Aftershock
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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 thousands of instrumentally detectable aftershocks, which steadily decrease in magnitude and frequency according to a consistent pattern. In some earthquakes the main rupture happens in two or more steps, resulting in multiple main shocks. These are known as doublet earthquakes, and in general can be distinguished from aftershocks in having similar magnitudes and nearly identical seismic waveforms.
Most aftershocks are located over the full area of fault rupture and either occur along the fault plane itself or along other faults within the volume affected by the strain associated with the main shock. Typically, aftershocks are found up to a distance equal to the rupture length away from the fault plane.
The pattern of aftershocks helps confirm the size of area that slipped during the main shock. In the case of the 2004 Indian Ocean earthquake and the 2008 Sichuan earthquake the aftershock distribution shows in both cases that the epicenter (where the rupture initiated) lies to one end of the final area of slip, implying strongly asymmetric rupture propagation.
The frequency of aftershocks decreases roughly with the reciprocal of time after the main shock. This empirical relation was first described by Fusakichi Omori in 1894 and is known as Omori's law.[1] It is expressed as
According to these equations, the rate of aftershocks decreases quickly with time. The rate of aftershocks is proportional to the inverse of time since the mainshock and this relationship can be used to estimate the probability of future aftershock occurrence.[4] Thus whatever the probability of an aftershock are on the first day, the second day will have 1/2 the probability of the first day and the tenth day will have approximately 1/10 the probability of the first day (when p is equal to 1). These patterns describe only the statistical behavior of aftershocks; the actual times, numbers and locations of the aftershocks are stochastic, while tending to follow these patterns. As this is an empirical law, values of the parameters are obtained by fitting to data after a mainshock has occurred, and they imply no specific physical mechanism in any given case.
The Utsu-Omori law has also been obtained theoretically, asthe solution of a differential equation describing the evolution of the aftershock activity,[5] where the interpretation of the evolution equation is based on the idea of deactivation of the faults in the vicinity of the main shock of the earthquake. Also, previously Utsu-Omori law was obtained from a nucleation process.[6] Results show that the spatial and temporal distribution of aftershocks is separable into a dependence on space and a dependence on time. And more recently, through the application of a fractional solution of the reactive differential equation,[7] a double power law model shows the number density decay in several possible ways, among which is a particular case the Utsu-Omori Law.
Land movement around the New Madrid is reported to be no more than 0.2 mm (0.0079 in) a year,[10] in contrast to the San Andreas Fault which averages up to 37 mm (1.5 in) a year across California.[11] Aftershocks on the San Andreas are now believed to top out at 10 years while earthquakes in New Madrid were considered aftershocks nearly 200 years after the 1812 New Madrid earthquake.[12]
Some scientists have tried to use foreshocks to help predict upcoming earthquakes, having one of their few successes with the 1975 Haicheng earthquake in China. On the East Pacific Rise however, transform faults show quite predictable foreshock behaviour before the main seismic event. Reviews of data of past events and their foreshocks showed that they have a low number of aftershocks and high foreshock rates compared to continental strike-slip faults.[13]
Following a large earthquake and aftershocks, many people have reported feeling \"phantom earthquakes\" when in fact no earthquake was taking place. This condition, known as \"earthquake sickness\" is thought to be related to motion sickness, and usually goes away as seismic activity tails off.[16][17]
Most large earthquakes are followed by additional earthquakes, called aftershocks, which make up an aftershock sequence. While most aftershocks are smaller than the mainshock, they can still be damaging or deadly. A small fraction of earthquakes are followed by a larger earthquake, in which case the first earthquake is referred to as a foreshock. For example, the 2011 M9.1 Japan earthquake and tsunami was preceded by a M7.3 foreshock two days before. When the M7.3 earthquake first occurred, it was called the mainshock, and then when the M9.1 earthquake occurred, that larger earthquake became the mainshock.
Following a significant earthquake, this aftershock forecast can provide situational awareness of the expected number of aftershocks, as well as the probability of subsequent larger earthquakes. Specifically, we forecast:
We forecast aftershock activity over future time intervals of a day,a week, a month, and a year. We use the behavior of past aftershock sequences to forecast the likelihood of future aftershocks. As an aftershock sequence progresses, our forecast also incorporates information about the behavior of that specific sequence.
Forecasts are posted for earthquakes of M5+ in the United States and U.S. territories. There are higher thresholds of M6 or 6.5 in some remote and poorly-instrumented areas. We also compute forecasts for some smaller earthquakes that are of particular public interest, for example earthquakes in densely-populated areas. We will not usually compute aftershock forecasts for earthquakes that are themselves aftershocks of a prior larger earthquake, or for earthquakes that occur as part of volcanic activity.
Forecasts are updated regularly. The rate of aftershocks changes with time, generally decreasing, although sometimes temporarily increasing after a significant aftershock. Therefore, the forecasts are updated to keep current with the changing aftershock rate. We also update the forecasts over time to incorporate more information about the specific behavior of the aftershock sequence. We update at least once within the first day, again within the first week, and again within the first month. The time that the current forecast was released, and the planned time of the next forecast update, are included in each forecast.
The Commentary tab describes the aftershock forecast in simple language, starting with the concept that larger earthquakes could follow and that aftershocks will be continuing for some time; and some safety information is included. The subsequent information is a simple summary of the forecast, followed by what has already happened, and ending with a more quantitative version of the forecast.
The Forecast tab presents the forecast as tables, covering a range of aftershock magnitudes and time frames. The first table shows the probability of at least one aftershock above a certain magnitude and within a certain time frame. The second table shows the likely number of aftershocks above a certain magnitude and within a certain time frame, given as range of numbers which represents the uncertainty of the forecast. If it is unlikely that there will be any aftershocks of that magnitude during that time frame, the table shows an asterisk, which means that an earthquake is possible but with a low probability.
Forecasts are currently made only with the Reasenberg-Jones (1989, 1994) model, which models the aftershock rate with a smooth decay with time following the mainshock. At this time we are not calculating spatial forecasts or providing maps to show areas with the highest likelihood of aftershocks. As a rule of thumb, aftershocks are most likely to occur near the mainshock fault plane and in areas already experiencing numerous aftershocks.
The initial forecast after an earthquake occurs is calculated using parameters that worked for previous earthquakes in that region or similar regions around the world. As time goes by and we observe how many aftershocks are happening we use parameters that are a combination of the initial parameters and parameters determined from the current sequence of earthquakes.
The initial forecast uses only the mainshock magnitude, and therefore can be released soon after the mainshock, and before many aftershocks have occurred. Because the initial forecast depends a lot on the mainshock magnitude, we wait at least 30 minutes after the event occurs before issuing a forecast, to allow the preferred mainshock magnitude to stabilize. We also update the forecast if the mainshock magnitude significantly changes after the initial forecast.
These additional quakes, called aftershocks, are common after a large earthquake; many aftershocks can be small relative to the main quake, but some have the potential to be severe and destructive, as was the case in Turkey.
A powerful 6.4 magnitude aftershock struck near Antakya city in southern Turkey on Feb. 20, two weeks after the main quake, killing at least six. Another 5.6 magnitude quake struck near Malatya on Feb. 27, killing one person, injuring 110 and causing 29 buildings to collapse.
Chart shows about ten thousand earthquakes that have been recorded in southern Turkey since a 7.8 magnitude earthquake occurred on February 6. The frequency and intensity of these aftershocks have slightly diminished over time.
More than 570 aftershocks were recorded within 24 hours of the main quake on Feb. 6, and more than 10,000 were recorded in the three weeks afterward. The frequency and intensity of the aftershocks has diminished since then, but the temblors continue to be a threat.
Seismologists define aftershocks as temblors triggered by a large earthquake, close in time and lo