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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. [1] 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.
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 Ms differs markedly from the moment magnitude Mw, 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. [2] 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 zones where there is a large accretionary wedge or where sediments are being subducted, as this weaker material leads to the slower rupture velocities. [2]
Analysis of tsunami earthquakes such as the 1946 Aleutian Islands earthquake shows that the release of seismic moment 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. [1] Slow rupture velocities are linked to propagation through relatively weak material, such as poorly consolidated sedimentary rocks. 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. [3]
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. [4]
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 Mt=8.2 while the earthquake shaking only indicated a magnitude of Ms=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 Mw=8.0–8.1. [5]
The 1992 Nicaragua earthquake was the first tsunami earthquake to be recorded with a broad-band seismic network. [6]
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 1946 Aleutian Islands earthquake occurred near the Aleutian Islands, Alaska on April 1, 1946. The shock measured 8.6, Mt 9.3 or 7.4. It had a maximum Mercalli intensity of VI (Strong). It resulted in 165–173 casualties and over US$26 million in damage. The seafloor along the fault was elevated, triggering a Pacific-wide tsunami with multiple destructive waves at heights ranging from 45–138 ft (14–42 m). The tsunami obliterated the Scotch Cap Lighthouse on Unimak Island, Alaska among others, and killed all five lighthouse keepers. Despite the destruction to the Aleutian Island Unimak, the tsunami had almost an imperceptible effect on the Alaskan mainland.
The 1957 Andreanof Islands earthquake occurred at 04:22 local time on March 9 with a moment magnitude estimated at 8.6 and a maximum Modified Mercalli intensity of VIII (Severe). It occurred south of the Andreanof Islands group, which is part of the Aleutian Islands arc. The event occurred along the Aleutian Trench, the convergent plate boundary that separates the Pacific plate and the North American plates near Alaska. A basin-wide tsunami followed, with effects felt in Alaska and Hawaii, and strong waves recorded across the Pacific rim. Total losses were around $5 million.
An earthquake occurred on July 17, 2006, at 15:19:27 local time 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 intensity of IV (Light) in Jakarta, the capital and largest city of Indonesia. There were no direct effects of the earthquake's shaking due to its low intensity, and the large loss of life from the event was due to the resulting tsunami, which inundated a 300 km (190 mi) portion of the Java coast that had been unaffected by the earlier 2004 Indian Ocean earthquake and tsunami that was off the coast of Sumatra. The July 2006 earthquake was also centered in the Indian Ocean, 180 kilometers (110 mi) from the coast of Java, and had a duration of more than three minutes.
In seismology, a supershear earthquake is when the propagation of the rupture along the fault surface occurs at speeds in excess of the seismic shear wave velocity. This causes an effect analogous to a sonic boom.
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 1896 Sanriku earthquake was one of the most destructive seismic events in Japanese history. The 8.5 magnitude earthquake occurred at 19:32 on June 15, 1896, approximately 166 kilometres (103 mi) off the coast of Iwate Prefecture, Honshu. It resulted in two tsunami waves which destroyed about 9,000 homes and caused at least 22,000 deaths. The waves reached a then-record height of 38.2 metres (125 ft); this would remain the highest on record until waves from the 2011 Tōhoku earthquake exceeded that height by more than 2 metres.
The Sunda megathrust is a fault that extends approximately 5,500 km (3300 mi) from Myanmar (Burma) in the north, running along the southwestern side of Sumatra, to the south of Java and Bali before terminating near Australia. It is a megathrust, located at a convergent plate boundary where it forms the interface between the overriding Eurasian plate and the subducting Indo-Australian plate. It is one of the most seismogenic structures on Earth, being responsible for many great and giant earthquakes, including the 2004 Indian Ocean earthquake and tsunami that killed over 227,000 people. The Sunda megathrust can be divided into the Andaman Megathrust, Sumatra(n) Megathrust and Java(n) Megathrust. The Bali-Sumbawa segment is much less active and therefore does not have the "megathrust" term associated with it.
The 1965 Rat Islands earthquake occurred at 05:01 UTC, on 4 February. It had a magnitude of 8.7 and triggered a tsunami of over 10 m on Shemya Island, but caused very little damage.
An earthquake occurred on June 3, 1994, at 01:17:37 local time off the coast of Indonesia. The epicenter was off the eastern part of the southern Java coast, near the east end of the Java Trench.
The 1959 Kamchatka earthquake occurred on May 4 at 19:15 local time with a moment magnitude of 8.0–8.3, and a surface wave magnitude of 8.25. The epicenter was near the Kamchatka Peninsula, Russian SFSR, USSR. Building damage was reported in Petropavlovsk-Kamchatsky. The maximum intensity was VIII (Damaging) on the Medvedev–Sponheuer–Karnik scale. The intensity in Petropavlovsk-Kamchatsky was about VIII MSK.
The November 1960 Peru earthquake occurred offshore northern Peru on November 20 at 17:02 local time. The magnitude of the earthquake was Ms 6.75 by using the conventional surface-wave magnitude measurement within a shorter duration of ~20 s. However, there is a large discrepancy between the magnitudes in Ms and Mw in this earthquake. The discrepancy was caused from the earthquake's long source duration of about 130 s, and by calculating the seismic moment, the magnitude would be Mw 7.6 or Mw 7.8, according to different sources. This earthquake belongs to a category of earthquakes with slow rupture velocities and potential of producing tsunamis larger than those expected from the moment magnitudes.
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.
The Nemuro-Oki earthquake in scientific literature, occurred on June 17 at 12:55 local time. It struck with an epicenter just off the Nemuro Peninsula in northern Hokkaidō, Japan. It measured 7.8–7.9 on the moment magnitude scale (Mw ), 8.1 on the tsunami magnitude scale (Mt ) and 7.4 on the Japan Meteorological Agency magnitude scale (MJMA ).
An tsunamigenic earthquake occurred on 26 August 2012 at 22:37 CST with an epcenter off the coast of Usulután, El Salvador. It measured 7.3 on the moment magnitude scale and had a focal depth of 16.0 kilometres (10 mi). It was felt along the country's coast and in San Salvador although there was no damage. Along the San Juan del Gozo Peninsula, a tsunami triggered by the shock left 40 people injured. Waves were measured with a maximum run-up of 6.3 m (21 ft). The earthquake and tsunami was caused by a rupture on a subduction zone associated with the Middle America Trench.
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 1604 Arica earthquake is an earthquake that occurred at 1:30 pm on November 24, 1604, offshore Arica, Chile. The estimated magnitude range is Ms8.0–8.5 and up to Mw8.7–9.0 and Mt8.8–9.0. It had a destructive tsunami that destroyed moust of Southern Peru, including Arica and Arequipa. 1,200–2,800 km (750–1,740 mi) of coastline was affected by the tsunami. The recorded effects of this earthquake are very similar to those for the 1868 Arica event, suggesting a similar magnitude and rupture area of the megathrust between the subducting Nazca plate and the overriding South American plate. Tsunami deposits have been identified on the Chatham Islands that are likely to have been caused by a trans-Pacific tsunami caused by the 1604 earthquake.
An earthquake occurred off the coast of the Alaska Peninsula on July 28, 2021, at 10:15 p.m. local time. The large megathrust earthquake had a moment magnitude of 8.2 according to the United States Geological Survey (USGS). A tsunami warning was issued by the National Oceanic and Atmospheric Administration (NOAA) but later cancelled. The mainshock was followed by a number of aftershocks, including three that were of magnitude 5.9, 6.1 and 6.9 respectively.
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