Primary on fault paleoseismic evidences from trench investigation along the Bathubasti fault, South Andaman, India

Abstract

Continued structural descent of the Indian lithosphere below the Sunda-Andaman trench produced several destructive large to great earthquakes in the region of Andaman and Nicobar Islands. The pattern of strain accumulation and its release in the context of great plate boundary mega thrust earthquakes and slip partitioning along active faults in the Andaman and Nicobar region during these events is unclear, owing to lack of on-fault primary paleoseismic evidences, structural complexity associated with subduction processes and lack of primary surface ruptures of large to huge magnitude earthquakes. In this context, preliminary paleoseismic trench investigations were performed across a 6-m high fault scarp at Bathubasti (South Andaman) to understand the slip partitioning and rupture dynamics of subduction zone earthquakes. Ground-penetrating radar (GPR) profiles and the trench exposure across the scarp suggest progressive deformation and several episodes of co-seismic scarp generation where Andaman Flysch Sandstone (Oligocene) is in fault contact with the late Holocene sediments. A total of eight stratigraphic units (a to g) including three fault strands (F0, F1, and F2) were observed and mapped in the trench stratigraphy. OSL ages constrain that event 1 occurred along F0 around 2312 ± 302 years BP and thrust unit (g) Andaman Flysch -bedrock over unit (f) paleosols. Event 2 has deformed unit (C)-medium sand bed and the basal portion of unit (b)-fine sand bed along F1 and F2 around 422 ± 57 years BP. Given the scarp height of 6 m and with, a vertical displacement of approximately 3 m of unit (g)- bed rock along low-angle fault (θ = 18°) implies an average slip rate of 4.12 mm/year, shortening rate of 3.88 mm/year, and an uplift rate of 4.54 mm/year.