This paper presents a timely and detailed study of significant injection-induced seismicity recently observed in the Sichuan Basin, China, where shale-gas hydraulic fracturing has been initiated and the aggressive production of shale gas is planned for the coming years. Multiple lines of evidence, including an epidemic-type aftershock sequence model, relocated hypocenters, the mechanisms of 13 large events (M _W > 3.5), and numerically calculated Coulomb failure stress results, convincingly suggest that a series of earthquakes with moment magnitudes up to M _W 4.7 has been induced by “short-term” (several months at a single well pad) injections for hydraulic fracturing at depths of 2.3 to 3 km. This, in turn, supports the hypothesis that they represent examples of injection-induced fault reactivation. The geologic reasons why earthquake magnitudes associated with hydraulic fracturing operations are so high in this area are discussed. Because hydraulic fracturing operations are on the rise in the Sichuan Basin, it would be beneficial for the geoscience, gas operator, regulator, and academic communities to work collectively to elucidate the local factors governing the high level of injection-induced seismicity, with the ultimate goal of ensuring that shale gas fracking can be carried out effectively and safely.

This paper describes the localization of deformation acceleration in the period prior to dynamic failure in hornblende schist rock under triaxial compression using acoustic emission (AE) monitoring. Rather than stabilize the failure process by controlling axial stress to maintain a constant rate of AE (for monitoring AE hypocenters) as in previous works [e.g., Lockner et al, 1991], we have instead developed a rapid multichannel data collection system. This enables us to elucidate the dynamics of fault nucleation under condition of constant stress (creep) loading, which is a better approximation to low strain rate condition in the Earth and allows both quasi‐static and dynamic crack growth to occur. The waveforms of more than 8000 AE events which occurred mainly during a 15 s period were recorded on 32 channels, with a sampling rate of 50 ns and mask time of 200 μs. Hypocentral locations of AE sources revealed that the fault initiated at one end of the core and then propagated into the unfaulted rock with a process zone (fault front) of intense cracking. We found that there were two different processes operating during the quasi‐static nucleation of a shear fault, namely, a process zone in front of the fault tip and a “wake” of damage zone following the process zone. The process zone had the following features: (1) major tensile cracking, (2) low b value and fewer larger events, and (3) strong self‐excitation. The mechanism of crack interaction and fault growth was, therefore, a mutual enhancement on dilatation due to tensile cracking. On the other hand, the damage zone was characterized by (1) major shear cracking, (2) low b value and more larger events, and (3) weak self‐excitation, indicating that in the damage zone, following the development of a shear fault, linkage between cracks became the major mechanism of crack interaction and fault development. The mutual changes of b value and self‐exciting strength observed in our experiments seem to occur as a result of the hierarchy of fault growth, which was not observed under slowed down loading conditions. Therefore our experimental results, under a realistic approximation of the dynamic condition of the Earth, are meaningful for the problems of earthquakes as well as rock bursts.

An $ML$ 5.7 earthquake struck Xingwen County, Sichuan Province, China (28.239° N, 104.922° E) on 16 December 2018. More than two weeks later on 3 January 2019, an $ML$ 5.3 earthquake occurred 8 km to the west. These were the largest and most damaging events in the Changning shale gas block and resulted in extensive damage to nearby farmhouses and other structures. Although the official hypocentral depths for both events are large than 5 km, the centroid moment tensor solutions based on the generated cut‐and‐paste method were very shallow (3 and 1.8 km for $ML$ 5.7 and 5.3, respectively). In addition, both events were very close to hydraulic‐fracturing (HF) zones of horizontal wells, in which HF was ongoing. A convening chain of evidence, including spatiotemporal correlation between earthquakes and HF zones, statistic parameters of seismicity, and estimated overpressure required to activate the unfavorably oriented source faults of the largest events, suggests that a series of earthquakes were induced by HF at a depth of $∼2.5–3km$⁠. In the Changning block, thus far, 11 $ML≥4.0$ events (including 3 $ML≥5.0$⁠) were observed since the systematic HF operation began in 2014. The Omori‐type aftershock productivity is very low as compared to normal tectonic seismicity. In addition, estimated overpressure ranged from $∼0.3$ to 5.8 MPa for $Mw>3.5$ events. The poroelastic stress changes are less than 1 MPa out of the treatment zones. Hence, overpressure‐driven reactivation of pre‐existing faults (most of them are unmapped and unfavorably oriented for slip under the present tectonic stress field) is considered to be the cause of these abnormal moderate‐size earthquakes.