Aftershocks are a series of earthquakes that occur following a greater mainshock on a fault. A swarm, on the other hand, is a series of primarily minor earthquakes with no discernible mainshock. Swarms can be local or widespread.
An earthquake cluster is a sequence of closely spaced earthquakes in different parts of the world within a short period of time. Most clusters are sequences of small quakes but they can also consist of large shocks. The largest recorded earthquake ever to have occurred on Earth was the Great Sumatran Earthquake of December 26, 2004. It had a magnitude of 9.1 and killed over 200,000 people.
The Great Chilean Earthquake of February 27, 2010 was one of the most powerful earthquakes ever recorded. It had a magnitude of 8.8 and killed over 600 people.
The New Madrid Seismic Zone is a region of southern Missouri and northern Kentucky where several major earthquakes have occurred in the past. The last major event in this zone was in 1812 when an estimated magnitude of 7.9 hit.
These terms are used interchangeably with the word "magnitude". They all refer to the size of the earthquake as measured by seismic instruments on the ground or in space.
Aftershocks occur in the fault zone where the mainshock ruptured and are part of the "readjustment process" following the fault's main slide. They are felt at distances beyond what would be expected from just geology alone, so they can provide evidence for any changes in depth or type of material beneath the surface.
The word "aftershock" comes from a French term meaning "again shock". These events are also called "secondary quakes" or "re-emissions". The term "mantle quake aftershock", or simply "mantle quakes", refers to aftershocks that occur within the Earth's mantle.
Mantle quakes are thought to originate in regions of weakness within the Earth's mantle. Because of their location deep under continents and oceans, they are not likely to be detected by seismic monitoring systems designed to look for ground vibrations caused by shallow earthquakes. However there have been a few exceptions; a series of large mantle quakes occurred in Japan in 1999 and again in 2016. These events were widely reported by the media due to their impact on coastal areas and because they showed clear evidence of hypocenters (the locations where the waves first arrive) far away from any known seismogenic activity.
Mantle quakes may be triggered by the same mechanisms that cause shallow earthquakes.
Aftershocks are minor earthquakes that occur in the same general location days to years after a bigger occurrence, known as a "mainshock." They occur within 1-2 fault lengths of one other and before the background seismicity level has returned. The largest recorded number of aftershocks in a single event was following the 2004 Indian Ocean earthquake, where more than 200 distinct shocks were detected by monitoring stations worldwide.
The term "aftershock" can be used for any seismic event that occurs at the same site as a larger one, but it is most commonly applied to smaller events detected after a major shock. Aftershocks can be divided into two categories based on magnitude: micro- (<=1) aftershock and macroseismic (>1). Micro-aftershocks account for a large majority of events (more than 90%) and are usually clustered around the main shock. Macroseismic aftershocks occur less frequently (about 10% of all aftershocks) and are spread out over a wider area. They can also have magnitudes greater than the original mainshock.
Macroseismic events can cause damage over a wide area if they are close together in time or if they occur away from population centers. Microseismic events cannot cause significant damage alone, but they can increase the likelihood of damage from macroseismic events through mechanisms such as preconditioning or stress redistribution.
Aftershocks are earthquakes that occur immediately after the greatest shock in an earthquake series. They are smaller than the mainshock and have 1-2 rupture lengths in common with it. More than one aftershock can happen at the same place and time as long as they are more than 4 hours apart. The term "aftershock" is used for earthquakes of any magnitude, but most are too small to be felt by humans.
The largest known earthquake in history was the Great Japanese Earthquake of March 11, 1911. The quake had a magnitude of 9.0 and can be seen today as a strong example of a tsunami. Tidal waves up to 30 meters (100 feet) high destroyed much of coastal Japan, killing over 20,000 people.
Smaller earthquakes cause buildings to collapse, roads to buckle, and trees to fall. They often but not always come in sequences after a large earthquake. If you are in an area prone to earthquakes, you should be prepared for any type of disaster to happen at any time. Get informed on how to react in different situations before anything happens.
An aftershock sequence usually starts with a small earthquake that can be felt by humans. This might not be the first aftershock but rather another part of the original fault line hitting something hard, such as an old building or rock formation.
An aftershock is a minor earthquake that occurs after a bigger earthquake in the same location as the main shock, produced by the displaced crust adjusting to the effects of the original shock. Aftershocks tend to be less violent than the initial quake.
People often report feeling an aftershock several minutes after the main shock has occurred. However, seismic activity dies down quickly after large events, so there may not be any aftershocks detected by people on the ground for some time after they happen.
Aftershocks can further divide into two categories based on magnitude: strong aftershocks are those that are equal to or exceed the threshold magnitude for that region. Weak aftershocks are all others.
The rate at which aftershocks occur after a major earthquake varies significantly from event to event and location to location. Smaller quakes are usually more frequent near the surface while larger ones produce fewer shocks deeper under the earth's skin. Aftershocks can also vary in intensity; some may be barely detectable while others might be strong enough to cause damage if they were felt alone, but many aftershocks are too weak to be noticeable.
Earthquake swarms are clusters of earthquakes that strike a certain location in rapid succession. They vary from earthquakes followed by a succession of aftershocks in that no single earthquake in the sequence is clearly the primary shock, and hence none has a significantly larger magnitude than another. Earthquakes often cause lights to go out and power lines to fall down, so people usually want to know what will happen next. When a large number of small earthquakes occur close together, it can be difficult for people to tell when or if the main earthquake happened.
An example of a seismic swarm is shown in data from 2010, when several groups of small quakes were detected near Mexico's Pacific Coast. The largest group was detected on February 19th and measured 5.7 on the richter scale. After this event scientists realized that it was part of a much larger movement called the Pacific Northwest Seismic Network (PNSN). The PNSN includes more than 10,000 sensors that monitor ground motion all around the region. These events indicate that there is a lot of activity under the Pacific Ocean at a depth of about 200 miles. It may be that a large portion of California lies over such an area!
Another type of seismic swarm occurs within active volcanoes. Smaller eruptions sometimes cause earthquakes, which then stop until another eruption begins the process again.