Determination of Focal Depths, Moment Magnitudes and Focal Mechanisms of Small Magnitude Local and Regional Earthquakes Recorded by a Sparse Seismic Network

Date: 4/25/2019

Time: 04:15 PM

Room: Cascade I

Determination of precise focal depths, moment magnitudes and focal mechanisms of small magnitude earthquakes using regional network stations is important in order to understand earthquake source processes and seismic hazard. We present a technique that can accurately determine focal mechanisms, moment magnitudes and focal depths of small magnitude earthquakes (2.5 < M < 4) using only a few regional seismic stations. We filter observed seismograms as well as synthetic seismograms through a higher frequency band like1-3 Hz and 1.5-2.5 Hz, which has a good signal-to-noise ratio (SNR) for the small earthquakes of the magnitude that we are working with. The waveforms are processed to their envelopes in order to make the waveforms relatively simple for the modeling. We have tested the method on 17 aftershocks of modified duration magnitude (M_D*) between 3.6 and 2.5 and 1 aftershock of magnitude M_w 3.14 of the 2011 Mineral, Virginia M_w5.7 earthquake. We have also tested the method on 5 aftershocks of 2013 Ladysmith, Quebec M_w4.5 earthquake of magnitudes between Mn 2.8 and Mn 3.7. Our depths, moment magnitudes and focal mechanisms are consistent with the depths, moment magnitudes and focal mechanisms that were previously reported for these events. This study confirms that the envelopes of the seismic waveforms can be used to extract the focal mechanisms and focal depths of an earthquake as low as M2.5 using only a few regional seismic network stations at epicentral distances of 200-500 km, and an accurate estimate of the focal depth and moment magnitude is possible using data from just one seismic station. This method is applied to study the aftershocks of Mw 7.8 2015 Nepal earthquake.

Presenting Author: Nawa Dahal

Authors