seismology Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

, a tsunami earthquake is an

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fro ...

which triggers a

tsunami A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater explo ...

of significantly greater magnitude, as measured by shorter-period

seismic wave A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic energy. ...

s. The term was introduced by Japanese seismologist

Hiroo Kanamori is a Japanese seismologist who has made fundamental contributions to understanding the physics of earthquakes and the tectonic processes that cause them. Career Kanamori and American seismologist Thomas C. Hanks developed the moment magnitud ...

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.

Characteristics

The distinguishing feature for a tsunami earthquake is that the release of seismic energy occurs at long periods (low frequencies) relative to typical tsunamigenic earthquakes. Earthquakes of this type do not generally show the peaks of seismic wave activity associated with ordinary events. A tsunami earthquake can be defined as an undersea earthquake for which the

surface wave magnitude The surface wave magnitude (M_s) scale is one of the magnitude scales used in seismology to describe the size of an earthquake. It is based on measurements of Rayleigh surface waves that travel along the uppermost layers of the Earth. This m ...

M_s differs markedly from the moment magnitude M_w, because the former is calculated from surface waves with a period of about 20 seconds, whereas the latter is a measure of the total energy release at all frequencies. The displacements associated with tsunami earthquakes are consistently greater than those associated with ordinary tsunamigenic earthquakes of the same moment magnitude, typically more than double. Rupture velocities for tsunami earthquakes are typically about 1.0 km per second, compared to the more normal 2.5–3.5 km per second for other megathrust earthquakes. These slow rupture speeds lead to greater directivity, with the potential to cause higher run-ups on short coastal sections. Tsunami earthquakes mainly occur at

subduction zone Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

s where there is a large

accretionary wedge An accretionary wedge or accretionary prism forms from sediments accreted onto the non-subducting tectonic plate at a convergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the do ...

or where sediments are being subducted, as this weaker material leads to the slower rupture velocities.

Cause

Analysis of tsunami earthquakes such as the

1946 Aleutian Islands earthquake The 1946 Aleutian Islands earthquake occurred near the Aleutian Islands, Alaska on April 1, 1946. The shock had a moment magnitude () of 8.6, a tsunami magnitude of 9.3, and a surface-wave magnitude () of only 7.4, and a maximum Mercalli inte ...

shows that the release of

seismic moment Seismic moment is a quantity used by seismologists to measure the size of an earthquake. The scalar seismic moment M_0 is defined by the equation M_0=\mu AD, where *\mu is the shear modulus of the rocks involved in the earthquake (in pascals (Pa), ...

takes place at an unusually long period. Calculations of the effective moment derived from surface waves show a rapid increase with decrease in the frequency of the seismic waves, whereas for ordinary earthquakes it remains almost constant with frequency. The duration over which the seabed is deformed has little effect on the size of the resultant tsunami for times up to several minutes. The observation of long period energy release is consistent with unusually slow rupture propagation velocities. Slow rupture velocities are linked to propagation through relatively weak material, such as poorly consolidated

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

s. Most tsunami earthquakes have been linked to rupture within the uppermost part of a subduction zone, where an accretionary wedge is developed in the hanging wall of the megathrust. Tsunami earthquakes have also been linked to the presence of a thin layer of subducted sedimentary rock along the uppermost part of the plate interface, as is thought to be present in areas of significant topography at the top of the oceanic crust, and where propagation was in an up-dip direction, possibly reaching the seafloor.

Identifying tsunami earthquakes

Standard methods of giving early warnings for tsunamis rely on data that will not typically identify a tsunami earthquake as tsunamigenic and therefore fail to predict possibly damaging tsunamis.

Examples

1896 Sanriku

On 15 June 1896 the Sanriku coast was struck by a devastating tsunami with a maximum wave height of 38.2 m, which caused more than 22,000 deaths. The residents of the coastal towns and villages were taken completely by surprise because the tsunami had only been preceded by a relatively weak shock. The magnitude of the tsunami has been estimated as M_t=8.2 while the earthquake shaking only indicated a magnitude of M_s=7.2. This discrepancy in magnitude requires more than just a slow rupture velocity. Modelling of tsunami generation that takes into account additional uplift associated with deformation of the softer sediments of the accretionary wedge caused by horizontal movement of the 'backstop' in the overriding plate has successfully explained the discrepancy, estimating a magnitude of M_w=8.0–8.1.

1992 Nicaragua

The 1992 Nicaragua earthquake was the first tsunami earthquake to be recorded with a broad-band seismic network.

Other tsunami earthquakes

* 1605 Nankai earthquake * 1677 Bōsō earthquake * 1771 Great Yaeyama earthquake * 1791 Okinawa earthquake *

1907 Sumatra earthquake The 1907 Sumatra earthquake occurred on January 4 at 05:19:12 UTC. The estimated magnitude is 7.5–8.0 , with an epicentre close to Simeulue, off Sumatra. It triggered a widespread and damaging tsunami that caused at least 2,188 deaths. The low o ...

* April 1923 Kamchatka earthquake and tsunami * 1934 Santa Cruz earthquake * 1932 Jalisco earthquakes *

* November 1960 Peru earthquake * 1963 Kuril Islands earthquake * 1975 Kuril Islands earthquake * 1982 Tonga earthquake * 1994 Java earthquake * 1996 Chimbote earthquake * 2002 Guerrero earthquake — A 6.7 earthquake off the coast of Mexico failed to trigger country's

earthquake warning system An earthquake warning system or earthquake early warning system is a system of accelerometers, seismometers, communication, computers, and alarms that is devised for notifying adjoining regions of a substantial earthquake while it is in progres ...

due to its extremely low peak acceleration. The near-trench earthquake ruptured the Guerrero Gap and may have generated an anomalously large tsunami. *

2006 Pangandaran earthquake and tsunami An earthquake occurred on July 17, 2006 at along a subduction zone off the coast of west and central Java, a large and densely populated island in the Indonesian archipelago. The shock had a moment magnitude of 7.7 and a maximum perceived int ...

* 2010 Mentawai earthquake and tsunami * 2012 El Salvador earthquake *2015 Torishima earthquake — A moderate 5.7 earthquake near Tori-shima generated waves with a maximum amplitude of 0.5 meters at

Hachijō-jima is a volcanic Japanese island in the Philippine Sea. It is about south of the special wards of Tokyo. It is part of the Izu archipelago and within the Fuji-Hakone-Izu National Park. Its only municipality is Hachijō. On 1 March 2018, its po ...

. * 2021 South Sandwich Islands earthquakes