Abstract
We developed a real-time tsunami forecast method using only pressure data collected from the bottom of the ocean via a dense offshore observation network. The key feature of the method is rapid matching between offshore tsunami observations and pre-calculated offshore tsunami spatial distributions. We first calculate the tsunami waveforms at offshore stations and the maximum coastal tsunami heights from any possible tsunami source model and register them in the proposed Tsunami Scenario Bank (TSB). When a tsunami occurs, we use multiple indices to quickly select dozens of appropriate tsunami scenarios that can explain the offshore observations. At the same time, the maximum coastal tsunami heights coupled with the selected tsunami scenarios are forecast. We apply three indices, which are the correlation coefficient and two kinds of variance reductions normalized by the L2-norm of either the observation or calculation, to match the observed spatial distributions with the pre-calculated spatial distributions in the TSB. We examine the ability of our method to select appropriate tsunami scenarios by conducting synthetic tests using a scenario based on “pseudo-observations.” For these tests, we construct a tentative TSB, which contains tsunami waveforms at locations in the Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench and maximum coastal tsunami heights, using about 2000 tsunami source models along the Japan Trench. Based on the test results, we confirm that the method can select appropriate tsunami scenarios within a certain precision by using the two kinds of variance reductions, which are sensitive to the tsunami size, and the correlation coefficient, which is sensitive to the tsunami source location. In this paper, we present the results and discuss the characteristics and behavior of the multi-index method. The addition of tsunami inundation components to the TSB is expected to enable the application of this method to real-time tsunami inundation forecasts in the near future.
Highlights
A wide variety of methods for near-field tsunami forecasts have been proposed
These examinations enabled us to confirm that appropriate tsunami scenarios could be selected by using the three indices, which consist of two kinds of variance reductions and a correlation coefficient, with offshore waveforms obtained by both pseudo-observation and by calculation, both of which are derived from oceanbottom pressure changes
Mathematical considerations and synthetic examinations revealed that two different kinds of variance reductions, variance reduction normalized by observation (VRO)(t) and variance reduction normalized by calculation (VRC)(t), which are normalized by the L2-norm of either the observed or calculated waveforms, are sensitive to overestimation or underestimation of the tsunami size
Summary
A wide variety of methods for near-field tsunami forecasts have been proposed. These methods depend on the purpose and type of observational data (e.g., Tsushima and Ohta 2014). We refer to the coastal indices as coastal R, coastal VRO, and coastal VRC, which are defined by the pseudo-observation distributions and the calculated distributions of maximum coastal tsunami heights that do not depend on time t.
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