South Pacific Tsunami Event 29 September 2009

Observed water heights and calculated tsunami travel times, 29 Sept 2009

Observed water heights and calculated tsunami travel times, 29 Sept 2009


The 29 September 2009 M7.9 earthquake generated a tsunami that caused deaths and casualties in Samoa, American Samoa, and Tonga, and damage in Wallis and Futuna. Strong shaking was felt for a period of at least 60 s with some eyewitnesses reporting more than two minutes. The tsunami arrived 10-20 min after earthquake shaking stopped in Samoa, American Samoa, and Tonga, and caused deaths in Samoa (149), Am Samoa (34), and Tonga (9). Maximum run-up’s of 16-17+ m were measured in all three countries. Extensive coastal damage to structures and marine / coral reef, and lagoon ecosystems occurred. The international alert system (PTWC) provided its first messages (preliminary earthquake observatory message) 11 minutes after the earthquake and an official warning 16 min after the earthquake. In order to improve on the timeliness of alerting, improvements in seismic networks are required, and these have been progressing actively for the Southwest Pacific since 2007.

However, through numerous Pacific Tsunami Warning and Mitigation System (PTWS) working groups over the last decade and other workshops and trainings by ITIC and PTWS countries, it is well-known that for local tsunamis, education, awareness, preparedness before the event and heeding of natural warnings when it does occur are the most important activities for saving lives. In other words, no one should be wait for an official alert to evacuate. Rather, if they are living near the coast and a strong earthquake occurs and/or unusual/abnormal sea level activities are seen or load roars are heard, everyone should go to higher ground and inland immediately!

This was clearly demonstrated in American Samoa and Samoa, where both countries had actively engaged in pre-event awareness and education. For last decade, this has been carried out continuously in American Samoa and September 2009 was American Samoa Disaster Preparedness month. Additionally, the Pacific Wave 2006 and 2008 helped to prepare countries, their agencies, and their peoples for the next tsunami.

Factors which helped to reduce vulnerability for this event were the time of day (many were already awake, but not on the road and the work day had not started), generally limited earthquake damage which reduced injuries and damage to transportation infrastructure, relative closeness and availability of high ground, pre-event plans (such as school evacuation plans in American Samoa) and school and community drills (National Drill in 2007 and 2008 in Samoa). The 2004 Indian Ocean tsunami, and more recently, the 19 March earthquake off Tonga, also served as reminders of what tsunamis can do, and how warning and response agencies need to coordinate and respond.

Today, the countries are working on recovery issues, and improving their early warning and alert systems, especially in hardening the communications infrastructures critical for providing information on warning and evacuation. Both traditional methods of alerting (bells), and modern media (sirens, mobile phone alerting, radio/TV broadcast), are being strengthened. In addition, at the community and village levels, education and preparedness continues such as the development and/or clarification of evacuation maps and routes to safe areas. As the countries build back, they are doing so in ways which acknowledge and recognize that traditional building practices (open fales) survived well.


For more technical information, visit this tsunami's event page (click here).


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