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This study analyzed and summarized in detail the spatial and temporal distributions of earthquakes, tidal responses, focal mechanisms, and stress field characteristics for the M 7.3 Haicheng earthquake sequence in February 1975. The foreshocks are related to the main fault and the conjugate faults surrounding the extension step-over in the middle. The initiation timing of the foreshock clusters and the original time of the mainshock were clearly modulated by the Earth's tidal force and coincided with the peak of dilational volumetric tidal strain. As a plausible and testable hypothesis, we proposed a fluid-driven foreshock model, by which all observed seismicity features can be more reasonably interpreted with respect to the results of existing models. Together with some other known examples, the widely existing step-over along strike-slip faults and associated conjugate faults, especially for extensional ones in the presence of deep fluids, favor the occurrence of short-term foreshocks. Although clustered seismicity with characteristics similar to those of the studied case is not a sufficient and necessary condition for large earthquakes to occur under similar tectonic conditions, it undoubtedly has a warning significance for the criticality of the main fault. Subsequent testing would require quantification of true/false positives/negatives. [ABSTRACT FROM AUTHOR]

Various earthquake models predict that aseismic slip modulates the seismic rupture process but actual observations of such seismic‐aseismic interaction are scarce. We analyze seismic and aseismic processes during the 2014 Iquique earthquake sequence. High‐rate Global Positioning System displacements demonstrate that most of the early afterslip is located downdip of the M 8.1 mainshock and is accompanied by decaying aftershock activity. An intriguing secondary afterslip peak is located ∼120 km south of the mainshock epicenter. The area of this secondary afterslip peak likely acted as a barrier to the propagating mainshock rupture and delayed the M 7.6 largest aftershock, which occurred 27 hr later. Interevent seismicity in this secondary afterslip area ended with a M 6.1 near the largest aftershock epicenter, kicking the largest aftershock rupture in the same area. Hence, the interevent afterslip likely promoted the largest aftershock nucleation by destabilizing its source area, favoring a rate‐dependent cascade‐up model. Plain Language Summary: Subduction zone faults host both fast (regular earthquakes, seismic) and slow (aseismic) slip. Simulation models predict that slow slip can affect fast slip processes. We explored such an interaction taking place during the 2014 Iquique earthquake offshore northern Chile using observation data of crustal deformation by Global Positioning System and earthquakes. We discovered that the fast mainshock slip was terminated by a slowly slipping fault zone, which prevented the simultaneous occurrence of the largest aftershock. Furthermore, afterslip, one type of slow slip following the mainshock, helped the occurrence of the largest aftershock 27 hr after the mainshock. Therefore, the sequential occurrence of large earthquakes can be controlled by slowly slipping faults. Key Points: Global Positioning System captured crustal deformation during 27 hr between the 2014 Iquique mainshock and its largest aftershockThe mainshock and the largest aftershock areas are separated by an aseismic area, likely preventing both from rupturing as a single eventThe largest aftershock nucleation is a mixture of seismicity and decelerating afterslip, favoring a rate‐dependent cascade‐up model [ABSTRACT FROM AUTHOR]

We search for earthquake swarms along the Chilean subduction, from −18° to −39° of latitude, 2003–2020 by developing an objective, model-based method that detects potential swarms as anomalous changes in earthquake rate. A set of 16 swarms is obtained after careful inspection of the candidate swarms; this assessment allows to reject cases for which model errors, rather than 'true' anomalous rate changes, are likely the cause of the detection. Averaging the activity over these 16 episodes, we find indirect evidence for a mostly aseismic driving mechanism, and a mean aseismic to seismic ratio estimated to range between 40 and 90 when using the seismicity rate as a proxy for slip. All the swarms are found in the 20–50 km depth range with the notable exception of one 60–100-km-deep swarm that occurs several days after the 2010 Maule earthquake and downdip of it. The dominant depth range (20–50 km) is in agreement with previous studies that suggest this range to be a transition zone from the shallower, locked part of the subduction, to the freely slipping interface at greater depth and intraslab earthquake activity. The swarms can be separated into three spatial groups, two of which being related to a subducting oceanic ridge. This structural control by fluid-rich geological features is modulated by stress control, that is swarms cluster in time with intermediate to large ruptures, both prior and following them, pointing to a close interplay between seismic slip and aseismic deformation in specific, well separated segments of the Chilean subduction. [ABSTRACT FROM AUTHOR]

Earthquakes come in clusters formed of mostly aftershock sequences, swarms and occasional foreshock sequences. This clustering is thought to result either from stress transfer among faults, a process referred to as cascading, or from transient loading by aseismic slip (pre-slip, afterslip or slow slip events). The ETAS statistical model is often used to quantify the fraction of clustering due to stress transfer and to assess the eventual need for aseismic slip to explain foreshocks or swarms. Another popular model of clustering relies on the earthquake nucleation model derived from experimental rate-and-state friction. According to this model, earthquakes cluster because they are time-advanced by the stress change imparted by the mainshock. This model ignores stress interactions among aftershocks and cannot explain foreshocks or swarms in the absence of transient loading. Here, we analyse foreshock, swarm and aftershock sequences resulting from cascades in a Discrete Fault Network model governed by rate-and-state friction. We show that the model produces realistic swarms, foreshocks and aftershocks. The Omori law, characterizing the temporal decay of aftershocks, emerges in all simulations independently of the assumed initial condition. In our simulations, the Omori law results from the earthquake nucleation process due to rate and state friction and from the heterogeneous stress changes due to the coseismic stress transfers. By contrast, the inverse Omori law, which characterizes the accelerating rate of foreshocks, emerges only in the simulations with a dense enough fault system. A high-density complex fault zone favours fault interactions and the emergence of an accelerating sequence of foreshocks. Seismicity catalogues generated with our discrete fault network model can generally be fitted with the ETAS model but with some material differences. In the discrete fault network simulations, fault interactions are weaker in aftershock sequences because they occur in a broader zone of lower fault density and because of the depletion of critically stressed faults. The productivity of the cascading process is, therefore, significantly higher in foreshocks than in aftershocks if fault zone complexity is high. This effect is not captured by the ETAS model of fault interactions. It follows that a foreshock acceleration stronger than expected from ETAS statistics does not necessarily require aseismic slip preceding the mainshock (pre-slip). It can be a manifestation of a cascading process enhanced by the topological properties of the fault network. Similarly, earthquake swarms might not always imply transient loading by aseismic slip, as they can emerge from stress interactions. [ABSTRACT FROM AUTHOR]

Foreshock analysis promises new insights into the earthquake nucleation process and could potentially improve earthquake forecasting. Well‐performing clustering models like the Epidemic‐Type Aftershock Sequence (ETAS) model assume that foreshocks and general seismicity are generated by the same physical process, implying that foreshocks can be identified only in retrospect. However, several studies have recently found higher foreshock activity than predicted by ETAS. Here, we revisit the foreshock activity in southern California using different statistical methods and find anomalous foreshock sequences, that is, those unexplained by ETAS, mostly for mainshock magnitudes below 5.5. The spatial distribution of these anomalies reveals a preferential occurrence in zones of high heat flow, which are known to host swarm‐like seismicity. Outside these zones, the foreshocks generally behave as expected by ETAS. These findings show that anomalous foreshock sequences in southern California do not indicate a pre‐slip nucleation process, but swarm‐like behavior driven by heat flow. Plain Language Summary: Many studies have observed that large earthquakes are preceded by smaller events, called foreshocks. If they have distinctive characteristics that make them recognizable in an ongoing sequence in real time, they can significantly improve the forecasting capability of large earthquakes. To investigate the nature of foreshocks, we compare actual seismicity with the expectation of the most skilled earthquake forecasting model, which assumes that foreshocks are not different from other earthquakes. We find that discrepancies between reality and expectation mostly affect foreshock sequences that anticipate moderate mainshocks with magnitudes below 5.5. We find that those anomalous foreshock sequences tend to occur more often where the heat flow is high. Those zones are already known for the occurrence of swarm‐like sequences, which are prolonged episodes of seismicity without a dominant event. Outside these zones, the observed foreshock activity is explained well by the forecasting model. These findings indicate that anomalous foreshock sequences are not indicating impending large earthquakes but are influenced by the heat flow. Key Points: We compare the foreshock activity in southern California with the expectation of the best‐performing class of earthquake clustering modelsSequences with an anomalous excess of foreshocks are associated mostly with moderate mainshocks and preferentially with high heat flowWe neither find evidence against the cascade nucleation hypothesis nor in favor of the pre‐slip nucleation hypothesis [ABSTRACT FROM AUTHOR]

We use the Japan Meteorological Agency (JMA) earthquake catalogue from January, 2001 to February, 2021 to investigate the spatiotemporal foreshock occurrence for shallow (within 30 km depth) onshore earthquakes (Mj3.0 to 7.2). We find clear peaks for the numbers of small earthquakes within 10 days and 3 km prior to the larger earthquakes, which are considered as our definition of foreshocks. After removing the aftershocks, earthquake swarms and possible triggered earthquakes by the 2011 Mw9.0 Tohoku-oki earthquake, we find that for the 2066 earthquakes (mainshocks), 783 (38%) have one or more foreshocks. There is a decreasing trend of foreshock occurrence rate with mainshock depth. Also, normal faulting earthquakes have higher foreshock occurrence rate than reverse faulting earthquakes. We calculate the earthquake occurrence rate as a function of the magnitudes of foreshocks and mainshocks, and we have found no clear trend between the magnitudes of foreshocks and mainshocks. [ABSTRACT FROM AUTHOR]

Understanding earthquake foreshocks is essential for deciphering earthquake rupture physics and can aid seismic hazard mitigation. With regional dense seismic arrays, we identify immediate foreshocks of 527 0.9 ≤M≤ 5.4 events of the 2019 Ridgecrest earthquake sequence, including 48 earthquakes with series of immediate foreshocks. These immediate foreshocks are adjacent to the mainshocks occurring within 100 s of the mainshocks, and their P waves share high resemblances with the mainshock P waves. However, attributes of the immediate‐foreshock P waves, including the amplitudes and preceding times, do not clearly scale with the mainshock magnitudes. Our observations suggest that earthquake rupture may initiate in a universal fashion but evolves stochastically. This indicates that earthquake rupture development is likely controlled by fine‐scale fault heterogeneities in the Ridgecrest fault system, and the final magnitude is the only difference between small and large earthquakes. Plain Language Summary: Understanding earthquake foreshocks have both scientific and societal implications regarding earthquake physics and seismic hazards. Using dense arrays in the Ridgecrest region, we find immediate foreshocks of 527 earthquakes that occurred within a month of the 2019 Mw 7.1 Ridgecrest earthquake. These immediate foreshocks are adjacent to their mainshocks and likely near‐instantaneously trigger the following slip within 100 s. Attributes of the P waves of these immediate foreshocks do not seem to correlate with the mainshock magnitudes. Our observations suggest that earthquakes may initiate via similar means and it remains challenging to use such foreshocks to predict the mainshock magnitudes. Key Points: Immediate foreshocks are observed for 527 Ridgecrest earthquakesCharacteristics of their P waves do not scale with the eventual earthquake magnitudesThese Ridgecrest earthquakes may have initiated as a rate‐dependent cascading process [ABSTRACT FROM AUTHOR]