Introduction — Doublet (multiplet/twin) earthquakes
Doublet or multiplet earthquakes are sequences in which two or more principal shocks originate from the same rupture zone and stress field and produce nearly identical seismic waveforms. They are distinguished from ordinary aftershocks by their similarity in size—typically within ~0.4 magnitude—and by their overlapping focal areas, which for very large events can extend up to ~100 km (e.g., magnitude ~7.5). Temporal spacing varies widely: shocks can follow within tens of seconds or be separated by months to years.
The physical mechanism invoked for multiplets emphasizes fault heterogeneity: large, stuck patches or asperities, geometric irregularities, or bends can arrest an advancing rupture so that only part of the accumulated tectonic stress is released. Continued stress transfer onto the arrested patch may eventually trigger its failure, producing a subsequent main shock at the same locus. This behavior contrasts with classical aftershock sequences, which typically follow Båth’s law (aftershocks beginning about 1.2 magnitude units smaller than the mainshock and decaying in size and rate according to statistical laws).
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Multiplets have important consequences for ground motion and damage. Nearly simultaneous strong shocks can substantially lengthen the duration and enlarge the area of intense shaking—illustrated by the 1997 Harnai sequence (Mw 7.0 followed by Mw 6.8 only 19 seconds later)—increasing the probability of cumulative structural failure and complicating rescue and recovery. Because multiplets share a common rupture zone, they are conceptually distinct from remotely triggered earthquakes, although practical classification can be sensitive to observational resolution.
Recognition of true multiplets arose from waveform-comparison studies in the 1970s–1980s, which established identification criteria based on waveform similarity, focal overlap, and magnitude closeness. Reported prevalence varies with dataset and criteria: estimates range from roughly 7% (narrow criteria) to substantially higher fractions in some analyses (e.g., 37–75% in selected datasets), with about 20% of very large events (>7.5) identified as doublets and regional studies (Solomon Islands) reporting ~10% of M ≥ 6.0 and ~25% of M ≥ 7.0 events as doublets.
Multiplets also bear on seismic hazard theory and practice: they challenge simple characteristic‑earthquake models that assume segmented faults limit rupture extent, and their inclusion—through multisegment or overlapping-rupture scenarios—alters expected size–frequency relationships and hazard forecasts (for example, revisions incorporated into UCERF3). Practically, engineering design, emergency planning, and post‑event operations must account for the possibility of repeated, near-equal main shocks that produce extended or cumulative strong shaking and elevated collapse risk.