Earthquake Source Characteristics and S-Wave Propagation Attenuation in the Junction of the Northwest Tarim Basin and Kepingtage Fold-and-Thrust Zone

Hongwei Wang,Ruizhi Wen

doi:10.3389/feart.2020.567939

Abstract

We separated the propagation path attenuation and source spectra from the S-wave Fourier amplitude spectra of the observed ground motions recorded during 46 small-to-moderate earthquakes in the junction of the northwest Tarim Basin and Kepingtage fold-and-thrust zone, mainly composed of two Jiashi seismic sequences in 2020 and 2018. Slow seismic wave decay was observed as the distance increased, while the quality factor regressed as 60.066f0.988for frequencyf= 0.254–30 Hz reflects the strong anelastic attenuation in the study region. We estimated the stress drops for the 46 earthquakes under investigation from the preferred corner frequencies and seismic moments by fitting the inverted source spectra and the theoretical ω-square model. The relationship between seismic moment and corner frequency and the dependence of the stress drop on the moment magnitude reveal the breakdown of earthquake self-similar scaling for the events in this study. The temporal variation in stress drops indicates that the mainshock plays a short-term role in the source characteristics of the surrounding earthquakes. Aftershocks immediately following the mainshock show a low stress release and then gradually recover in a short time. The healing process for the fractured fault in the mainshock may be one reason for the stress drop recovery of the aftershock. The foreshock with the low stress release occurring in the high-heterogeneity fault zone may motivate the following occurrence of the largest magnitude mainshock with a high stress drop. We inferred that the foreshock-mainshock behavior, including several moderate events, may be predisposed to occur in our study region characterized by an inhomogeneous crust.

Highlights

A moderate earthquake of Ms 5.4 abruptly shook the Jiashi county of the Xinjiang region in northwest China on January 18, 2020, at 00:05 Beijing time (Figure 1), arousing the 2020 Jiashi seismic sequence
The Kepingtage thrust fault is the southernmost margin of the Kepingtage fold-and-thrust zone, Cenozoic compressive structures neoformed above a Paleozoic basal decollement level at a depth of 4–6 km, predominantly thrusting toward the interior of the northwest Tarim Basin to the south (Allen and Vincent, 1999; Turner et al, 2010; Yang et al, 2018; Laborde et al, 2019)
The hinged bilinear geometrical spreading model was used, and the inverted path attenuation curve was modeled by ln A fm, Rk,m n1 ln R0 Rk,m + fm Q fm

Summary

Introduction

A moderate earthquake of Ms 5.4 abruptly shook the Jiashi county of the Xinjiang region in northwest China on January 18, 2020, at 00:05 Beijing time (Figure 1), arousing the 2020 Jiashi seismic sequence. This sequence rapidly reached its climax on the following night as the largest magnitude mainshock, measured as Ms 6.4, occurred ∼2.5 km to the east of the Ms 5.4 foreshock, and a great number of aftershocks followed immediately, including the largest one measured as Ms 5.2 about 22 km to the east of the mainshock. Much deeper focal depths implied that this sequence was more likely to occur in the basement structures below the decollement level, rather than the thrust sheets that grew above the decollement level (Gao et al, 2013; Laborde et al, 2019)

Methods

Results

Conclusion