Joint inversion of teleseismic and InSAR datasets for the rupture process of the 2010 Yushu, China, earthquake
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2012
Received: 12 March 2012
Accepted: 16 April 2012
Published: 26 November 2012
We have examined the rupture process of the 2010 Yushu, China, earthquake, which occurred in the eastern Tibetan plateau. Based on the results of centroid location grid searches, the aftershock distribution, and InSAR observations, we first constructed a left-lateral strike-slip bending fault model. We then carried out joint inversions of the teleseismic waveform data and InSAR data using various rupture front velocities. We obtained the optimum source model with a seismic moment of 2.3 × 1019 N m (Mw ∼ 6.8), where the rupture velocity was found to be slightly faster than the shear wave velocity of the crust in this region. This supershear rupture velocity resulted in severe damage in and around the county seat of Yushu, which is located in the forward rupture direction of the source model. The features of this event, such as the shallow and fast rupture propagation may represent a seismic feature of the tectonic activity in the eastern Tibetan plateau.
The 2010 Yushu, China, earthquake occurred at 7:49 a.m. on April 14 local time (11:49 p.m. on April 13 UT), 2010, in Yushu County, Yushu Tibetan Autonomous Prefecture, Qinghai Province, China, which is located in the eastern Tibetan plateau (ETP). The Global CMT Project (available at http://www.globalcmt.org/) reported the moment magnitude (Mw) and centroid depth to be 6.9 and 20 km, respectively, indicating that this earthquake was a medium-sized crustal event. Yushu County suffered severe damage from this earthquake. The Xinhua News Agency reported that approximately 3000 people had died or were missing (available at http://english.news.cn/, 2010), which was larger than the several hundred estimated by USGS (http://earthquake.usgs.gov/earthquakes/pager/events/us/2010vacp/index.html). The source characteristics of the earthquake were, at least in part, responsible for the large number of casualties.
The source models of this earthquake were assumed using teleseismic data by Zhang et al. (2010) and other studies (the Yagi model; Y. Yagi, http://www.geol.tsukuba.ac.jp/~yagi-y/EQ/20100414China/, 2010, and the Wang model; W. Wang, L. Zhao, and Z. Yao, http://www.csi.ac.cn/manage/html/4028861611c5c2ba0111c5c558b00001/_content/10_04/14/1271250181605.html, 2010), and using InSAR data by Tobita et al. (2011). However, the features of these slip models are somewhat different from each other. In particular, Zhang et al. (2010) only suggested that this rupture propagated at a supershear speed. In the present study, we show the precise slip distribution and rupture propagation by a joint inversion of all available data, i.e., teleseismic and InSAR datasets.
We first determine the location of the centroid moment by the grid search method using W-phase and P-wave data. Next, we perform a joint inversion for the rupture process and assume the rupture velocity. Finally, we discuss the relevance of the inversion results to the severe damage and the tectonic activity in the ETP.
2. Centroid Location Grid Search
We first searched for the location of the centroid moment using the grid search method around the Yushu fault. This grid search was performed on a grid with a spacing of 0.2° at a depth of 10 km. We used the two point source inversion methods of W-phase and P-wave waveforms developed by Kanamori and Rivera (2008) and Kikuchi and Kanamori (1991), respectively. The W-phase and P-wave parts of the teleseismic waveform data from the IRIS DMC were filtered with passbands of 0.002–0.01 Hz and 0.02–0.5 Hz, respectively. Figure 1(c) shows the solutions and the misfit percentages on the grids. Each best-fitting solution is located not around the surface rupture trace (Chen et al., 2010; Lin et al., 2011) but in the vicinity of the USGS hypocenter.
We obtained two point source solutions from the above grid searches. The strike, dip, and rake in the first solution from the W-phase grid search are 119°, 72°, and 3°, respectively, whereas those in the second solution from the P-wave grid search are 119°, 84°, and 3°. Therefore, both solutions suggest an overall focal mechanism of left-lateral strike slip on a near-vertical fault plane and an Mw of 6.8 ~ 6.9, which is similar to the solution obtained by the Global CMT Project (2010).
3. Joint Inversion
Next, we performed a joint inversion of the teleseismic and InSAR data using the method of Yoshida et al. (1996) with the revisions of Hikima and Koketsu (2005). Based on the grid searches and InSAR data (Fig. 1(b)), we determined the hypocenter as (latitude: 33.18°N, longitude: 96.65°E, depth: 10 km below ground surface). In addition, based on the aftershock distribution determined by the China Earthquake Networks Center (available at http://www.csndmc.ac.cn/newweb/large_quakes/qinghai2010/qinghai.htm, 2010; Fig. 1(a)), we constructed a 72 km × 24 km fault model with a dip of 84°. This fault model was divided into 108 subfaults of 4 km × 4 km. The strikes of these subfaults were set to 107° and 119° in the western six columns and the eastern twelve columns, respectively. Strike-slip vectors, represented by linear combinations of two components in the directions of 0°±45°, were placed at the centers of the subfaults. Ten triangle functions with a rise time of 1.0 s were used as the temporal basis functions for ten time windows at each subfault.
We then prepared the teleseismic and InSAR datasets. We used the 60-s-long P-wave parts of the teleseismic waveform data filtered with a passband of 0.02–0.5 Hz. Considering the data quality and the azimuthal coverage, we chose the waveforms observed at 23 stations with epicentral distances between 30° and 100° (Fig. 1(d)). The interferogram data (Fig. 1(b)) were acquired on 15 January, 2010, and 17 April, 2010. For the inversion, we resampled this data using a quadtree decomposition method (Jonsson et al., 2002).
For the teleseismic dataset, the Green’s functions were computed using the method of Kikuchi and Kanamori (1991), which includes an extension to reflect the velocity structures near the source and stations. For the In-SAR dataset, the Green’s functions were computed using the method of Zhu and Rivera (2002). We adopted the CRUST 2.0 model by Laske, Masters, and Reif (available at http://igppweb.ucsd.edu/~gabi/crust2.html) for deriving these velocity structures and applied the rigidity based on this model.
Finally, we performed the finite source inversion for inferring the rupture process of the 2010 Yushu earthquake. In order to stabilize the inversion, we used a smoothness constraint of the slip distribution with a discrete Laplacian in space, and the weight of the constraint was determined by minimizing Akaike’s Bayesian Information Criterion (ABIC) (Akaike, 1980). The weight for each dataset was applied so that the datasets were equally weighted.
Figure 3(e) shows the resultant slip distribution. There is a large slip area in the neighborhood of the hypocenter, which is consistent with the results of the grid searches. This slip area spans 30 km to the southeast and extends to the surface. This resultant slip value and slip distribution are consistent with a source model assumed by Tobita et al. (2011) using the InSAR data only.
This shallow slip area caused strong ground motions in and around the earthquake source region. Those to the southeast were further enhanced due to the rupture directivity effect (e.g., Koketsu and Miyake, 2008). In particular, in cases of supershear rupture propagation, such as in the 2010 Yushu earthquake, the large S-wave energy is radiated around the rupture front (Dunham and Archuleta, 2005; Dunham and Bhat, 2008). Since the county seat of Yushu is located around this rupture front, as shown in Fig. 1(a), the supershear rupture propagation may be one reason for the severe damage from the earthquake.
4. Relation to Tectonic Activity
When the accumulated strain in a bonded region reaches its maximum, an earthquake occurs. Therefore, in zone B, large earthquakes were generated repeatedly in bonded regions. An event close to the Yushu fault occurred in 1738 (Chen et al., 2010). Because the average rate of the relative motion along the Yushu fault is about 7 mm/yr (Zhou et al., 1997), the total accumulated movement from 1738 along the Yushu fault can be assumed to be about 1.9 m. Our resultant slip is consistent with this value. Using the InSAR data only, Tobita et al. (2011) showed that their resultant maximum slip was also in agreement with this value. From these results, the recurrence interval of this repeating earthquake is about 300 years.
In zone B, the shallow and supershear rupture propagation was observed during the 2001 Central Kunlun earthquake (Bouchon and Vallee, 2003; Lin et al., 2003). Koketsu et al. (2010) also observed the near-shear rupture propagation during the 2008 Wenchuan earthquake in zone C. Even in the case of this earthquake, a shallow rupture propagation was assumed (Koketsu et al., 2010; Xu et al., 2010; Zhao et al., 2010). Our results suggest that the 2010 Yushu earthquake had features similar to these earthquakes. These kinds of rupture propagation may represent a seismic feature of the tectonic activity in the ETP.
We have examined the rupture process of the 2010 Yushu earthquake by the joint inversion of teleseismic and InSAR datasets. The obtained inversion results suggest that the rupture directivity effect and supershear rupture propagation caused the severe damage in Yushu County. These results are also consistent with the tectonic activity in the ETP, exhibiting the seismic features of zones B and C, such as shallow and fast rupture propagation. These features may enhance seismic damage in this region.
We thank Dr. Eric M. Dunham and an anonymous reviewer for their helpful comments. The teleseismic waveform data and information on the earthquakes and aftershocks were provided by the IRIS DMC, Global CMT Project, USGS, and China Earthquake Networks Center.
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