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In seismology, the mainshock is the largest earthquake in a sequence, sometimes preceded by one or more foreshocks, and almost always followed by many aftershocks. [1] [2] [3]
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 sequence of events has happened. [4]
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 known laws. 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.
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 known laws. 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.
In seismology, an earthquake swarm is a sequence of seismic events occurring in a local area within a relatively short period of time. The length of time used to define the swarm itself varies, but may be of the order of days, months, or even years. Such an energy release is different from what happens commonly when a major earthquake (mainshock) is followed by a series of aftershocks: in earthquake swarms, no single earthquake in the sequence is obviously the mainshock. In particular, a cluster of aftershocks occurring after a mainshock is not a swarm.
A foreshock is an earthquake that occurs before a larger seismic event 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 sequence of events has happened.
The 1857 Fort Tejon earthquake occurred at about 8:20 a.m. on January 9 in central and Southern California. One of the largest recorded earthquakes in the United States, with an estimated moment magnitude of 7.9, it ruptured the southern part of the San Andreas Fault for a length of about 225 miles, between Parkfield and Wrightwood.
The 1930 Salmas earthquake occurred on May 7 at 01:34:26 IRST in West Azerbaijan Province, Iran. The earthquake, which was among Iran's largest, measured 7.1 on the moment magnitude scale and had a maximum Mercalli intensity of IX (Violent). A damaging foreshock occurred fifteen hours prior to the main event and served as a warning to the people that felt it strongly. Reports from seismologists and seismological organizations indicate that up to 3,000 fatalities may have occurred in northwest Iran and southeast Turkey.
The 1997 Umbria and Marche earthquake occurred in the regions of Umbria and Marche, central Italy on the morning of September 26. It was preceded by a foreshock almost as strong as the main quake. The foreshock occurred at 02:33 CEST, rated Mw5.7, and the second – the main shock – occurred at 11:40 CEST, rated Mw 6.0. Their epicentre was in Annifo. The mainshock was assigned X (Extreme) and foreshock VIII (Severe) on the Mercalli intensity scale.
In seismology, doublet earthquakes – and more generally, multiplet earthquakes – were originally identified as multiple earthquakes with nearly identical waveforms originating from the same location. They are now characterized as single earthquakes having two main shocks of similar magnitude, sometimes occurring within tens of seconds, but sometimes separated by years. The similarity of magnitude – often within four-tenths of a unit of magnitude – distinguishes multiplet events from aftershocks, which start at about 1.2 magnitude less than the parent shock, and decrease in magnitude and frequency according to known laws.
The 1986 Chalfant Valley earthquake struck southern Mono County near Bishop and Chalfant, California at 07:42:28 Pacific Daylight Time on July 21. With a moment magnitude of 6.2 and a maximum Mercalli intensity of VI (Strong), the shock injured two people and caused property damage estimated at $2.7 million in the affected areas. There was a significant foreshock and aftershock sequence that included a few moderate events, and was the last in a series of three earthquakes that affected southern California and the northern Owens Valley in July 1986.
The 1995 Gulf of Aqaba earthquake occurred on November 22 at 06:15 local time and registered 7.3 on the Mw scale. The epicenter was located in the central segment of the Gulf of Aqaba, the narrow body of water that separates Egypt's Sinai Peninsula from the western border of Saudi Arabia. At least 8 people were killed and 30 were injured in the meizoseismal area.
The 1872 North Cascades earthquake occurred at 9:40 p.m. local time on December 14 in central Washington state. A maximum Mercalli intensity of VIII (Severe) was assessed for several locations, though less intense shaking was observed at many other locations in Washington, Oregon, and British Columbia. Some of these intermediate outlying areas reported V (Moderate) to VII shaking, but intensities as high as IV (Light) were reported as far distant as Idaho and Montana. Due to the remote location of the mainshock and a series of strong aftershocks, damage to structures was limited to a few cabins close to the areas of the highest intensity.
In May 2012, two major earthquakes struck Northern Italy, causing 27 deaths and widespread damage. The events are known in Italy as 2012 Emilia earthquakes, because they mainly affected the Emilia region.
This is a list of different types of earthquake.
The 2014 Murmuri earthquake occurred on August 18 in the Zagros Mountains of Iran with a moment magnitude of 6.2 and a maximum Mercalli intensity of VIII (Severe). The dip-slip shock injured 60–330 people and was followed by a number of high intensity aftershocks.
The 1979 Coyote Lake earthquake occurred at 10:05:24 local time on August 6 with a moment magnitude of 5.7 and a maximum Mercalli Intensity of VII. The shock occurred on the Calaveras Fault near Coyote Lake in Santa Clara County, California and resulted in a number of injuries, including some that required hospitalization. Most of the $500,000 in damage that was caused was non-structural, but several businesses were closed for repairs. Data from numerous strong motion instruments was used to determine the type, depth, and extent of slip. A non-destructive aftershock sequence that lasted throughout the remainder of the month was of interest to seismologists, especially with regard to fault creep, and following the event local governments evaluated their response to the incident.
The 1992 Murindó earthquake occurred on October 18 at 15:12 with an epicenter in the Department of Chocó, northern Colombia. The shallow magnitude 7.2 earthquake struck northwest of the town of Murindó, killing ten and injured more than a hundred. Thirty three municipalities were severely damaged.
The 2019 Ridgecrest earthquakes of July 4 and 5 occurred north and northeast of the town of Ridgecrest, California located in Kern County and west of Searles Valley. They included three initial main shocks of Mw magnitudes 6.4, 5.4, and 7.1, and many perceptible aftershocks, mainly within the area of the Naval Air Weapons Station China Lake. Eleven months later, a Mw 5.5 aftershock took place to the east of Ridgecrest. The first main shock occurred on Thursday, July 4 at 10:33 a.m. PDT, approximately 18 km (11.2 mi) ENE of Ridgecrest, and 13 km (8.1 mi) WSW of Trona, on a previously unnoticed NE-SW trending fault where it intersects the NW-SE trending Little Lake Fault Zone. This quake was preceded by several smaller earthquakes, and was followed by more than 1,400 detected aftershocks. The M 5.4 and M 7.1 quakes struck on Friday, July 5 at 4:08 a.m. and 8:19 p.m. PDT approximately 10 km (6 miles) to the northwest. The latter, now considered the mainshock, was the most powerful earthquake to occur in the state in 20 years. Subsequent aftershocks extended approximately 50 km (~30 miles) along the Little Lake Fault Zone.
The 1833 Nepal–India earthquake, also known as the 1833 Bihar–Nepal earthquake occurred on August 26 at 22:58 local time (NPT). This earthquake had an estimated moment magnitude of 7.6–7.9 and struck with an epicenter somewhere in or near the Kathmandu Valley. The earthquake caused major damage and deaths in numerous towns and villages in Nepal, northern India and Tibet. The earthquake was so powerful that it was also felt in Chittagong, Bangladesh. Despite the extent of the damage, the number of fatalities resulted from the earthquake was very low, at around 500. This was because the mainshock was preceded by two smaller but intense foreshocks earlier that day, causing many residents to make refuge outside their homes.
The 1995 Menglian earthquake or 1995 Myanmar–China earthquake occurred on 12 July at 05:46:43 local time in the Myanmar–China border region. The earthquake had an epicenter on the Myanmar side of the border, located in the mountainous region of Shan State. It registered 7.3 on the Chinese surface wave magnitude scale (Ms ) and 6.8 on the moment magnitude scale (Mw ). With a maximim Mercalli intensity assigned at VIII, the quake killed eleven people and left another 136 injured. Over 100,000 homes in both countries were destroyed and 42,000 seriously damaged. Some damage to structures were also reported in Chiang Mai and Chiang Rai, Thailand. The low death toll from this earthquake was attributed to an early warning issued prior to it happening. Precursor events including foreshocks and some seismic anomalies led to an evacuation of the area before the mainshock struck. It is thought to be one of the few successfully predicted earthquakes in history.
The 1941 Sa'dah earthquake or the Jabal Razih earthquake occurred on January 11 in the Razih District of Yemen. The earthquake had a surface wave magnitude of 5.8 or 6.2/6.5 and a shallow focal depth. Despite the moserate size of this earthquake, an estimated 1,200 people perished and at least 200. With a maximum MSK-64 intensity assigned at VIII, it destroyed many villages and collapsed homes in the region of North Yemen.
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