Complex Multi-Fault Dynamics in Sikkim Himalaya: New Insights From Local Earthquake Analysis

Anomalously overturned thrust faults, lineaments and segmentation causing cross-cutting basement structures characterize the tectonic setting of Sikkim Himalaya. However, its seismotectonics is poorly constrained along with the speculated northward extension of the Dhubri-Chungthang Fault Zone (DCFZ) causing segmentation. Here, we utilize the precise location of newly acquired local earthquake data and fault plane solutions using full-waveform moment tensor inversion to better constrain seismically active zones. Transtensional shearing along the Main Himalayan Thrust in central Sikkim is possibly incited by fluid-rich upper-crust. Cessation of the mapped 20 km wide mid-crustal seismogenic zone of DCFZ at Chungthang and its northward discontinuation into the Higher Himalayas is confirmed by the striking variation in focal mechanisms. Earthquakes along imbricated segments in the lower-crust originate possibly in response to crustal shortening. Extensional shearing along the Moho triggers seismicity to the northwest of Sikkim. Such complex tectonic dynamics instigating persistent seismicity indicates high potential for future great earthquakes in Sikkim Himalaya. A dense seismic network operational from April 2019 to May 2023 has been continuously recording earthquakes occurring in and around Sikkim Himalaya. In this study, we have located the earthquakes occurring within 200 km radius of Sikkim Himalaya to determine which regions are seismically active. We further analyzed these earthquakes to deduce deformation in the source region, which comprises of how the accumulated stress in the Earth gets released. This study aided in mapping the fault zones and its sense of motion, which are responsible for the persistent occurrence of earthquakes. We provide the possible extent of the debated mid-crustal Dhubri-Chungthang Fault Zone, which traverses across the study region and is stated to orthogonally divide the arcuate Himalayas. Our study also provides evidence for earthquakes originating in the lower-crust due to overlapping of parallelly dipping layers of the Moho. Earthquakes occurring in upper-crust and along the Moho boundary attest to previously obtained results. Such complex multi-fault dynamics increases the seismic hazard potential in Sikkim Himalaya. High resolution local earthquake catalog for Sikkim Himalaya recorded at the longest running seismic network Delineation of possible seismogenic extent of the highly debated Dhubri-Chungthang Fault Zone Evidence of lower crustal earthquakes along imbricated Moho