The attenuation mechanism of high-frequency seismic waves in the Palghar swarm earthquake source area in Western India

Abstract

We study attenuation of seismic waves in the source area of Palghar swarm in western part of India. The intrinsic and scattering loss are separated using the Wennerberg method in the assumption of co-located source and receiver. The coda-normalization and single back-scattering model of coda wave generation are used for determining Q factors describing attenuation for body (Q_P and Q_S) and coda (Q_C) wave, respectively. The data set includes 1419 high-quality earthquakes (1.5 < M_L<4.7) during intense swarm from 2019 to 2022 recorded by six stations in the Palghar region. Analysis is performed at five central frequencies 1.5, 3, 6, 12, and 24 Hz for varying lapse time windows of 5 to 40 s. The frequency-dependent P- and S-wave attenuations are expressed as ${Q_P=\left(8.7\pm 1\right)f}^{\left(0.85\pm 0.01\right)}$ and ${Q_S=\left(28.4\pm 0.4\right)f}^{\left(0.87\pm 0.004\right)}$, respectively in 1.5–24 Hz. The spatially averaged frequency-dependent coda Q_C(f) relations are ${Q_C=\left(26.9\pm 13\right)f}^{\left(0.93\pm 0.04\right)}$ and ${Q_C=\left(138.8\pm 41\right)f}^{\left(1.08\pm 0.05\right)}$ for 5 and 40 s, respectively. The S-waves attenuate faster than the coda waves in 1.5–24 Hz. The Q_P/Q_S ratio is greater than unity in the analysed frequencies. Intrinsic attenuation dominates the scattering attenuation in the whole frequency range. Dominant intrinsic absorption with its strong frequency dependence requires the presence of fluids in the shallow crust, as shown from other geophysical methods in the Palghar swarm area. Attenuation mechanisms are found to be similar for other swarm areas worldwide. The attenuation results could be useful while finding the earthquake source parameters to correct the path in the modelling for accurately estimating source scaling relations of swarm-related earthquakes.