Remotely triggered earthquakes — Introduction

Remotely triggered earthquakes are seismic events that occur outside the immediate aftershock zone of a large earthquake, separated in space and/or time from the initiating rupture such that causal linkage becomes increasingly uncertain with greater distance and elapsed time. Their study rests on two complementary physical mechanisms. First, static stress changes produced by permanent deformation of the crust can alter the steady-state stress field on distant faults, bringing critically stressed patches closer to failure. Second, transient dynamic perturbations carried by seismic waves can briefly change stress, pore pressure, or frictional strength on faults and thereby initiate slip on faults that were already near critical stress.

Empirical cases have helped to distinguish these mechanisms. The 1992 Mw 7.3 Landers earthquake in southern California produced regional redistributions of static stress that coincided with heightened seismicity across a broad area, demonstrating how permanent deformation can modulate fault failure at distance. Conversely, the 2004 Mw 9.1 Indian Ocean earthquake generated seismic waves that have been causally linked to triggered earthquakes at transoceanic distances, including documented activation as far away as Alaska; such examples illustrate the capacity of dynamic stresses to operate over very long ranges.

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Mechanistic support for a wide-area influence comes from both numerical and analogue experiments. Discrete element models that represent brittle crustal material as interacting blocks show that a single perturbation in a critically stressed assemblage can produce cascading failure across an extended region. At a smaller scale, slope-instability analogues—where a modest disturbance in an already near-failure slope initiates a landslide that propagates to engulf a larger mass—provide an instructive physical analogy for cascading seismic triggering.

Interpreting remotely triggered activity requires distinguishing among a variety of rupture styles and temporal patterns (e.g., mainshocks, foreshocks, aftershocks, doublets, interplate vs intraplate events, megathrusts, slow earthquakes, swarms, submarine ruptures, supershear events) because each class carries implications for stress transfer and susceptibility to remote perturbation. It is also necessary to consider proximate causes that can create or modify stress fields—tectonic slip, volcanic processes, and anthropogenic activities such as mining, reservoir filling, or fluid injection—which can either generate local seismicity or prime distant faults for triggering.

Seismological analysis of triggered events relies on basic location and wave-propagation concepts: epicentral distance and hypocentral depth to quantify spatial separation from the source; wave types (P and S) and their propagation characteristics to assess dynamic stress transmission; and phenomena such as shadow zones that affect energy delivery. Instrumentation and operational frameworks underpin observation and forecasting: networks of seismometers, magnitude and intensity scales, regional cataloguing schemes, and institutional bodies and methodologies that aim to quantify changes in seismic hazard, including elevated probabilities associated with remote triggering.

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Finally, a suite of geophysical tools and concepts informs interpretation of triggering processes: shear-wave splitting as an indicator of stress/anisotropy, the Adams–Williamson relation for density–pressure structure with depth, regionalization schemes such as Flinn–Engdahl for cataloguing global seismicity, engineering practice for mitigation, and sedimentary seismites as palaeoseismic evidence. Together, these observational, theoretical, and modelling components form the scientific framework for detecting, attributing, and assessing the consequences of remotely triggered earthquakes.