Stress drops calculated from seismic Lg-waves and their applications for investigating the typical earthquake sequences in the eastern margin of the Tibetan Plateau

Stress drop measures the stress release level over a fault during an earthquake rupture. As one of the important parameters for characterizing source mechanisms and predicting strong ground motions, the stress drop is controlled by the tectonic environment, focal mechanism, and type of earthquake sequence. The stress drop measurements from the seismic data also depend on the observation frequency band. Therefore, the values obtained from various seismic phases, such as body and surface waves, may be different. Previous studies have often used indirect methods to remove the attenuation effects along the propagation path to obtain the source spectra and then estimate the stress drops. Using a broadband high-resolution Lg-wave attenuation model, the attenuation effect can be directly corrected to obtain the Lg-wave source excitation spectra. By fitting the observed spectra to the theoretical source model, we can calculate the seismic moments and corner frequencies from which the stress drops can be calculated. Taking a typical tectonic earthquake and a potentially induced earthquake, that is, the 2017 M_S7.0 Jiuzhaigou earthquake sequence and the 2019 M_S 6.0 Changning earthquake sequence, in the eastern margin of the Tibetan Plateau as examples, we explored the potential physical differences between tectonic and induced earthquakes. The stress drop in the 2017 Jiuzhaigou mainshock was approximately 27 MPa. The stress drops, and the magnitude of its aftershocks rapidly decay. However, for the 2019 Changning earthquake sequence, the temporal variation of stress drops declined slowly, with two aftershocks having stress drops comparable to the main shock. For an area with long-term water injection, it takes a long time to recover to its equilibrium status once a large earthquake disturbs the regional stress field. Given that no apparent difference in the absolute level of stress drops can be observed between the two types of earthquake sequences, the stress drops alone cannot be used to distinguish between induced and tectonic earthquakes in this area. The increasingly accumulated underground water may have created pathways linking multiple fault systems in the changing salt mining. Thus, the possibility of future induced earthquakes cannot be ruled out.