In seismology, an earthquake rupture is the extent of slip that occurs during an earthquake in the Earth's crust. Earthquakes occur for many reasons that include: landslides, movement of magma in a volcano, the formation of a new fault, or, most commonly of all, a slip on an existing fault. [1]
A tectonic earthquake begins by an initial rupture at a point on the fault surface, a process known as nucleation. The scale of the nucleation zone is uncertain, with some evidence, such as the rupture dimensions of the smallest earthquakes, suggesting that it is smaller than 100 m while other evidence, such as a slow component revealed by low-frequency spectra of some earthquakes, suggest that it is larger. [2] The possibility that the nucleation involves some sort of preparation process is supported by the observation that about 40% of earthquakes are preceded by foreshocks. However, some large earthquakes, such as the M8.6 1950 India – China earthquake., [3] have no foreshocks and it remains unclear whether they just cause stress changes or are simply a result of increasing stresses in the region of the mainshock. [4]
Once the rupture has initiated, it begins to propagate along the fault surface. The mechanics of this process are poorly understood, partly because it is difficult to recreate the high sliding velocities in a laboratory. Also the effects of strong ground motion make it very difficult to record information close to a nucleation zone. [2]
Following nucleation, the rupture propagates away from the hypocentre in all directions along the fault surface. The propagation will continue as long as there is sufficient stored strain energy to create new rupture surface. Although the rupture starts to propagate in all directions, it often becomes unidirectional, with most of the propagation in a mainly horizontal direction. Depending on the depth of the hypocentre, the size of the earthquake and whether the fault extends that far, the rupture may reach the ground surface, forming a surface rupture. The rupture will also propagate down the fault plane, in many cases reaching the base of the seismogenic layer, below which the deformation starts to become more ductile in nature. [2]
Propagation may take place on a single fault, but in many cases the rupture starts on one fault before jumping to another, sometimes repeatedly. The 2002 Denali earthquake initiated on a thrust fault, the Sutsina Glacier Thrust, before jumping onto the Denali Fault for most of its propagation before finally jumping again onto the Totschunda Fault. The rupture of the 2016 Kaikōura earthquake was particularly complex, with surface rupture observed on at least 21 separate faults. [5]
Some ruptures simply run out of sufficient stored energy, preventing further propagation. [2] This may either be the result of stress relaxation due to an earlier earthquake on another part of the fault or because the next segment moves by aseismic creep, such that the stress never builds sufficiently to support rupture propagation. In other cases there is strong evidence for persistent barriers to propagation, giving an upper limit to earthquake magnitude. Rupture length correlates with earthquake magnitude and varies from an order of magnitude of kilometers in the single digits for a magnitude 5–6 earthquake up to hundreds of kilometers for stronger earthquakes (magnitude 7–9), although the correlation is not exact and outliers exist. [6]
Most ruptures propagate at speeds in the range of 0.5–0.7 of the shear wave velocity, with only a minority of ruptures propagating significantly faster or slower than that.
The upper limit to normal propagation is the velocity of Rayleigh waves, 0.92 of the shear wave velocity, typically about 3.5 km per second. Observations from some earthquakes indicate that ruptures can propagate at speeds between the S-wave and P-wave velocity. These supershear earthquakes are all associated with strike-slip movement. The rupture cannot accelerate through the Rayleigh wave limit, so the accepted mechanism is that supershear rupture begins on a separate "daughter" rupture in the zone of high stress at the tip of the propagating main rupture. [7] All observed examples show evidence of a transition to supershear at the point where the rupture jumps from one fault segment to another.
Slower than normal rupture propagation is associated with the presence of relatively mechanically weak material in the fault zone. This is particularly the case for some megathrust earthquakes, where the rupture velocity is about 1.0 km per second. These tsunami earthquakes are dangerous because most of the energy release happens at lower frequencies than normal earthquakes and they lack the peaks of seismic wave activity that would alert coastal populations to a possible tsunami risk. Typically the surface-wave magnitude for such an event is much smaller than moment magnitude as the former does not capture the longer wavelength energy release. [8] The 1896 Sanriku earthquake went almost unnoticed, but the associated tsunami killed more than 22,000 people.
Extremely slow ruptures take place on a time scale of hours to weeks, giving rise to slow earthquakes. These very slow ruptures occur deeper than the locked zone where normal earthquake ruptures occur on the same megathrusts. [9]
An earthquake – also called a quake, tremor, or temblor – is the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they cannot be felt, to those violent enough to propel objects and people into the air, damage critical infrastructure, and wreak destruction across entire cities. The seismic activity of an area is the frequency, type, and size of earthquakes experienced over a particular time. The seismicity at a particular location in the Earth is the average rate of seismic energy release per unit volume.
The epicenter, epicentre, or epicentrum in seismology is the point on the Earth's surface directly above a hypocenter or focus, the point where an earthquake or an underground explosion originates.
The moment magnitude scale is a measure of an earthquake's magnitude based on its seismic moment. Mw was defined in a 1979 paper by Thomas C. Hanks and Hiroo Kanamori. Similar to the local magnitude/Richter scale (ML ) defined by Charles Francis Richter in 1935, it uses a logarithmic scale; small earthquakes have approximately the same magnitudes on both scales. Despite the difference, news media often use the term "Richter scale" when referring to the moment magnitude scale.
Megathrust earthquakes occur at convergent plate boundaries, where one tectonic plate is forced underneath another. The earthquakes are caused by slip along the thrust fault that forms the contact between the two plates. These interplate earthquakes are the planet's most powerful, with moment magnitudes (Mw) that can exceed 9.0. Since 1900, all earthquakes of magnitude 9.0 or greater have been megathrust earthquakes.
The 1999 Düzce earthquake occurred on 12 November at 18:57:22 local time with a moment magnitude of 7.2 and a maximum Mercalli intensity of IX (Violent), causing damage and at least 845 fatalities in Düzce, Turkey. The epicenter was approximately 100 km (62 mi) to the east of the extremely destructive 1999 İzmit earthquake that happened nearly three months earlier. Both strike-slip earthquakes were caused by movement on the North Anatolian Fault.
A slow earthquake is a discontinuous, earthquake-like event that releases energy over a period of hours to months, rather than the seconds to minutes characteristic of a typical earthquake. First detected using long term strain measurements, most slow earthquakes now appear to be accompanied by fluid flow and related tremor, which can be detected and approximately located using seismometer data filtered appropriately. That is, they are quiet compared to a regular earthquake, but not "silent" as described in the past.
In seismology, a supershear earthquake is an earthquake in which the propagation of the rupture along the fault surface occurs at speeds in excess of the seismic shear wave (S-wave) velocity. This causes an effect analogous to a sonic boom.
An earthquake occurred in China on 14 November 2001 at 09:26 UTC, with an epicenter near Kokoxili, close to the border between Qinghai and Xinjiang in a remote mountainous region. With a magnitude of 7.8 Mw, it was the most powerful earthquake in China for 5 decades. No casualties were reported, presumably due to the very low population density and the lack of high-rise buildings. This earthquake was associated with the longest surface rupture ever recorded on land, ~450 km.
The 2002 Denali earthquake occurred at 22:12:41 UTC November 3 with an epicenter 66 km ESE of Denali National Park, Alaska, United States. This 7.9 Mw earthquake was the largest recorded in the United States in 37 years. The shock was the strongest ever recorded in the interior of Alaska. Due to the remote location, there were no fatalities and only one injury.
The 1995 Antofagasta earthquake occurred on July 30 at 05:11 UTC with a moment magnitude of 8.0 and a maximum Mercalli intensity of VII. The Antofagasta Region in Chile was affected by a moderate tsunami, with three people killed, 58 or 59 injured, and around 600 homeless. Total damage from the earthquake and tsunami amounted to $1.791 million.
The 2002 Sumatra earthquake occurred at 01:26 UTC on 2 November. It had a magnitude of 7.4 on the moment magnitude scale with an epicenter just north of Simeulue island and caused three deaths. This earthquake is regarded as a foreshock of the 2004 Indian Ocean earthquake, which had an epicenter about 60 km to the northwest.
In seismology, a tsunami earthquake is an earthquake which triggers a tsunami of significantly greater magnitude, as measured by shorter-period seismic waves. The term was introduced by Japanese seismologist Hiroo Kanamori in 1972. Such events are a result of relatively slow rupture velocities. They are particularly dangerous as a large tsunami may arrive at a coastline with little or no warning.
This is a list of different types of earthquake.
On 9 January 2018, at approximately 8:51 p.m. local time, a magnitude 7.5 earthquake struck in the Yucatán Basin of the Caribbean Sea, 44 kilometres (27 mi) east of Great Swan Island off the coast of Honduras. The earthquake was felt across Central America, and rattled windows in Tegucigalpa. The earthquake was also felt in the Cayman Islands.
The 1907 Sumatra earthquake occurred on January 4 at 05:19:12 UTC. The re-estimated moment magnitude (Mw) is 8.2 to 8.4, with an epicentre close to Simeulue, off Sumatra. An earlier study re-estimated a surface-wave magnitude (Ms) of 7.5 to 8.0. It triggered a widespread and damaging Indian Ocean wide tsunami that caused at least 2,188 deaths on Sumatra. The low observed intensity compared to the size of the tsunami has led to its interpretation as a tsunami earthquake. Higher levels of shaking observed on Nias are attributed to a large aftershock, less than an hour later. The tsunami gave rise to the S'mong legend, which is credited with saving many lives during the 2004 earthquake.
On July 17, 2017, an earthquake struck near the Komandorski Islands, east of the Kamchatka Peninsula in the Bering Sea at. Although there were no casualties from this earthquake, it was notable for a rare characteristic known as supershear, and is one of the few times a large supershear earthquake has been observed. It was preceded by a few foreshocks months earlier, and aftershocks that continued for nearly six months.
The 2013 Craig, Alaska earthquake struck on January 5, at 12:58 am (UTC–7) near the city of Craig and Hydaburg, on Prince of Wales Island. The Mw 7.5 earthquake came nearly three months after an Mw 7.8 quake struck Haida Gwaii on October 28, in 2012. The quake prompted a regional tsunami warning to British Columbia and Alaska, but it was later cancelled. Due to the remote location of the quake, there were no reports of casualties or damage.
An earthquake occurred on 26 August 2012 at 22:37 local time. The earthquake located off the coast of El Salvador measured 7.3 on the moment magnitude scale and had a focal depth of 16.0 kilometres (10 mi). No deaths were reported, however more than 40 people were injured when they were caught in a tsunami generated by the earthquake. Waves from the tsunami were unusually large for an earthquake of this size. The large waves were attributed to the earthquake's unique rupture characteristic. In addition to the absence of fatalities, damage caused by the earthquake and tsunami was minimal as a result of the sparse population around the affected region and the slow rupture characteristic of the event.
On April 13, 1923, at 15:31 UTC, an earthquake occurred off the northern coast of the Kamchatka Peninsula in the USSR, present-day Russia. The earthquake had a surface-wave magnitude (Ms ) of 6.8–7.3 and an estimated moment magnitude (Mw ) of 7.0–8.2. This event came just two months after a slightly larger earthquake with an epicenter struck south of the April event. Both earthquakes were tsunamigenic although the latter generated wave heights far exceeding that of the one in February. After two foreshocks of "moderate force", the main event caused considerable damage. Most of the 36 casualties were the result of the tsunami inundation rather than the earthquake.
The 2021 South Sandwich Islands earthquakes were a pair of powerful earthquakes, followed by many strong aftershocks which struck along the South Sandwich Trench in August 2021. The quakes measured 7.5 and 8.1 on the moment magnitude scale, according to the United States Geological Survey. The mainshock is tied with another event in 1929 as the largest earthquake ever recorded in the South Atlantic region, and is tied with the 2021 Kermadec Islands earthquake as the second largest earthquake of 2021.
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