Earthquake focal mechanisms and stress field in Sichuan-Yunnan area determined using P wave polarity and short period P and S waveform data

Abstract

CLC number: P315.3+3 Document code: A Based on waveform data, several methods to determine focal mechanisms of small earthquakes were developed since 1980. Kisslinger (1980) and Julian, Foulger (1996) proposed an approach to determine solution by using amplitude ratio of P and S wave. Schwartz (1995) devised a method to determine solutions by the use of polarity data and amplitudes of seismogram envelopes. Amplitudes of short period seismic waves propagating in an inhomogeneous medium are sensitive to the variation in velocity and Q structure. Nakamura, et al (1999) took medium inhomogeneity into account in determining focal mechanisms of small earthquakes using waveform data. If the locations of small earthquakes are concentrated in a small region, we can assume that the raypaths from the events to a given station are almost the same. So P and S wave attenuations are independent of event locations. In this case it is convenient to determine focal mechanisms of these events by using short period P and S wave data Q. Focal mechanism solutions of small earthquakes in 5 regions, i.e., Rongchang, Mabian-Muchuan, Ya'an, Baoxing and Mianzhu, which are covered by the Chengdu Telemetered Network, are obtained by analyzing the P polarity and short body wave amplitude data recorded in the network since 1992. According to the method proposed by Gephart and Forsyth (1984), based on well determined focal mechanism solutions in 15 sub-zones of Sichuan and Yunnan area, three principal stress tensors cry, Crz, and crj, instead of averages of P, B, and T axis of the solutions, are determined to represent the regional stress field distribution. Method Determination of focal mechanismFor P or S wave, the spectrum amplitude U!j(co) from i-th event observed atj-th station can be written as Uij (co) = m i ( co)Ri/C~ (co) / rij (1)where mi(co) is seismic moment, Ri/is source radiation factor, r,) is source distance, co is angular frequency. Here only *

Spatiotemporal Variations of Focal Mechanism Solutions and Stress Field of the 2019 Changning Ms 6.0 Earthquake Sequence

“…Among the various methods to compute FMS, Compared with the P-wave first-motion method (Chen et al, 2001), the inversion of body waves (Fan and Wallace, 1991;Dreger and Helmerger, 1993) and the inversion of surface waves (Patton, 1980;Thio and Kanamori, 1995), the "cut and paste" (CAP) method (Zhao and Helmberger, 1994;Zhu and Helmberger, 1996) can be regarded as a full waveform inversion method. It has the advantages of requiring fewer stations, and the inversion results are relatively independent of the selected velocity model and the lateral variations in the crustal structure (Tan et al, 2006;Zheng et al, 2009;Luo et al, 2015;Zhang et al, 2015a;Zhao et al, 2021).…”

Spatio-temporal variation of the stress field in the Wenchuan aftershock region

“…From the stress tensor of earthquakes at depths shallower than 12 km (Figure 3a), the segmentation feature of aftershock area is obvious: the dominant orientation of the S 1 axis on the southern segment is NW-WNW, and close to that of the regional stress field (Cheng, 1981;Xu et al, 1987;Chen et al, 2001), indicating that this segment is under NW-WNW directed horizontal compressive stress state. Since less shallow earthquakes occurred on the Wenchuan-Mianyang and Beichuan-Pingwu segments, we did not get the stress tensor solution there.…”