Correlation of change in magnetotelluric parameters before and after Sikkim Himalayan earthquakes associated with stress variation

Abstract

MT stations that were recorded before the moderate 5.3 Mw, 2006 Sikkim Himalaya earthquake had been reoccupied after 2007 (Mw 5.0) and 2011 (Mw 6.9) seismic activities. We analyzed the broadband MT data (0.001–1000 s) within the 60 km radius from epicenters location and within the focal depth range of 9–30 km. The observed change in apparent resistivity in the period range of 0.001–10 s has shown a considerable post-seismic decrease of the order 10–30%. The 2D geoelectric model constructed with post seismic acquired data also reveals the feature of a decrease in resistivity (C1). The relative change (decrease) of observed apparent resistivity ((Δρ/ρobs) after 2006, 2007 and 2011 earthquakes find a correlation with combined stress drop 11–18.2 MPa values estimated by Arms-Δσ approach. The correlation is further strengthened by a similar change in relative apparent resistivities ((Δρ/ρ2D_model) derived from 2D model with stress drop. Integrated stress drop values attribute to the accumulated apparent stresses (σa = 0.23 ∆σ) that reveals the area might be subjugated to 0.6–1.2 MPa stress before 2006, 1–1.5 MPa stress before 2007 and 1.7–2.9 MPa before 2011 earthquakes occurrence. Development of micro-cracks and generation of aqueous fluids is possible due to such stressed and slow rate of relentless thrust/shear environment within the area of crust. Therefore, resistivity could have been decreased with a fluid transition from compression to dilatancy zones at different depth levels before each earthquake and further decreased upon subsequent stress drops associated with strong ground motions. We interpreted that maximum stress might be dropped in dilatational zones oblique to the maximum compression stress directions along N200E, N-S and NNW-SSE before 2006, 2007 and 2011 earthquakes and resulted in a considerable decrease in apparent resistivity along E-W (TE-mode), i.e., ρyx component.