Doublet earthquake

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In seismology, doublet earthquakes—and more generally, multiple earthquakes or twin earthquakes—were originally identified as multiple earthquakes with nearly identical waveforms originating from the same location. They are now characterized as distinct earthquake sequences having two (or more) main shocks with similar/slightly different magnitudes that occurred twice (or more) in a single moment, sometimes occurring within tens of seconds, but sometimes separated by years. The similarity of magnitude—often within 0.4 magnitude—distinguishes multiplet events from aftershocks, which start at about 1.2 magnitude less than the parent shock (Båth's law) and decrease in magnitude and frequency according to known laws. Doublet/multiplet events also have nearly identical seismic waveforms, as they come from the same rupture zone and stress field, whereas aftershocks, being peripheral to the main rupture, typically reflect more diverse circumstances of origin. Multiplet events overlap in their focal fields (rupture zones), which can be up to 100 kilometers across for magnitude 7.5 earthquakes. Doublets have been distinguished from triggered earthquakes, where the energy of the seismic waves triggers a distant earthquake with a different rupture zone, although it has been suggested such a distinction reflects "imprecise taxonomy" more than any physical reality. Multiplet earthquakes are believed to result when asperities, such as large chunks of crust stuck in the rupturing fault, or irregularities or bends in the fault, temporarily impede the main rupture. Unlike a normal earthquake, where it is believed the earthquake releases enough of the tectonic stress driving it that it will take decades to centuries to accumulate enough stress to drive the next earthquake (per the elastic rebound theory), the initial multiplet quake only releases part of the pent-up stress when the rupture hits the asperity. This increases the stress across the asperity, which may fail within seconds, minutes, months, or even years. In the 1997 Harnai earthquake,...