Seismicity and Seismotectonics in the Himalaya of Bhutan: Insights from the GANSSER Seismic Network

Abstract:

The seismicity of Bhutan is characterized by the apparent lack of great earthquakes and a significantly lower activity compared to most other parts of the Himalayan arc. To better understand the underlying mechanisms of this anomalously low activity and to relate it with possible along-strike differences in the structure of the orogenic belt, a temporary network with 38 broadband seismometers was installed in the Kingdom of Bhutan between January 2013 and April 2014.

In this work we present a first catalog of local and regional earthquakes detected and located with the GANSSER (Geodynamics ANd Seismic Structure of the Eastern-Himalaya Region) network. Events were automatically detected using STA/LTA triggers with a global 1-D velocity model. A subset of more than 800 automatic locations were manually reviewed and more than 400 events in Bhutan and surrounding regions have been manually repicked and accurately relocated. Magnitudes of these hypocenters range from ML 0.5 to 5.6. Seismicity along the Main Frontal and Main Boundary Thrusts in south-central Bhutan is very low. The highest activity is clustered along a NE-SW oriented structure in southwest Bhutan, which might be associated with the Goalpara lineament. In eastern Bhutan a cluster of seismicity is identified in the vicinity of the 2009 Mw=6.2 event and the embayment of the Main Central Thrust, near the town of Mongar.

For two events in Bhutan a moment tensor was derived. Both solutions indicate strike-slip mechanisms and an Mw of 3.9 and 4.0. The majority of well-constrained focal depths ranges between 10 to 30 km and might be associated with the Main Himalayan Thrust (MHT). One exception is the Mw=4.0 event in northern Bhutan in June 2013, which occurred at about 70 km depth, most likely in the underthrusting Indian lower crust.

To further improve the accuracy of locations, about 100 events were used to simultaneously invert for seismic velocities and hypocenters. We will discuss possible seismotectonic models for Bhutan based on the relocated seismicity, focal mechanisms, seismic velocities, and interfaces (Moho and intra-crustal structures like the MHT) imaged by receiver functions.