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[HDS19-23] 2015 Torishima tsunami earthquake: Ray tracing analysis of dispersive tsunami wave
Keywords:tsunami, dispersion, tsunami earthquake, ray tracing, volcanic earthquake
Tsunami waves caused by the 2015 earthquake were recorded by a temporary ocean bottom array of water pressure gauges, 100 km to the NNE from the epicenter. The tsunami traces at the array gauges started with a positive onset and showed dispersion effects. It is notable that the measured slowness orientation of wavefront approximated by a plane wave varies as a function of frequency (Fukao et al., 2016, JpGU).
In this study, we investigated frequency-dependent ray paths of the tsunami by a ray tracing method considering the dispersion effects. We first iteratively calculated two-dimensional phase- and group-velocity fields dependent on frequency, using the theoretical formula of gravity waves and a smoothed bathymetry. These velocity fields enable us to measure travel times of both of wavefronts and wave packets. We assumed a tsunami source on the Smith caldera and applied the ray equations for seismic surface waves on spherical Earth (e.g. Sobel and Seggern, 1978, BSSA; Jobert and Jobert, 1983, GRL) to ray tracing of tsunamis.
Ray paths show that longer-period waves are more affected by bathymetry variations. We note that wavefronts toward NNE at the source change their direction to the north, furthermore to the NNW in case of longer-period waves. This trend explains well the frequency-dependent slowness orientation of wavefront at the array gauges. Travel times of wave packets are also consistent with frequency-dependent arrival times observed at the gauges. In addition, ray paths show an intensity of energy northward along the shallow ridge, regardless of frequency, which might have contributed to high amplitudes at Hachijo Island.
Our ray tracing method including dispersion effects allows us to investigate ray paths for shorter waves out of applicable range of the linear long-wave theory. Therefore we can consider path effects for shorter waves following primary longer waves. This method may be also applicable for tsunami forecasting including subsequent shorter waves due to dispersive effects with small numerical computation costs.