Search

Stress features, particularly local stress field and earthquake stress drops, are important to understand mechanism of induced earthquakes. Since shale gas exploitations in 2015, the Weiyuan shale gas block has experienced frequent earthquakes. In this paper, we determine focal mechanisms of 257 events with ML > 1.5 by fitting three‐component waveforms, invert for direction of maximum horizontal stress for two dense earthquake clusters, and then calculate stress drops for 17 earthquakes with moment magnitudes between 2.2 and 2.75 through the spectral ratio method. The focal mechanisms of all earthquakes are reverse faulting. The orientation of local maximum horizontal compressive stress is in the ESE direction, consistent with crustal movement indicated by GPS measurements. Both the focal mechanisms and microseismicity locations suggest the existence of steeply dipping faults with dip angles of ∼70°, which are relatively rare and difficult to slip under usual conditions, unlike major induced earthquakes with strike‐slip or low‐angle thrusting faults in the midwestern United States and western Canada. However, the steeply dipping reverse events can be induced by large pore pressure from hydraulic fracturing. The stress drops range from 2.5 to 54.7 MPa, comparable to those of potentially induced earthquakes in the midwestern United States. Our results imply earthquakes in the Weiyuan area are controlled by local tectonic stress and induced by large pore pressure from hydraulic fracturing, which advances our knowledge of reactivation of steeply dipping reverse faults. Plain Language Summary: Earthquakes in the Weiyuan area in southwestern China are thought to be induced by hydraulic fracturing due to shale gas production. Local stress field and earthquake stress drops provide crucial information to understanding their physical mechanisms. Here we determine source parameters of 257 events with magnitude greater than 1.5, invert for direction of maximum horizontal stress, and calculate stress drops for 17 representative events. The results suggested that all the earthquakes occurred on reverse faults and the inferred maximum horizontal stress orientations agree with local GPS measurements. Surprisingly, we find some steeply dipping faults with dip angle of ∼70°, which are assumed to be stable under usual stress conditions. The stress drops range from 2.5 to 54.7 MPa, which are comparable to those of potentially induced earthquakes in the midwestern United States. We infer that the steeply dipping reverse events are induced by high pore pressure from hydraulic fracturing. Our results imply earthquakes in the Weiyuan area are controlled by local tectonic stress and induced by hydraulic fracturing with high‐pressure fluid injection. Key Points: Focal mechanisms of 257 ML > 1.5 earthquakes suggest consistent maximum horizontal stress with local tectonic settingsSteeply dipping reverse faults may be reactivated by high pore pressure fluid during hydraulic fracturingStress drops of the induced earthquakes range from 2.5 to 54.7 MPa [ABSTRACT FROM AUTHOR]

Hydraulic fracturing has induced small‐to‐moderate‐size earthquakes around the world. Identifying spatio‐temporal evolution of microseismicity is important for understanding the physical processes that control hydraulic fracturing‐induced seismicity. In this study, we build an enhanced earthquake catalog from continuous seismic data recorded by 1‐year temporary deployment with an automatic procedure and relocate 18,663 earthquakes in the Weiyuan shale gas block in the southern Sichuan Basin, China. Our catalog, with a maximum ML of 3.5, has a completeness magnitude (Mc) of 0.4, which is ∼23 times more events than listed in the standard National Earthquake Data Center (NEDC) of China. Most earthquakes are clustered near hydraulic fracturing wells and delineate many pre‐existing faults in the north‐south direction. The space‐time evolution of microearthquakes indicates fluid diffusion processes as the primary drivers for seismicity in this region. The fast earthquake migration patterns show that permeability within a fault zone could be affected by fault‐valve behaviors and enhanced by earthquake rupture process. We find that the Gutenberg‐Richter b‐values increase systematically with depth, and b‐values further away from hydraulic fracturing pads are generally low, especially for three strands with relatively high migration velocities. Our results confirm that earthquake clusters in Weiyuan are induced by hydraulic fracturing and the reactivated faults act as conduit networks for fluid flow, which promote triggering of earthquakes further away from the hydraulic fracturing pad. This study provides additional evidence for hydraulic fracturing‐induced earthquake in the southern Sichuan Basin and advances our understanding of injection induced earthquakes. Plain Language Summary: The Weiyuan area in the stable southern Sichuan Basin, China recently has experienced increasing seismicity since shale gas mass production. Spatio‐temporal evolution of these earthquakes is important for understanding the physical processes that control hydraulic fracturing induced seismicity. In this study, we build an enhanced earthquake catalog with a completeness magnitude (Mc) of 0.4 from continuous seismic data recorded by 1‐year temporary deployment and relocate 18,663 events in the Weiyuan area. Most events are clustered near hydraulic fracturing wells and delineate many pre‐existing faults in the north‐south direction. We then analyze spatio‐temporal migration and b‐value changes of the seismicity, and find that earthquake clusters in the Weiyuan area are induced by hydraulic fracturing mainly through pore fluid diffusion. The reactivated faults act as conduit networks for fluid flow, which promote triggering of earthquakes further away from hydraulic fracturing pads. Fault‐zone permeability may also be affected by fault‐valve behaviors and can be enhanced by recent fault ruptures. This study provides additional evidence for hydraulic fracturing induced earthquake in the southern Sichuan Basin and advances our understanding of injection induced earthquakes. Key Points: Earthquakes in the Weiyuan shale gas block delineate complex fault systems near hydraulic fracturing wellsSpatio‐temporal migration of the seismicity reveals pore fluid diffusion as the primary triggering mechanismEarthquake b‐values systematically decrease with increasing depth and their variations may be affected by fault behaviors [ABSTRACT FROM AUTHOR]

On 7 September 2019, an ML5.4 earthquake struck the Weiyuan area in southwestern China. This is the largest and most damaging event in the Weiyuan shale gas field. In the next 4 months, two earthquakes with ML4.3 and ML5.2 rattled the same area. In this study, we obtain source parameters of these earthquakes (e.g., centroid location, focal mechanism, and rupture directivity) to investigate whether they are induced by hydraulic fracturing of shale gas production. We first invert for high‐resolution centroid locations and focal mechanisms of two M ~ 3 earthquakes recorded by a temporary dense seismic array. We then adopt the M ~ 3 events as references to obtain reliable centroid locations and rupture directivity of those moderate‐size earthquakes. The results show that all earthquakes have centroid depths of no more than 3 km and are within 2 km to injection wells of shale gas production, which suggest that these earthquakes may be induced by local hydraulic fracturing. The ML5.2 earthquake has the same rupture directivity to NE as the ML5.4 event and is located in the zone of positive Coulomb stress change caused by the latter. However, the long time lapse of the ML5.2 earthquake and the existence of an injection well within 2 km suggest that it is more likely induced by hydraulic fracturing than triggered statically by the ML5.4 earthquake. Key Points: The shallow thrusting earthquakes are located within 2 km to injection wells in the Weiyuan areaThe ML5.2 earthquake is 5–7 km to NE of the ML5.4 earthquake, and both events rupture to NEThe ML5.2 earthquake is more likely induced by hydraulic fracturing than triggered statically by the ML5.4 earthquake [ABSTRACT FROM AUTHOR]

We applied joint inversion of multifrequency receiver functions and surface wave dispersion to image crustal S-wave velocities and discontinuities beneath 29 temporary seismic stations with ~5 km station spacing in the Weiyuan shale gas field. The results show similarities and differences in the crust between the anticline and syncline areas and a widespread low-velocity layer in the synclinal sedimentary rocks. Similar crustal S-wave velocities and high velocities in the middle-lower crust and a thickness of ~12 km in the upper crust are observed in both the anticline and syncline areas. The sedimentary thickness changes mainly from 3 km to 4 km, and the average crustal thickness is ~42.4 km in the anticline area, while sedimentary thickness varies mainly from 4 km to 5 km and the average crustal thickness is ~41.0 km in the syncline area. These results suggest that the Weiyuan anticline should be a thin-skinned structure and that the study region maintains a typical cratonic structure in the crust. The observed 1–2 km-thick low-velocity layer at depths of 3–4 km in the syncline area may be related to Ordovician and Silurian shale gas strata with high porosity and low density. The observed characteristics of alternating high−/low-velocity layers and long-term hydraulic fracturing and water injection may have induced the occurrence of M L > 4.0 earthquakes in the Weiyuan shale gas field since 2018. Our study suggests that dense temporary seismic data can not only delineate small-scale structural characteristics but also provide valuable references for oil-gas exploration and earthquake mitigation. • A widespread low-velocity layer in sedimentary cover in the syncline area. • Low-velocity layer corresponds to shale gas strata. • Weiyuan anticline should be a thin-skinned structure. • Weiyuan shale gas field maintains a typical cratonic structure. • Alternating velocity layers and hydraulic fracturing induce M L > 4.0 earthquakes. [ABSTRACT FROM AUTHOR]