Seismotectonics of Siang Valley and Adjoining Region Inferred from Focal Mechanism Solutions Using Waveform Inversion

The Siang Valley of Arunachal Pradesh, North-East India, is one of the seismotectonically active region that lie in the eastern Himalayan Syntaxis (EHS); we have investigated seismicity, fault plane solutions (FPS) and P (Pressure) axis orientation in this region. We have analyzed 756 local earthquakes of magnitude range (1.0 ≤ M_L ≤ 5.9) in the region during the period from January 2019 to December 2021. From the spatial distribution of local seismicity, it is estimated that the concentration of seismicity is in Namcha-Barwa, western and eastern flanks of Siang Antiform, respectively. The depth distribution of seismicity extends upto a focal depth of 60 km with a higher concentration in the upper crustal part. Further, we determined 15 fault plane solutions (FPS) using waveform inversions (ISOLA) for events with a magnitude range of 3.5 to 5.9. The waveform inversion has been performed for the events with maximum azimuthal coverage. The frequency band used for the inversion is in the 0.01–0.1 Hz range corresponding to the maximum signal to noise ratio to precise crustal velocity structures, hypocenter positions, and appropriate frequency ranges were used to obtain reliable FPS. The FPS obtained for the shallow focused earthquakes shows Normal faulting with Strike-slip components. The compressional axes orientations of the thrust FPS show a north-east direction. The intense seismic activity and compressional axis orientation in this study area is due to the collision between Indian and Eurasian plate in the north and and east-ward subduction of Indian plate below the Burmese plate.