge of the earth model. Thus, an easier and probably more accurate way of removing these effects is through deconvolution of the records of a smaller event located close enough to the examined event and having similar focal mechanism. The small event is treated as point source in both space and time and as long as this assumption holds the resulting function is a realistic estimate of the true STF of the large event. In the case where the point source assumption does not hold, the calculated function is the source time function of the target event relative to the source time function of the small event (RSTF). Therefore the accuracy of the estimated STFs depends on the choice of the small event. The event should be small enough to ensure that the point source assumption will not be violated, but it should be also large enough to provide a good signal-to-noise ratio at the examined regional distances. In the applied method, the entire waveforms of both the large and the small event are used, assuming that they are dominated by S-wave phases. Such an assumption has been proven to work well at regional distances, giving slip distribution models that are consistent with models developed using theoretical Green s functions and near-source data (e.g. Dreger 1994). The use of the entire waveform is considered an advantage of the method, as isolation of particular phases is usually very difficult to be done at regional distance records. In the subsequent step the estimated STFs at the available stations are inverted to reveal the characteristics of the source process. The applied inversion technique (Mori & Hartzell 1990) assumes that the variations in the shape of the STFs can be mapped onto the spatial and temporal slip history of the event. The absolute amplitudes of the STFs are not taken into account to avoid errors due to differences in the focal mechanisms of the two events. Therefore, prior to the inversion the STFs are normalized to unit area. In this way, we also equalize the weighting of individual stations in the inversion.

3 Source process of the 2001 Skyros Island earthquake 543 Table 1. Source parameters for the Skyros mainshock (Benetatos et al. 2002) and the aftershock (Swiss Seismological Service) used as empirical Green s function. Event Date Time Latitude Longitude Depth M Mo Plane1 Plane2 DD/MM/YR h:m:s ( N) ( E) (km) Nt m Strike/Dip/Rake ( ) Strike/Dip/Rake ( ) Main 26/07/01 00:21: /80/8 54/82/170 EGF 26/07/01 04:53: /85/15 65/75/175 Figure 2. Comparison of displacement waveforms (north south component), at selected stations, for the Skyros mainshock and the employed empirical Green s function. The name and azimuth of each station are noted on the top. Figure 3. Source time functions at station APE derived from the deconvolution of the north south (top left), east west (middle left) and vertical (bottom left) EGF components from the mainshock are compared to their stack (right part).

4 544 Z. Roumelioti, A. Kiratzi and D. Dreger Figure 4. Azimuthal variation of the STFs obtained for the Skyros mainshock. The name and azimuth of each station is denoted at the top right of each STF. A map with the distribution of relocated aftershock epicentres (Roumelioti et al. 2003) is shown in the middle. The star symbol corresponds to the mainshock epicentre.

5 Source process of the 2001 Skyros Island earthquake 545 The source parametrization employed is that of a radially expanding rupture front with constant rupture velocity, with the slip confined to one of the nodal planes as determined by the focal mechanism of the examined event. The fit to each observed STF is determined by summing synthetics from different parts along the fault, which are positioned at equal intervals. These synthetics are appropriately delayed by taking into account the time needed for the wave to propagate from the hypocentre to the recording station and the time delay due to the propagation of the rupture front. Distances to the different subfaults are estimated using a half-space ray trajectory approximation. It is also assumed that the contribution to the examined event s STF from each subfault has the form of a boxcar function. The length of the boxcar represents the slip duration of each subfault and is fixed in each inversion. The optimal values for the rupture velocity and the dislocation rise time, which are also fixed in each inversion, are found by performing a series of inversions testing the parameter space. The subfault STFs (B), which have been assigned the shape of boxcar functions, are related to the observed STFs (D) through a system of equations of the form: m D i (t) = STF i (t) = B j (t τ ij )w j (2) j where τ is the time delay due to wave and rupture propagation, i is a station index, j is a subfault index and w is a weight proportional to fault slip. Two constraints are imposed on the inversion procedure. The first one is a positivity constraint, which allows only those subfaults with positive slip to contribute to the solution and the second one is a spatial derivative minimization constraint to smooth the resulting slip model. Taking into account the two constraints, eq. (2) can be rewritten in matrix form as: [ B λs ] w = [ D 0 ] where S is the matrix of first spatial derivatives and λ is a constant controlling the weight of the smoothing equation. The slip weight vector is obtained by standard least squares. The timing of slip on each subfault is controlled by the radially expanding rupture front and each part is allowed to rupture only once. As mentioned before, the inversion yields slip weights and not absolute slip amplitudes. To derive the absolute slip at each subfault, u j,an independent estimate of the seismic moment of the examined event, M 0, is required. The relation used for this purpose has the following form: (3) Seismological Service ( An aftershock of M 4.9, which also occurred on 2001 July 26 at 04:53 UTM, was identified as the most suitable for our study. The adopted parameters of the selected aftershock are shown in Table 1, along with the parameters employed for the mainshock. The hypocentral parameters of the two events were taken from a relocation study of the 2001 Skyros sequence (Roumelioti et al. 2003). In Fig. 2, we compare horizontal displacement seismograms at selected stations for the mainshock and the chosen aftershock. The source time functions of the Skyros mainshock were estimated using broadband waveforms recorded at the stations depicted in Fig. 1. The EGF recordings were deconvolved from the mainshock recordings in the frequency domain, using a 1 per cent water level (Clayton & Wiggins 1976) to stabilize the process. Each component was treated separately during the deconvolution, although the resulting STFs of all three components were stacked in order to reduce the background noise. An example of the stacking procedure at one selected station (APE) is shown in Fig. 3. The computed STFs, normalized to unit area, are depicted in Fig. 4 as a function of the azimuth of the stations. The shapes and u j = M 0 w j (4) A µ where A is the subfault area and µ is the shear modulus, usually taken equal to Pa. Thus, the final slip amplitudes are subject to the same errors and uncertainty included in the determination of the seismic moment of the examined event. 4 APPLICATION The choice of the appropriate empirical Green s function to be used in the study of the Skyros mainshock was primarily based on a visual comparison of the moment tensor solutions and the broadband waveforms of the largest aftershocks to the corresponding moment tensor and waveforms of the mainshock. The focal mechanism of the mainshock was taken from Benetatos et al. (2002), while the mechanisms of the largest aftershocks were retrieved on-line from the Swiss Figure 5. Inversion results for the two nodal planes of the 2001 July 26 Skyros earthquake. Diamonds correspond to the NW SE striking plane and circles to the NE SW plane. Top panel shows results for different values of rupture velocity and lower panel the corresponding results for different values of the rise time. Dashed lines outline our preferred values for the examined parameters.

6 546 Z. Roumelioti, A. Kiratzi and D. Dreger amplitudes of the STF do not clearly exhibit the emergence of any directivity effects during the Skyros earthquake, suggesting that the rupture was a bilateral one. However, a careful look at stations KZN, NEO and PLG indicate a component of NW directivity, although for the most part the rupture is bilateral. At least two distinct pulses can be distinguished in most of the computed STF, which imply a heterogeneous distribution of slip and probably correspond to two distinct areas of slip concentration. The STF shapes are in good agreement with the average STF computed from teleseismic waveform data (Benetatos et al. 2002), which presents a trapezoidal shape and duration of about 8 s. In the inversion of the STF shapes we assumed a planar fault model with dimensions km, discretized into 1 km 2 square subfaults. The dimensions of the fault model were chosen to be larger than those expected for an earthquake of M 6.5, according to the empirical relations applicable in Greece and worldwide (Papazachos & Papazachou 1997; Wells & Coppersmith 1994) in order to avoid an imposed confinement of the slip. Even though the distribution of the aftershocks of the Skyros event suggests that the NW SE plane of the focal mechanism is the one that ruptured, we successively tested both nodal planes in our inversions. The preferred values for the rupture velocity, V r, and the rise time, τ, which remain fixed in each inversion, were investigated by a thorough search of the model space. The effectiveness of the examined values was evaluated by means of the variance reduction function. The inversion results are illustrated in Fig. 5. Circles correspond to the results for the NE SW plane and diamonds to the NW SE striking plane. The top panel shows the variance reduction curves for inversion along each nodal plane using different values for the rupture velocity within the range from 2.0 to 3.6 km s 1, while the bottom panel shows the corresponding curves for different values of the rise time spanning the range 0.1 to 1.5 s. The NW SE nodal plane systematically results in larger variance reduction throughout, although the difference between the two nodal planes is small in terms of absolute values ( 3 per cent in most cases). Nevertheless, an F-test on the calculated values reveals that this difference is statistically significant at the 95 per cent confidence level, which means that the applied method outlines the NW SE nodal plane as the one that ruptured during the Skyros earthquake. This result is in accordance with the faulting orientation inferred by the aftershock distribution (Melis et al. 2001; Drakatos et al. 2002; Karakostas et al. 2003; Roumelioti et al. 2003) and other studies (Zahradnik 2002). Regarding the optimum values for the rupture velocity and the rise time, we found a shallow maximum in variance reduction for V r = 2.4 km s 1 and τ = 0.7 s. Actually, the inversions result in a preferred range of values for both the examined parameters rather than in one single value. Therefore, the two values given above can be considered as averages since the specific data set and/or the specific slip distribution pattern does not allow any further resolution. The best-fitting values for the rupture velocity and the rise time were used to obtain the final slip distribution model, for which we obtained a variance reduction of 96.8 per cent. Fig. 6 shows a map of the resulting slip and Fig. 7 the comparison between observed and synthetic STFs at the 12 examined stations. Relocated hypocentres of 445 aftershocks of the 2001 Skyros earthquake (Roumelioti et al. 2003) are also plotted in Fig. 6. The slip, averaged across the ruptured fault surface, is 23 cm although it locally reaches much larger Figure 6. Slip distribution model for the 2001, M6.5 Skyros mainshock. Contours are for 25 cm intervals of slip beginning at 5 cm. The white star symbol depicts the adopted hypocentral location. Aftershock hypocentres are also plotted as cubes. Black stars correspond to the two largest aftershocks that occurred a few minutes after the mainshock.

7 Source process of the 2001 Skyros Island earthquake 547 Figure 7. Comparison between observed (continuous lines) and synthetic (dotted lines) source time functions at the 12 examined stations. values. A peak value of about 1.8 m was obtained very close to the adopted hypocentre. Generally most of the slip is concentrated around the hypocentre and the rupture appears to have propagated almost in the same way towards both upwards and downwards. A relatively larger extent of the slip appears towards N NW but this conclusion is subject to large uncertainty since the low-slip patch at the northwestern part of the fault might have not been well resolved. Another slip patch was also found at relatively large depths (18 24 km). It is interesting to note that some of the large aftershocks that occurred a few minutes after the mainshock have hypocentral depths of this order. The hypocentres of two large aftershocks that occurred min after the mainshock (at 00:31:54 and 00:34:59 UTM) are depicted as solid stars in Fig. 6. These aftershocks may be related to the redistribution of stresses caused by the nearby deep slip patch. A generally good agreement between the distribution of the aftershock hypocentres and the margins of areas with high slip concentration was found for the entire slip model. This result is consistent with the observations of Mendoza & Hartzell (1988) in a large number of earthquakes. 5 CONCLUSIONS The source process of the 2001 July 27 M 6.5 Skyros (North Aegean, Greece) earthquake was investigated through a source time function inversion technique. Regional broadband waveforms of an aftershock, which was treated as an empirical Green s function, were deconvolved from the corresponding waveforms of the mainshock to obtain the source time functions of the latter event. Subsequently,

8 548 Z. Roumelioti, A. Kiratzi and D. Dreger the shapes of these functions were back-propagated onto the fault plane through an inversion procedure, to reveal the spatial and temporal characteristics of the distribution of slip. The resulting slip distribution model indicates that the slip is well confined in an area of 30 km length and less than 25 km width. The slip model is rather inhomogeneous, with at least two distinctive areas of slip concentration. Peak values of slip appear very close to the hypocentre location, while the rupture was found to be mainly a bilateral one, even though a component of NW directivity may be detected. A slip patch was found at relatively large depth (18 to 24 km), whose existence may be further supported by the fact that some of the large aftershocks following the main event, whose depth is 12 km, occurred at large depths. Under certain conditions the applied method has been shown to correctly identify which conjugate plane has ruptured (e.g. Dreger 1994; Antolik 1996). Thus, from this test our results clearly suggest the NW SE trending plane as the fault plane, in accordance with aftershock distribution studies (Melis et al. 2001; Drakatos et al. 2002; Roumelioti et al. 2003). The Skyros 2001 event with its sinistral strike-slip faulting and its epicentre in the Northern Aegean Sea where the strands of the well-known dextral North Anatolian fault (NAF) end has its scientific significance. Because it occurred in the area of interaction between the dextral shear transferred from the east (NAF) and the normal faulting system of continental Greece. The region is complex and is associated with numerous large earthquakes. The transition from strike-slip faulting to normal faulting is terminating against the NW SE striking structures of Evia Island and Magnesia (see inset of Fig. 4 for locations). In general the structures that strike NW SE in eastern continental Greece are inherited from past alpine deformations (Tranos 1998). The Skyros mainshock with its clear NW SE plane indicates that these faults can be reactivated under the presently acting stress field with a sinistral strike-slip component or normal component depending on their orientation in respect to the stress field (Kiratzi 2002). ACKNOWLEDGMENTS The Seismological Institute of the National Observatory of Athens provided part of the data used in the present study, and thanks are due to Dr George Stavrakakis, director of the Institute and to Dr Nikos Melis. This work was supported by the General Secretariat of Research and Technology (GSRT) of Greece. 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