[S-SS07] Environmental Seismology: from deep earth to surface process

[SSS07-P06] Stress drops estimated by regional Lg-waves and its applications to typical earthquake sequences in the eastern margin of the Tibetan Plateau

*Xu Chang¹, Lin Shen¹, Lian-Feng Zhao¹, Xiao-Bi Xie², Zhen-Xing Yao¹ (1.Institute of Geology and Geophysics Chinese Academy of Sciences, 2.Institute of Geophysics and Planetary Physics, University of California at Santa Cruz)

Keywords:Source parameters, Stress drop

The stress drop measures the average shear stress difference before and after an earthquake, which is influenced by the tectonic environment, focal mechanism, and type of earthquake sequence. Some studies showed that the stress drops of induced earthquakes tend to be lower than those of typical tectonic earthquakes, possibly due to the reduction in effective normal stress caused by fluid injection. However, some researchers argued that the induced earthquakes are primarily driven by tectonic stresses, and their stress drop values are comparable to those of tectonic earthquakes. To investigate the potential physical differences between tectonic and induced earthquakes in the eastern margin of the Tibetan plateau, we estimate the stress drops of a potentially induced earthquake sequence and two tectonic earthquake sequences: the 2019 Ms 6.0 Changning earthquake sequence, the 2017 Ms 7.0 Jiuzhaigou earthquake sequence, and the 2021 Mw 7.4 Madoi earthquake sequence. Based on an established regional high-resolution broadband Lg-wave attenuation model, we directly correct the attenuation effect along the propagation path for observed spectra. Fitting observed spectra with a theoretical source model, we estimate seismic moments, corner frequencies, and high-frequency falloff rates for these earthquake sequences, from which the stress drops are calculated. Our results show no significant difference in the absolute stress drops among these earthquakes with the same magnitude, indicating the stress drops may be related to the tectonic environment. However, when comparing the temporal variation of stress drops, we observe that the 2019 Changning earthquake sequence exhibits a slower decline in stress drop over time, with two aftershocks having stress drops comparable to the main shock. Given that the Changning area has experienced over 20 years of water injection and elevated pore pressure underground, it may take a long time for the regional stress field to return to equilibrium following a large earthquake. This work is supported by the National Natural Science Foundation of China (42474084).