JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, B08306, doi: /2004jb002980, 2004

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /2004jb002980, 2004 Analysis of the 2001 Geiyo, Japan, earthquake using high-density strong ground motion data: Detailed rupture process of a slab earthquake in a medium with a large velocity contrast Y. Kakehi Faculty of Science, Kobe University, Nada, Kobe, Japan Received 18 January 2004; revised 25 May 2004; accepted 14 June 2004; published 11 August [1] Detailed rupture process of the 2001 Geiyo, Japan, earthquake (M w = 6.8), which is a slab earthquake in the Philippine Sea slab, is investigated from the inversion of strong motion waveforms. In the inversion, a realistic curved fault model is assumed, and the one-dimensional underground structure models considering the deep slab structure and the shallow surface layers are used. The estimated seismic moment and the maximum slip are N m and 2.4 m, respectively. The total rupture duration is 10 s. The obtained rupture process is very complex. The rupture changed gradually from a normal fault type into a more strike-slip type during its propagation. The slip distribution is heterogeneous, and the rupture area extends over both of the slab oceanic crust and the slab oceanic mantle. In the medium of the source area, there exists a large velocity contrast at the slab oceanic crust-mantle boundary. The largest slip is seen in the low-velocity (low-rigidity) oceanic crust. It is reasonable from the viewpoint of rupture dynamics that large slip occurs in the low-rigidity layer, which is easier to deform. Thus the obtained slip model suggests that the rupture process of the 2001 Geiyo earthquake was strongly influenced by the heterogeneity of the background medium. INDEX TERMS: 7209 Seismology: Earthquake dynamics and mechanics; 7215 Seismology: Earthquake parameters; 7212 Seismology: Earthquake ground motions and engineering; 7230 Seismology: Seismicity and seismotectonics; KEYWORDS: 2001 Geiyo earthquake, rupture process, slab earthquake Citation: Kakehi, Y. (2004), Analysis of the 2001 Geiyo, Japan, earthquake using high-density strong ground motion data: Detailed rupture process of a slab earthquake in a medium with a large velocity contrast, J. Geophys. Res., 109,, doi: /2004jb Introduction [2] At 0627 UT on 24 March 2001, a M w = 6.8 (M JMA = 6.7) earthquake occurred beneath the Seto Inland Sea of Japan. This earthquake was a slab event, which occurred in the Philippine Sea slab subducting beneath the southwest Japan. Its focal mechanism was a normal fault type. Though the focal depth was as deep as km according to the Japan Meteorological Agency (JMA) unified catalogue, this earthquake brought heavy damages: two people were killed, 288 people were injured, and 70 houses collapsed [Fire and Disaster Management Agency, 2002]. In this study, the rupture process is investigated in detail from waveform inversion of high-density strong ground motion records, assuming a realistic curved fault model. This earthquake occurred in the area where a large seismic velocity contrast exists in the slab as shown later. The relation between the rupture process and the background slab structure heterogeneity will be discussed from the Copyright 2004 by the American Geophysical Union /04/2004JB002980$09.00 viewpoint of rupture dynamics. Investigation of the rupture process of this event will also produce essential information for discussing the stress field in this area of the Philippine Sea slab. Additionally, this study also has a significance in the strong motion seismology. As was shown by the 2001 Geiyo earthquake itself, a large slab earthquake can be damaging to nearby cities. Therefore strong ground motion prediction for the future slab earthquakes is a very important task of seismology. The detailed rupture process of the 2001 Geiyo earthquake will be basic information for the source modeling of slab earthquakes for quantitative strong ground motion prediction. [3] In this study, I use the strong ground motion data of K-NET [Kinoshita, 1998], KiK-net [Aoi et al., 2000], and F-net [Fukuyama et al., 1996], all of which are the observation networks of National Institute of Earth Science and Disaster Prevention (NIED) of Japan. High density of station distribution of these networks has made it possible to select stations surrounding the source area for the waveform inversion. This work has demonstrated the power and significance of high-density strong motion observation 1of14

2 3. Construction of Structure Models for the Green s Functions [6] For the calculation of theoretical Green s functions, one-dimensional underground structure models are constructed. A common structure model among all the stations is assumed for the deeper part (slab and crust). Different models are used for different stations for the shallow surface layers. The discrete wave number method [Bouchon, 1981] and the reflection/transmission method [Kennett and Kerry, 1979] are used to calculate the Green s functions for the one-dimensional structure models. Details of the structure model construction are described in sections Figure 1. Fourteen stations (seven KiK-net stations, six K-NET stations, and one F-net station) whose strong ground motion data are used as the waveform inversion data. The KiK-net stations have the names of XXXHXX. TGW000 is the F-net station. The other stations with the names of XXX0XX are the K-NET stations. The white star is the main shock epicenter. The epicenters of the aftershocks occurring during the 24 hours after the main shock are shown with solid circles. network, in addition to the main purpose of investigating the rupture process of the 2001 Geiyo earthquake. 2. Data Used for the Waveform Inversion [4] For the waveform inversion, the strong ground motion data at the 14 stations (seven KiK-net stations, six K-NET stations, and one F-net station) surrounding the source area are used (Figure 1). These stations are selected with the condition that one-dimensional structure model can reproduce path and site effect well. Details of the onedimensional structure models will be described in section 3. Though there are some other stations located closer to the source area than the selected stations, they are not used for the waveform inversion because of strong site effects, for which one-dimensional structure models no longer seem appropriate. The station distribution of the selected fourteen stations lacks stations located to the southwest of the source area. These stations are not selected for the same reason. [5] The observed acceleration waveforms were numerically integrated into velocity waveforms except for F-net station TGW000, whose data is originally velocity waveform. The velocity waveforms were band-pass filtered from 0.1 to 0.5 Hz and resampled with a sampling interval of 0.1 s. The S wave part with a time length of 16 s of the three components at each station is used for the inversion Structure Model of the Philippine Sea Slab [7] Miyoshi and Ishibashi [2004] obtained the depth contours of the top surface of the Philippine Sea slab from a detailed analysis of seismicity using the JMA unified hypocenter catalogue. According to their results, the dip of the Philippine Sea slab is close to horizontal in the source area of the 2001 Geiyo earthquake. Therefore the assumption of using one-dimensional structures for the calculation of Green s functions is reasonable. Their results show the depth of the slab surface in this area to be 40 km. Ohkura and Seno [2002] showed the existence of a low-velocity oceanic crust overlying a high-velocity oceanic mantle in the source area of the 2001 Geiyo earthquake from the investigation of the head waves on the observed aftershock waveforms. If the aftershock hypocenter is located in the oceanic mantle of the slab, the ray down going from the source and propagating along the top surface of the highvelocity oceanic mantle can be observed as head wave before direct P wave (Figure 2, left). Therefore the existence of head wave arrival before direct P wave can be used as an index to judge whether that aftershock occurred in the slab oceanic crust or in the slab oceanic mantle. It should be noted that the head waves are observed at the stations that are located in Shikoku (located to the southeast of the aftershock region) and have epicentral distances larger than 80 km according to Ohkura and Seno [2002]. These epicentral distances are much larger than those of the stations used for inversion in this study, which are within 60 km. From the investigation of head waves, Ohkura and Seno [2002] identified the aftershocks occurring in the oceanic crust and the ones occurring in the oceanic mantle. Figure 2 (right) shows their results in depth sections, where hypocenter depths are based on the JMA unified catalogue. The aftershocks with head waves are located shallower than the ones without head waves, and there is a boundary at 46 km depth between them. This means that the depth of the slab oceanic crust-mantle boundary is 46 km. Ohkura and Seno [2002] reported that the main shock hypocenter was located in the oceanic mantle since no head wave was observed. This agrees with the main shock hypocenter depth being at km depth and slightly deeper than the oceanic crust-mantle boundary. Referring to these results, I adopt 40 km as the depth of slab surface and 46 km as the depth of the slab oceanic crust-mantle boundary. [8] For the seismic velocities and Q values, I refer to Ohkura [2000] and Koketsu and Furumura [2002]. Shibutani [2001] estimated the velocity structure beneath the eastern Shikoku Island using the receiver function method. Takahashi et al. [2002] estimated the velocity structure beneath the western end of Nankai trough on the basis of the seismic experiment using air guns and ocean bottom seismometers. Though their target areas are different from the target of this study, both of their velocity structure 2of14

3 Figure 2. (left) Schematic illustration of the head wave from an event in the slab oceanic crust (gray star). The seismic ray from an event in the slab oceanic mantle (white star) is also shown. (right) The existence/nonexistence of head waves [after Ohkura and Seno, 2002] and the depth sections of the aftershocks along N S and E W directions. The focal depths are after the JMA unified catalogue. The depth boundary of the existence/nonexistence of head waves corresponds to the slab oceanic crustmantle boundary. models have two layers in the oceanic crust of the Philippine Sea slab. The oceanic crust model used in this study is divided into two layers considering it. Table 1 shows the slab structure model that is constructed in this manner Structure Model of Crust [9] The structure model of the crust used in this study is shown in Table 1. It is based on the work by Asano et al. [1986] except for the shallow part. The layer top depths of V p = 5500 m/s and V p = 6100 m/s are assumed to be variable depending on the stations in order to add surface layers. In the structure model constructed in this study, the continental lower crust is in contact with the slab oceanic crust. The seismic velocities of the continental lower crust are very close to those of the slab oceanic crust. Therefore the constructed structure model is, so to speak, just like a model made by modifying an ordinary inland crust model by moving Moho deeper to the depth of the oceanic crustmantle boundary. [10] Koketsu and Furumura [2002] reported that the low Q values of the mantle wedge, which exists to the northwest and west of the source region of the 2001 Geiyo earthquake, brought significant attenuation of the high-frequency waves observed at the northwest and west strong motion stations. However, as Koketsu and Furumura [2002] have shown, this attenuation was seen in the frequency range of 3 5 Hz. Since the frequency range of the waveforms used in the inversion is Hz, the data set used in this study is free from this significant attenuation Structure Model of Surface Layers [11] Generally, the geological conditions of surface layers are very different depending on the station locations. Therefore different one-dimensional structure models of surface layers are constructed for different stations in this study. An aftershock whose source is regarded as a point source is selected, and the observed waveform at each station is used as fitting target of the structure model search. The preferred one-dimensional structure model of shallow Table 1. One-Dimensional Underground Structure Model of the Slab and the Crust Layer Layer Top Depth, m V P, m/s V S, m/s Density, kg/m 3 Q P Q S Continental Plate Upper crust Upper crust Lower crust 16, Slab Oceanic crust 1 40, Oceanic crust 2 42, Oceanic mantle 46, of14

4 Table 2. One-Dimensional Structure Models of the Surface Layers of All the Stations Layer Top Depth, m V P,m/s V S,m/s Density, kg/m 3 Q P Q S HRS017 (K-NET, LD = 10 m a ) HRSH08 (KiK-net, LD = 103 m) HRSH02 (KiK-net, LD = 103 m) HRSH12 (KiK-net, LD = 153 m) HRS012 (K-NET, LD = 11 m) HRS008 (K-NET, LD = 11 m) HRSH04 (KiK-net, LD = 203 m) YMGH04 (KiK-net, LD = 105 m) YMGH03 (KiK-net, LD = 203 m) Table 2. (continued) Layer Top Depth, m V P,m/s V S,m/s Density, kg/m 3 Q P Q S YMG019 (K-NET, LD = 10 m) EHM006 (K-NET, LD = 10 m) EHM007 (K-NET, LD = 20 m) TGW000 (F-net, no logging data) KOCH02 (KiK-net, LD = 103 m) a LD, depth of logging data. layers is determined by trial and error so that the theoretical waveform fit the observed at each station. [12] The aftershock used for the modeling of surface layers is the M w 5.2 event which occurred at 2040 UT on 25 March 2001 close to the rupture starting point of the main shock. In calculating the theoretical waveforms, the focal mechanism and seismic moment of NIED F-net solution is used: (strike, dip, rake) = (181, 62, 77 ) and M 0 = N m. The velocity waveforms in the same frequency band as for the waveform inversion are used for the waveform fitting. [13] From the forward modeling of the aftershock waveforms by trial and error, following two kinds of layer parameters were determined: (1) layer top depths of V p = 5500 m/s and V p = 6100 m/s layers and (2) number of layers, layer top depths, P and S wave velocities, and density of the layers shallower than V p = 500 m/s layer. Q values were given based on the experience. KiK-net stations used in this study have logging data of about 100 m or 200 m depth. The logging data include P wave velocity, S wave velocity, and density, and they provide very important information in structure modeling. These logging data 4of14

5 Figure 3. (left) Theoretical waveform of the M w 5.2 aftershock (25 March, 2040 UT) at EHM006 in case of a structure model without soft surface layers shown with a solid line. The observed waveform is shown with a dotted line for comparison. The waveforms are band-pass filtered between 0.1 and 0.5 Hz. (right) Same as Figure 3 (left) but in case of the structure model with soft surface layers shown in Table 2. were basically used for the parameters of the shallowest layers from the surface to the logging data depth. Though their depths are not deep enough to provide the layer parameters from a few hundred meters to a few kilometers depth, which are also important for waveform modeling, the significance of KiK-net logging data in the structure modeling should be emphasized. In case of K-NET stations, the depths of the logging data are only about 10 or 20 m. Therefore the logging data do not necessarily provide information available for the waveform modeling. However, in some cases, the shallowest layer with the parameters of the logging data was preferred in the structure model search. Since the F-net station has no logging data, no constraining information was available in the model search. However, F-net stations are usually located on the sites with geologically good conditions. Actually, the preferred structure model for TGW000 has relatively hard surface layers. The depths of the logging data of all the stations are shown in Table 2. [14] Table 2 shows the obtained structure models of surface layers at all the stations. Soft surface layers are necessary at many stations. While the stations in Hiroshima Prefecture (HRS stations), located in the north of the source area, have relatively hard surface layers, the stations in Ehime and Kochi Prefectures (EHM and KOC stations), located in the south of the source area, have relatively soft surface layers. Figure 3 shows the difference between the theoretical waveforms based on the structure models without or with soft surface layers for EHM006. In the structure model without soft surface layers, V p = 5500 m/s layer is outcropping on the surface with other deeper layers being the same as the ones in Table 2. These theoretical waveforms are for the aftershock mentioned above, and the observed waveform is also shown for comparison. The observed waveform has a large amplitude and the S wave part has a long-duration coda, indicating reverberation within the soft surface layers. These are the typical features of the waveforms observed at the stations with soft sedimentary layers. In case of the structure model without soft surface layers, the theoretical waveform shows a smaller amplitude and too simple and short S wave part lacking long-duration coda. In contrast, the synthetic waveform for the structure model with soft surface layers reproduces the S wave part with a large amplitude and a longduration coda well, and the waveform fitting is greatly improved. This comparison shows the significant effect of soft surface layers on waveform. Though E W component of the theoretical waveform matches the observed relatively well, phase shifts are seen between the observed and synthetic waveforms on the other two components, especially on U-D component. Generally, in the waveform simulation for structure model tuning, in-plane waves (radial and vertical components) are more difficult to match than the antiplane waves (transverse component), because in-plane waves are affected not only by S wave velocities but also by P wave velocities. Since EHM006 is located nearly to the south of the source area, E W and N S components roughly correspond to transverse and radial components, respectively. Therefore, while E W component is mainly composed of antiplane waves, N S and U-D components are mainly composed of in-plane waves. Probably this is one of the reasons for the worse waveform fitting on the N S and U-D components. Additionally, the actual underground structure has more or less two-dimensional or three-dimensional heterogeneity. The deviation of the actual structure from one-dimensional model can be another reason. Nevertheless, the structure model listed in Table 2 is much preferable than the model with V p = 5500 m/s layer outcropping on the surface, which is usually used in the hypocenter determination, since the waveform fitting of the aftershock has been greatly improved by introducing the soft surface layers. [15] If the structure models neglecting soft surface layers are used for the stations that actually have soft surface layers in the source inversion, the amplification brought by the site effect will be attributed to the source factor. In that case, the source factor may be overestimated. From this point of view, in section 7.3, I compare the inversion solutions for two cases: one is the case using the structure models with soft surface layers in Table 2 and the other is the case using the model neglecting soft surface layers with V p = 5500 m/s layer being the surface layer. 4. Fault Model for the Waveform Inversion [16] In this section, a curved fault model, which will be used for the waveform inversion, is constructed based on the aftershock distribution and the preliminary moment tensor analysis. The aftershock distribution is based on the JMA unified catalogue. This data set is estimated to be reliable, since the aftershock hypocenters of this catalogue show almost no difference from those relocated using double difference method [Waldhauser and Ellsworth, 2000] even though the aftershock region is relatively deep (T. Miyoshi, personal communication, 2001). Figure 4 shows the aftershock distribution during the 24 hours after the main shock and the constructed curved fault model. In the northern part of the source area, aftershock distribution shows a westward dipping distribution with a strike of a 170 and a dip of 60 (Figure 4a). This agrees well with the P wave polarity mechanism of the main shock (strike 169, dip59 ) in the JMA unified catalogue. On the other 5of14

6 Figure 4. Curved fault model composed of four planar segments assumed for the waveform inversion. (a) Surface projection of the curved fault model, subfaults, and epicenters of the aftershocks (solid circles) that occurred during the 24 hours after the main shock. The white star is the rupture starting point (main shock hypocenter). The subfault centers are shown with small gray circles. Depth sections along the strike of segment 1 (170 ) are also shown. In the depth sections, only the aftershock hypocenters and the subfault centers inside the gray dotted rectangle are shown. (b) Same as Figure 4a, but the depth sections along the strike of segment 4 (190 ) are shown. In the depth sections, only the aftershock hypocenters and the subfault centers inside the gray dotted rectangle are shown. hand, the aftershocks in the southern area show more scattered distribution. In the scattered distribution, there is a sharp vertical planar cluster of aftershocks with 170 strike and 90 dip (shown clearly later in Figure 12). The aftershocks with this sharp cluster excluded show less scatter and are well described with the fault plane with a 190 strike and 70 dip. The depth sections of the southern aftershocks along the strike of 190 is shown in Figure 4b, which also includes the sharp vertical cluster. Figure 5 shows the P wave polarity mechanisms of the main shock and large aftershocks. While the northern two events, one of which is the initial rupture of the main shock, show almost pure normal fault type mechanisms, many events show strike-slip-like mechanisms in the southern part. Thus both the aftershock distribution and the aftershock mechanisms suggest that the strike and dip angles differ between the northern and southern parts of the fault. Therefore the fault cannot be described as a single plane for this earthquake. [17] Then I performed a moment tensor inversion using low-frequency ( Hz) waveforms at the near-source stations assuming two point sources, one of which is located in the northern part of the source area and the other is located in the southern part. The obtained mechanisms (strike, dip, rake) of the northern and southern point sources are (159, 65, 112 ) and (200, 74, 41 ), respectively. This result suggests a mechanism change during the rupture process. While the northern part shows SSE striking, the southern part shows SSW striking. The dip angle is larger and the rake angle has a larger horizontal component in the southern area. This means that the northern part had a normal fault type mechanism but the southern part had a mechanism close to a strike-slip type. The mechanism change during the main shock agrees well with the aftershock distribution and the aftershock mechanisms mentioned above. [18] On the basis of these analyses, a curved fault model with the strike changing from 170 to 190 and the dip changing from 60 to 70 is constructed (Figure 4). This model is composed of four planar segments with gradually changing strike and dip angles to simulate a continuous curved fault surface. Hereinafter the four segments are called segments 1, 2, 3, and 4 from north to south. The lengths (along-strike direction) of segments 1, 2, 3, and 4 are 15, 3, 3, and 9 km, respectively. The widths (along-dip 6of14

7 [19] This curved fault model is constructed by neglecting the sharp vertical aftershock cluster in the southern area mentioned above. Later in the discussion, waveform inversion will be performed assuming another fault model that has the southern half plane corresponding to this sharp vertical aftershock cluster. Figure 5. Focal mechanisms of the main shock and large aftershocks (M JMA 3.9) determined from P wave polarities (after the JMA catalogue). For the 25 March, 1716 UT, aftershock, NIED F-net mechanism solution is shown since P wave polarity mechanism is not determined for this event. direction) are all 18 km. The strike angles of the four segments are 170, 177, 183, and 190, and the dip angles are 60, 63, 67, and 70, respectively. As seen in Figure 4, most of the aftershocks are concentrated in the narrow depth range, which is 6 km width from 42 to 48 km in depth. However, there are some aftershocks at the shallower and deeper depths, though their number is small. The constructed fault model covers these shallow and deep aftershocks, too. Note that the depths of the upper edges of the fault segments are taken to be almost the same as that of the slab surface (40 km), since it is rather unlikely that the fault would extend beyond the slab surface. According to Miyoshi and Ishibashi [2004], neither the background seismicity nor the aftershock activity is seen at the depths shallower than 40 km. This indicates that the medium shallower than 40 km is completely nonseismogenic. This also supports that the fault top depth is taken to be 40 km. Each fault segment is divided into 3.0 km 3.0 km subfaults for waveform inversion. The number of subfaults along strike for the fault segments 1, 2, 3, and 4 is 5, 1, 1, and 3, respectively. The number of the subfaults along-dip direction is 6 for all the four segments. The total number of subfaults is ( ) 6 = 60. The rupture starting point is the (i x, i y ) = (2, 3) subfault, where i x and i y are the subfault number along strike and dip, respectively. The rupture starting point location is ( , , km), which is the hypocenter location of the main shock after the JMA unified catalogue. The i y = 3 subfaults of all the segments have a common depth of km and are in contact with the neighboring segment subfault. 5. Inversion Method and Related Parameters [20] The waveform inversion is done using the multiple time window analysis [Hartzell and Heaton, 1983]. The size and number of subfaults into which the fault surface is divided for inversion have been explained in section 4. The number of time windows is six. Each time window has a duration of 1.0 s, and one time window is put after the previous one with a time lag of 0.5 s. These time windows allow a maximum rupture duration of 3.5 s. The model parameters of the waveform inversion are the weights of the two orthogonal slip vectors given in each time window in each subfault. By giving a nonnegative constraint [Lawson and Hanson, 1974] to these weights, the rake angle of the slip vector of each subfault is allowed to vary within the central angle ±45. On the basis of the result of the preliminary analysis that the rake angle is close to pure dip in the northern part of the fault but has a large horizontal component in the southern part, 70, 60, 60, and 50 are used as the rake central angles for the fault segments 1, 2, 3, and 4, respectively. [21] The discretized observation equation of the waveform fitting is Gm ¼ d; where G is the differential kernel matrix, m is the model parameter vector, and d is the data vector. The waveform amplitudes are normalized with the maximum amplitude of the observed waveform at each station. A spatial and temporal smoothing constraint is also given to the model parameters by adding a minimum spatiotemporal Laplacian 2 m ¼ 0 in the inversion formula, where x is along-strike direction and y is along-dip direction. Then, the least squares problem to be solved becomes ð1þ ð2þ G Am A; ð3þ ls 0 where S is the discretized Laplacian and l represents the relative weight of the smoothing constraint. The value of l was determined to be 1.7 by the minimum Akaike s Bayesian information criterion (ABIC) condition following Sekiguchi et al. [2000]. [22] Five values of 3.00, 3.23, 3.47, 3.70, and 3.93 km/s were tested as the propagation velocity of the first time window. They correspond to 65, 70, 75, 80, and 85% of the S wave velocity of the slab oceanic mantle, respectively. 7of14

8 Figure 6. Comparison between the observed waveforms (solid line) and the synthetic waveforms obtained from the inversion (gray line). The numbers beside the station names are the maximum amplitudes of the observed waveforms. I adopted 3.47 km/s which gave the minimum residual, and the result of this case is described in detail in section Inversion Results [23] Figure 6 shows the comparison between the observed waveforms and the synthetic waveforms obtained from the inversion. The variance reduction is 72% and the waveform fitting is quite well. Figure 7 shows the moment release on the fault surface obtained from the inversion. The vertical axis of Figure 7 is depth, not down dip distance, because the four fault segments have different dip angles. Figure 8 shows the slip distribution on the fault. Note that the slip distribution is different from the moment release Figure 7. Moment release distribution and slip vectors (motion of the hanging wall relative to the footwall) on the fault obtained from the inversion. The vertical axis is the depth. The star is the rupture starting point (main shock hypocenter). 8of14

9 Figure 8. Slip distribution and slip vectors (motion of the hanging wall relative to the footwall) on the fault obtained from the inversion. The slab surface and the slab oceanic crust-mantle boundary are also shown with thick gray lines. The star is the rupture starting point (main shock hypocenter). distribution, since the rigidity varies with depth in the assumed slab structure model. The result shows that the rupture area of the 2001 Geiyo earthquake extends over both of the slab oceanic crust and the slab oceanic mantle of the slab. Large slip is seen in the shallowest part (in the oceanic crust) and in the deepest part (in the oceanic mantle) of the southern part of the fault surface, and the largest slip is seen in the oceanic crust. Relatively large slip is also seen at the rupture starting point in the northern part and deep in the central part of the fault. The estimated total seismic moment is Nm(M w = 6.8) and the maximum slip is 2.4 m. In Figure 8, the slip vectors are also shown. While the slip vectors show almost pure dip slip in the northern part of the fault, the slip vectors become closer to horizontal in the southern deep part. Remember that the dip angle becomes steeper in the southern part of the fault. This means that the rupture started in the northern part with a normal fault character but gradually changed to more of a strike-slip type when propagating to the south. The source process of the 2001 Geiyo earthquake can be concluded to be very complex from various viewpoints such as slip distribution heterogeneity, change of fault orientation, and change of slip type (from normal fault to strike slip) during the rupture process. [24] Figure 9 shows the snapshots of the rupture propagation on the fault. The total duration of the rupture process is 10 s. Complexity of the source process is clearly seen in these snapshots. Globally, the rupture propagated almost unilaterally from north to south. The large percentage of seismic waves was radiated in the later stage of rupture process, namely, from 4.5 s to 7.5 s of the snapshots. Figure 10 shows the source time function in each subfault on the fault surface. Source time function durations vary between 2 and 3.5 s. The largest slip velocity is seen in the southern shallowest subfault, which is the neighboring subfault of the largest slip subfault. The shapes of the source time functions are relatively complex and it is in accordance with the complexity of the spatial distribution of slip. [25] In order to see the contributions of slip of different areas on the fault surface to the waveforms, four areas with large slip are selected as shown in Figure 11. In Figure 11, the observed and synthetic waveforms of the N S compo- Figure 9. Snapshots of the rupture propagation on the fault. Slip for every 1.5 s is shown in each panel. Total slip distribution is also shown. The star is the rupture starting point. Though the fault surface is composed of four segments with different strike and dip angles, it is shown as one rectangle for simplicity. The segment boundaries are denoted with dotted lines. The vertical axis is up dip distance in km. 9of14

10 Figure 10. Source time function in each subfault obtained from the inversion. The star shows the subfault corresponding to the rupture starting point. Though the fault surface is composed of four segments with different strike and dip angles, it is shown as one rectangle for simplicity. nent at three stations, HRS008, YMGH04, and EHM006, are shown. The synthetic waveforms are for the contribution from each of the four areas with large slip. These three stations are located to the north, to the west, and to the south of the source area, respectively. Since the rupture propagated from north to south, backward and forward directivity effects are expected for HRS008 (north) and EHM006 (south). Actually, at the backward station HRS008, the waves coming from slip area 1 and those from area 4 arrive separately with a large time lag. On the other hand, in the case of the forward station EHM006, the waves from the four areas all arrive in a short time window. At HRS008, the maximum Figure 11. Four areas with large slip on the fault surface and the waveforms at three stations HRS008 (north), YMGH04 (west), and EHM006 (south). The solid lines show the N S component synthetic waveforms corresponding to the contribution of each of these four areas. The observed waveforms are also shown with dotted lines for comparison. 10 of 14

11 Figure 12. Waveform inversion assuming another fault model is tried. This fault model has a vertical southern segment that corresponds to the vertical aftershock cluster. The fault model, subfaults, and epicenters of the aftershocks occurring during the 24 hours after the main shock are shown. In the depth sections, only the aftershock hypocenters and the subfault centers inside the gray dotted rectangle are shown. Since the southern vertical segment is purely vertical, it is shown with a solid line in the map view. amplitude is seen in the later part of S wave, and it is caused by the waves from slip areas 2 and 3. At EHM006, the maximum amplitude is seen at the initial part of S wave. It is caused by the almost simultaneous arrivals of the waves from slip areas 1, 2, and 3. The large pulse with a duration of 5 s observed at YMGH04 is caused by the sum of contributions from slip areas 2, 3, and Discussion 7.1. Slab Structure and the Rupture Process [26] In Figure 8 the slab surface and the slab oceanic crust-mantle boundary in this area are also shown. The derived slip distribution on the fault shows that the largest slip is seen in the slab oceanic crust. This is a very interesting result from the viewpoint of earthquake rupture dynamics. Since the oceanic crust has a low seismic wave velocity, i.e., is easier to deform elastically, it is very reasonable that the large slip occurs there once the rupture propagates into it. Mikumo et al. [1987] performed a numerical simulation of dynamic rupture process of inland crustal earthquake in the horizontally layered medium using finite difference method. Their simulation showed that a large slip occurs when the rupture goes into the low-velocity surface layer. The large slip in the oceanic crust of the 2001 Geiyo earthquake may be comprehended by referring to their simulation result. That is, the rupture process of the 2001 Geiyo earthquake is estimated to have been strongly influenced by the heterogeneity of the medium where it occurred. The large slip in the oceanic crust is interpreted to have occurred because of the low rigidity of the oceanic crust. [27] What should be emphasized here is that the velocity contrast between the oceanic crust and the oceanic mantle is remarkably large. Actually, it is so large that the head wave along the boundary ( Moho in the slab) can be observed, as explained in section 3.1. Such large velocity contrast or rigidity contrast is rarely seen in other tectonic environments for large earthquakes such as inland crust and subduction plate boundaries. Slab earthquakes that occur in areas of a large velocity contrast will be good targets for the study of the influence of medium heterogeneity on the rupture complexity. Of course, there are various controlling factors for earthquake rupture other than rigidity of medium, such as regional stress, strength excess, stress drop, and critical slip. However, in case that the rigidity contrast is remarkably large as in this case, the rigidity of medium may be a dominant controlling factor. [28] Though the oceanic crust is relatively soft, it is stiff enough to be seismogenic. Ohkura and Seno [2002] reported that the aftershock activity of the 2001 Geiyo earthquake is seen in the oceanic crust. Miyoshi and Ishibashi [2004] reported that the background seismicity is also seen in the oceanic crust in this area. Therefore it was very natural that the rupture of the 2001 Geiyo earthquake, which started just below the oceanic crust-mantle boundary, propagated into the seismogenic oceanic crust. [29] It should be noted that the various conditions are different between the shallow inland crustal earthquake and the deep slab earthquake. For example, the effect of free surface can be an important controlling factor in the rupture of crustal earthquake. In case of slab earthquake, free surface has almost no effect, since source fault is located very deep away from the surface. As is known well, very shallow part of the crust is often nonseismogenic. In contrast, as far as the source area of the 2001 Geiyo earthquake is concerned, the slab oceanic crust is seismogenic as mentioned above. Therefore, in a strict sense, the simulation result by Mikumo et al. [1987] may not be applied directly in interpreting the rupture dynamics of the 2001 Geiyo slab earthquake, considering these differences. In order to confirm the influence of slab structure heterogeneity on rupture process of slab earthquake, it will be inevitable to perform a numerical simulation of rupture propagation in a realistic structure model of slab environments. [30] Since the source area is relatively deep, there may be some ambiguity in the depth information. Therefore it is better to give a comment on the spatial relation between the slab structure and the fault model assumed for the inversion. As mentioned in section 3.1, the slab surface depth and the slab oceanic crust-mantle boundary depth are both based on the hypocentral data of the JMA unified catalogue. The fault 11 of 14

12 Figure 13. Comparison between the observed waveforms (solid line) and the synthetic waveforms obtained from the inversion (gray line) based on the fault model with a vertical southern segment. The waveform fitting is clearly worse compared with the case of the curved fault model. model assumed for the waveform inversion is constructed based on the aftershock data of the same catalogue. Consequently, the comparison of the slab structure with the slip distribution on the fault is valid, at least for the relative spatial relation Inversion Assuming a Fault Model With a Vertical Southern Fault Segment [31] As mentioned in section 4, the curved fault model used in the inversion was constructed by neglecting the sharp vertical aftershock cluster in the southern area. Here, waveform inversion is performed assuming another fault model that has a vertical plane for the southern part corresponding to this cluster. The assumed fault model is shown in Figure 12. Since the strike of the vertical aftershock cluster is 170, which is the same as the strike of the northern segment of the fault, the strike and dip angles are taken to be 170 and 90 for the vertical southern fault segment. The length and width of the vertical segment are 15 and 18 km. The northern segment is the same as segment 1 of the former curved fault model. Figure 13 shows the result of the waveform inversion based on this fault model. The waveform fitting is much worse (variance reduction is 52%) and it is clear that the fault model with a vertical southern segment cannot reproduce the observed waveforms. This result shows the curved fault model is preferred. [32] Though the vertical plane is shown to be inappropriate for the southern main fault, there remains another possibility that fault branching corresponding to the vertical aftershock cluster occurred during the rupture process. It is difficult, however, to examine the possibility of fault branching based on the data set used in this study. As already shown, the fault model without a branch can reproduce the observed waveforms very well. An additional fault branch corresponding to the vertical cluster is not necessary to explain the observed waveforms. However, this does not mean that branching did not occur in the actual rupture of the 2001 Geiyo earthquake. It just means that the data set used in this study does not have enough resolution to tell whether branching occurred or not. It should be noted that the possibility of fault branching in this earthquake remains an important problem to be solved in the future Importance of Modeling of Soft Surface Layers in Source Inversion [33] As mentioned in section 3.3, the observed aftershock waveforms and the logging data clearly indicate that the surface layers at some stations are soft sedimentary layers. In order to check the effect of soft surface layers on source inversion, a waveform inversion is performed using the Green s functions calculated for the structure model with surface layer harder than those shown in Table 2. In the tested case, the structure models with V p = 5500 m/s layer outcropping on the surface are used at all the stations. The deeper layers are the same as in the previous structure models. In this case, the estimated seismic moment and the maximum slip are N m and 3.2 m. These values are 14% and 33% overestimated, respectively, compared 12 of 14

13 with the values of N m and 2.4 m for the models with soft surface layers. Thus, if structure models with hard surface layers are used for the stations that actually have soft surface layers, the source factor is overestimated by attributing the amplification effect by the soft surface layers to the source factor. This shows the importance of modeling of shallow structure in the waveform inversion of source process. Calibration of velocity structure model using actual waveform data such as aftershock records, which approach was adopted in this study, is effective for it Contribution to Strong Ground Motion Study [34] Modeling of heterogeneous source process is one of the important issues for the quantitative strong ground motion prediction for scenario earthquakes. From this viewpoint, Somerville et al. [1999] characterized the heterogeneous slip models of 15 crustal earthquakes and obtained scaling relations between various source parameters and seismic moment. Irikura and Miyake [2001] proposed a procedure to predict the strong ground motions from scenario earthquakes (the procedure is referred to as recipe in their manuscript), which includes the procedure to characterize heterogeneous source models. As was shown by the 2001 Geiyo earthquake, slab earthquake can be damaging and should be included in the targets of strong ground motion prediction. Therefore characters of source heterogeneity of slab earthquakes should be investigated in detail and systematically, as was done by Somerville et al. [1999] for crustal earthquakes. [35] Takeo et al. [1993] investigated the source process of the 1993 Kushiro-Oki, Japan, earthquake (M w = 7.6), which was a slab earthquake in the subducting Pacific plate, from the waveform inversion. They obtained a slip model with large slip concentrated in the center of the fault plane. Kakehi and Irikura [1996] performed an envelope inversion of acceleration waveforms for this earthquake and found the high-frequency (2 10 Hz) waves had been mainly radiated from the periphery of the fault plane. These results suggest that the 1993 Kushiro-Oki slab earthquake had a relatively simple, crack-like source model. On the other hand, the inversion result in this study clearly shows that the source model of the 2001 Geiyo earthquake is heterogeneous. Since the number of the studies on the detailed rupture process of slab earthquakes is still very small at present, we have to say that our knowledge on the degree of heterogeneity of the slab earthquake source is very limited. It will be inevitable to increase the number of analyses of slab earthquakes in order to construct a reliable scaling relation on the source heterogeneity in the future. At the same time, theoretical approach to the rupture dynamics of slab earthquake will be also important to give physically reasonable constraints to scenarios. For example, a source model with large slip in the seismogenic low-rigidity slab oceanic crust will be a candidate of reasonable source model, if the relation between the slab structure heterogeneity and rupture process suggested by this study is valid. 8. Conclusions [36] From the waveform inversion of near-source strong ground motions recorded by the high-density observation network, the detailed rupture process of the 2001 Geiyo earthquake has been investigated. The rupture changed gradually from a normal fault type into a more strike-slip type during its propagation. The slip distribution is heterogeneous and the rupture area extends over both of the slab oceanic crust and the oceanic mantle. The largest slip seen in the slab oceanic crust, which has a remarkably lower rigidity than that of the slab oceanic mantle, suggests that the heterogeneity of the background medium had a strong influence on the rupture process. Importance modeling of soft surface layers in source inversion has been also demonstrated. For the quantitative strong ground motion prediction of future slab earthquakes, it is important to increase the number of rupture process analyses and accumulate knowledge on the source characters of slab earthquakes. [37] Acknowledgments. The author is very grateful to the following organizations and people. The strong ground motion data recorded by K-NET, KiK-net, and F-net of National Institute of Earth Science and Disaster Prevention and the hypocenter data of the unified hypocenter catalogue by the Japan Meteorological Agency were used for the analysis. Takahiro Ohkura kindly gave me the information of the head waves of the aftershocks. Haruko Sekiguchi kindly gave me useful information of the velocity structure models. The program for calculating the Green s function was originally coded by Olivier Coutant. Discussions with K. Ishibashi and the students in the seismological laboratory of Kobe University were helpful throughout this study. The valuable and helpful comments from the Associate Editor Kelin Wang and the two anonymous reviewers greatly improved the manuscript. GMT [Wessel and Smith, 1995] was used for drawing the figures. This study was partly supported by Grant-In-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan, and by the Special Project for Earthquake Disaster Mitigation in Urban Areas, which is promoted by the Ministry of Education, Culture, Sports, Science, and Technology, Japan. References Aoi, S., K. Obara, S. Hori, K. Kasahara, and Y. Okada (2000), New strongmotion observation network: KiK-net, Eos Trans. AGU, 81(48), Fall Meet. Suppl., Abstract S71A-05. Asano, S., K. Miura, Y. Inoue, R. Miura, Y. Ishiketa, and T. Yoshii (1986), Recent seismic activity in Chugoku district and its vicinity, western Japan as revealed by the telemetering network of Shiraki micro-earthquake observatory (in Japanese with English abstract), J. Seismol. Soc. Jpn., 39, Bouchon, M. (1981), A simple method to calculate Green s function for elastic layered media, Bull. Seismol. Soc. Am., 71, Fire and Disaster Management Agency (2002), The 2001 Geiyo earthquake [in Japanese], final report, Tokyo. Fukuyama, E., M. Ishida, S. Hori, S. Sekiguchi, and S. Watada (1996), Broadband seismic observation conducted under the FREESIA Project, Rep. Natl. Res. Inst. Earth Sci. Disaster Prev., 57, Fukuyama, E., M. Ishida, D. S. Dreger, and H. Kawai (1998), Automated seismic moment tensor determination by using on-line broadband seismic waveforms (in Japanese with English abstract), J. Seismol. Soc. Jpn., 51, Hartzell, S. H., and T. Heaton (1983), Inversion of strong ground motion and teleseismic waveform data for the fault rupture history of the 1979 Imperial Valley, California, earthquake, Bull.Seismol.Soc.Am., 73, Irikura, K., and H. Miyake (2001), Prediction of strong ground motions for scenario earthquakes (in Japanese with English abstract), J. Geogr., 110, Kakehi, Y., and K. Irikura (1996), Estimation of high-frequency wave radiation areas on the fault plane by the envelope inversion of acceleration seismograms, Geophys. J. Int., 125, Kennett, B. L., and N. J. Kerry (1979), Seismic waves in a stratified half space, Geophys. J. R. Astron. Soc., 57, Kinoshita, S. (1998), Kyoshin Net (K-NET), Seismol. Res. Lett., 69, Koketsu, K., and T. Furumura (2002), The distribution of strong ground motion from the 2001 Geiyo earthquake and the deep underground structure (in Japanese with English abstract), J. Seismol. Soc. Jpn., 55, Lawson, C. L., and R. J. Hanson (1974), Solving Least Squares Problems, 340 pp., Prentice-Hall, Old Tappan, N. J. 13 of 14

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