[1] They are now characterized as distinct earthquake sequences having two (or more) main shocks of similar magnitude, sometimes occurring within tens of seconds, but sometimes separated by years.
[5] Multiplet events overlap in their focal fields (rupture zones), which can be up 100 kilometers across for magnitude 7.5 earthquakes.
[6] 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.
This increases the stress across the asperity, which may fail within seconds, minutes, months, or even years.
[18] Newer forecasts of seismic hazard, such as UCERF3, factor in a greater likelihood of multisegment ruptures, which changes the relative frequency of different sizes of earthquakes.