April 1 marked the 69th anniversary of the 1946 tsunami that devastated the Big Island, causing widespread damage and taking 159 lives. To honor the people lost in the 1946 and subsequent tsunamis, the state of Hawaii proclaimed April as “Tsunami Awareness Month” — a time to focus attention on preparing for and taking action to mitigate the impacts of future destructive tsunamis.
The 1946 tsunami was generated from a large earthquake in the Aleutian Islands, thousands of miles north of Hawaii. The Aleutian Islands are located along the northern extent of the “Ring of Fire,” a tectonically and volcanically active region that circles the Pacific Ocean. Because Hawaii is located in the middle of the Pacific, we are uniquely exposed to tsunamis originating from large earthquakes anywhere around this Ring of Fire.
In Hawaii, we consider an event such as the April 1, 1946, Aleutian earthquake a teleseism (distant earthquake). Likewise, we can refer to the accompanying tsunami as a tele-tsunami (one that originates from a source usually more than 1000 km, or 600 mi, away).
In addition to concerns about tele-tsunamis, Hawaii also faces the threat of local tsunamis generated by earthquakes that occur on or near the islands. In 1868 and 1975, the Big Island experienced deadly tsunamis associated with large local earthquakes centered on the island’s southeastern coastline. In both cases, the fatalities occurred close to where the tsunamis originated — essentially within the area where the earthquake slip occurred.
Seismic waves travel through earth faster than water waves cross the ocean. Using this knowledge, Thomas A. Jaggar Jr., founder of the Hawaiian Volcano Observatory, pioneered forecasting tsunamis in Hawaii in 1922. Results of his predictions in 1923 and 1927 were mixed, but in 1933, after identifying a teleseism from Japan, Jaggar and his colleagues agreed a tsunami was possible and issued warnings for Hilo and Kona. People heeded these warnings and moved out of harm’s way prior to the arrival of the tsunami, so no lives were lost.
In May 1960, a tsunami produced by the largest earthquake ever recorded — a magnitude 9.5 quake that ruptured a 1000-km- (600-mi-) long section of Chile’s coast — devastated Hilo and took thousands of lives throughout the Pacific region. This disaster prompted the global community to establish the International Tsunami Warning System in 1965.
Today, the Pacific Tsunami Warning Center on Oahu (http://ptwc.weather.gov/) provides warnings for Pacific basin tele-tsunamis to coastal populations throughout the Pacific. When a large, tsunami-producing earthquake occurs, PTWC’s principal goal is to locate the earthquake and forecast the tsunami behavior with adequate time to issue warnings to potentially affected areas. The farther a tsunami has to travel, the more time there is between the warning and arrival of the tsunami.
In 1946, the tsunami generated by the Aleutian earthquake — some 3,700 km (2,300 mi) from Hawaii — took slightly less than five hours to cross the ocean and reach Hawaiian coastlines. In 1960, the devastating tsunami from the Chilean earthquake — roughly 10,900 km (6,800 mi) from Hawaii — took about 15 hours to strike Hilo. While the Aleutian earthquake and tsunami provided less warning time, those hours of advance notice can be critical and do afford people an opportunity to evacuate.
Tsunamis produced by local earthquakes are quite different because their travel distances are much shorter. Rather than taking hours to cross the ocean, locally generated tsunamis can strike Hawaiian shorelines within minutes. In these cases, survival depends on being aware and taking quick action. For instance, if you feel a strong earthquake while at the beach, you should immediately move to higher ground — a tsunami wave can come ashore before warning sirens can alert you to the potential danger.
You can learn more about tsunami awareness and preparedness in “Surviving a Tsunami — Lessons from Chile, Hawaii, and Japan” (http://pubs.usgs.gov/circ/c1187/) and “Hawaii Tsunami Preparedness and Safety Information” (http://www.prh.noaa.gov/hnl/pages/tsunami_safety.php). For additional information about Tsunami Awareness Month and tsunami safety, visit these websites: Hawaii Emergency Management Agency (www.scd.hawaii.gov), National Weather Service “TsunamiReady” (http://www.tsunamiready.noaa.gov/), and the International Tsunami Information Center (itic.ioc-unesco.org).
Kilauea activity update
Kilauea’s East Rift Zone lava flow continues to feed three areas of breakouts near Pu‘u ‘O‘o. The front of the breakout farthest downslope had advanced very little during the past week and was still about 7 km (just over 4 mi) northeast of Pu‘u ‘O‘o when mapped Thursday.
There have been no major changes at Kilauea’s summit vent, which continues to host an active lava lake. The lava lake level continues to fluctuate and was 37 m (120 ft) below the rim of the Overlook crater Thursday.
There were two earthquakes reported felt on the the Big Island in the past week. At 12:16 a.m. Sunday, April 5, a magnitude 3.3 earthquake occurred and was located 10.6 km (6.6 mi) northeast of Kawaihae at a depth of 25.8 km (16.0 mi). At 3:23 a.m. the same day, a magnitude 4.5 earthquake occurred and was located 6.3 km (10.1 mi) northwest of Kailua at a depth of 10.0 km (6.2 mi).
Visit the HVO website (http://hvo.wr.usgs.gov) for past Volcano Watch articles, Kilauea daily eruption updates and other volcano status reports, current volcano photos, recent earthquakes and more; call (808) 967-8862 for a Kilauea summary update; email questions to askHVO@usgs.gov.
Volcano Watch (http://hvo.wr.usgs.gov/volcanowatch/) is a weekly article and activity update written by scientists at the U.S. Geological Survey`s Hawaiian Volcano Observatory.
