An impr ov ed estimation of stress dr op and its application on induced earthquakes in the Weiyuan Shale Gas Field in China

Abstract

Stress drop is a proxy of understanding earthquake source process, and it is controversial whether the stress drops of induced earthquakes associated with hydraulic fracturing and injection activities are similar to those of tectonic earthquakes. The measurement of stress drops is usually biased due to the limitations of observation means, or hidden issues in the estimation approaches. Utilizing a local short-period seismic network, we investigate the stress drops of induced earthquakes in Weiyuan Shale Gas Field in Sichuan Province, China from 2019 to 2020. Totally 11 844 earthquakes are involved in the analysis, and their stress drops are obtained using an improved approach on the basis of the traditional spectral decomposition method combined with a global optimization algorithm to avoid stacking of spectra that is found leading to source parameter underestimation. We divide the studied area into three subareas, and the results show strong stress drop heterogeneity across the entire region. We obtain an average stress drop of 2.29 MPa, piecewise stress drop dependence to earthquake magnitude, and complex depth dependence pattern. Our results indicate that stress drops of induced earthquakes are overall consistent with the induced earthquakes in other areas as well as tectonic earthquakes in different environments. Meanwhile, the complexity in the stress drop dependence to depth possibly reflects the variability of stress drops for different earthquake triggering mechanisms.