Evidence of Surface Rupture from the 1897 Chedrang Valley Earthquake (Mw ~ 8.1) on the Shillong Plateau: Insights from MASW, Resistivity Sounding, and Fluvial Geomorphology.

The significant Shillong earthquake of 1897 in the western part of the Shillong Massif profoundly impacted the entire Northeast India region, resulting in extensive devastation at both surface and subsurface levels. Numerous land fissures, sand veins, swampy areas, and land tilting and upliftment were formed during the earthquake in the western part of the Shillong plateau. This study adopts an integrated approach, combining seismic, geophysical, and fluvial geomorphology methods, to quantify the co-seismic subsurface rupture. Multichannel Surface Wave Analyzer (MASW) and resistivity surveys are conducted at fourteen locations along three profiles crossing the Krishnai and Chedrang Rivers in the Chedrang valley. In certain specified locations, significant surface rupture is observed, and the co-seismic surface uplift is measured to be 5 + 1 m on vertical planes at the western end of the rupture of the Oldham fault. The shear wave velocities along the profiles range from 184 to 466 m/s, indicating a platform for intense shallow seismicity triggered by fracture. The region experiences intense seismic activity, with the bottom of the seismogenic zone estimated to be 40 km, mainly dominated by thrust with strike-slip components. The local stress pattern in the Chedrang valley, oriented NE-SW, differs from the regional stress pattern, NNW-SSE, indicating the influence of geodynamic controls. Fluvial-morphometric measurements of the Krishnai and Chedrang Rivers for six different stretches in each case during 1964, 1991, and 2014 reveal a westward shifting of the Krishnai River ranging from 190 to 269 m, while the Chedrang River exhibits variable south-westward avulsion along these stretches, shifting approximately 132–312 m. Detailed fluvial morphological mapping highlights a key site where the stratigraphy of the fault scarp, reaching a height of 13ft, indicates the presence of modern channels, colluviums, gravel, and pebble beds with sand lenses, representing a portion of a mega thrust event. Simultaneously, high-resolution topographic profiles are analyzed using Total Station (GPS based) to observe co-seismic surface uplifts induced by the 1897 earthquake. The simplest deformation model suggests that a majority of the intra-seismic elastic loading east of the Krishnai and Chedrang rivers was uplifted during the 1897 earthquake due to co-seismic and post-seismic rupture, primarily in the western part of the Shillong plateau beneath the Chedrang valley. This energy transfer affected the north–south direction of the Krishnai valley, where the Krishnai river and the 'Mori' (dead) Krishnai river exist, as evidenced by subsurface deformation and subsequent avulsion of the Krishnai river. The parameters investigated in this study contribute to a better understanding of the geodynamics and seismic hazard assessment of the region, providing valuable insights into the consequences of the 1897 earthquake and its implications for future seismic events. [ABSTRACT FROM AUTHOR]