Offshore forecasting of Hawaiian tsunamis generated in Alaskan-Aleutian Subduction Zone

This report describes an R&D activity conducted during FY 1998 to develop tsunami forecasting tools for the Pacific Disaster Center (PDC). The activity included analytical and numerical sensitivity studies of tsunami wave characteristics offshore of Hawaii, for ranges of earthquake source parameters in the Alaska-Aleutian Subduction Zone (AASZ); this region is a major source of destructive tsunamis that strike Hawaii. A set of tsunami numerical simulation scenarios was designed to be the basis for the sensitivity analysis, using the 1996 Andreanov Island earthquake/tsunami as a reference. The analysis shows that the first waves are relatively insensitive to the details of the earthquake fault plane parameters. Simulation results for unit sources have been stored as an online database with a WWW interface. A database user can quickly obtain a model prediction of the offshore tsunami wave heights at selected locations for a wide variety of AASZ earthquake scenarios. This database provides a useful offshore tsunami forecasting tool for hazard mitigation managers. 1. Background and Motivation This study has been conducted as part of an effort to provide the Pacific Disaster Center (PDC) with tsunami forecasting capabilities. PDC is a joint Department of Defense and State of Hawaii activity, initially established in February 1996. The purpose of this center is to provide a source of information and guidance for all disaster-related events. It is currently in the process of acquiring tools to provide this capability for selected categories of disasters, including tsunamis [see URL http://www.pdc.org]. Potentially, the best approach to forecasting the impact of a particular tsunami event on a specific coastal site is to perform real-time tsunami model simulations that include real-time data assimilation. With such a capability, forecasts of tsunami impact would be computed immediately following a potentially tsunamigenic earthquake. As a community, we should work toward this difficult “Holy Grail” of tsunami forecasting. However, insufficient real-time earthquake data and a lack of confidence in tsunami generation and inundation modeling presently render the implementation of such an operational capability imprudent. Fortunately, however, existing numerical models are good enough to provide useful guidance, if exercised with care by an experienced tsunami modeler. Eventand site-specific inundation estimates can be computed far in advance of the earthquake, thoroughly tested and scrutinized for reasonableness and sensitivity to errors, and then stored as a database. When an event occurs, the appropriate pre-computed results can be recalled, modified by data assimilation schemes that utilize a user-friendly interface to incorporate real-time tsunami measurements, and made available to aid hazard assessment and evacuation decision-making. Such a database of pre-computed results cannot contain all possible tsunami events, simply because there is too much variability in the mechanism of tsunamigenic earthquakes. The database must contain a finite