The Pacific Ocean Tsunami Threat is Higher Than Expected

Potential of Future Tsunamis

The potential for a huge Pacific Ocean tsunami on the West Coast of North America may be greater than previously thought.

The new study of geological evidence along the Gulf of Alaska coast suggests that future tsunamis could reach a scale far beyond that suffered in the tsunami generated by the great 1964 Alaskan earthquake. Official figures put the number of deaths caused by the earthquake at around 130: 114 in Alaska and 16 in Oregon and California. The tsunami killed 35 people directly and caused extensive damage in Alaska, British Columbia, and the US Pacific region*.

The 1964 Alaskan earthquake - the second biggest recorded in history with a magnitude of 9.2 - triggered a series of massive waves with run up heights of as much as 12.7 metres in the Alaskan Gulf region and 52 metres in the Shoup Bay submarine slide in Valdez Arm.

The study suggests that rupture of an even larger area than the 1964 rupture zone could create an even bigger tsunami. Warning systems are in place on the west coast of North America but the finding

Evaluation of Ancient Earthquakes

The research team from Durham University in the UK, the University of Utah and Plafker Geohazard Consultants, gauged the extent of earthquakes over the last 2,000 years by studying subsoil samples and sediment sequences at sites along the Alaskan coast. The team radiocarbon-dated peat layers and sediments, and analysed the distribution of mud, sand and peat within them. The results suggest that earthquakes in the region may rupture even larger segments of the coast and sea floor than was previously thought.

The study published in the academic journal Quaternary Science Reviews and funded by the National Science Foundation, NASA, and the US Geological Survey shows that the potential impact in terms of tsunami generation, could be significantly greater if both the 800-km-long 1964 segment and the 250-km-long adjacent Yakataga segment to the east were to rupture simultaneously.

Larger Ancient Earthquakes

Lead author, Professor Ian Shennan, from Durham University's Geography Department said: "Our radiocarbon-dated samples suggest that previous earthquakes were fifteen per cent bigger in terms of the area affected than the 1964 event. This historical evidence of widespread, simultaneous plate rupturing within the Alaskan region has significant implications for the tsunami potential of the Gulf of Alaska and the Pacific region as a whole."

"Peat layers provide a clear picture of what's happened to the Earth. Our data indicate that two major earthquakes have struck Alaska in the last 1,500 years and our findings show that a bigger earthquake and a more destructive tsunami than the 1964 event are possible in the future. The region has been hit by large single event earthquakes and tsunamis before, and our evidence indicates that multiple and more extensive ruptures can happen."

Causes of a Tsunami

Tsunamis can be created by the rapid displacement of water when the sea floor lifts and/or falls due to crustal movements that accompany very large earthquakes. The shallow nature of the sea floor off the coast of Alaska could increase the destructive potential of a tsunami wave in the Pacific.

Earthquake behaviour is difficult to predict in this region which is a transition zone between two of the world's most active plate boundary faults; the Fairweather fault, and the Aleutian subduction zone. In 1899 and 1979, large earthquakes occurred in the region but did not trigger a Tsunami because the rupturing was localized beneath the land instead of the sea floor.

Prof Ron Bruhn from the University of Utah said: "If the larger earthquake that is suggested by our work hits the region, the size of the potential tsunami could be signficantly larger than in 1964 because a multi-rupture quake would displace the shallow continental shelf of the Yakutat microplate."

"In the case of a multi-rupture event, the energy imparted to the tsunami will be larger but spread out over a longer strike distance. Except for the small communities at the tsunami source in Alaska, the longer length will have more of an effect on areas farther from the source such as southeastern Alaska, British Columbia, and the US west coast from Washington to California."

Tsunami Warning Systems

Warning systems have been in place on the US western seaboard and Hawaii since the 1946 Aleutian Islands tsunami. Improvements were made following the 2004 earthquake under the Indian Ocean that triggered the most deadly tsunami in recorded history, killing more than 230,000 people.

Prof Shennan said: "Earthquakes can hit at any time of the day or night, and that's a big challenge for emergency planners. A tsunami in this region could cause damage and threaten life from Alaska to California and beyond; in 1964 the effects of the tsunami waves were felt as far away as southern California and were recorded on tide gages throughout the Pacific Ocean."

Dr George Plafker from Plafker Geohazard Consultants said: "A large scale earthquake will not necessarily create a large wave. Tsunami height is a function of bathymetry, and the amount of slip and dip of the faults that take up the displacement, and all these factors can vary greatly along the strike."

"Tsunamis will occur in the future. There are issues in warning and evacuating large numbers of people in coastal communities quickly and safely. The US has excellent warning systems in place but awareness is vital."

Mega Tsunamis

How Mega Tsunamis Are Formed?

An "Ultrasound" of the Nankai Trough

Research by a team of United States and Japanese geoscientists may help explain why part of the seafloor near the southwest coast of Japan is particularly good at generating devastating tsunamis, such as the 1944 Tonankai event, which killed at least 1,200 people. The findings will help scientists assess the risk of mega tsunamis in other regions of the world. Geoscientists from The University of Texas at Austin and colleagues used a commercial ship to collect three-dimensional seismic data that reveals the structure of Earth’s crust below a region of the Pacific seafloor known as the Nankai Trough. The resulting images are akin to ultrasounds of the human body. The results, published in the journal Science, address a long standing mystery as to why earthquakes below some parts of the seafloor form large tsunamis while earthquakes in other regions do not. How Mega Tsunamis Are Formed The 3D seismic images allowed the researchers to reconstruct how layers of rock and sediment have cracked and shifted over time. They found two things that contribute to mega tsunamis. First, they confirmed the existence of a major fault that runs from a region known to unleash earthquakes about 10 kilometers (6 miles) deep right up to the seafloor. When an earthquake happens, the fault allows it to reach up and move the seafloor up or down, carrying a column of water with it and setting up a series of tsunami waves that spread outward. Second, and most surprising, the team discovered that the recent fault activity, probably including the slip that caused the 1944 event, has shifted to landward branches of the fault, becoming shallower and steeper than it was in the past. “That leads to more direct displacement of the seafloor and a larger vertical component of seafloor displacement that is more effective in forming tsunamis,” said Nathan Bangs, senior research scientist at the Institute for Geophysics at The University of Texas at Austin who was co-principal investigator on the research project and co-author on the Science article. The Nankai Trough The Nankai Trough is a subduction zone, an area where tectonic plates converge with one sinking as it passes below the other. Where steady movement is somehow impeded, elastic rock strains build up to the point that rupture occurs with the release of seismic energy. Subduction zones are the sites of the world's largest earthquakes. In 2002, a team of researchers led by Jin-Oh Park at Japan Marine Science and Technology Center (JAMSTEC) had identified the fault, known as a megathrust or megasplay fault, using less detailed two-dimensional geophysical methods. Based on its location, they suggested a possible link to the 1944 event, but they were unable to determine where faulting has been recently active. Fault Slip & the 1944 Tonankai Earthquake “What we can now say is that slip has very recently propagated up to or near to the seafloor, and slip along these thrusts most likely formed the large tsunami during the 1944 Tonankai 8.1 magnitude event,” said Bangs. The images produced in this project will be used by scientists in the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), an international effort designed to, for the first time, “drill, sample and instrument the earthquake-causing, or seismogenic portion of Earth’s crust, where violent, large-scale earthquakes have occurred repeatedly throughout history.” Determining Tsunami Potential “The ultimate goal is to understand what’s happening at different margins,” said Bangs. “The 2004 Indonesian tsunami was a big surprise. It’s still not clear why that earthquake created such a large tsunami. By understanding places like Nankai, we’ll have more information and a better approach to looking at other places to determine whether they have potential. And we’ll be less surprised in the future.” Bangs’ co-principal investigator was Gregory Moore at JAMSTEC in Yokohama and the University of Hawaii, Honolulu. The other co-authors are Emily Pangborn at the Institute for Geophysics at The University of Texas at Austin, Asahiko Taira and Shin'ichi Kuramoto at JAMSTEC and Harold Tobin at the University of Wisconsin, Madison. Funding for the project was provided by the National Science Foundation, Ocean Drilling Program and Japanese Ministry of Education, Culture, Sports and Technology.

World's Biggest Tsunami

The largest recorded tsunami was a wave 1720 feet tall in Lituya Bay, Alaska

On the night of July 9, 1958 an earthquake along the Fairweather Fault in the Alaska Panhandle loosened about 40 million cubic yards (30.6 million cubic meters) of rock high above the northeastern shore of Lituya Bay. This mass of rock plunged from an altitude of approximately 3000 feet (914 meters) down into the waters of Gilbert Inlet (see map below). The impact generated a local tsunami that crashed against the southwest shoreline of Gilbert Inlet. The wave hit with such power that it swept completely over the spur of land that separates Gilbert Inlet from the main body of Lituya Bay. The wave then contiuned down the entire length of Lituya Bay, over La Chaussee Spit and into the Gulf of Alaska. The force of the wave removed all trees and vegetation from elevations as high as 1720 feet (524 meters) above sea level. Millions of trees were uprooted and swept away by the wave. This is the highest wave that has ever been known.

Detail Map: Lituya Bay, Alaska

Lituya Bay is an ice-scoured tidal inlet on the northeast shore of the Gulf of Alaska. It is about seven miles long (11.3 kilometers) and up to two miles wide (3.2 kilometers). It has a maximum depth of about 720 feet (219 meters) but a sill of only 32 feet (9.7 meters) in depth separates it from the Gulf of Alaska between La Chaussee Spit and Harbor Point. The Fairweather Fault trends across the northeast end of the Bay and is responsible for the T-shape of the bay. Glacial scour has exploited the weak zone along the fault to produce a long linear trough known as the Fairweather Trench. The Lituya Glacier and North Crillon Glacier have scoured portions of the Fairweather Trench in the area of Lituya Bay. Gilbert Inlet and Crillon Inlet occupy the Fairweather Trench on the northeast end of Lituya Bay. The rock fall of July 9, 1958 occurred on steep cliffs above the northeast shore of Gilbert Inlet. It is marked on the map above in red. The rocks fell from an elevation of about 3000 feet (914 meters). The impact of 40 million cubic yards (30.6 million cubic meters) of rock hitting the water produced a local tsunami that swept the entire length of the Lituya Bay and over the La Chaussee Spit. This wave stripped all vegetation and soil from along the edges of the bay. This damaged area is shown in yellow on the map above. The numbers are elevations (in feet) of the upper edge of the wave damage area and represent the approximate elevation of the wave as it traveled through the bay. Map redrawn from data included in United States Geological Survey Professional Paper 354-C.