Interseismic Coupling beneath the Sikkim–Bhutan Himalaya Constrained by GPS Measurements and Its Implication for Strain Segmentation and Seismic Activity

Shuiping Li,Jianwei Huang,Yongchao Zhu,Qi Wang,Xiaochuan Qu,Fei Gao,Tingye Tao

doi:10.3390/rs12142202

Abstract

The Sikkim–Bhutan seismic gap has witnessed a long earthquake quiescence since the 1714 M7.5~8.5 earthquake. The state of stress accumulation beneath the Sikkim–Bhutan Himalaya and its spatial correlation with seismicity remains unclear due to the lack of geodetic measurements and the low levels of seismic activity. We compile Global Positioning System (GPS) measurements in southern Tibet with the available velocities in the Sikkim–Bhutan Himalaya to reveal the characteristics of strain buildup on the Main Himalayan Thrust (MHT). We correct non-tectonic hydrological loading effects in a GPS time series to accurately determine the Three-Dimensional (3D) velocities of each continuous station. Extensive GPS measurements yield convergence rates of 16.2~18.5 mm/y across the Sikkim–Bhutan Himalaya, which is quite consistent with that observed elsewhere in the Himalaya. Based on a double-ramp structure of the MHT, a refined 3D coupling image is inverted using a dense network of GPS velocities. The result indicates significant along-strike variations of fault coupling beneath the Sikkim–Bhutan Himalaya. The locking width (coupling > 0.5) of western Bhutan reaches ~100 km, which is 30~40% wider than Sikkim and eastern Bhutan. An obvious embayment of decoupling zone near the border between Sikkim and western Bhutan is recognized, and coincides spatially with the rupture terminates of the 1934 Mw8.2 and the 1714 M7.5~8.5 earthquakes, indicating that the large megathrust earthquakes along the Sikkim–Bhutan Himalaya are largely segmented by the spatial variation of frictional properties on the MHT. Using a new compilation of seismic records in the Sikkim–Bhutan Himalaya, we analyze the spatial correlation between fault coupling and seismic activity. The result suggests that the seismicity in the Bhutan Himalaya is broadly distributed, instead of restricted in the lower edge of the interseismic locking zone. This implies that the seismic activity in the Bhutan Himalaya is not uniquely controlled by the stress accumulation at the downdip end of the locked portion of the MHT.

Highlights

The collision and ongoing convergence of India with Eurasia, initiated ~50 Ma ago, have formed the giant Himalayan subduction zone and given rise to the highest mountains on Earth [1,2]
The interseismic crustal deformation in the Sikkim–Bhutan Himalaya is assumed to result from the5j.o1i.nCtoenffveercgtesnocef RstaetaesdaycrbolsosctkhemSoiktkiiomn,BthhuetannoHnriemcaolavyearable intrablock strain and the locking of the block boundary faults [53]
The results provide very good constraints on the convergence rate across the Sikkim–Bhutan Himalaya, while the locking depth and the fault dip are not very tightly constrained which could attribute to the large uncertainty of the velocities at some campaign sites

Summary

Introduction

The collision and ongoing convergence of India with Eurasia, initiated ~50 Ma ago, have formed the giant Himalayan subduction zone and given rise to the highest mountains on Earth [1,2]. The Himalayan arc, with total length of ~2500 km, has experienced more than eight large megathrust earthquakes during the past centuries and shown a few seismic gaps with elapsed time of the latest large earthquake over 300 years (Figure 1b) [3,4]. The Sikkim–Bhutan segment, showing the most prominent seismic gaps in the HRiemmoatelaSeynas.n20e20a,r1t2h, xqFuOaRkPeEbERelRt,EVhIaEsWattracted extensive attentions to the state of stress accu3mofu21lation and its future seismic risk [5,6,7]. Over the past half millennium, the only reported large event in the Sikkiwmi–dBthhuoftathneHLeimssearlaHyiamiaslathyae sMho7w.5s~o8b.v5ioeuasrtlhatqeuraalkveairniat1i7on14s athloantgp-setrrihkaep.

Results

Discussion

Conclusion