An official website of the United States government
SUMMARY 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.
more »
« less
A laboratory perspective on accelerating preparatory processes before earthquakes and implications for foreshock detectability
Abstract Dynamic failure in the laboratory is commonly preceded by many foreshocks which accompany premonitory aseismic slip. Aseismic slip is also thought to govern earthquake nucleation in nature, yet, foreshocks are rare. Here, we examine how heterogeneity due to different roughness, damage and pore pressures affects premonitory slip and acoustic emission characteristics. High fluid pressures increase stiffness and reduce heterogeneity which promotes more rapid slip acceleration and shorter precursory periods, similar to the effect of low geometric heterogeneity on smooth faults. The associated acoustic emission activity in low-heterogeneity samples becomes increasingly dominated by earthquake-like double-couple focal mechanisms. The similarity of fluid pressure increase and roughness reduction suggests that increased stress and geometric homogeneity may substantially shorten the duration of foreshock activity. Gradual fault activation and extended foreshock activity is more likely observable on immature faults at shallow depth.
more »
« less
- Award ID(s):
- 2142489
- PAR ID:
- 10520680
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Nature Communications
- Volume:
- 15
- Issue:
- 1
- ISSN:
- 2041-1723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Earthquake swarms attributed to subsurface fluid injection are usually assumed to occur on faults destabilized by increased pore-fluid pressures. However, fluid injection could also activate aseismic slip, which might outpace pore-fluid migration and transmit earthquake-triggering stress changes beyond the fluid-pressurized region. We tested this theoretical prediction against data derived from fluid-injection experiments that activated and measured slow, aseismic slip on preexisting, shallow faults. We found that the pore pressure and slip history imply a fault whose strength is the product of a slip-weakening friction coefficient and the local effective normal stress. Using a coupled shear-rupture model, we derived constraints on the hydromechanical parameters of the actively deforming fault. The inferred aseismic rupture front propagates faster and to larger distances than the diffusion of pressurized pore fluid.more » « less
-
Abstract While many large earthquakes are preceded by observable foreshocks, the mechanism responsible for the occurrence of these smaller‐scale seismic events remains uncertain. One physical explanation of foreshocks with growing support is that they are produced by the interaction of slow slip with fault heterogeneity. Inspired by the suggestion from laboratory experiments that foreshocks occur on fault asperities (bumps), we explore rate‐and‐state fault models with patches of higher normal stress embedded in a larger seismogenic region by conducting 3‐D numerical simulations of their behavior over long‐term sequences of aseismic and seismic slips. The models do produce smaller‐scale seismicity during the aseismic nucleation of larger‐scale seismic events. These smaller‐scale events have reasonable stress drops, despite the highly elevated compression assigned to the source patches. We find that the two main factors contributing to the reasonable stress drops are the significant extent of the rupture into the region surrounding the patches and the aseismic stress release just prior to the seismic events. The smaller‐scale seismicity can only occur if a sufficient separation in nucleation scales between the foreshock‐like events and mainshocks is achieved. Our modeling provides insight into the conditions conducive for generating foreshocks on both natural and laboratory faults.more » « less
-
Abstract Foreshocks are the only currently widely identified precursory seismic behavior, yet their utility and even identifiability are problematic, in part because of extreme variation in behavior. Here, we establish some global trends that help identify the expected frequency of foreshocks as well the type of earthquake most prone to foreshocks. We establish these tendencies using the global earthquake catalog of the U.S. Geological Survey National Earthquake Information Center with a completeness level of magnitude 5 and mainshocks with Mw≥7.0. Foreshocks are identified using three clustering algorithms to address the challenge of distinguishing foreshocks from background activity. The methods give a range of 15%–43% of large mainshocks having at least one foreshock but a narrower range of 13%–26% having at least one foreshock with magnitude within two units of the mainshock magnitude. These observed global foreshock rates are similar to regional values for a completeness level of magnitude 3 using the same detection conditions. The foreshock sequences have distinctive characteristics with the global composite population b-values being lower for foreshocks than for aftershocks, an attribute that is also manifested in synthetic catalogs computed by epidemic-type aftershock sequences, which intrinsically involves only cascading processes. Focal mechanism similarity of foreshocks relative to mainshocks is more pronounced than for aftershocks. Despite these distinguishing characteristics of foreshock sequences, the conditions that promote high foreshock productivity are similar to those that promote high aftershock productivity. For instance, a modestly higher percentage of interplate mainshocks have foreshocks than intraplate mainshocks, and reverse faulting events slightly more commonly have foreshocks than normal or strike-slip-faulting mainshocks. The western circum-Pacific is prone to having slightly more foreshock activity than the eastern circum-Pacific.more » « less
-
Foreshocks are the most obvious signature of the earthquake nucleation stage and could, in principle, forewarn of an impending earthquake. However, foreshocks are only sometimes observed, and we have a limited understanding of the physics that controls their occurrence. In this work, we use high-resolution earthquake catalogs and estimates of source properties to understand the spatiotemporal evolution of a sequence of 11 foreshocks that occurred ~ 6.5 hours before the 2020 Mw 4.8 Mentone earthquake in west Texas. Elevated pore-pressure and poroelastic stressing from subsurface fluid injection from oil-gas operations is often invoked to explain seismicity in west Texas and the surrounding region. However, here we show that static stresses induced from the initial ML 4.0 foreshock significantly perturbed the local shear stress along the fault and could have triggered the Mentone mainshock. The majority (9/11) of the earthquakes leading up to the Mentone mainshock nucleated in areas where the static shear stresses were increased from the initial ML 4.0 foreshock. The spatiotemporal properties of the 11 earthquakes that preceded the mainshock cannot easily be explained in the context of a preslip or cascade nucleation model. We show that at least 6/11 events are better classified as aftershocks of the initial ML 4.0. Together, our results suggest that a combination of physical mechanisms contributed to the occurrence of the 11 earthquakes that preceded the mainshock, including static-stressing from earthquake-earthquake interactions, aseismic creep, and stress perturbations induced from fluid injection. Our work highlights the role of earthquake-earthquake triggering in induced earthquake sequences, and suggests that such triggering could help sustain seismic activity following initial stressing perturbations from fluid injection.more » « less
