Geophys. J. Int. (2009) 177, 555–570 doi: 10.1111/j.1365-246X.2008.04063.x Validation of the rupture properties of the 2001 Kunlun, China (M s = 8.1), earthquake from seismological and geological observations ∗ Yi-Ying Wen,1 Kuo-Fong Ma,1 Teh-Ru Alex Song,2 and Walter D. Mooney3 1Graduate Institute of Geophysics, National Central University, Taiwan. E-mail: [email protected] 2Seismological Laboratory, California Institute of Technology, USA 3US Geological Survey, Menlo Park, CA, USA Accepted 2008 October 31. Received 2008 October 31; in original form 2008 April 30 SUMMARY We determine the finite-fault slip distribution of the 2001 Kunlun earthquake (M s = 8.1) by inverting teleseismic waveforms, as constrained by geological and remote sensing field observations. The spatial slip distribution along the 400-km-long fault was divided into five segments in accordance with geological observations. Forward modelling of regional surface waves was performed to estimate the variation of the speed of rupture propagation during faulting. For our modelling, the regional 1-D velocity structure was carefully constructed for each of six regional seismic stations using three events with magnitudes of 5.1–5.4 distributed along the ruptured portion of the Kunlun fault. Our result shows that the average rupture velocity is about 3.6 km s−1, consistent with teleseismic long period wave modelling. The initial rupture was almost purely strike-slip with a rupture velocity of 1.9 km s−1, increasing to 3.5 km s−1 in the second fault segment, and reaching a rupture velocity of about 6 km s−1 in the third segment and the fourth segment, where the maximum surface offset, with a broad fault zone, was observed. The rupture velocity decelerated to a value of 3.3 km s−1 in the fifth and final segment. Coseismic slip on the fault was concentrated between the surface and a depth of about 10 km. We infer that significant variations in rupture velocity and the observed fault segmentation are indicative of variations in strength along the interface of the Kunlun fault, as well as variations in fault geometry. GJI Seismology Key words: Satellite geodesy; Earthquake source observations; Dynamics and mechanics of faulting. different segments of the Kunlun fault (Fig. 1): the 1937 M = 7.5 1 INTRODUCTION Huashi Canyon earthquake, the 1963 M s = 7.1 Dulan earthquake, The roughly east–west sinistral strike-slip Kunlun fault, one of the the 1973 M s = 7.3 Manyi earthquake and the 1997 M w = 7.5 Manyi faults that accommodates the eastward extrusion of Tibet plateau, earthquake. The surface rupture of all the events are believed to have is an example of large scale slip partitioning in the continental crust been greater than 150 km, and the focal mechanisms all show dis- (Tapponnier & Molnar 1997; Meyer et al. 1998; Tapponnier et al. tinct left-lateral strike-slip motion (Molnar & Deng 1984; Gu et al. 2001; Wang et al. 2001; Van Der Woerd et al. 2002a,b). On 2001 1989). The ruptured segment of the 2001 Kunlun earthquake lies November 14 (09:26:10 GMT), an M s = 8.1 earthquake struck between the four previous large events. Seismicity that was recorded in this active fault zone. According to the US Geological Survey 1 yr before the 2001 Kunlun earthquake by the China Seismic Net- (USGS), the epicentre of the Kunlun earthquake was located in work DMC (CSNDMC) indicated that there was almost no activity Qinghai Province, northwest China, at 35.946◦N and 90.541◦E, and in the rupture area of the 2001 event. The locations of aftershocks the surface rupture extended laterally more than 400 km (Lin et al. in the first year following the main shock were concentrated on 2002, 2003; Xu et al. 2002; Van Der Woerd et al. 2002a) (Fig. 1). the eastern end of the ruptured fault, with depths reaching nearly Since 1937, several large (M > 7) earthquakes have occurred along 35 km (Fig. 1). Similar to previous large historical events that oc- curred along the Kunlun fault, the 2001 Kunlun earthquake showed a distinct left-lateral strike-slip motion according to the Harvard CMT solution. ∗ Now at: Department of Terrestrial Magnetism, Carnegie Institution of Fig. 1 also shows the epicentres and focal mechanisms deter- Washington, Washington, DC, USA. mined by various institutes, namely, USGS, Earthquake Research C 2009 The Authors 555 Journal compilation C 2009 RAS 556 Y.-Y. Wen et al. Figure 1. Locations (asterisks) and fault plane solutions (green beach balls) of the 2001 Kunlun earthquake determined by USGS, ERI, CCDSN and HARVARD, respectively. The diamonds indicate the locations of historic large events along the Kunlun fault: the 1937 M7.5 Huashi Canyon earthquake, the 1963 M s7.1 Dulan earthquake, the 1973 M s7.3 Manyi earthquake, and the 1997 M w7.5 Manyi earthquake, with the focal mechanism in black from the Harvard CMT solution, and the grey ones from Molnar & Deng (1984). The thick black line represents the surface rupture during the main shock. The triangles and circles show the foreshocks and aftershocks, recorded by CSNDMC, within 1 yr before and after the main shock, respectively. The lower panel shows the depth distribution of aftershocks near the ruptured fault. The inset map at the right upper corner shows the major faults around Tibet, and the dashed rectangle indicates the region of the Kunlun fault. KLF, Kunlun fault; ATF, Altyn Tagh fault and HYF, Haiyuan fault. Institute in Tokyo (ERI), China Center of Digital Seismic Network eral similar historical events, and report a maximum slip of about (CCDSN) and Harvard CMT. The fault plane solution of ERI and 7.6m(+/– 0.4 m). The surface slip along the ruptured fault defined Harvard CMT are similar, with almost east–west strike-slip focal from InSAR data (Lasserre et al. 2005) or measured at high res- mechanisms, in agreement with the strike direction of the Kunlun olution using optical correlation of satellite images (Klinger et al. fault. The reported hypocentres for the main shock are all located 2006) are consistent with the observations of Xu et al. (2006), as close to the western terminus of the ruptured fault, while the cen- shown in Fig. 2. Regardless of the precise amount of the maxi- troidal solution determined by the Harvard CMT is located near the mum slip, the investigations all showed maximum slip at a loca- middle of the rupture fault. The hypocentral location exhibits the ini- tion 240–280 km to the east of the epicentre. The surface rupture tial point of fault rupture, while the centroidal solution corresponds consists of shear, transtensional, transpressional, extensional and to the region of maximum energy release. In our study, we use the thrusting fractures, as well as tension gashes, pull-aparts, releasing USGS hypocentre, which had been carefully relocated, and has been or compressing jogs and mole tracks (Lin et al. 2002, 2003; Xu widely adopted in previous works (Bouchon & Vallee´ 2003; Lin et al. 2002, 2006; Van Der Woerd et al. 2002a; Li et al. 2005). et al. 2003; Antolik et al. 2004). The fact that the hypocentre is lo- Reverse components of slip are also observed, but they are much cated at the western end of the rupture suggests an almost unilateral smaller than the horizontal slip at the same locations (Xu et al. rupture of the earthquake. 2002, 2006). Post-earthquake geological field investigations were carried out In addition to significant variations in the magnitude of slip along by four groups, as reported by Lin et al. (2002), Xu et al. (2002, the 2001 rupture, a large variation in rupture velocity has been 2006), Li et al. (2005) and Klinger et al. (2005). All studies have reported. Bouchon & Vallee´ (2003) used regional surface wave data revealed significant left-lateral slip along the Kunlun fault, but the to determine a rupture velocity of ∼5.0 km s−1, greatly exceeding estimated maximum slips of 16.3 and ∼8 m are very different the shear wave velocity, for most sections of the fault. However, (Fig. 2). To resolve this discrepancy in maximum slip for the Hjorleifsdottir et al. (2003) modelled long-period body waves and 2001 Kunlun earthquake, Xu et al. (2006) re-investigated sev- concluded that the average rupture velocity proposed by Bouchon eral sites where the maximum slip of 16.3 m was reported by Lin &Vallee´ (2003) was likely too high. On the other hand, Robinson et al. (2002). They concluded that the reported maximum slip of et al. (2006) obtained a rupture velocity that even exceeded the 16.3 m almost certainly represents the cumulative slip of sev- P-wave velocity in the region of highest slip. Recently, Tocheport C 2009 The Authors, GJI, 177, 555–570 Journal compilation C 2009 RAS Validation of the rupture properties of the 2001 Kunlun, China (Ms = 8.1), earthquake 557 Figure 2. Coseismic horizontal displacements from field investigation and InSAR data. The red dashed line indicates the field data modified from Lin et al. (2003); note that the maximum estimated slip of 16.3 m was not confirmed by the later field study of Xu et al. (2006). The grey bars indicate the field measurements of Xu et al. (2006), and the blue line represents the coseismic horizontal displacements modelled from InSAR data of Lasserre et al. (2005). The green line indicates measured left lateral offsets of Klinger et al. (2006). The red asterisk indicates the epicentre. The fault ruptures were classified into five segments as S-1 to S-5, shown by vertical dashed lines, according to Xu et al.