Tsunami earthquake
Encyclopedia
A tsunami earthquake is an earthquake
that triggers a tsunami
of a magnitude that is very much larger than the magnitude of the earthquake as measured by shorter-period seismic waves. The term was introduced by 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 neighbouring coast with little or no warning.
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. 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.
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
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.
=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.
was the first tsunami earthquake to be recorded with a broad-band seismic network.
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
that triggers a tsunami
Tsunami
A tsunami is a series of water waves caused by the displacement of a large volume of a body of water, typically an ocean or a large lake...
of a magnitude that is very much larger than the magnitude of the earthquake as measured by shorter-period seismic waves. The term was introduced by Hiroo Kanamori
Hiroo Kanamori
is a Japanese American seismologist who has made fundamental contributions to understanding the physics of earthquakes and the tectonic processes that cause them....
in 1972. Such events are a result of relatively slow rupture velocities. They are particularly dangerous as a large tsunami may arrive at a neighbouring coast 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 magnitudeSurface wave magnitude
The surface wave magnitude scale is one of the magnitude scales used in seismology to describe the size of an earthquake. It is based on measurements in Rayleigh surface waves that travel primarily along the uppermost layers of the earth...
Ms differs markedly from the moment magnitude
Moment magnitude scale
The moment magnitude scale is used by seismologists to measure the size of earthquakes in terms of the energy released. The magnitude is based on the seismic moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of...
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. 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
Accretionary wedge
An accretionary wedge or accretionary prism is formed from sediments that are accreted onto the non-subducting tectonic plate at a convergent plate boundary...
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 shows that the release of seismic momentSeismic moment
Seismic moment is a quantity used by earthquake seismologists to measure the size of an earthquake. The scalar seismic moment M_0 is defined by the equationM_0=\mu AD, where*\mu is the shear modulus of the rocks involved in the earthquake...
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 rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
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.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 Mt=8.2 while the earthquake shaking only indicated a magnitude of MsSurface wave magnitude
The surface wave magnitude scale is one of the magnitude scales used in seismology to describe the size of an earthquake. It is based on measurements in Rayleigh surface waves that travel primarily along the uppermost layers of the earth...
=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.
1992 Nicaragua
The 1992 Nicaragua earthquake1992 Nicaragua earthquake
The 1992 Nicaragua earthquake occurred off the coast of Nicaragua on September 2, 1992. Some damage was also reported in Costa Rica. At least 116 people were killed and several more were injured. The quake was located in an active zone of stress and deformation...
was the first tsunami earthquake to be recorded with a broad-band seismic network.
Other tsunami earthquakes
- 1605 Keichō Nankaidō earthquake1605 Keichō Nankaidō earthquakeThe 1605 Keichō Nankaidō earthquake occurred at about 20:00 local time on 3 February. It had an estimated magnitude of 7.9 on the surface wave magnitude scale and triggered a devastating tsunami that resulted in thousands of deaths in the Nankai and Tōkai regions of Japan. It is uncertain whether...
- 1946 Aleutian Islands earthquake
- 1960 Peru earthquake
- 1963 Kuril Islands earthquake1963 Kuril Islands earthquakeThe 1963 Kuril Islands earthquake occurred at 05:17 UTC, on October 13. The earthquake had a magnitude of 8.5 and was followed by a Mw=7.8 event seven days later...
- 1975 Kuril Islands earthquake
- 1994 Java earthquake1994 Java earthquakeThe 1994 Java earthquake, or the 1994 Java tsunami earthquake, occurred on June 3, 1994 at 01:18 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. This earthquake occurred in a region which is normally...
- 1996 Chimbote earthquake1996 Chimbote earthquakeThe 1996 Chimbote earthquake occurred on February 21, 1996 at 07:51 local time about 130 km off the coast of northern Peru, near the Peru-Chile Trench. It was an earthquake of magnitude Mw 7.4.- Geology :...
- July 2006 Java earthquakeJuly 2006 Java earthquakeThe July 2006 Java earthquake was a magnitude 7.7 earthquake off the southwestern coast of Java, Indonesia. It occurred on July 17, 2006, at 08:24 UTC ....