Mapping Coherent, Time-Varying Wavefronts from the Tohoku Tsunami into Enhanced, Time-Dependent Warning Messages

Abstract

Recent results are presented to illustrate how predictions of tsunami wave impact and tsunami warning mes-sages can be improved by including information about multiple, large-amplitude wave arrivals over longer time durations and at refined spatial resolution. A deployment of ocean bottom seismometers off the coast of southern California recorded the March 2011 Tohoku tsunami on 22 differential pressure gauges. The pressure gauge tsu-nami records across the entire array show multiple large-amplitude, coherent phases arriving one hour to more than 36 hours after the initial tsunami phase. Analysis of the pressure gauge recordings reveals possible locations of the geographical sources that contributed to secondary tsunami arrivals in southern California. A beamform-ing technique is applied to the pressure gauge data to determine the azimuths and arrival times of scattered wave energy. In addition, a backward ray-tracing procedure is applied to a wide range of back azimuth starting values from the pressure gauge array to map possible scattering source locations. The results show several possible candidates of secondary tsunami source structures. These include: (1) southeastern Alaska producing a tsunami arrival 1–2 hours after the first arrival; and elongated bathymetry structures near: (2) the northern Hawaiian Is-land chain producing an arrival 1–2 hours, (3) Papua New Guinea producing an arrival 8–9 hours, and (4) French Polynesia producing an arrival 10–11 hours, all after the first arrival. These results are then incorporated into tsunami warning messages to improve clarity of the hazard threat and protective action guidance, and the specificity of impact location over time. Revised tsunami messages have been tested through online experiments with the public in order to determine how changes in message clarity and specificity affects message receiver understanding, believing, and personalizing, all of which are pre-decisional sense-making activities. The geo-physical results are mapped into modified tsunami warning messages to show how a time-varying hazard could be communicated with more effective message format and content. The results are demonstrating the effects of including clearly described locations, time of impact, and hazard impact consequences on message perception among the public.