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.
The New Madrid Seismic Zone, sometimes called the New Madrid Fault Line, is a major seismic zone and a prolific source of intraplate earthquakes in the Southern and Midwestern United States, stretching to the southwest from New Madrid, Missouri.
Peak ground acceleration (PGA) is equal to the maximum ground acceleration that occurred during earthquake shaking at a location. PGA is equal to the amplitude of the largest absolute acceleration recorded on an accelerogram at a site during a particular earthquake. Earthquake shaking generally occurs in all three directions. Therefore, PGA is often split into the horizontal and vertical components. Horizontal PGAs are generally larger than those in the vertical direction but this is not always true, especially close to large earthquakes. PGA is an important parameter for earthquake engineering, The design basis earthquake ground motion (DBEGM) is often defined in terms of PGA.
In seismology, an earthquake swarm is a sequence of seismic events occurring in a local area within a relatively short period. The time span used to define a swarm varies, but may be days, months, or years. Such an energy release is different from the situation when a major earthquake is followed by a series of aftershocks: in earthquake swarms, no single earthquake in the sequence is obviously the main shock. In particular, a cluster of aftershocks occurring after a mainshock is not a swarm.
Induced seismicity is typically earthquakes and tremors that are caused by human activity that alters the stresses and strains on Earth's crust. Most induced seismicity is of a low magnitude. A few sites regularly have larger quakes, such as The Geysers geothermal plant in California which averaged two M4 events and 15 M3 events every year from 2004 to 2009. The Human-Induced Earthquake Database (HiQuake) documents all reported cases of induced seismicity proposed on scientific grounds and is the most complete compilation of its kind.
The Fukuoka earthquake struck Fukuoka Prefecture, Japan at 10:53 am JST on March 20, 2005, and lasted for approximately 1 minute. The Japan Meteorological Agency (JMA) measured it as peaking at a magnitude of 7.0, whereas the United States Geological Survey (USGS) reported a magnitude of 6.6. The quake occurred along a previously unknown fault in the Genkai Sea, North of Fukuoka city, and the residents of Genkai Island were forced to evacuate as houses collapsed and landslides occurred in places. Investigations subsequent to the earthquake determined that the new fault was most likely an extension of the known Kego fault that runs through the centre of the city.
The 1886 Charleston earthquake in South Carolina occurred about 9:50 p.m. local time August 31. It caused 60 deaths and $5–6 million in damage to 2,000 buildings in the Southeastern United States. It is one of the most powerful and damaging earthquakes to hit the East Coast of the United States.
The Wabash Valley Seismic Zone is a tectonic region located in the Midwest of the United States, centered on the valley of the Lower Wabash River, along the state line between southeastern Illinois and southwestern Indiana.
The 2008 Chino Hills earthquake occurred at 11:42:15 am PDT on July 29 in Southern California. The epicenter of the magnitude 5.4 earthquake was in Chino Hills, c. 28 miles (45 km) east-southeast of downtown Los Angeles. Movement on an oblique-slip fault resulted in a maximum Mercalli intensity of VI (Strong). Though there were no deaths, eight people were injured, and it caused considerable damage in numerous structures throughout the area and caused some amusement park facilities to shut down their rides. The earthquake led to increased discussion regarding the possibility of a stronger earthquake in the future.
The 2010 Baja California earthquake occurred on April 4 with a moment magnitude of 7.2 and a maximum Mercalli intensity of VII. The shock originated at south of Guadalupe Victoria, Baja California, Mexico.
The 2011 Oklahoma earthquake was a 5.7 magnitude intraplate earthquake which occurred near Prague, Oklahoma on November 5 at 10:53 p.m. CDT in the U.S. state of Oklahoma. The epicenter of the earthquake was in the vicinity of several active wastewater injection wells. According to the United States Geological Survey (USGS), it was the most powerful earthquake ever recorded in Oklahoma; this record was surpassed by the 2016 Oklahoma earthquake. The previous record was a 5.5 magnitude earthquake that struck near the town of El Reno in 1952. The quake's epicenter was approximately 44 miles (71 km) east-northeast of Oklahoma City, near the town of Sparks and was felt in the neighboring states of Texas, Arkansas, Kansas and Missouri and even as far away as Tennessee and Wisconsin. The quake followed several minor quakes earlier in the day, including a 4.7 magnitude foreshock. The quake had a maximum perceived intensity of VIII (Severe) on the Mercalli intensity scale in the area closest to the epicenter. Numerous aftershocks were detected after the main quake, with a few registering at 4.0 magnitude.
The 1993 Klamath Falls earthquakes took place in Klamath Falls, Oregon, beginning on Monday, 20 September at 8:28 p.m. The doublet earthquake registered respective magnitudes of 6.0 and 5.9 on the moment magnitude scale. The earthquakes were located at a depth of 5.6 miles (9 km) and tremors continued to be felt more than three months after the initial shocks.
The 2014 Orkney earthquake occurred at 12:22:33 SAST on 5 August, with the epicentre near Orkney, a gold mining town in the Klerksdorp district in the North West province of South Africa. The shock was assigned a magnitude of 5.5 on the Richter scale by the Council for Geoscience (CGS) in South Africa, making it the biggest earthquake in South Africa since the 1969 Tulbagh earthquake, which had a magnitude of 6.3 on the Richter scale. The United States Geological Survey (USGS) estimated a focal depth of 5.0 km (3.1 mi). The CGS reported 84 aftershocks on 5 August and 31 aftershocks on 6 August, with a magnitude of 1.0 to 3.8 on the Richter scale. According to the CGS, the earthquake is the biggest mining-related earthquake in South African history.
The 2016 Gyeongju earthquake occurred on September 12 near Gyeongju, South Korea. Measuring 5.8 on the local magnitude scale, it was the strongest earthquake in the country until the following year. The 2017 Pohang earthquake caused more damage despite being equal in magnitude. An hour before, at 07:44:32 local time, a ML 5.1 foreshock occurred.
On May 4, 2018, an earthquake with a magnitude of 6.9 struck Hawaii island in the Hawaii archipelago at around 12:33 p.m. local time. The earthquake's epicenter was near the south flank of Kīlauea, which has been the site of seismic and volcanic activity since late April of that year. According to the United States Geological Survey the quake was related to the new lava outbreaks at the volcano, and it resulted in the Hilina Slump moving about two feet. It was the largest earthquake to affect Hawaii since the 1975 earthquake, which affected the same region, killing two people and injuring another 28.
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 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 2020 Central Idaho earthquake occurred in the western United States on March 31, 2020, at 5:52 PM MDT, near Ruffneck Peak in the Sawtooth Mountains of central Idaho, 72 miles (116 km) northeast of Boise and 19 miles (31 km) northwest of Stanley. It had a magnitude of 6.5 and was felt with a maximum intensity of VIII.
The 2020 Sparta earthquake was a relatively uncommon intraplate earthquake that occurred near the small town of Sparta, North Carolina, on August 9, 2020 at 8:07 am local time. The earthquake had a moment magnitude of 5.1, and a shallow depth of 7.6 kilometers (4.7 mi). Shaking was reported throughout the Southern, Midwestern, and Northeastern United States. It was the strongest earthquake recorded in North Carolina in 104 years, the second-strongest in the state's history, and the largest to strike the East Coast since the 2011 Virginia earthquake.
An earthquake struck approximately 53 kilometres SSE of the town of Mansfield, in the Victorian Alps of Australia on 22 September 2021, at 09:15 local time. The earthquake measured 5.9 on the moment magnitude scale. The earthquake caused minor structural damage in parts of Melbourne and left one person injured. The earthquake was also felt in New South Wales, Australian Capital Territory, South Australia and Tasmania. The earthquake was substantially stronger than the 1989 Newcastle earthquake that measured 5.6 and killed 13 people.
The 1992 Joshua Tree earthquake occurred at 9:50:25 p.m. PDT on April 22 in Southern California. The magnitude 6.2 earthquake struck under the Little San Bernardino Mountains, near the town of Joshua Tree, California. Though no deaths were reported, the earthquake caused 32 injuries. A maximum Mercalli intensity of VII was observed in Joshua Tree and caused light to moderate damage. The event preceded the Landers and Big Bear earthquakes by two months but is now recognized as the beginning of a series of major earthquakes that culminated in two events on June 28, 1992.
