What Is A Tsunami?
Tsunami (soo-NAH-mee): a Japanese word that means harbor wave; a sea
wave of local or distant origin that results from large-scale seafloor
displacements associated with large earthquakes, major submarine slides,
or exploding volcanic islands. Typically generated by seismic or volcanic
activity or by underwater landslides, a tsunami consists of a series of
high-energy waves that radiate outward like pond ripples from the area in
which the generating event occurred.
Not all earthquakes produce tsunamis. To generate a tsunami, an earthquake
must occur underneath or near the ocean, be very large (approximately Richter
magnitude 7 or greater), and create vertical movement of the sea floor. However,
recent studies regarding the potential for a great Cascadia Subduction zone
earthquake off the Washington, Oregon, and Northern California coastlines
indicate the local tsunami waves may reach nearby coastal communities within
minutes of the earthquake thereby giving little or no time to issue
warnings.
Tsunamis in Hawaiʻi
Hawaiʻi is at risk from tsunamis caused by both distant ("teleseismic") and local sources. PTWC issues different messages types for tsunamis generated by these two types of sources. In either case, a large, shallow earthquake originating beneath the ocean floor has the potential to generate a tsunami.
- Tsunamis from Distant Sources: Most tsunamis that affect Hawaiʻi originate from seismically active areas around the Pacific. In particular, areas where tectonic plates are in collision (subduction zones), such as Alaska's Aleutian Island chain and the west coast of South America, generate most of the world's tsunamigenic earthquakes. When this occurs, residents of Hawaiʻi have ample time to prepare for an incoming tsunami (4 hours if it's from Alaska, 10 hours from Chile). In this case people should stay tuned to local radio and TV to determine when and if they should evacuate to higher ground. Distant tsunamis with runups in Hawaiʻi exceeding 3 meters have occurred in 1868 (Chile), 1877 (Chile), 1896 (Japan), 1906 (Chile), 1923 Kamchatka, 1933 (Japan), 1946 (Aleutians), 1952 (Kamchatka), 1957 (Aleutians), 1960 (Chile), 1964 (Alaska).
- Tsunamis from Local Sources: Because Hawaiʻi is seismically active, a shallow undersea earthquake can reach sufficient size to generate a local tsunami. While destructive local tsunamis are less frequent, there is little time to react to such an event. Waves from the tsunami caused by the 1975 Kalapana earthquake killed two campers in the Halape area about a minute after they experienced the strong shaking. Therefore, if you feel strong shaking and are near the water, you should immediately move to higher ground. For example, a tsunami generated from the southeast coast of the Big Island will only take 5-10 minutes to reach Hilo or Kona, so you should act fast. Residents of Maui have about 15-20 minutes, and Oʻahu has about 30-40 minutes warning. Local tsunamis with runups exceeding 3 meters have occurred in 1868 (Ka`u) and 1975 (Kalapana).
More Resources
- Hawaiʻi State Civil Defense
- Hawaiʻi County Civil Defense
- Maui County Civil Defense
- Oʻahu Civil Defense
- Kauaʻi County Civil Defense
- Pacific Disaster Center
- International Tsunami Information Center
- Pacific Tsunami Museum
- Natural Hazards in Hawaiʻi
- USGS Tsunami History on the Island of Hawaiʻi
How will I know if a Tsunami is coming?
The West Coast/Alaska Tsunami Warning Center (WC/ATWC) is responsible for
tsunami warnings for California, Oregon, Washington, British Columbia and
Alaska.
The Pacific Tsunami Warning Center (PTWC) provides warnings to
international authorities, Hawaii, and U.S. Territories within the Pacific
Basin.
The WC/ATWC and PTWC may issue the following bulletins:
- Information: A message with information about an earthquake
that is not expected to generate a tsunami.
- Advisory: An earthquake has occurred in the Pacific Basin, which
might generate a tsunami.
- Watch: A tsunami was or may have been generated, but is at
least two hours travel time to the area in Watch status.
- Warning: A tsunami was, or may have been generated, which could cause
damage; therefore, people in the warned area are strongly advised to evacuate.
Hawaii Message Definitions:
- Tsunami Warning
A tsunami warning is issued when a potential tsunami with significant widespread inundation is imminent or expected. Warnings alert the public that widespread, dangerous coastal flooding accompanied by powerful currents is possible and may continue for several hours after arrival of the initial wave. Warnings also 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. To provide the earliest possible alert, initial warnings are normally based only on seismic information.
- 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 in the WC/ATWC AOR) - 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
Advisories are issued to coastal populations within areas not currently in either warning or watch status when a tsunami warning has been issued for another region of the same ocean. An Advisory indicates that an area is either outside the current warning and watch regions or that the tsunami poses no danger to that area. The PTWC will continue to monitor the event, issuing updates at least hourly. As conditions warrant, the Advisory will either be continued, upgraded to a watch or warning, or ended.
- Tsunami Information Statement
A Tsunami Information Statement is issued to inform emergency management officials and the public that an earthquake has occurred, or that a tsunami warning, watch or advisory has been issued for another section of the ocean. In most cases, information statements are issued to indicate there is no threat of a destructive tsunami and to prevent unnecessary evacuations as the earthquake may have been felt in coastal areas. An information statement may, in appropriate situations, caution about the possibility of destructive local tsunamis. Information statements may be re-issued with additional information, though normally these messages are not updated. However, a watch, advisory or warning may be issued for the area, if necessary, after analysis and/or updated information becomes available.
- Warning Cancellation
A final product indicating the end of the damaging tsunami threat. A cancellation is usually issued after an evaluation of sea level data confirms that a destructive tsunami will not impact the warned area.
If you're near a coastal beach, here are ways to know a tsunami may be
imminent and you need to seek higher ground:
- A warning siren may sound.
- Seawater may recede quickly.
- The ground may shake, indicating an earthquake has occurred.
- Your NOAA Tone Alert radio issues a warning that a tsunami may be headed to your area.
- Sign up for free
Email and Text Mesage Tsunami Alerts from the Pacific Tsunami Warning Center
Tsunami Evacuation Maps
Additional information on evacation routes for Hawai'i can be found at the following link; Hawai'i Evacuation routes.
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