Direct energy estimation of the 2011 Japan tsunami using deep‐ocean pressure measurements

Abstract

[1] We have developed a method to compute the total energy transmitted by tsunami waves, to the case where the earthquake source is unknown, by using deep-ocean pressure measurements and numerical models (tsunami source functions). Based on the first wave recorded at the two closest tsunameters (Deep-Ocean Assessment and Reporting of Tsunamis (DART)), our analysis suggests that the March 11, 2011 Tohoku-Oki tsunami generated off Japan originated from a 300-400 km long and 100 km wide area, and the total propagated energy is 3 Â 10 15 J (with 6% uncertainty). Measurements from 30 tsunameters and 32 coastal tide stations show excellent agreement with the forecasts obtained in real time. Our study indicates that the propagated energy and the source location are the most important source characteristics for predicting tsunami impacts. Interactions of tsunami waves with seafloor topography delay and redirect the energy flux, posing hazards from delayed and amplified waves with long duration. Seafloor topography also gives its spectral imprint to tsunami waves. Travel time forecast errors are path-specific and correlated to the major wave scatterers in the Pacific. Numerical dissipation in the propagation modeling highlights the need of high-resolution inundation models for accurate coastal predictions. On the other hand, it also can be used to account for physical dissipation to achieve efficiency. Our results provide guidelines for the earliest reliable tsunami forecast, warnings of long duration tsunami waves signals and enhancement of the experimental tsunami forecast system. We apply the method to quantify the energy of 15 past tsunamis, independently from earthquake magnitudes. The small tsunami to seismic radiation energy ratios, and their variability (0.01-0.8%), reinforce the importance of using deep-ocean tsunami data, the direct measures of tsunamis, for estimates of tsunami energy and accurate forecasting.

Dispersion and nonlinear effects in the 2011 Tohoku‐Oki earthquake tsunami

“…Figure 9 shows that the early stage of the tsunami energy (< elapsed time of 150 min) is characterized by a decay time t d of 78 min. In contrast, the energy decay at a much later stage (elapsed time of $48 hr) is characterized by a much longer decay time (t d $ 24 h) [see Saito et al 2013, Figure 11;Tang et al, 2012]. This value is almost the same as that obtained for the Pacific Ocean by Van Dorn [1984].…”

“…Some studies reported that the wave dispersion (or physical dispersion) could be modeled by creating a numerical dispersion by setting the grid spacing artificially wide enough to create the numerical dispersion. This method works well for rapid tsunami prediction purposes [e.g., Tang et al, 2012;Zhou et al, 2012]. However, this method is inappropriate when attempting to reproduce the detailed features of the tsunami waveform, because the grid spacing for the numerical dispersion is too coarse to represent the short wavelength components of the tsunami source or the initial tsunami-height distribution.…”

“…For the Tohoku tsunami, many studies considered the nonlinear process and the inundation to successfully simulate the tsunami waveforms [e.g., Tang et al, 2012;Wei et al, 2013;Yamazaki et al, 2013]. However, the most dominant factor contributing to the tsunami waveforms that were observed offshore was not obvious.…”

“…However, the analysis failed to accurately estimate the earthquake and tsunami sizes, because the long-period (> $100 s) seismic waves were not properly included for the rapid magnitude estimation [e.g., Kanamori, 2012]. Conversely, according to a tsunami waveform analysis, the tsunami forecasting by the DART system estimated the total tsunami energy within 1.5 h and successfully forecasted the tsunami arrivals and heights in the far-field coasts [Tang et al, 2012]. Numerous studies have been conducted to develop tsunami-forecasting techniques for near-field tsunami [e.g., Tsushima et al, 2011;Ohta et al, 2012;Gusman and Tanioka, 2013;Melgar and Bock, 2013;Wei et al, 2013].…”

“…The inundation heights and area were investigated widely along the Pacific coast of northeastern Honshu, Japan, by the 2011 Tohoku Earthquake Tsunami Joint Survey Group [e.g., Mori et al, 2011] and also by aerial photographs [Nakajima and Koarai, 2011]. The Deep-ocean Assessment and Reporting of Tsunamis (DART) system recorded the tsunami propagation across the Pacific Ocean and it successfully predicted the tsunami arrival for various far-field coasts [Tang et al, 2012]. A number of high-quality data were recorded for this event.…”