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Hypocenter distances measured from the first event as a function of earthquake occurrence time for each sub‐fault enclosed by ellipses in Figure 4. The solid lines show the theoretical propagating pore pressure front and the numbers indicate the hydraulic diffusivities. In each case, diffusivity curves of double and half of the hydraulic diffusivity are plotted with dashed lines for reference. The red and green lines in (d, f) indicate the north front and the south front, respectively.
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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‐y...
Citations
... Studies on the seismicity migration (Lei et al., 2020;Meng et al., 2018), focal mechanism (Chu & Sheng, 2023;Dai et al., 2023;Liu & Zahradník, 2020;Sheng et al., 2020;Yi et al., 2020), Coulomb stress changes (Lei et al., 2017), finite fault models (S. Wang et al., 2022), statistical analysis (e.g., b-values and ETAS models, etc.) (Sheng et al., 2022;Tan et al., 2023), and numerical simulation (H. Yang et al., 2024; in southern Sichuan suggested that industrial activities could alter the subsurface fluid state, affect the local crustal stress, accelerate the fracturing of tectonic formations, and finally induce earthquakes. ...
... Fluid injection operations can propagate pore pressure and stress through fluid pressure diffusion, different types of sliding, and pore-elastic effect (Eyre et al., 2019;Gori et al., 2021). Researches have shown that fluid pressure diffusion typically occurs within a few kilometers from the injection point (Shapiro et al., 1997;Sheng et al., 2022;F. Zhang, Wang, et al., 2022), while significant stress transfer resulting from injection-induced aseismic slip can propagate at higher speeds and more easily trigger distant earthquakes (Cebry et al., 2022;Guglielmi et al., 2015;. ...
Unprecedented levels of seismicity have been seen in southern Sichuan, China, since the large‐scale exploitation of shale gas. Fluid and pore pressure transported through hydrological channel are thought as pivotal elements in the induction of earthquakes. Our high‐resolution tomography results reveal two inclined seismic anomalies featured by low Vs and high Vp/Vs at different depth range. The deeper anomaly extends 15 km from NE to SE and connects the well g048 from 3 km depth to the vicinity of the Ms 4.7 Gongxian earthquake 5.4 km deep, which is hinted to be a hydrological channel inferred from the high fluid overpressure of 28 Mpa calculated from focal mechanism solution. The injection operation of multiple shale gas wells along the channel may potentially accumulate the pore pressure and cause the fault near the end of the channel to reach critical stress state through various mechanisms.
... We can see that over the first 6 days, a diffusivity of 0.25 m 2 /s is able to fit most of the seismicity relatively well. This is on the same order of magnitude as the diffusivity we used for our simulations, and is consistent with some estimates of the this region 28,29 . There is also a group of seismicity in Fig. 5e after Day 5 that does not fit under the diffusion curve, and they can be reasonably well fit by a linear migration curve with a speed of 250 m/day. ...
As concerns rise over damaging earthquakes related to industrial activities such as hydraulic fracturing, geothermal energy extraction and wastewater disposal, it is essential to understand how subsurface fluid injection triggers seismicity even in distant regions where pore pressure diffusion cannot reach. Previous studies suggested long-range poroelastic stressing and aseismic slip as potential triggering mechanisms. In this study, we show that significant stress transfer far ahead of injection-induced aseismic slip can travel at much higher speeds and is a viable mechanism for distant earthquake triggering. It could also explain seismicity migration that is much faster than aseismic slip front propagation. We demonstrate the application of these concepts with seismicity triggered by hydraulic fracturing operations in Weiyuan shale gas field, China. The speed of stress transfer is dependent on the background stress level and injection rate, and can be almost an order of magnitude higher than that of the aseismic slip front.
... We propose that these earthquakes result from the response of pre-existing faults to fracturing activities. Previous studies have indicated that fault activation during and after hydraulic fracturing can be triggered by various mechanisms, including stress changes due to the elastic response of the rock-mass to hydraulic fracturing or pore-pressure changes due to fluid diffusion along a permeable fault zone (Bao and Eaton, 2016;Sheng et al., 2022). Stressrelated triggering tends to diminish shortly after operations, while a fluid-pressurized fault may be susceptible to persistent seismicity for several months at least (Bao and Eaton, 2016;Sheng et al., 2022). ...
... Previous studies have indicated that fault activation during and after hydraulic fracturing can be triggered by various mechanisms, including stress changes due to the elastic response of the rock-mass to hydraulic fracturing or pore-pressure changes due to fluid diffusion along a permeable fault zone (Bao and Eaton, 2016;Sheng et al., 2022). Stressrelated triggering tends to diminish shortly after operations, while a fluid-pressurized fault may be susceptible to persistent seismicity for several months at least (Bao and Eaton, 2016;Sheng et al., 2022). It appears that the outbreak period of these earthquakes occurred towards the end of our observation period. ...
... Based on the spatial-temporal statistics, we found that they occurred primarily at nighttime (local time from 8:00 pm to 8:00 am of the next day) (Fig. 7g) and exhibit continuous strip distribution, which started from northeast to southwest and then turning back to northeast (Fig. 3a, b). These events share the same characteristics as those observed in the Weiyuan shale gas block (~100 km away from the study region), which were suggested to be explosions for a 3D seismic survey (Zhou et al., 2021;Sheng et al., 2022). The preliminary P/S ratio analysis shows that these events exhibit distinct characteristics from natural earthquakes (Fig. S2), although more comprehensive investigations are necessary to differentiate between them (Wang et al., 2020b(Wang et al., ,c, 2021. ...
... Besides, the earthquake is close to the Moxi gas field, an important energy production base in the Sichuan-Chongqing region, thus raising possibility of the earthquakes induced by energy production processes (Keranen et al., 2013;Wang et al., 2016;Kim et al., 2018). Indeed, Earthquakes in the southern part of Sichuan Basin occurred near wastewater reinjection, well salt production and shale gas development, with some earthquakes above magnitude 5 (Sheng et al., 2020(Sheng et al., , 2022Lei et al., 2020). As one of the most destructive events occurred in Sichuan Basin, a detailed study of the Suining earthquake may help to understand the source characteristics and seismogenic mechanism of induced earthquakes in Sichuan Basin (Lei et al., 2020). ...
Although the Sichuan basin is a stable block with low historical seismicity, the Suining M5.0 earthquake on January 31, 2010 occurred near the center of the basin, causing casualty and substantial damage. Previous studies have shown that the earthquake is very shallow and may occur in the sedimentary cover rocks, but its causative fault has not been identified. Based on local broadband seismic waveform data as well as a pair of ALOS PALSAR ascending orbit data, we explore the seismogenic mechanism via further constraining the source depth and the ruptured fault. The earthquake caused ground uplift in the southeast of the epicenter area, with a maximum line of sight displacement of about 13.6 cm, much larger than the displacement caused by a M5 earthquake at a typical depth of 10 km, which indicates that the earthquake is very shallow. Through joint inversion of seismic waveform and InSAR data, we obtain the moment magnitude of Suining earthquake as MW4.5, with the strike, dip, and rake of its fault plane as 17°, 66° and 90°, respectively, and the centroid depth less than 1 km, supporting that the earthquake occurred at the shallow part of a high angle thrust fault dipping to the southeast. It is further confirmed that the earthquake may be triggered by the diffusion of high-pressure fluid migrating from the underside gas reservoir.
... The pore pressure diffusion front with hydraulic diffusivity 0.1 m 2 /s is plotted in dark blue. This is the value we use for our simulations and is consistent with some estimates of the same region26,27 However, outside the diffusion and aseismic slip front, there exist many earthquakes that migrate faster and could not be explained by just these two mechanisms. To compare, we also added 250 m/day, 500 m/day and 800 m/day trends in dotted red lines. ...
As concerns rise over damaging earthquakes related to industrial activities such as hydraulic fracturing, geothermal energy extraction and wastewater disposal, it is essential to understand how subsurface fluid injection triggers seismicity even in distant regions where pore pressure diffusion cannot reach. Previous studies suggested long-range poroelastic stressing and aseismic slip as potential triggering mechanisms. In this study, we show that significant stress transfer originating from injection-induced aseismic slip can travel at much higher speeds and is a viable mechanism for distant earthquake triggering. It could also explain microseismicity migration that is much faster than aseismic slip front propagation. We demonstrate the application of these concepts with seismicity triggered by hydraulic fracturing operations in Weiyuan Shale Gas Field, China. The speed of stress transfer is dependent on the background stress level and injection rate, and can be almost an order of magnitude higher than that of the aseismic slip front.
... Instead, a swarm of isolated deep earthquakes is distributed over a depth range of 15-25 km ( Figure 5a). A relatively continuous cluster of earthquakes is observed at the eastern limb of the Weiyuan anticline (WY) (e.g., G. Liu et al., 2023;Sheng et al., 2022;Wong et al., 2021). These earthquakes are located in Zones IV and VII (Figures 5b and 5c). ...
The Sichuan basin, located adjacent to the eastern margin of the Tibetan Plateau, serves as an ideal marker for testing the extrusion process of the plateau. The basin is seismically active, with the strongest earthquake, the 2008 Mw 7.9 Wenchuan earthquake, occurring in the Longmen Shan range along its northwestern edge. A new regional compilation of focal mechanism solutions of earthquakes in and surrounding the basin reveals that a large fraction of the events have focal depths ranging between 8 and 25 km, corresponding to the crystalline basement of the basin. Seismic deformation involves right‐lateral oblique reverse faults, mostly trending northeast–southwest, similar to the kinematics of the mainshock of the Wenchuan earthquake. Shallow earthquakes (3–8 km) suggest that some of the seismic faults rupturing the crystalline basement are growing toward the surface. To the southwest, the seismicity transitions to activity along the left‐lateral Xianshuihe–Xiaojiang fault zone. The spatial relationship between these two sets of fault zones is consistent with a model in which the Sichuan basin responds to the southeast extrusion of the Chuan‐Dian block at the southeast margin of the plateau by a counterclockwise bookshelf rotation of the crystalline basement. This deformation pattern initiated ∼4–2 Ma, as shown by the age of the Xiaojiang fault segment. The history of left‐lateral movement along the Xianshuihe‐Xiaojiang fault can be traced back to ∼12 Ma when the eastward growth of the Tibetan Plateau resulted in the shortening of both the Longmen Shan thrust belt and the sedimentary over the Sichuan basin.
... Earthquake swarms are seismic sequences without distinct mainshocks (Vidale and Shearer, 2006), which have been frequently observed in volcanic and geothermal regions (e.g., Fischer et al., 2014;Shelly et al., 2016;Yukutake et al., 2022), during fluid injection into the crust (e.g., Shapiro et al., 1997;Sheng et al., 2022), and at plate interfaces (e.g., Nishikawa and Ide, 2017). Each swarm exhibits unique progress in time and space, but they usually initiate from a very local area, then gradually extend outward (Deng et al., 2003). ...
3D fault geometry has recently been shown to play a key role in controlling the kinematics of earthquake swarms. With the aid of a temporary dense seismic array, machine-learning-based detection, and accurate relocation, we outline a complex 3D fault zone accommodating a small earthquake swarm near Yunlong city, western Yunnan, China from February to May 2013. Our results show that the swarm initiated from a compressive stepover zone and subsequently activated a complex fault zone including six planar fault segments. The migration front of the swarm can be well-modelled by fluid diffusion, indicating the swarm was primarily driven by pressurized fluid. A nearby GPS site exhibits a slight displacement that started immediately before the swarm, then gradually decayed in the following months, implying the stress fluctuations caused by a possible aseismic slip likely initiated the swarm and, along with fluid diffusion, drove the migration of the swarm. Within the stepover zone, complex and dense fractures act as conduits connecting the reservoir and fault zone, facilitating fluid flow. Meanwhile, the stress in the stepover zone tends to increase in response to the compression of the two boundary faults, which not only makes the stepover zone more susceptible to be triggered by those transient stresses but also forms a fluid pumping mechanism that drives fluids from the stepover zone into the complex fault zone. We suggest the complexity of the initiation and evolution of the 2013 Yunglong swarm is strongly controlled by 3D fault geometry.
... Therefore, it remains unclear whether hydraulic fluids could have transferred this far to activate fault segment 1. Further evidence is obtained from the focal mechanism analysis from both our nodal array Seismological Research Letters 7 and the regional broadband array (Sheng et al., 2022): using S/P ratios and the P-wave first motions (e.g., HASH, Hardebeck and Shearer, 2003),we resolved a few candidate fault planes for which the strike from the best-ranked solution (310°) agrees with the aftershock distribution (315°; see Fig. 9a). These seismological observations agree with the recent regional 3D seismic survey that reveals conjugate faults aligned along northeastsouthwest and northwest-southeast in northern Sichuan basin (Yang Zhao, personal comm., 2023). ...
... The final catalog and station locations (in relative scale) have been included in the supporting information, together with all the earthquake phase arrivals; they are also openly (and permanently) available at doi: 10.5281/zenodo.5867580. The broadband waveform of the mainshock is openly available from Sheng et al. (2022). All locations and time have been switched to relative scale, consistent with our presentation in this study. ...
The development of portable nodal array in the recent years greatly improved the seismic monitoring ability across multiple scales. The dense arrays also directly benefit microseismic monitoring by providing relatively low-cost surface recordings. However, the rapid growth of seismic data is accompanied by the increased demand for efficient seismic phase picking. On the other hand, machine learning-based phase picking techniques achieved high stability and accuracy, showing promising potential to replace human labors and traditional automatic pickers. In this study, we applied a state-of-the-art package on newly collected nodal array data around a hydraulic fracturing well in southwestern China. The array consists of up to 85 nodes with an average station spacing of less than a kilometer. Within the hydraulic fracturing stimulation periods, we detected ∼3000 seismic events with magnitude down to ∼−2. After waveform cross-correlation-based relocation, the 1979 relocated events clearly light up a 1 km long fault structure and several fractures. Furthermore, the frequency–magnitude distribution of the catalog exhibits weak bilinear features with relatively low b-value (0.88) and a moderate coefficient of variation (Cv ∼2). The nature and origin of the observed earthquake cluster are then discussed and defined based on the industrial information, high-resolution earthquake catalog, and basic statistics. Finally, we summarized our experience and provided recommendations for applying similar approaches to other local scale, surface microseismic monitoring scenarios.
... Source parameters of the two earthquakes indicate that they are likely induced by hydraulic fracturing (Lei et al., 2020;Sheng et al., 2020). There are evidences that some induced earthquakes in the Weiyuan area occur on steeply dipping reverse faults, with a dip angle of ∼70° (Sheng et al., 2022). Because steeply dipping reverse induced events are relatively rare and difficult to slip under usual conditions, it will be of significance to investigate their source properties, particularly local stress field and stress drops. ...
... Liu & Wang, 2016;Zou et al., 2015). Since the beginning of mass shale gas production in 2015, seismicity in the Weiyuan block has been increasing significantly (Sheng et al., 2022). ...
... High-resolution earthquake locations are helpful for focal mechanism inversion and seismogenic structure discrimination (Qin et al., 2019;Schoenball & Ellsworth, 2017). Sheng et al. (2022) built a high-resolution earthquake catalog for 18,663 events from continuous waveform data recorded by the same temporary dense array in the Weiyuan area. The root-mean-square of P-wave travel-time residuals decrease from 0.2015 s of initial locations to 0.0294 s of double-difference relocations. ...
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.
... This result is similar to the frequency-magnitude distributions of several induced seismic clusters due to hydraulic fracturing in western Canada , with b values varying from 0.74 (Fox Creek cluster) to 1.6 (Dawson Creek cluster) and an average value of 1.1 (Dokht et al., 2021), as well as similar cases reported from the United States (Holland, 2013;Skoumal et al., 2015). In contrast, the large b value at greater depths deviates significantly from those at shallow depths (Sheng et al., 2022). We speculate that these events are in more accordance with the microseismicity that occurred on newly created hydraulic fractures considering (a) their close spatiotemporal affinity with HF operations, (b) relatively small (<−1) magnitudes, and (c) the consistent elongation direction of each subcluster that agrees with the orientation of the maximum horizontal stress (SHmax; Reiter et al., 2014). ...
Dense arrays deployed near the hydraulic fracturing (HF) wells greatly enhance the understanding of injection‐induced seismicity. In this study, we revisit the continuous recordings that are acquired by 69 three‐component nodes at an HF site in Alberta, Canada, taking advantage of a machine learning‐based seismic detection and location workflow. The obtained new earthquake catalog contains 21,619 events with relative location errors of <1 m, which exceeds ∼20% of the number of earthquakes (18,040) reported previously (Igonin et al., 2021). This high‐resolution catalog reveals the distribution of earthquake sequences at much improved spatiotemporal resolution and illustrates several previously unmapped faults/fractures. Earthquake frequency‐magnitude distribution reveals that the average b value increases with depth from ∼1.1 above 3.5 km to ∼2.5 at greater depths. Further spatial analysis of seismic clusters indicates that the b value varies laterally (∼1–1.7) at shallow depths and is inversely related to the proximity to injection wells in conjunction with the change in structural types (i.e., reactivated fault and pre‐existing fracture). The seismic sequence on the north‐south oriented faults also shows a distinctive occurrence pattern and temporal affinity to the fracturing network reactivation between the two stages of HF operations. The change in faulting behavior could reflect a shift of dominating triggering mechanisms and physical processes from (a) the rapid diffusion of pore fluid pressure along pre‐existing fracture corridors to (b) the cascade migration of earthquake sequences in response to the cumulative Coulomb stress perturbation on the fluid‐lubricated, critically stressed faults.
