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Abstract

Hydraulic fracturing (HF) is a viable technique to stimulate the productivity of unconventional hydrocarbon resources. However the mobility of injected fluid in HF plays a critical role in controlling fault reactivation and consequently induced seismicity. Yet tracking the injected fluids underground remains challenging because of the limited resolution in traditional approaches. In this study, we attempt to investigate the possible controlling mechanisms of induced seismicity in the Weiyuan shale gas field in Sichuan, China using a dense temporary seismic network that was deployed between April to June 2020. We apply the frequency index to analyze the impact of injected fluid on event waveforms and classify them based on the low/high-frequency components. We assess the potential controlling mechanism of these waveforms, including source, path, or injected fluid, and evaluate with waveform analysis. Our results reveal that low-frequency waveforms (LFW) are prominent in a certain azimuth range which rules out the possibility of source effects. Meanwhile, most of these LFW are constrained temporally and spatially and unlikely to be path effects. Furthermore, waveforms from nearly collocated earthquakes at different depths have distinct frequency contents. P phases are relatively unaffected, compared to highly attenuated S phases which suggest that absorption might be a key factor, instead of scattering. We identify a zone with congregated fluid near hydraulic fracturing platforms which might be responsible for LFW when ray paths encounter. Such fluid may trigger delayed seismicity or upward fluid migration which poses challenges to establishing an effective mitigation strategy for potential seismic risk.