Examining Seismites using Anisotropy of magnetic susceptibility in and around the Kopili Fault Zone, Northeast India: A Characterization Study

 

The North East Region (NER) of India is a Tectonically Active Zone with Noteworthy Intraplate Seismic Activity over the Past 200 Years. The tectonics and seismicity of large intraplate earthquakes in NER are poorly understood. As a result, the Kopili Fault (KF) has a complex tectonic setting with a history of past two large earthquakes of 1869 Cachar earthquake (M_w-7.4) and 1943 Hajoi earthquake (M_w-7.2) are being observed. Paleoseismological evidence reveals valuable insights into seismic hazard and the historical occurrence of earthquakes, as manifested in preserved liquefaction features. Field studies carried out by excavating trenches in five sites have uncovered secondary evidence of significant liquefaction-induced deformation features, known as seismites, occurring at a depth of approximately 2-3 meters below the surface. These features manifest as sand dykes and sills, exhibiting variations in colour, grain size, and sediment indurations across the sites. The findings from the excavated trenches have been summarized, incorporating multiple analyses to differentiate the seismites from depositional features. To aid in this distinction, the study utilized the Anisotropy of Magnetic Susceptibility (AMS) technique.

The stereographic projection and bootstrap plots for the host sediment at the NB site clearly depict a vertical orientation for Kmin, while Kint and Kmax are distributed around the horizontal plane, indicative of sediment formation through fluvial activity. In the doublet liquefaction dykes NBRD and NBLD, all three axes exhibit random scattering. Notably, Kmin is sub-vertical, and Kmax is sub-horizontal in the southeast direction. The Degree of Anisotropy (Pj), Lineation (L), and Shape parameter (T) plots for the host specimen fall within the oblate field, whereas liquefaction dykes exhibit a distribution ranging from prolate to triaxial. In our presentation, we delve into a detailed discussion on how Anisotropy of Magnetic Susceptibility (AMS) serves as a valuable tool for comprehending seismite behavior and their occurrences, particularly in relation to large to great earthquakes.

Understanding the tectonic and seismic characteristics of the Kopili Fault is crucial for assessing and managing earthquake risk in Northeastern India. Our studies will likely contribute to improved earthquake preparedness and resilience in the region for future prospecting.