Abstract
Megathrust earthquakes that occur repeatedly along the plate interface of subduction zones can cause severe damage due to strong ground motion and the destructive tsunamis they can generate. We developed a set of scenario earthquakes to evaluate tsunami hazards and tsunami early warning systems for such devastating earthquakes. Although it is known that the slip distribution on a fault strongly affects the tsunami height distribution in near-field coastal areas, the slip distribution of future earthquakes cannot be exactly predicted. One way to resolve this difficulty is to create a set of scenario earthquakes in which a set of heterogeneous slip distributions on the source fault is stochastically generated based on a given slip probability density function (SPDF). The slip distributions generated in this manner differ from event to event, but their average over a large ensemble of models converges to a predefined SPDF resembling the long-term average of ruptures on the target fault zone. We created a set of SPDF-based scenario earthquakes for an expected future Mw 8.2 Tonankai earthquake in the eastern half of the Nankai trough, off southwest Japan, and computed the ensuing tsunamis. We found that the estimated peak coastal amplitudes among the ensemble of tsunamis along the near-field coast differed by factors of 3 to 9, and the earliest and latest arrivals at each observation site differed by 400 to 700 s. The variations in both peak tsunami amplitude and arrival time at each site were well approximated by a Gaussian distribution. For cases in which the slip distribution is unknown, the average and standard deviation of these scenario datasets can provide first approximations of forecast tsunami height and arrival time and their uncertainties, respectively. At most coastal observation sites, tsunamis modeled similarly but using a uniform slip distribution underpredicted tsunami amplitudes but gave earlier arrival times than those modeled with a heterogeneous slip distribution. Use of these earlier arrival times may be useful for providing conservative early warnings of tsunami arrivals. Therefore, tsunami computations for both heterogeneous and uniform slip distributions are important for tsunami disaster mitigation.Graphical abstract
Highlights
Megathrust earthquakes along the Nankai trough off southwestern Japan, such as the 1944 Tonankai and 1946 Nankai events, have caused severe damage due to strong ground motion and large tsunamis
3 Results and discussion We measured the peak near-shore tsunami amplitude (PNTA) and arrival time of each of the tsunamis generated by the 200 scenario earthquakes
We generated 200 scenario earthquakes for an anticipated Mw 8.2 Tonankai earthquake in the Nankai trough and found that the estimated nearfield tsunami amplitudes and coastal arrival times were strongly dependent on slip distribution
Summary
Megathrust earthquakes along the Nankai trough off southwestern Japan, such as the 1944 Tonankai and 1946 Nankai events, have caused severe damage due to strong ground motion and large tsunamis. Tsunami warning systems generally estimate coastal tsunami heights and arrival times on the basis of earthquake source parameters, crustal deformation, and initial tsunami heights estimated by inversion of seismic, geodetic, or offshore ocean-bottom pressure gauge (OBP) records, respectively (Tatehata 1997; Titof et al 2005; Blewitt et al 2009; Tsushima et al 2009, 2011, 2012, 2014). Yamamoto et al (2016b) developed a method to rapidly estimate initial tsunami heights by using dense OBP networks. Maeda et al (2015) developed a method to use real-time OBP data to estimate coastal tsunami heights based on data assimilation
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