aftershock
-
What is an aftershock in geology?
In geology, an aftershock is a lower-magnitude earthquake that follows a larger earthquake, known as the main shock. Aftershocks occur due to the transfer of energy to nearby rocks, adding to existing stresses. When these stresses become too great, the rocks fracture, causing smaller tremors.
Aftershocks originate within the fault of the main earthquake or within one fault length from its epicenter. Their magnitude and frequency decrease over time, with the most severe occurring in the hours and days after the main quake. Although generally weaker, some aftershocks can cause significant damage, like the 1976 aftershock in Luanxian, China, and the 2011 aftershock in Christchurch, New Zealand.
2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
When do aftershocks usually happen?
Aftershocks are most severe and frequent in the hours and days after an earthquake. They occur because the energy released from the main earthquake is transferred to nearby rocks, creating new stresses or increasing existing ones. When these stresses cause the rocks to fracture, it results in smaller tremors or aftershocks.
The frequency and magnitude of aftershocks decrease over time. The duration of an aftershock sequence can range from a few weeks to several decades, though some can last centuries.
3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How are aftershocks different from the main earthquake?
Aftershocks are lower-magnitude tremors that occur after a main earthquake, resulting from the transfer of energy to nearby rocks, which adds to the stress on them. This can lead to new fractures and faults, causing a series of smaller tremors. Aftershocks occur within the fault of the main earthquake or within one fault length from its epicenter and decrease in magnitude and frequency over time. While generally less intense than the main quake, aftershocks can still destabilize structures and impede rescue efforts. The frequency of aftershocks decreases roughly in inverse proportion to the time since the main earthquake.
3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
Why do aftershocks occur after an earthquake?
Aftershocks are lower-magnitude earthquakes that occur after a main earthquake as a result of the sudden change in stress within and between rocks. When a principal earthquake occurs, the energy released from the fracturing of rocks is transferred to nearby rocks, adding to the stress. If the stress is too much, the rocks break, releasing energy and creating new faults, causing aftershocks. Aftershocks occur in rocks near the epicenter or along the fault of the main quake. Their magnitude and frequency decrease over time, with the most severe occurring in the hours and days after the earthquake.
2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How can aftershocks affect people and buildings?
Aftershocks are lower-magnitude earthquakes that follow a main shock, resulting from stress changes in rocks after the principal earthquake. They can destabilize buildings and structures, hampering rescue efforts. The shaking intensity of aftershocks, though typically smaller than the main earthquake, can still be large enough to cause further damage. For residents, these can be stressful as they cope with loss and damage from the initial quake. Aftershocks are most severe and frequent soon after the earthquake, decreasing in magnitude and frequency over time.
3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How do scientists predict and measure aftershocks?
Scientists use several methods to predict and measure aftershocks. Aftershocks are small tremors that follow an earthquake within one full fault length from the earthquake’s epicenter.
Seismographs are used to measure ground motion in both earthquakes and microseisms (small oscillations). They record oscillations in the ground caused by seismic waves traveling from their origin through the Earth or along its surface.
By analyzing the frequency and magnitude of aftershocks, seismologists can estimate how the rate of these tremors declines over time. The decrease in magnitude between the main earthquake and the largest aftershock is often predicted using Båth’s Law. Also, the use of Global Positioning Systems (GPS) at satellite ground stations provides quantitative data on a millimetre scale concerning the relative movement of crustal blocks across seismic faults.
5 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
aftershock, any of several lower-magnitude earthquakes that follow the main shock of a larger earthquake. An aftershock results from the sudden change in stress occurring within and between rocks and the previous release of stress brought on by the principal earthquake. Aftershocks occur in rocks located near the epicentre or along the fault that harboured the principal quake. Although the shaking intensity associated with most aftershocks is small compared with that of the principal earthquake, many are large enough to hamper rescue efforts by further destabilizing buildings and other structures and can be stressful for local residents coping with the damage and loss of life wrought by the principal quake.
Geophysical process
The sudden release of energy resulting from the fracturing of rocks relieves much of the stress at the earthquake’s focus; however, much of this energy is transferred to nearby rock. This transference either creates stresses where none existed before or increases the stress within or between rocks. When the sudden buildup of stress is great enough to fracture these rocks, thereby relieving the stress between them, a series of smaller tremors are produced.
Small tremors that follow an earthquake are considered aftershocks if they originate within the fault harbouring the earthquake or if they occur outside the fault within one full fault length (the measurement of the fault end to end) from the earthquake’s epicentre. (This measure will vary according to the length of the fault.) Aftershocks decrease in magnitude and frequency over time. Overall, this decay is inversely proportional to the amount of time passing since the principal earthquake. Once the rate at which these tremors occur has declined to pre-earthquake levels, the sequence of aftershocks ends. The typical aftershock sequence may be as short as a few weeks or as long as a few decades. Some aftershock sequences, however, may last centuries, such as the sequence resulting from the New Madrid earthquakes of 1811–12, which continues to the present.
" height="20" alt="HEW Logo">
In general, aftershocks are most severe and happen more frequently in the hours and days that follow an earthquake. Larger earthquakes tend to produce larger aftershocks. When attempting to predict the decrease in moment magnitude (the measure of the total amount of energy released during an earthquake) between the principal earthquake and the largest aftershock, seismologists often refer to Båth’s Law, which notes that the average difference in size between the two events is 1.2 orders of magnitude. The actual difference in size, however, ranges from 0.1 to 3 orders of magnitude. Small aftershocks occur with a greater frequency than large ones; however, aftershock frequency falls off with the passage of time.
Notable aftershocks
Although aftershocks tend to be weaker events relative to the power of the main quake, some aftershocks have caused significant damage. One of the most devastating aftershocks was a magnitude-7.1 event that struck the city of Luanxian, China, on July 28, 1976. Some hours earlier, a magnitude-7.5 earthquake had struck the nearby city of Tangshan. The aftershock was responsible for additional damage and casualties, as well as for delaying efforts to free people already trapped beneath rubble. There are also examples of large aftershocks’ causing more damage and loss of life than the earthquakes they are associated with. One of the most famous was the magnitude-6.3 aftershock that struck Christchurch, N.Z., on Feb. 22, 2011. Because of the aftershock’s shallow focus and the location of the tremor’s epicentre within Christchurch’s metropolitan area, it proved to be more devastating and deadly than the principal quake, which was centred in a rural area.