Inversion of Non-Double-Couple Source Mechanisms for Single-Well Microseismic Events Considering Converted Waves during Hydraulic Fracturing in the Weiyuan Area

We propose an approach for estimating microseismic source mechanisms in single-well monitoring by introducing converted-wave data to resolve Non-Double-Couple (non-DC) components. This method incorporates converted wave phases in addition to conventional first-arrival waves, compensating for the limitations of azimuthal coverage and the limited number of stations in single-well monitoring, thereby offering more accurate Double-Couple (DC) and non-DC components. By calculating the norm of the waveform fitting error, we used grid search to determine the optimal solution. Synthetic testing indicates that this method can accurately invert for non-DC components in single-well observation systems. We applied this method to hydraulic fracturing induced seismic clusters in the Weiyuan area and found that the non-DC component differs between fault-reactivation seismicity, which resembles tectonic earthquakes, and fracture-driven microseismicity typical of hydraulic fracturing. By comparing borehole inversion results with those from surface stations with better azimuthal coverage, we validated the method’s accuracy and advanced the understanding of the physical processes driving hydraulic fracturing-induced seismicity.