Source parameter and tectonic implications of small earthquakes originating in South India

Abstract

A data collection of 80 local earthquakes (1.8 < M_L < 3.5) recorded by several seismic stations beneath South India from February 2009 to October 2012 was studied to estimate the source parameter characteristics. The result shows that the seismic moments ($M_0$) vary from 8.95$\times {10}^{11}$to 6.56$\times {10}^{13}$ Nm, while source radii (r) are between 120 and 150 m. The source radius seems to be independent of magnitude and smaller within a major part of the region. This can be due to local earthquakes that may originate in the region from either the brittle shear-failure mechanism on faults or the presence of weakened zones in this region. The estimated stress drops values range from 0.20 to 3 MPa for most of the events and shows an increasing trend with the seismic moment, indicating a wide range of strength of crustal rocks. Few lower crustal event exhibits slightly elevated stress drop (4–10 MPa) values, and these cannot be solely attributed to a single model; instead, it appears that the potential contributing factors vary area wise. The corner ($f_c$) and high cut ($f_{\max }$) frequency values are bit scattered with the seismic moment, and the possible explanation would be either a complex rupture process or the involvement of a long period spectrum in the component. Both $f_{\max }$ and $f_c$ show a decreasing trend against seismic moments, indicating that both are caused by a source process and independent of epicentral distances and focal depths. However, source displacement $\left(D\right)$(0.006 and 0.04 m) and radiated seismic energy ($E_s$) increases linearly with the seismic moment and is an indication of the size dependency feature. We established various empirical relationships between source parameters, including M_W - M_L and log(M₀) - M_L and proposed the M_L- M_W relationship for the study region, which is M_W $\propto$ 0.62M_L. Overall, the present study indicates that most source parameters tend to vary with the size of the earthquake and generally follow the global model of small magnitude earthquakes. The information, we gained through this study provides insight into earthquake size, source physics, and that can help the scientific community significantly, to better understand, mitigate, and respond to the seismic hazards posed by earthquakes in the studied area.