The first wave may not be the largest in the series of waves. For example, in several different recent tsunami the first, third, and fifth waves were the largest. There is an average of two destructive tsunami per year in the Pacific basin. Pacific wide tsunami are a rare phenomenon, occurring every 10 - 12 years on the average. Most of these tsunami are generated by earthquakes that cause displacement of the seafloor, but, as we shall see, tsunami can be generated by volcanic eruptions, landslides, underwater explosions, and meteorite impacts.
Earthquakes cause tsunami by causing a disturbance of the seafloor. Thus, earthquakes that occur along coastlines or anywhere beneath the oceans can generate tsunami. The size of the tsunami is usually related to the size of the earthquake, with larger tsunami generated by larger earthquakes. But the sense of displacement is also important.
Tsunami are generally only formed when an earthquake causes vertical displacement of the seafloor. Thus, tsunami only occur if the fault generating the earthquake has normal or reverse displacement.
Because of this, most tsunami 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, tsunami are frequently generated by earthquakes around the margins of the Pacific Ocean.
Although the December Indian Ocean tsunami is by far the best well known and most deadly and will be featured in a video in class , we here discuss other disastrous tsunami generated by earthquakes.
May 22, - A moment magnitude9. Because the population of Chile is familiar with earthquakes and potential tsunami, most people along the coast moved to higher ground. 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. This wave crushed boats along the coast and destroyed coastal buildings. The resulting causalities listed dead with missing.
In Hawaii, a tsunami warning system was in place and the tsunami was expected to arrive at AM. It hit at AM and 61 people died, mostly sightseers that wanted to watch the wave roll in at close range obviously they were too close. The tsunami continued across the Pacific Ocean, eventually reaching Japan where it killed an additional people. This earthquake also occurred along the subduction zone, and as we saw in our study of earthquakes, caused deformation of the crust where huge blocks where dropped down as much as 2.
Because the coastline of Alaska is sparsely populated, only people died from the tsunami in Alaska. With a tsunami warning system in place in Crescent City, California, all the townspeople 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. September 2, - 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 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. Seawater apparently absorbed some of the energy and sent a tsunami onto the coast. Residents had little warning, people died and 13, people were left homeless.
Volcanoes that occur along coastal zones, like in Japan 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 generated at least three tsunami that killed 36, 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.
A loud explosive blast was heard as far away as Australia. This blast was likely caused by a phreatic explosion that occurred as a result of seawater coming in contact with the magma. The explosion could have generated at least one of the tsunami. At some point during the eruption a caldera formed by collapse of the volcanic island. Areas that were once more than m above sea level were found m below sea level after the eruption.
The sudden collapse of the volcano to form this caldera could have caused one or more tsunami. Earthquakes were felt throughout the eruption. Geological Survey Scientific Investigation Report —, 32 p.
Year Published: The Great Alaska Earthquake and tsunamis: a modern perspective and enduring legacies The magnitude 9. Brocher, Thomas M. Year Published: Tsunami hazards— A national threat In December , when a tsunami killed more than , people in 11 countries around the Indian Ocean, the United States was reminded of its own tsunami risks.
Filter Total Items: 7. Date published: May 31, Date published: February 1, Date published: September 18, Attribution: Science Application for Risk Reduction. Date published: May 20, Date published: September 5, Date published: October 29, Attribution: Region 9: Columbia-Pacific Northwest. Date published: May 4, Filter Total Items: List Grid. February 1, January 26, Attribution: Natural Hazards. December 31, Large storm waves crashing on the rocks near Santa Cruz, California.
November 28, Southeast Asia in May The warning centers analyze this information to determine if the earthquake could have generated a tsunami and if a tsunami message is necessary. Water-Level Networks —If an earthquake meets certain criteria, the warning centers turn to water-level information, looking for changes in water-level height that could indicate the existence and size of a tsunami.
The primary sources of information about water-level change are a network of Deep-ocean Assessment and Reporting of Tsunami DART systems and an extensive array of coastal water-level stations.
Each system consists of a bottom pressure recorder BPR anchored on the ocean floor and a separately moored companion surface buoy. When a tsunami passes over a BPR, the instrument detects and records the changes in the overlying water pressure.
An acoustic link transmits information from the BPR to the surface buoy, which then relays it via satellite to the warning centers where the information is incorporated into tsunami forecast models. See how a DART system works video. Coastal water-level stations collect important information about the height of the ocean at specific coastal locations. Their primary purpose is to monitor tides for navigation purposes, thus they are located on the coast in contrast to the DART systems, which are in deep water , generally on piers in harbors.
Information from these stations is relayed via satellite to the warning centers where it is used to confirm tsunami arrival time and height and is incorporated into tsunami forecast models. Coastal water-level stations are owned and operated by a number of national and international organizations.
In the United States, most of the tsunami-capable coastal water-level stations i. In most cases, the first sign of a potential tsunami is an earthquake. Seismic waves travel about times faster than tsunamis, so information about an earthquake is available before information about any tsunami it may have generated. Three key pieces of information about an earthquake help the Tsunami Warning Centers determine if it was capable of generating a tsunami: location, depth, and magnitude.
The warning centers use this preliminary seismic information to decide if they should issue a tsunami message and at what alert level s. Once a message is issued, the warning centers conduct additional seismic analysis and run tsunami forecast models using information from the seismic and water-level networks as it becomes available.
These numerical models use the real-time information and pre-established scenarios to simulate tsunami movement across the ocean and estimate coastal impacts, including wave height and arrival times, the location and extent of coastal flooding, and event duration. The resulting forecasts, combined with historic tsunami information and additional seismic analysis, help the warning centers decide if they should issue an updated or cancellation message.
It is more difficult to forecast nonseismic tsunamis like landslide and volcanic tsunamis and meteotsunamis , which can arrive with little to no warning. Even if a nonseismic tsunami is detected by a DART system or coastal water-level station, there may not be time to develop a detailed forecast. In the case of meteotsunamis, NWS Weather Forecast Offices, with decision support from the warning centers, can notify the public of the potential coastal threat given the presence of or potential for certain weather conditions along with observed water-level measurements.
Tsunami messages are issued by the Tsunami Warning Centers to notify emergency managers and other local officials, the public, and other partners about the potential for a tsunami following a possible tsunami-generating event. There are four levels of tsunami alerts: warning, advisory, watch, and information statement. Initial tsunami messages include alert level s , preliminary information about the earthquake, and an evaluation of the threat.
If a tsunami is already suspected, the message may also include wave arrival times, recommended life safety actions, and potential impacts. Subsequent messages, both updates and cancellations, are based on additional seismic analysis and results from the tsunami forecast models and may feature more refined, detailed, and targeted information.
A tsunami warning is issued when a tsunami with the potential to generate widespread inundation is imminent, expected, or occurring. Warnings alert the public that dangerous coastal flooding accompanied by powerful currents is possible and may continue for several hours after initial arrival. Warnings alert emergency management officials to take action for the entire tsunami hazard zone.
Appropriate actions to be taken by local officials may include the evacuation of low-lying coastal areas and the repositioning of ships to deep waters when there is time to safely do so. Warnings may be updated, adjusted geographically, downgraded, or canceled based on updated information and analysis. A tsunami advisory is issued when a tsunami with the potential to generate strong currents or waves dangerous to those in or very near the water is imminent, expected, or occurring.
The threat may continue for several hours after initial arrival, but significant inundation is not expected for areas under an advisory. Appropriate actions to be taken by local officials may include closing beaches, evacuating harbors and marinas, and the repositioning of ships to deep waters when there is time to safely do so.
Advisories may be updated, adjusted geographically, upgraded to a warning, or canceled based on updated information and analysis. A tsunami watch is issued when a tsunami may later impact the watch area. The watch may be upgraded to a warning or advisory or canceled based on updated information and analysis.
Emergency management officials and the public should prepare to take action. A tsunami information statement is issued when an earthquake or tsunami has occurred of interest to the message recipients. In most cases, information statements are issued to indicate there is no threat of a destructive basin-wide tsunami and to prevent unnecessary evacuations. Information statements for distant events requiring evaluation may be upgraded to a warning, advisory, or watch based on updated information and analysis.
A tsunami threat message is a tsunami message for international partners in the Pacific and Caribbean. The United States does not issue alerts for these partners. The primary purpose of these messages is to help national authorities understand the threat to their coasts so they can determine which alerts to issue for their coastlines, if any.
A threat message describes tsunami threats according to the potential hazard and impact to people, structures, and ecosystems on land or in nearshore marine environments. National authorities will determine the appropriate level of alert for each country and may issue additional or more refined information and instructions. A threat message may be updated based on new information, data, and analysis.
An information statement may be issued following an earthquake or tsunami of interest to the message recipients if there is little to no threat, but may be upgraded to a tsunami threat message if warranted. The Tsunami Warning Centers prepare and issue tsunami messages for their respective designated service areas.
Coast Guard, the U. Each of these recipients is responsible for forwarding the message to its own constituents. The Tsunami Warning Centers issue a cancellation after they determine that a destructive tsunami will not affect an area under a warning, advisory, or watch or that a tsunami has diminished to a level where additional damage is not expected. However, the cancellation of a message does not mean the area is safe.
The final decision that an area is safe is up to local and state emergency management officials. Pacific and Caribbean territories, and the British Virgin Islands and is the primary international forecast center for the warning systems of the Intergovernmental Oceanographic Commission of the United Nations Educational, Scientific, and Cultural Organization in the Pacific and the Caribbean and Adjacent Regions.
The Tsunami Warning Centers base their initial tsunami messages on the preliminary earthquake information location, depth, and magnitude received from seismic networks since that is all the information available within the first few minutes after an earthquake. They use preset criteria Atlantic , Pacific to decide when to issue a tsunami message and what alert s to include. Subsequent messages and alerts are based on impact estimation resulting from additional seismic analysis, water-level measurements, tsunami forecast model results, and historical tsunami information.
Tsunami warnings are typically issued following coastal earthquakes magnitude 6. Tsunami height also affects alert selection. In general, the warning centers issue a tsunami warning if the forecast or observed tsunami height exceeds 1. In regions of high seismic network density, the warning centers can issue messages within five minutes.
In areas of lower seismic network density, response time increases to minutes. They may also come through outdoor sirens, local officials, emails and text message alerts from state and local opt-in systems, and telephone notifications. There may not always be enough time for an official warning, so it is important to understand natural warnings. Learn how to get alerts from the Tsunami Warning Centers.
A tsunami is one the most powerful and destructive natural forces. It can produce unusually strong currents, rapidly flood land, and devastate coastal communities. Low-lying areas such as beaches, bays, lagoons, harbors, river mouths, and areas along rivers and streams leading to the ocean are the most vulnerable.
Most tsunami damage and destruction is caused by flooding, wave impacts, strong currents, erosion, and debris. The water can be just as dangerous as it returns to the sea, taking debris and people with it.
In addition to loss of life and mass injuries, other potential impacts include damage to and destruction of homes and businesses, cultural and natural resources, infrastructure, and critical facilities. Flooding and dangerous currents can last for days. Even small tsunamis can pose a threat.
Much like a stone thrown into the water creates ripples, tsunami waves spread out in all directions from a point of disturbance such as an earthquake. The waves of a tsunami are relatively small and far apart when they are in the deep ocean, and they can travel as fast as a jet airplane. As they approach a shoreline, the coastal shallowing effect slows the waves down, but they increase in height and come closer together.
This series of waves inundates the shoreline, but not always like a huge wave breaking over it — often more like an extremely rapid encroachment of a very turbulent high tide. The drawback of tsunami waves from the flooded coast is also extremely destructive, as it drags everything in its path into the sea. Thus, there are widespread local, regional and international efforts to develop and improve warning systems as well as to educate the public about how to be alert, informed and safe.
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