Today is World Tsunami Awareness Day — designated by the United Nations General Assembly, and according to the United Nations Office for Disaster Risk Reduction (UNISDR), on average, tsunami events have a higher mortality rate than any other hazard. Over the past 20 years (1998-2017) tsunamis have claimed more than 250,000 lives and are also attributable for US$280 billion of the US$661 billion of total recorded economic losses for earthquakes and tsunamis. Between 1978-1997, tsunamis claimed 998 lives, and US$2.7 billion in losses. Overall, tsunamis are rare, but as the UN points out, when they occur they are deadly and hugely damaging. This infrequency makes building awareness and preparedness more of a challenge.
The UN has promoted World Tsunami Awareness Day since 2015, and the UN Secretary-General’s Special Representative for Disaster Risk Reduction, Mami Mizutori, stated that “…it is an occasion to promote greater understanding of tsunami risk to avoid future loss of life. This year we also want to bring attention to the economic losses tsunamis can inflict as a result of damage to critical infrastructure located along vulnerable, densely populated coastlines.”
Thankfully, seven years have now passed since the last “mega-tsunami” caused by the shifts in the seafloor that accompanied the Mw9 earthquake offshore from the Pacific coast of Japan. But since that time, there have been local tsunamis accompanying earthquakes in Indonesia, as with the tragic Sulawesi earthquake of September 28, 2018. The historical roll-call of countries that have been hit by local tsunamis is long and includes: Italy, Greece, Lebanon, Israel, Puerto Rico, Jamaica, Taiwan, Newfoundland, and New Zealand, but very often a single stretch of coastline has only been hit a single time in several hundred years.
The challenge of the largest regional tsunamis is that, like the 2011 Japanese tsunami, they may have return periods of a thousand years or more. And there are, in an average century, only three or four Magnitude 9 earthquakes anywhere on the planet; read more in an RMS report from 2015 entitled “Coastlines at Risk of Giant Earthquakes and the Mega-Tsunami”. Even the local damaging tsunamis are quite scarce. As a result, we have a huge challenge to preserve a memory long enough to ensure that our grandchildren’s grandchildren will be aware of the risk. Telling the fishermen of Sri Lanka not to build at the coast may simply not be relevant, when the next tsunami will be five hundred, or more, years in the future, by which time sea-level could be five meters (16 feet) higher.
For impact and loss modeling, tsunami hazard is increasingly available in RMS earthquake catastrophe models for Japan, New Zealand, and western North America and will become included in every well-populated region where Mw9 earthquakes can occur, as along the west coasts of South and Central America as well as the coastlines of the Caribbean. In a single coastline we will still have the challenge of keeping alive the knowledge that one day a great tsunami will rise ten or twenty meters (32 to 65 feet) high engulfing the towns and villages. Through these models, and how they get communicated, working with local insurers and cities, we can help to keep alive the knowledge of the risk.