An improved geodetic source model for the 1999 Mw 6.3 Chamoli earthquake, India

Geophysical Journal International Geophys. J. Int. (2016) 205, 236–242 doi: 10.1093/gji/ggw016 GJI Gravity, geodesy and tides An improved geodetic source model for the 1999 M w 6.3 Chamoli earthquake, India Wenbin Xu, 1,2 Roland B¨urgmann 1 and Zhiwei Li 2 1 Department 2 School of Earth and Planetary Science, University of California, Berkeley, California, USA. E-mail: wenbin.xu@kaust.edu. sa of Geosciences and Info-Physics, Central South University, Changsha 410083, Hunan, China Accepted 2016 January 11. Received 2016 January 9; in original form 2015 August 28 Key words: Radar interferometry; Earthquake source observations; Crustal structure; Asia. 1 I N T RO D U C T I O N The Himalayan range formed due to the collision of the Indian plate and southern Tibet. Recent GPS-constrained convergence rates range from ∼13.3 ± 1.7 mm yr −1 in northwest India to ∼21.2 ± 2.0 mm yr −1 in Assam (Stevens & Avouac 2015). As a result of the crustal shortening, several thrust faults have devel- oped in the Himalayan range, such as the Main Central Thrust (MCT), the Main Boundary Thrust (MBT) and the Main Frontal Thrust (MFT). Currently, most slip appears to involve the MFT (Lav´e & Avouac 2001), but there is geologic evidence for some out- of-sequence faulting further north (e.g. Wobus et al. 2005). These faults cut the entire crust independently but are rooted into a com- mon basal detachment termed the Main Himalayan Thrust (MHT; Ni & Barazangi 1984; Yin 2006), which has a flat-ramp-flat ge- ometry. Recent geodetic observations using interseismic coupling models reveal that the MHT is fully locked from the frontal flat to the mid-crustal ramp at ∼20 km depth (Feldl & Bilham 2006; Ader et al. 2012; Avouac 2015; Stevens & Avouac 2015). As slip deficit and stress are accumulating on the fault ramp, this portion of the MHT has the potential to generate great earthquakes and is also responsible for mountain building (Banerjee & B¨urgmann 2002; Grandin et al. 2012). The Garhwal region lies in an 800-km-long seismic gap between the rupture zones of the 1905 M s 7.8 Kangra and 1934 M w 8 Bihar earthquakes, where great earthquakes have rarely occurred during the past ∼500 years (Rajendran et al. 2015). GPS measurements indicate that the region may have a slip potential of nearly 10 m, implying that the moment deficit and equivalent magnitude of a gap-filling event can be greater than 8.5 (Bilham et al. 2001). Re- cently, the M w 7.8 Gorkha, Nepal earthquake took place in the eastern section of this seismic gap, causing significant damage and casualties (Avouac et al. 2015; Galetzka et al. 2015; Lindsey et al. 2015; Wang & Fialko 2015). To the west, the Garhwal region has experienced several moderate-sized earthquakes in the 1990 s, the most prominent being the 1991 M w 6.8 Uttarkashi and the 1999 M w 6.3 Chamoli earthquakes. In this work, we focus on the Chamoli earthquake that occurred on 1999 March 29. The main shock was widely felt in Uttarakhand, damaging tens of thousands of houses and causing ∼103 casual- ties (Sarkar et al. 2001). No apparent foreshocks had been recorded prior to the main shock, but intense aftershocks have been observed, including several events with M w > 5. Most of the aftershocks were located to the east of Chamoli village (Fig. 1). The 1999 Chamoli earthquake has been studied using geodetic and seismic methods (Mukhopadhyay & Kayal 2003; Rajput et al. 2005; Satyabala & C Crown copyright 2016. Downloaded from http://gji.oxfordjournals.org/ at University of California, Berkeley on February 17, 2016 SUMMARY We present a distributed slip model for the 1999 M w 6.3 Chamoli earthquake of north India using interferometric synthetic aperture radar (InSAR) data from both ascending and descend- ing orbits and Bayesian estimation of confidence levels and trade-offs of the model geometry parameters. The results of fault-slip inversion in an elastic half-space show that the earthquake ruptured a 9 ◦+3.4 −2.2 northeast-dipping plane with a maximum slip of ∼1 m. The fault plane is located at a depth of ∼15.9 +1.1 −3.0 km and is ∼120 km north of the Main Frontal Thrust, implying that the rupture plane was on the northernmost detachment near the mid-crustal ramp of the Main Himalayan Thrust. The InSAR-determined moment is 3.35 × 10 18 Nm with a shear modulus of 30 GPa, equivalent to M w 6.3, which is smaller than the seismic moment estimates of M w 6.4–6.6. Possible reasons for this discrepancy include the trade-off between moment and depth, uncertainties in seismic moment tensor components for shallow dip-slip earthquakes and the role of earth structure models in the inversions. The released seismic energy from recent earthquakes in the Garhwal region is far less than the accumulated strain energy since the 1803 M s 7.5 earthquake, implying substantial hazard of future great earthquakes.