Moment Tensor Inversion of Moderate Earthquakes and the Locally Perturbed Stress Field in the Jiashi Source Region

Abstract

Moment tensors of 52 moderate sized earthquakes occurred in Jiashi source region, Xinjiang, from 1997 to 2004 were determined in time domain by using the long period waveforms recorded by regional broadband stations. The results demonstrate significant variations of the orientation and plunge of P, T and N axis from west to east in the source region. Therefore, we further divided the whole region into two parts, which are called the western part and the eastern part, and performed the stress tensor inversion respectively. The results show that in the eastern part, the maximum principal axis σ1 is almost horizontal and trends N321°E, the minimum principal axis σ3 trends 68° with a plunge of 40°, and σ2 trends 213°; with a plunge about 40°;. Till 2004, the stress within the western part remains stable, with the σ1 and σ3 nearly horizontal and trending to N12°E and N282°E respectively, the middle axis σ2 almost vertical. Thus the maximum principal stress σ1 significantly rotates counter clockwise from N10°E in the west part to N39°W in the east part across the whole studied region. Compared with the stress field in the eastern part, the western part is under a stronger horizontal extension effect. This lateral perturbation of stress field is consistent with the pattern of crustal deformation rate from GPS observation, as well as the topography map at the structure boundary between Tarim basin and Tienshan. We infer that both the sharp deformation in the deep crust and the large‐scale NNW trending Puchang and Selibuya fault structures may play a crucial role in resulting in the local stress perturbation pattern.

Active Structural Geometries and Their Correlation With Moderate (M 5.5‐7.0) Earthquakes in the Jiashi‐Keping Region, Tian Shan Southwestern Front

“…Furthermore, two additional profiles are interpreted to constrain fault geometries associated with the 2020 M w 6.0 and the 2003 M w 6.2 events. For the faults associated with the 1997‐2000 earthquake swarm (Figure 1b), they are too deep (>10 km) to be resolved by seismic‐reflection profiles; their geometries are determined using seismologic parameters and relocated epicenters (Guo et al., 2002; Huang et al., 2017; Zhao et al., 2008). All above efforts provide an improved knowledge of the 3D architecture of active systems in the region.…”

“…Seismicity in this region, however, is characterized by frequent moderate (M 5.5‐7.0) events instead of strong (M ≥7.5) events as observed in other Tian Shan regions. For example, between 1997 and 2000, nine events with M w ≥5.5 and two of them with M w ≥6.0 occurred in this region (Figure 1b, Table 1; e.g., Huang et al., 2017; Sloan et al., 2011; Zhao et al., 2008). In 2003, a M w 6.2 event shocked the region again, followed later by the most recent 2020 M w 6.0 event.…”

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

“…where β is the shear-wave velocity set to 3.60 km/s at depths of 15-20 km near the source (Zhao et al, 2008), R 1 is the transition distance, and n 1 and n 2 represent the decay rates at the first and second segmentation, respectively. The SVD method was applied to solve Eq.…”

“…where M 0 and f c are the seismic moment and corner frequency, respectively. R ΘΦ is the average radiation pattern over a suitable range of azimuths and take-off angles [set to 0.55 according to Boore and Boatwright (1984)], V 1/√2 represents the partitioning of the total S-wave energy into horizontal components, F 2 accounts for the free surface amplification effect, and ρ is the density in the vicinity of the source (set to 2,600 kg/m 3 according to Zhao et al (2008)).…”

Source Rupture Processes of 3 Jiashi Ms6 Events (1998~2003) and Its Correlation with the Aftershock Activity

“…In terms of the relocation and mechanisms of the aftershocks, the relation of the aftershock locations and their mechanisms with the rupture process of the main shock is discussed for 2 M s 6 events occurred in 1998. Some of the aftershock's mechanisms are from the moment tensor inversion [9,10] , and some are obtained from applying the FOCMEC using both the polarity and amplitude ratio data [9] . There were more than 20 P-wave and 10 SH-wave polarities, and more than 10 P-wave/SH-wave ratios for each event in the calculation, which guarantied a precisely focal mechanism to be obtained.…”