Moment Tensor Determination of the September 2, 2009 Tasikmalaya, West Java Earthquake Using the Waveform Inversion Method of Near Field Data* (T2-P45) I. Suardi 1, Y. Yagi 2, Afnimar 3, S. Widiyantoro 3 INTRODUCTION 1 Faculty of Earth Sciences and Technology, Institut Teknologi Bandung, Bandung, Indonesia On Wednesday, September 2, 2009, a strong earthquake struck the southern coast of West Java, i.e. about 142 km 2 southwest of the Tasikmalaya District (Figure 1). Earthquake parameters released by the MCGA are as follows: origin time Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Japan O O 3 = 02/09/2009 07:55:00 UTC; epicenter = 8.24 S, 107.32 E; depth = 30 km; and moment magnitude M W = 7.3. Faculty of Mining and Petroleum Engineering, Institut Teknologi Bandung, Bandung, Indonesia DATA We used 18 components of the near field data from 5 stations of the MCGA broadband seismometer network and 1 The source mechanism of the September 2, 2009 Tasikmalaya, West Java earthquake is not consistent with the station of the Incorporated Research Institution for Seismology of the Data Management Center (IRIS-DMC) broadband Figure 6. Comparison of observed characteristic of the tectonic stress in this region in which the strike direction in general parallels to the present seismometer network (Figure 2). waveform (black curves) with day trench. In fact, the strike of the September 2, 2009 Tasikmalaya earthquake is nearly perpendicular to the To calculate the Green’s function, we used several velocity models, i.e. the Jeffreys -Bullen (Jeffreys and Bullen , 1940), theoretical waveform (gray curves). trench. We determined the moment tensor using the near field data from the regional network of the AK135 (Kennett et al., 1995), Koulakov et al. (Koulakov et al., 2007), and modified Koulakov et al. models (Laksono, 2011), Numbers below the station codes Meteorological, Climatological and Geophysical Agency (MCGA) of Indonesia, and from the IRIS-DMC seismic respectively (Table 2). depict the maximum amplitude of network. The frequency of band pass filter and the velocity structure model are determined by referring to For each pair of the range of band pass filters and the velocity structure model we applied a grid search method for the the observed waveform . previous study results, as well as by trial and error. The band pass filter and the velocity structure model that hypocenter depth determination. The range of centroid hypocenter depth used in the grid search started from a depth of produce the smallest variance of 0.2402 is 0.01 to 0.03 Hz and the Jeffreys-Bullen model, respectively. The 10 km to 100 km with an increment of 1 km. Green's functions were calculated using the extended reflectivity method for the near field data. Our inversion RESULTS AND DISCUSSION results show that the earthquake is an interplate earthquake type, which is located at the border around the Figure 4 and Table 3. The table shows the smallest variance of 0.2402, that was obtained by using the band pass filter with plate interface at a depth of 44 km. The strike is almost perpendicular to the trench, which may be related to a cut off frequencies of 0.01 Hz and 0.03 Hz (this study), and using the Jeffreys-Bullen velocity model. The source strong slab pull beneath the region. parameters of the September 2, 2009 Tasikmalaya earthquake are as follows: Moment = 0.2788x10 20 (Nm), Mw = 6.9 Depth = 44 km, variance = 0.2402.

Figure 1. Tectonic setting and seismicity around Java . Seismicity is marked with small gray circles. The yellow star depicts the MCGA Figure 3. Example of near field waveform data from epicenter of the September 2, 2009 event, and the aftershocks for the MCGA regional network used in the moment Figure 4. Results of the grid search process to determine the Table 4: Comparison of source mechanisms of the one week after the mainshock are depicted by red circles. The tensor inversion . best solution of focal mechanism as a function of ∆t and September 2, 2009 Tasikmalaya earthquake average rate of plate motion is marked by arrows (modified from variance for each velocity model (JB Model, AK135, Koulakov et Bock et al., 2003). al., and Modified Koulakov et al.) and the range of band pass filter as shown in Table 1. The solutions at ∆t = 1 second (the Figure 2. Configuration of lowest focal mechanism in each square) are the best and the MCGA broadband displayed in Table 3. seismic network. The yellow star depicts the CONCLUDING REMARKS epicenter of the September We have analyzed the moment tensor of the September 2, 2009 Tasikmalaya 2, 2009 Tasikmalaya earthquake by employing the waveform inversion method of near field data in Figure 5. Map view (upper panel) and earthquake. The red detail. The best result with the smallest variance of 0.2402 was obtained by using vertical cross section (lower panel) with the triangles and the blue the band pass filter with cut off frequencies of 0.01 Hz and 0.03 Hz, and the source mechanism of the September 2, triangle in the circle depict Jeffreys-Bullen velocity model. The focal mechanism resulting from this study is 2009 Tasikmalaya earthquake. The star and the MCGA near field in excellent agreement with those from the Global CMT, USGS and MCGA, as open circles, indicate the relocated stations and the IRIS -DMC shown in Table 4. The waveform fitting for the near field data used in this study mainshock hypocenter and the relocated station, respectively showed a good result. From the waveform data inversion, the centroid depth of aftershocks, respectively (Laksono, 2011). the September 2, 2009 Tasikmalaya earthquake is shallow i.e. 44 km. This Small gray circles indicate the seismicity suggests that the earthquake type was interplate. The strike that is almost relocated by PDE . The curved thick line perpendicular to the trench is interpreted to be related to a strong slab pull depicts the USGS global slab 1.0 model. beneath Java. Notice that the aftershocks distribution plotted in the lower panel, to some extent, depicts a splay fault .

*in press. Description: The values listed above are the cut off low frequency, the cut off high frequency, and the stop band frequency in Hz, respectively Table 3: The grid search results of source mechanisms