Doublet earthquake
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__NOTOC__ In
seismology Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...
, doublet earthquakes – and more generally, multiplet earthquakes – were originally identified as multiple
earthquakes An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...
with nearly identical
waveforms In electronics, acoustics, and related fields, the waveform of a signal is the shape of its graph as a function of time, independent of its time and magnitude scales and of any displacement in time.David Crecraft, David Gorham, ''Electronics ...
originating from the same
location In geography, location or place are used to denote a region (point, line, or area) on Earth's surface or elsewhere. The term ''location'' generally implies a higher degree of certainty than ''place'', the latter often indicating an entity with an ...
. They are now characterized as single earthquakes having two (or more) main shocks of similar magnitude, 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 In seismology, an aftershock is a smaller earthquake that follows a larger earthquake, in the same area of the main shock, caused as the displaced crust adjusts to the effects of the main shock. Large earthquakes can have hundreds to thousand ...
, 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 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 __NOTOC__ In geology, the elastic-rebound theory is an explanation for how energy is released during an earthquake. As the Earth's crust deforms, the rocks which span the opposing sides of a fault are subjected to shear stress. Slowly they def ...
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 The 1997 Harnai earthquake occurred on February 27 at 21:08 UTC near Harnai, Pakistan, and felt throughout much of central Balochistan, with a magnitude estimated at 7.0 on the scale. A 2016 study determined that this was a doublet earthquake, t ...
the initial shock was followed by an shock just 19 seconds later. The effect of such powerful shocks so close in time was to double the duration of ground shaking (bringing more structures to the point of collapse), and to double the area affected by the strongest shaking. When a subsequent, and possibly stronger, shock comes hours or days later it may suffice to collapse structures weakened by the previous shock, with serious consequences to rescue and recovery efforts. Although there have been numerous earthquakes with two or even three primary shocks of such similar magnitude that picking one as the main shock can be somewhat arbitrary, it was not until the 1970s and 1980s that studies of seismograms showed that some of these were not simply unusually large
foreshocks A foreshock is an earthquake that occurs before a larger seismic event (the mainshock) and is related to it in both time and space. The designation of an earthquake as ''foreshock'', ''mainshock'' or aftershock is only possible after the full seq ...
and aftershocks. Other studies have shown that about 20% of very large earthquakes (magnitude above 7.5) are doublets, and that, in some cases, 37 to 75 percent of earthquakes are multiplets. A theoretical study found about one earthquake in 15 (~7%) to be a doublet (using a narrow criterion of "doublet"), but also found that in the
Solomon Islands Solomon Islands is an island country consisting of six major islands and over 900 smaller islands in Oceania, to the east of Papua New Guinea and north-west of Vanuatu. It has a land area of , and a population of approx. 700,000. Its capita ...
six of 57 M ≥ 6.0 earthquakes were doublets, and 4 of 15 M ≥ 7.0 earthquakes, showing that approximately 10% and 25% of those quakes were doublets. Doublet earthquakes pose a challenge to the
characteristic earthquake Earthquake prediction is a branch of the science of seismology concerned with the specification of the time, location, and magnitude of future earthquakes within stated limits, and particularly "the determination of parameters for the ''next'' ...
model used for estimating
seismic hazard A seismic hazard is the probability that an earthquake will occur in a given geographic area, within a given window of time, and with ground motion intensity exceeding a given threshold. With a hazard thus estimated, risk can be assessed and includ ...
. This model assumes that faults are segmented, limiting the extent of rupturing, and therefore the maximum size of an earthquake, to the length of the segment. Newer forecasts of seismic hazard, such as
UCERF3 The 2015 Uniform California Earthquake Rupture Forecast, Version 3, or UCERF3, is the latest official earthquake rupture forecast (ERF) for the state of California, superseding UCERF2. It provides authoritative estimates of the likelihood and seve ...
, factor in a greater likelihood of multisegment ruptures, which changes the relative frequency of different sizes of earthquakes..


See also

* :Doublet earthquakes. Only a partial listing.


Notes


Sources

*. *. *. *. *. *. *. *. *. *. *. {{div col end Types of earthquake Doublet earthquakes