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[HDS10-P08] Decay process of the tsunami caused by the 2011 Tohoku earthquake
Keywords:tsunami, decay process
An accurate prediction of the tsunami decay process is necessary to cancel tsunami warnings, in particular, for far-field and giant tsunamis. This study evaluated the tsunami decay process observed at the tide gauges and the GPS wavemeters for the 2011 Tohoku tsunami. We removed the tidal components and unrelated fluctuations in the bay from the observed data using a bandpass filter to create tsunami records within 48 hours after the earthquake. We calculated moving root mean square (MRMS) amplitude following Hayashi et al. (2010)'s method with the time window width of 64 minutes. We also subtracted the average of MRMS amplitude for 3 hours before the earthquake for the zero-level shift. We fitted a function to the MRMS amplitude after 32 minutes since the arrival of the maximum tsunami. We analyzed the same stations with Hayashi et al. (2010). The tide gauges used in this study were Hanasaki, Mera, Okada, Chichishima, Awayuki, and Amami. The GPS wavemeters were Miyagi-hokubu and Miyagi-chubu.
We fitted an exponential function, M(t)=ke^(-t/τ), where τ is an attenuation parameter, t is time, k is a coefficient to the MRMS amplitude waveforms. The exponential function approximated the MRMS amplitude waveforms well at the tide gauges The attenuation parameters, τ, are almost the same among the tide gauges, about 20 hours. This result is consistent with Hayashi et al. (2010) for the 2006 and 2007 Kuril Island tsunamis. The attenuation parameter, τ, is not dependent on the observation locations and the tsunami sources.However, the exponential function did not approximate at the GPS wavemeters because the MRMS amplitude of the GPS wavemeters decayed more rapidly. Therefore, we used a power function, M(t)=kt^(-α), α is an attenuation parameter for the GPS wavemeters. The power functions approximate well the decay processes of tsunamis recorded by the GPS wavemeters. The presumed attenuation parameters, α, are almost identical between the two GPS wavemeters.
We attempted to simulate the MRMS amplitude decay process. Numerical tsunami simulations used the tsunami source model in Satake et al. (2013) and the terrain data with nesting domains of 1 arc-minute, 20 arc-second, 20/3 arc-second, and 20/9 arc-second. The calculated MRMS amplitude was as same as the observed one in Hanasaki. However, the calculated MRMS was smaller than the observations in the other tide gauge stations. The exponential function could generally approximate the attenuation of the MRMS amplitude of the calculated tsunami waveform. The attenuation parameters, τ, were between 11.1 hours and 21.0 hours, which is a variation larger than the attenuation parameter estimated in the observed data. We also calculated tsunamis from the 2006 and 2007 Kuril Islands earthquakes (Fujii and Satake, 2008). We got the same feature in the above regarding the arrival of the maximum tsunami height. We will conduct tsunami calculations with a more detailed dataset for the next step.
We fitted an exponential function, M(t)=ke^(-t/τ), where τ is an attenuation parameter, t is time, k is a coefficient to the MRMS amplitude waveforms. The exponential function approximated the MRMS amplitude waveforms well at the tide gauges The attenuation parameters, τ, are almost the same among the tide gauges, about 20 hours. This result is consistent with Hayashi et al. (2010) for the 2006 and 2007 Kuril Island tsunamis. The attenuation parameter, τ, is not dependent on the observation locations and the tsunami sources.However, the exponential function did not approximate at the GPS wavemeters because the MRMS amplitude of the GPS wavemeters decayed more rapidly. Therefore, we used a power function, M(t)=kt^(-α), α is an attenuation parameter for the GPS wavemeters. The power functions approximate well the decay processes of tsunamis recorded by the GPS wavemeters. The presumed attenuation parameters, α, are almost identical between the two GPS wavemeters.
We attempted to simulate the MRMS amplitude decay process. Numerical tsunami simulations used the tsunami source model in Satake et al. (2013) and the terrain data with nesting domains of 1 arc-minute, 20 arc-second, 20/3 arc-second, and 20/9 arc-second. The calculated MRMS amplitude was as same as the observed one in Hanasaki. However, the calculated MRMS was smaller than the observations in the other tide gauge stations. The exponential function could generally approximate the attenuation of the MRMS amplitude of the calculated tsunami waveform. The attenuation parameters, τ, were between 11.1 hours and 21.0 hours, which is a variation larger than the attenuation parameter estimated in the observed data. We also calculated tsunamis from the 2006 and 2007 Kuril Islands earthquakes (Fujii and Satake, 2008). We got the same feature in the above regarding the arrival of the maximum tsunami height. We will conduct tsunami calculations with a more detailed dataset for the next step.