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Tsunami Warning and Preparedness: An Assessment of the U.S. Tsunami Program APPENDIX I Samoa Tsunami The Samoa tsunami on September 29, 2009, had a devastating effect on the islands. The tsunami brought destruction to the small nation only 10-20 minutes after an earthquake about 120 km offshore shook the ground (Figure I.1). However, the tsunami had only a minor effect on Hawaii. The event illustrates (a) how a Tsunami Warning Center (TWC) is unlikely to effectively alert people of a near-field tsunami and (b) how valuable a TWC can be for monitoring a near- and far-field tsunami. It further demonstrates the importance of pre-event education to save lives from near-field tsunamis and the manner in which alert information is disseminated when a tsunami occurs. The earthquake occurred at 7:48 AM Hawaii Standard Time (HST) and the first Pacific Tsunami Warning Center (PTWC) tsunami warning was sent to the Intergovernmental Oceanographic Commission (IOC) listserv at 8:04 AM HST, 16 minutes after the earthquake1 (Table I.1). It announced the earthquake’s preliminary parameters with an origin time of 7:48 AM HST, coordinates 15.3 South and 171.0 West, located at Samoa Islands Region, and a magnitude of 7.9. The 7.9 magnitude was later increased to 8.3 in subsequent messages.1 The time of release of the first TWC product gave only 8 minutes of warning to American Samoa and 28 minutes of warning to Samoa, based on tsunami wave arrivals reported at the gauge stations. The delay between the earthquake and the initial message is relatively long. A preliminary earthquake assessment could have been completed as early as 2 minutes, and in the past few years the TWCs have been issuing these messages after less than 8 minutes (TWC presentation to the committee, 2008). Nevertheless, even if the message had been sent 2 minutes after the earthquake, it is questionable whether the message would have alerted the public at risk in time to allow for effective evacuation. In addition, even with a warning message reaching the public prior to wave arrival time, pre-event education would still be required to ensure proper protective action in such a short amount of time. At 8:05 AM HST on Tuesday, September 29, 2009 the first alert message was issued from the PTWC to the Hawaiian Civil Defense Authority. Tsunami Message Number 1 announced that “A Tsunami Watch is issued for the state of Hawaii effective at 8:05 AM HST.” The evaluation included in the text of this message declared that “Based on all available data a tsunami may have been generated…” and gave an estimated time arrival of a possible tsunami to reach Hawaii at 1:11 PM HST.2 It was understood that the PTWC would send subsequent alerts as its investigation continued. Tsunami Message Number 2 was issued 52 minutes later, at 8:57 AM HST. This message announced that “A Tsunami Watch continues in effect for the state of Hawaii” and documented an increase in the magnitude of the earthquake from 7.9 to 8.3, but indicated that the time, location, and coordinates remained the same. Additional changes between the initial message and the second message included reports of tsunami wave activity from gauge stations with
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Tsunami Warning and Preparedness: An Assessment of the U.S. Tsunami Program FIGURE I.1 Map of Samoa tsunami source. SOURCE: http://news.bbc.co.uk/2/hi/8281616.stm; with permission from the BBC. TABLE I.1 Timeline for Key Events During Samoa Tsunami Arrival time (HR:MIN:SEC) received by USGS QDDS system Elapsed time (HR:MIN:SEC) What 17:48:10 0:00:00 Earthquake origin time 17:48:37 0:00:27 P ARRIVAL at AFI (179km) 17:48:58 0:00:48 S ARRIVAL at AFI (179km) 17:49:50 0:01:40 Strong shaking abates at AFI 17:56:24 0:08:14 PTWC M7.1 Hypocenter 17:57:15 0:09:05 WC/ATWC M7.9 Hypocenter 18:03:15 0:15:05 WC/ATWC Tsunami Information Statement (message states 18:02 release) 18:05:10 0:17:00 PTWC Expanding Regional Warning (message states 18:04 release and M7.9) 18:07:15 0:19:05 NEIC M7.9 Hypocenter SOURCE: Committee staff.
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Tsunami Warning and Preparedness: An Assessment of the U.S. Tsunami Program wave amplitude listed, which confirmed the generation of a tsunami. Nevertheless, the official evaluation stated that “Based on available data a tsunami may have been generated….”2 After the second message, 86 minutes later, Tsunami Message Number 3 was issued at 10:23 AM HST. The tsunami watch was officially canceled in this announcement for the entire state of Hawaii, and was replaced with a tsunami advisory. This message was updated with a significant amount of information available from additional gauge stations. It was determined, as listed in the evaluation, that a major tsunami would not strike the state of Hawaii, but that a sea level change and stronger currents were possible, which could be hazardous to people along the coastlines. The estimated time that the effects from the sea level change and stronger currents could be expected to begin was 1:00 PM HST. Based on these hazards, it was stated that the tsunami advisory would remain in effect until 7:00 PM HST.2 Due to the timely delivery of this message, an evacuation was avoided. An announcement issued by Civil Defense was delivered by police, fire, and lifeguards to the public warning it of potential risks on beaches and suggesting that people remain out of the water.3,4 Despite Civil Defense’s announcement that beach goers should remain out of the water during the time of the possible arrival of the first wave, from 1:11 p.m. and onward, it was reported that Waikiki Beaches were still packed with people during this time.3 Because Civil Defense knew and communicated that a widespread evacuation was not necessary, people were unresponsive to suggestions that they remain out of the water. The fourth and final tsunami message, issued at 4:12 PM HST, announced the continuation of the tsunami advisory for the state of Hawaii. This message provided more detail from additional gauge stations with latitude, longitude, time, amplitude, and the wave period for tsunami wave activity. The evaluation in this message declared that “Small tsunami waves from this earthquake are now crossing the Hawaiian Islands. While these waves are not expected to cause any significant coastal flooding they can produce small changes of sea level at the coast and strong or unusual currents that can be hazardous to swimmers.”2 REFERENCES 1. National Oceanic and Atmospheric Administration. 2009. Pacific Tsunami Warning Center Tsunami Bulletins to IOC listserv. Pacific Tsunami Warning Center, National Oceanic and Atmospheric Administration, Ewa Beach, Hawaii. 2. National Oceanic and Atmospheric Administration. 2009. Pacific Tsunami Warning Center Tsunami Messages to Civil Defense. Pacific Tsunami Warning Center, National Oceanic and Atmospheric Administration, Ewa Beach, Hawaii. 3. KITV. 2009. Many Beachgoers in Waikiki Ignore Warnings. [Online]. Available: http://www.kitv.com/news/21150223/detail.html [2010, February, 3]. 4. Hawaii News Now. 2009. Tsunami Watch Canceled for Hawaii After 8.3 Samoa Quake. [Online]. Available: http://www.hawaiinewsnow.com/Global/story.asp?S=11225197 [2010, February, 3].
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