Influence of the Great Megathrust Earthquakes of the Past Decade on Risk Assessment and Outreach Programs

Abstract:

Four subduction zone earthquakes of magnitude ≥ 8.6 occurred between 2004 and 2013. No earthquakes of this size were reported anywhere in the world in the preceding 36 years. The wealth of seismic, geodetic, geologic and tsunami data from these great megathrust events has advanced the understanding of subduction zones and challenged a number of previously accepted ideas. This talk focuses on how they have also influenced risk assessment and preparedness programs. Megathrust earthquakes differ from other large damaging earthquakes. The size of the megathrust source means a much larger area may be impacted by earthquake shakingaffecting not only the amount of damage, but posing response and recovery challenges. A second factor is tsunami generation. About a third of the 760,000 casualties in the decade were caused by the four mega-earthquakes. All four produced deadly tsunamis and over 95% of the death total was attributed to tsunami. Even when the extraordinarily deadly 2004 Andaman Sumatra tsunami is removed from the data set, 85% of the casualties in the remaining three earthquakes were caused by tsunami. In contrast, in the non-megathrust events caused over two-thirds of the decade's casualties but less than 1 % were caused by tsunami. The Cascadia subduction zone along the coast of northern California, Oregon, Washington and southern British Columbia is the only location in the contiguous 48 states where a great megathrust earthquake will someday occur. Assessing the risk posed by Cascadia and developing effective preparedness programs pose a number of challenges. Awareness of Cascadia is relatively recent and assessing the magnitude, recurrence and nature of past events depends primarily on paleoseismology. The megathrust events of the past decade provide a proxy for and a general picture of the likely impacts of a future Cascadia earthquake and have influenced preparedness efforts throughout the Cascadia region. The recent events have also posed problems for Cascadia. Each event has changed perceptions and risk assessments among the Cascadia states and provinces that has led to differences in standards for hazard maps, sirens and education protocol. A challenges for the next decade is to bridge these differences to develop more consistency throughout the Cascadia region.