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Abstract:
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Hydraulic fracturing has been inferred to trigger the majority of injection‐induced seismicity in the Zhaotong and Changning shale gas field, Sichuan basin of China, in contrast to the Midwestern United States, where massive wastewater disposal has been the dominant triggering mechanism. More than 15,000 earthquakes, with magnitudes ranging up to MwMw 4.7, were recorded by a temporary network in the Sichuan basin for three years, with a completeness magnitude of MLML 1.1. A catalog of earthquakes relocated with code tomoDD, combined with comprehensive injection data during an eight day period, shows that the earthquakes were highly correlated spatiotemporally with hydraulic fracturing activities mostly from a single well pad. Three ML≥4.0ML≥4.0 events occurred during hydraulic fracturing operations from 12 to 19 January 2017, followed by the fourth and largest event, with moment magnitude (MwMw) 4.7, on 28 January. The hypocenters of the four largest events were located in dolomite of Cambrian age, between a shale gas reservoir and the top of the crystalline basement rocks. This was found to be similar to 60% of the smaller earthquakes in this cluster, at depths from 2.5 to 4.0 km. |
