Probabilistic Tsunami Hazard Assessment Considering the Sequence of the First and Second Earthquakes Along the Nankai Trough

The Earthquake Research Committee (ERC) of the Headquarters for Earthquake Research Promotion conducted a probabilistic tsunami hazard assessment due to large earthquakes along the Nankai Trough for the next 30 years. Utilizing the basic data of earthquake source models and calculated maximum tsunami heights, the authors propose a method to evaluate the hazard curves of the first and second earthquakes separately, considering the sequence of earthquake occurrence in one cycle of large earthquake activities along the Nankai Trough. First, based on the relative weights allocated to the 176 occurrence patterns of 79 earthquake source regions, the weights for the 2,720 characterized earthquake fault models (CEFMs) are calculated. The hazard curve of the first earthquake is evaluated using the 2,720 sets of maximum tsunami heights and weights under the condition that one of the CEFMs causes an earthquake. Next, the conditional hazard curves for the possible second earthquakes conditional on each individual first earthquake are calculated. Finally, the hazard curve for the second earthquake is evaluated as a weighted average of the conditional hazard curves. Numerical examples are shown for 15 sites. The first earthquake accounts for about 60% or more of the total hazard evaluated by ERC, and its contribution increases with increasing maximum tsunami height. The first and second earthquakes account for 80%–90% of the total hazard.