Tsunamis

Characteristics


Tsunami is a set of ocean waves caused by any large, abrupt disturbance of the sea-surface. If the disturbance is close to the coastline, local tsunamis can demolish coastal communities within minutes. A very large disturbance can cause local devastation and export tsunami destruction thousands of miles away.

Tsunami Wave
Tsunamis have very long wavelengths and periods and can have an average speed of 450 miles per hour. They can travel unnoticed in deep ocean waters sometimes with a wave height of only twelve inches. However, when the waves reach shallower water the wave speed slows and the wave height increases significantly. Some tsunamis can reach 100 feet in height and can cause devastation to a coastline.

An indication of an approaching tsunami would be rapid change in water levels on the coastline. The successive crests and troughs can occur from five to ninety minutes apart. Typically, the first wave is not the biggest one; therefore, it is not safe to return to the area until authorities deem it safe to return. 

Damages and Danger


Tsunamis rank high on the scale of natural disasters. Since 1850 alone, tsunamis have been responsible for the loss of over 420,000 lives and billions of dollars of damage to coastal structures and habitats. Most of these casualties were caused by local tsunamis that occur about once per year somewhere in the world.

Predicting when and where the next tsunami will strike is currently impossible. Once the tsunami is generated, forecasting tsunami arrival and impact is possible through modeling and measurement technologies.


Tsunami Development


Tsunamis are most commonly formed from earthquakes in marine and coastal areas. Most are produced by large, shallow earthquakes.

Underwater landslides, often associated with smaller earthquakes, are also capable of generating tsunamis as seen in 1998 in the Papua New Guinea event. Underwater volcanoes and asteroid impacts are also capable of creating tsunamis.

Occurrence


Tsunamis are most common in the Pacific region where dense oceanic plates slide under the lighter continental plates. Areas less than fifty feet above sea level and one mile inland would be at greatest risk for the impact of a tsunami.

According to a research paper published by the National Geophysical Data Center (Tsunamis and Tsunami-Like Waves of the Eastern United States, Lockridge, et al., 2002) since the 1600s there have been only 40 cataloged tsunami and tsunami-like wave events that have occurred in the Eastern United States.

The most notable events were the 1755 Queen Anne's earthquake, the New Madrid earthquakes of 1811-1812, the Charleston earthquake of 1886 and the Grand Banks event of 1929.

While East Coast tsunamis are rare the paper discusses two areas of investigation off of the coasts of North Carolina and Virginia. Fault-like cracks in the outer continental shelf are situated in areas with large deposits of methane hydrate and pressurized water that could create unstable shelf wall leading to an underwater landslide. 


Warning System


There are two types of bulletins to inform an area of the possibility of a tsunami. A Tsunami Watch Bulletin is released following an earthquake of a 6.75 or greater and a Tsunami Warning Bulletin is released when information from a tidal station indicates that the characteristics of the sea match those of a destructive tsunami. 

Fortunately, 75% of all warnings since 1948 have been false alarms.

Tsunami Safety

  1. Before a Tsunami
  2. During a Tsunami
  3. After a Tsunami

Before the Tsunami


The following are things you can do to protect yourself, your family and your property from the effects of a tsunami:

  • To begin preparing, you should build an emergency kit and make a family communications plan.
    • Talk to everyone in your household about what to do if a tsunami occurs. Create and practice an evacuation plan for your family. Familiarity may save your life. Be able to follow your escape route at night and during inclement weather. Practicing your plan makes the appropriate response more of a reaction, requiring less thinking during an actual emergency.
    • If the school evacuation plan requires you to pick your children up from school or from another location. Be aware telephone lines during a tsunami alert may be overloaded and routes to and from schools may be jammed.
    • Knowing your community's warning systems and disaster plans, including evacuation routes.
  • If you are a tourist, familiarize yourself with local tsunami evacuation protocols. If you are concerned that you will not be able to reach a safe place in time, ask your local emergency management office about vertical evacuation. Some strong (e.g., reinforced concrete) and tall buildings may be able to provide protection if no other options are available.
  • If an earthquake occurs and you are in a coastal area, turn on your radio to learn if there is a tsunami warning.

Know the Terms


  • Tsunami Watch: A tsunami watch is issued to alert emergency management officials and the public of an event which may later impact the watch area. The watch area may be upgraded to a warning or advisory - or canceled - based on updated information and analysis. Therefore, emergency management officials and the public should prepare to take action. Watches are normally issued based on seismic information without confirmation that a destructive tsunami is underway.
  • Tsunami Advisory: 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 or expected. 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 re-positioning of ships to deep waters when there is time to safely do so. Advisories are normally updated to continue the advisory, expand/contract affected areas, upgrade to a warning, or cancel the advisory. 
  • Tsunami Warning: A tsunami warning is issued when a tsunami with the potential to generate widespread inundation is imminent or expected. 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 re-positioning of ships to deep waters when there is time to safely do so. Warnings may be updated, adjusted geographically, downgraded, or canceled. To provide the earliest possible alert, initial warnings are normally based only on seismic information.

Additional Resources



Want to learn more about tsunamis and how to prepare for them? Visit these site for additional information: