Simultaneous estimation of earthquake source parameters and crustal Q value from broadband data of selected aftershocks of the 2001 M w 7.7 Bhuj earthquake

Abstract

This paper presents the simultaneous estimation of source parameters and crustal Q values for small to moderate-size aftershocks (M w 2.1–5.1) of the $M_{w }$ 7.7 2001 Bhuj earthquake. The horizontal-component S-waves of 144 well located earthquakes (2001–2010) recorded at 3–10 broadband seismograph sites in the Kachchh Seismic Zone, Gujarat, India are analyzed, and their seismic corner frequencies, long-period spectral levels and crustal Q values are simultaneously estimated by inverting the horizontal component of the S-wave displacement spectrum using the Levenberg–Marquardt nonlinear inversion technique, wherein the inversion scheme is formulated based on the ω-square source spectral model. The static stress drops (Δσ) are then calculated from the corner frequency and seismic moment. The estimated source parameters suggest that the seismic moment (M 0) and source radius (r) of aftershocks are varying from 1.12 × 1012 to 4.00 × 1016 N-m and 132.57 to 513.20 m, respectively. Whereas, estimated stress drops (Δσ) and multiplicative factor (E mo) values range from 0.01 to 20.0 MPa and 1.05 to 3.39, respectively. The corner frequencies are found to be ranging from 2.36 to 8.76 Hz. The crustal S-wave quality factor varies from 256 to 1882 with an average of 840 for the Kachchh region, which agrees well with the crustal Q value of the seismically active New Madrid region, USA. Our estimated stress drop values are quite large compared to the other similar size Indian intraplate earthquakes, which can be attributed to the presence of crustal mafic intrusives and aqueous fluids in the lower crust as revealed by the earlier tomographic study of the region.