Seismicity in the Kingdom of Bhutan (1937–2003): Evidence for crustal transcurrent deformation

Abstract

[1] We investigate moderate-sized historic to recent earthquakes in the Bhutan Himalaya spanning the years from 1937 to 2003. We find that few moderate-sized earthquakes occurred in the region during this time period. In order to better characterize the seismicity, we relocate all earthquakes and estimate focal mechanisms for events for which we have adequate data. We use first motion data for older events and waveform model digital seismograms for three earthquakes that occurred in 1980 (M w = 6.4), 1995 (M w = 5.4), and 2003 (M w = 5.4). For the modern events, we utilize three techniques for focal mechanism and depth determination: regional full waveform time domain modeling of the two moderate (M w = 5.4) events, and surface wave spectral method and body wave depth modeling approach for the M w = 6.4 event. We find that the first motion and digital data focal mechanisms are mostly strike-slip, with midcrustal to deep crustal depths. Although no recent great event has occurred along the main Himalayan thrust, the Indian plate is undergoing significant, midcrustal to deep crustal transcurrent deformation, likely due to oblique convergence of the Indian-Asian collision in this region. Unlike oceanic-continent oblique subduction, strike-slip and thrust partitioning occurs throughout the crust, not in distinct zones. The geological and geophysical nature of the Bhutan Himalaya appears significantly different than other Himalayan regions due to the strikeslip nature of many of the events, an out of sequence thrust fault, and the implications of deformation in the Shillong Plateau.

Active tectonics in Eastern Lunana (NW Bhutan): Implications for the seismic and glacial hazard potential of the Bhutan Himalaya

“…[38] Our observations and interpretations of seismically active strike-slip faults fit well with earthquake relocations and inversion results of SW Bhutan indicating strike-slip motions with midcrustal to deep-crustal depths for at least two seismic events [Drukpa et al, 2006]. These authors conclude that the Indian plate is undergoing significant, transcurrent deformation throughout its depth extent, likely due to oblique convergence of the Indian-Asian collision in this region.…”

“…The stress field for these conjugate sets of approximately N-S trending active faults is consistent with widespread observations throughout southern Tibet and the north Himalaya of young E-W expansion (e.g., the graben system of Armijo et al [1986]). We speculate that the occurrence of these strike-slip faults mark an important transition in the geodynamics between the western and the eastern Himalaya, respectively, and strike-slip faulting might be explained by an increasing degree of oblique subduction toward the Eastern Himalaya Syntax as suggested by Drukpa et al [2006].…”

“…Studies on recent seismic activity in the eastern Himalaya and in Bhutan have shown that earthquakes are not only related to thrusting, but also to midcrustal to deepcrustal strike-slip faulting at a depth of 10 -40 km [Drukpa et al, 2006;De and Kayal, 2003;Kayal et al, 1993;Wu et al, 1998]. …”

Geologic correlation of the Himalayan orogen and Indian craton: Part 2. Structural geology, geochronology, and tectonic evolution of the Eastern Himalaya

“…Importantly, structural geometry in Figure 4C requires the Himalayan interior to experience active crustal shortening and thus upward warping. This may explain widespread seismicity in the eastern Himalayan interior (Drukpa et al, 2006;Velasco et al, 2007).…”

“…1). In Bhutan, the work of Jangpangi (1974) and Gansser (1983) laid a foundation for the general geology that led to prolifi c studies across the country in the past three decades (Ray et al, 1989;Swapp and Hollister, 1991;Ray, 1995;Bhargava, 1995;Edwards et al, 1996;Grujic et al, 1996Grujic et al, , 2002Grujic et al, , 2006Davidson et al, 1997;Stüwe and Foster, 2001;Wiesmayr et al, 2002;Daniel et al, 2003;Tangri et al, 2003;Baillie and Norbu, 2004;Carosi et al, 2006;Meyer et al, 2006;Richards et al, 2006;Drukpa et al, 2006;Hollister and Grujic, 2006;McQuarrie et al, 2008). Following the traditional defi nition of major Himalayan structures and lithologic units by Heim and Gansser (1939), the Bhutan Himalaya is divided into the Lesser Himalayan Sequence, Greater Himalayan Crystalline Complex, and Tethyan Himalayan Sequence units bounded by the Main Boundary thrust below, the Main Central thrust in the middle, and the later discovered South Tibet detachment at the top ( Fig.…”