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Tsunamis

 

Tsunamis

Following the recent destructive tsunamis (2004 Sumatra tsunami and 2011 Japan tsunami), caused widespread damage and fatalities the public awareness of tsunamis has been intensified. The term tsunami comes from the Japanese, meaning "harbor" (tsu, ) and "wave" (nami, ). Tsunamis are sometimes referred to as tidal waves. A tsunami or tidal wave is a series of water waves (called a tsunami wave train) caused by the displacement of a large volume of a body of water. The wavelength of tsunamis are so great (often 100 Km or more) that even in the deepest oceans they are described essentially by shallow water waves. Thus, the leading wave of a tsunami travels with speed (gh)0.5 governed only by the depth of the water h and independent of its wavelength. Here g is, of course, the acceleration of gravity.

As the tsunami wave approaches the shallow waters of shore, its time period remains the same, but its wavelength decreases rapidly, thus causing the water to pile up to form tremendous crests, in an effect known as shoaling. It is in these shallow waters that a large tsunami can crest to heights exceeding 30 meter and strike devastating force, while in deep water its height is in the order one meter (Figure 1).


Figure 1: Tsunami evolution from source to shore.

 

Generally underwater earthquakes, volcanic eruptions and other underwater explosions (including detonations of underwater nuclear devices), submarine landslides and other mass movements, meteorite ocean impacts or similar impact events, and other disturbances above or below water all have the potential to generate a tsunami.

While in a few cases tsunamis probably have been generated by landslides or any submarine slumps, it has been well established that most tsunamis around the world are generated directly by the coseismic tectonics deformation of the ocean floor due to the accompanying earthquake. Nevertheless often just earthquakes happen at earth subduction zones can lead to tsunami formation; For example the huge earthquake occurred around the Sumatra's coasts in 2011 couldn’t make any tsunami, because it wasn't located at subduction zone. While the 2004 Indian Ocean earthquake occurred at subduction zone (between Sunda and India plate) and generated huge tsunami caused widespread damage and more than 225,000 fatalities within a few hours (Fig 2).


Figure 2: 2004 Indian Ocean tsunami impact damage in Banda Aceh, Sumatra, Indonesia.

Two main Indian Ocean subduction zones able to generate seismic tsunamis are the Indonesian subduction zone in the east and the Makran subduction zone in the north. Makran subduction zone has been formed in vicinity of southern coasts of Iran and Pakistan, as a result of the conflict of Eurasia and Arabian plate in northwest of Indian Ocean and extends about 800 km across the northern Arabian Sea from near the Strait of Hormoz  to in vicinity of Karachi in Pakistan (Figure 3).

Investigation on collected information achieved by historical events and regional seismicity shows that earthquakes tsunamis in the Makran zone have repeatedly happened in the past and are possible here in the future. The most important case of happened tsunamis is the 1945 tsunami; which possibly can be considered as the second deadliest tsunami in the Indian Ocean region occurred on 28 November, 1945 off the southern coast of Pakistan and lead to killing more than 4000 people (Heck, 1947).

 


Figure 3: Tectonic map of the MSZ at the northwestern Indian Ocean (Heidarzadeh et al, 2002).

 

Thus, can conclude the hazard of tsunami for coastal areas of neighboring countries of Makran subduction zone, like Pakistan, Iran, India and Oman is relatively high and it seems necessary to predict the effect of next possible tsunamis in order to mitigate the casualties and save lives and property. So, for assessment of Makran subduction zone tsunamis hazard on coasts` of Iran numerical model has been applied at Iranian National Center for Ocean Hazard (INCOH) to simulate different scenarios of earthquake tsunamis at Oman Sea and estimate the effect of these tsunamis.

Figure 4: Boulder deposits in Chabahar coast plucked by tsunami waves

Investigation of historical records and seismic activity of the region showed seismic tsunamis don’t occur at Caspian Sea, because no subduction zone exists there. In other word, earthquakes happen at Caspian Sea (more frequently at central part of the sea) can’t lead to displacement of seabed and generation of tsunami’s initial wave. Hence, it can be concluded the hazard of seismic tsunamis at Caspian Sea is negligible.

Based on new data of Caspian seabed`s profiles achieved in vicinity of Iran's coasts, a submarine landslide was detected around estuary of Sefidrood River. The submarine landslide was considered as the possible tsunami source at Caspian Sea. The tsunami caused duo to this source has been simulated using GEOWAVE model and its effect on coasts of Iran has been evaluated.

 

 

Tsunamis

Following the recent destructive tsunamis (2004 Sumatra tsunami and 2011 Japan tsunami), caused widespread damage and fatalities the public awareness of tsunamis has been intensified. The term tsunami comes from the Japanese, meaning "harbor" (tsu, ) and "wave" (nami, ). Tsunamis are sometimes referred to as tidal waves. A tsunami or tidal wave is a series of water waves (called a tsunami wave train) caused by the displacement of a large volume of a body of water. The wavelength of tsunamis are so great (often 100 Km or more) that even in the deepest oceans they are described essentially by shallow water waves. Thus, the leading wave of a tsunami travels with speed (gh)0.5 governed only by the depth of the water h and independent of its wavelength. Here g is, of course, the acceleration of gravity.

As the tsunami wave approaches the shallow waters of shore, its time period remains the same, but its wavelength decreases rapidly, thus causing the water to pile up to form tremendous crests, in an effect known as shoaling. It is in these shallow waters that a large tsunami can crest to heights exceeding 30 meter and strike devastating force, while in deep water its height is in the order one meter (Figure 1).


Figure 1: Tsunami evolution from source to shore.

 

Generally underwater earthquakes, volcanic eruptions and other underwater explosions (including detonations of underwater nuclear devices), submarine landslides and other mass movements, meteorite ocean impacts or similar impact events, and other disturbances above or below water all have the potential to generate a tsunami.

While in a few cases tsunamis probably have been generated by landslides or any submarine slumps, it has been well established that most tsunamis around the world are generated directly by the coseismic tectonics deformation of the ocean floor due to the accompanying earthquake. Nevertheless often just earthquakes happen at earth subduction zones can lead to tsunami formation; For example the huge earthquake occurred around the Sumatra's coasts in 2011 couldn’t make any tsunami, because it wasn't located at subduction zone. While the 2004 Indian Ocean earthquake occurred at subduction zone (between Sunda and India plate) and generated huge tsunami caused widespread damage and more than 225,000 fatalities within a few hours (Fig 2).


Figure 2: 2004 Indian Ocean tsunami impact damage in Banda Aceh, Sumatra, Indonesia.

Two main Indian Ocean subduction zones able to generate seismic tsunamis are the Indonesian subduction zone in the east and the Makran subduction zone in the north. Makran subduction zone has been formed in vicinity of southern coasts of Iran and Pakistan, as a result of the conflict of Eurasia and Arabian plate in northwest of Indian Ocean and extends about 800 km across the northern Arabian Sea from near the Strait of Hormoz  to in vicinity of Karachi in Pakistan (Figure 3).

Investigation on collected information achieved by historical events and regional seismicity shows that earthquakes tsunamis in the Makran zone have repeatedly happened in the past and are possible here in the future. The most important case of happened tsunamis is the 1945 tsunami; which possibly can be considered as the second deadliest tsunami in the Indian Ocean region occurred on 28 November, 1945 off the southern coast of Pakistan and lead to killing more than 4000 people (Heck, 1947).

 


Figure 3: Tectonic map of the MSZ at the northwestern Indian Ocean (Heidarzadeh et al, 2002).

 

Thus, can conclude the hazard of tsunami for coastal areas of neighboring countries of Makran subduction zone, like Pakistan, Iran, India and Oman is relatively high and it seems necessary to predict the effect of next possible tsunamis in order to mitigate the casualties and save lives and property. So, for assessment of Makran subduction zone tsunamis hazard on coasts` of Iran numerical model has been applied at Iranian National Center for Ocean Hazard (INCOH) to simulate different scenarios of earthquake tsunamis at Oman Sea and estimate the effect of these tsunamis.

Figure 4: Boulder deposits in Chabahar coast plucked by tsunami waves

Investigation of historical records and seismic activity of the region showed seismic tsunamis don’t occur at Caspian Sea, because no subduction zone exists there. In other word, earthquakes happen at Caspian Sea (more frequently at central part of the sea) can’t lead to displacement of seabed and generation of tsunami’s initial wave. Hence, it can be concluded the hazard of seismic tsunamis at Caspian Sea is negligible.

Based on new data of Caspian seabed`s profiles achieved in vicinity of Iran's coasts, a submarine landslide was detected around estuary of Sefidrood River. The submarine landslide was considered as the possible tsunami source at Caspian Sea. The tsunami caused duo to this source has been simulated using GEOWAVE model and its effect on coasts of Iran has been evaluated.

 

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