How Tsunamis are Generated


Most of tsunamis are generated by earthquakes that cause displacement of the seafloor, but tsunamis also can be generated by volcanic eruptions, landslides, underwater explosions, and meteorite impacts.

Earthquakes

Earthquakes cause tsunamis by causing a disturbance of the seafloor. Thus, earthquakes that occur along coastlines or anywhere beneath the oceans can generate tsunamis.  The size of the tsunami is usually related to the size of the earthquake, with larger tsunamis generated by larger earthquakes.  But the sense of displacement is also important.  Tsunamis are generally only formed when an earthquake causes vertical displacement of the seafloor.  The 1906 earthquake near San Francisco California had a Richter Magnitude of about 7.1, yet no tsunami was generated because the motion on the fault was strike-slip motion with no vertical displacement.  Thus, tsunami only occur if the fault generating the earthquake has normal or reverse displacement.  Because of this, most tsunamis are generated by earthquakes that occur along the subduction boundaries of plates, along the oceanic trenches.  Since the Pacific Ocean is surrounded by plate boundaries of this type, tsunamis are frequently generated by earthquakes around the margins of the Pacific Ocean.

Although the December 2004 Indonesian Ocean tsunami is by far the best well known and most deadly, there are other disastrous tsunamis generated by earthquakes.

  • April 1, 1946 – A magnitude 7.3  earthquake occurred near Unimak Island in the Aleutian Islands west of Alaska, near the Alaska Trench. Sediment accumulating in the trench slumped into the trench and generated a tsunami.  A lighthouse at Scotch Gap built of steel reinforced concrete was located on shore at an elevation of 14 m above mean low water. The first wave of the tsunami hit the Scotch Gap area 20 minutes after the earthquake, had a run-up 30 m and completely destroyed the lighthouse.  4.5 hours later the same tsunami reached the Hawaiian Islands after traveling at an average speed of 659 km/hr.  As it approached the city of Hilo on the Big Island, it slowed to about 47 km/hr and had a run up of 18 m above normal high tide. It killed 159 people (90 in Hilo) and caused $25 million in property damage.
  • May 22, 1960 – A moment magnitude 9.5 earthquake occurred along the subduction zone of South America.  Because the population of Chile is familiar with earthquakes and potential tsunami, most people along the coast moved to higher ground.  15 minutes after the earthquake, a tsunami with a run up of 4.5 m hit the coast.  The first wave then retreated, dragging broken houses and boats back into the ocean.  Many people saw this smooth retreat of the sea as a sign they could ride their boats out to sea and recover some of the property swept away by the first wave.  But, about 1 hour later, the second wave traveling at a velocity of 166 km/hr crashed in with a run up of 8 m.  This wave crushed boats along the coast and destroyed coastal buildings. This was followed by a third wave traveling at only 83 km/hr that crashed in later with a run up of 11 m, destroying all that was left of coastal villages.  The resulting causalities listed 909 dead with 834 missing.  In Hawaii, a tsunami warning system was in place and the tsunami was expected to arrive at 9:57 AM.  It hit at 9:58 AM and 61 people died, mostly sightseers that wanted to watch the wave roll in at close range. The tsunami continued across the Pacific Ocean, eventually reaching Japan where it killed an additional 185 people.
  • March 27, 1964 – The Good Friday Earthquake in Alaska had a moment  magnitude of 9.2.  This earthquake also occurred along the subduction zone, and caused deformation of the crust where huge blocks were dropped down as much as 2.3 m. Because the coastline of Alaska is sparsely populated, only 122 people died from the tsunami in Alaska. With a tsunami warning system in place in Crescent City, California, all the town’s people moved to higher ground.  After watching four successive waves destroy their town, many people returned to the low lying areas to assess the damage to their property.  The fifth wave had the largest run up of 6.3 m and killed 12 people.
  • September 2, 1992 – A magnitude 7 earthquake off the coast of Nicaragua in Central America occurred along the subduction zone below the Middle America Trench. The earthquake was barely felt by the residents of Nicaragua and was somewhat unusual.  A 100 km long segment of the oceanic lithosphere moved 1 m further below the over riding plate over a period of two minutes.  Much energy was released but the ground did not shake very much.  Sea water apparently absorbed some of the energy and sent a tsunami onto the coast.  Residents had little warning, 150 people died and 13,000 people were left homeless.

Volcanic Eruptions

Volcanoes that occur along coastal zones, like in Indonesia and island arcs throughout the world, can cause several effects that might generate a tsunami. Explosive eruptions can rapidly emplace pyroclastic flows into the water, landslides and debris avalanches produced by eruptions can rapidly move into water, and  collapse of volcanoes to form calderas can suddenly displace the water.

The eruption of Krakatau in the Straights of Sunda, between Java and Sumatra, in 1883 generated at least three tsunamis that killed 36,417 people.  It is still uncertain exactly what caused the tsunami, but it is known that several events that occurred during the eruption could have caused such tsunami.

Landslides

Landslides moving into oceans, bays, or lakes can also generate tsunamis. Most such landslides are generated by earthquakes or volcanic eruptions.  As previously mentioned, a large landslide or debris avalanche fell into Lituya Bay, Alaska in July 9, 1958 causing a wave with a run up of about 60 m as measured by a zone completely stripped of vegetation.

Underwater Explosions

Nuclear testing by the United States in the Marshall Islands in July 1946 at Bikini Atoll and 1958 had generated tsunamis.

Asteroid Impacts (also known as ‘bolide’ tsunami)

While no historic examples of meteorite impacts are known to have produced a tsunami, the apparent impact of a meteorite at the end of the Cretaceous Period, about 65 million years ago near the tip of what is now the Yucatan Peninsula of Mexico, produced tsunami that left deposits all along the Gulf coast of Mexico and the United States.

 

 source: The Influence of Building’s Height and Lay-out to Tsunami’s Run Up

 

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