11:00 AM - 11:15 AM
[HDS12-08] Tsunami inversion for sea surface displacement from far-field data of the 2011 Tohoku tsunami
Keywords:tsunami inversion, far-field, phase correction, 2011 Tohoku earthquake, trans-ocean tsunami
We investigate the initial tsunami sea surface displacement of the 2011 Tohoku earthquake by performing the tsunami waveform inversion with the data of far-field stations. Tsunami travel time difference up to 15 min and polarity reversal of initial phase between observed and synthetic waveforms have been pointed out for the stations with travel time larger than 3 hours (noted as “far-field”). The differences were mostly explained by the phase correction method (Watada et al., 2014, JGR), which accounts for the dispersion effects of tsunami loadings on the Earth, seawater compression, and gravitational potential change. By applying the phase correction method, we can convert synthetic waveforms of linear long wave into dispersive waveforms.
In this study, we attempt to improve the phase correction method by including the effect of ocean density stratification and calculating the realistic tsunami ray path, not along the great circle, which minimizes the tsunami travel time from the source to the station. The characteristic ocean depth 4 km is replaced by the corresponding depths along each path. We apply the improved phase correction method to waveforms of near-field stations (travel time < 3 hours, including wave gauges, GPS stations, and OBP-type stations) and far-field stations (only DART stations) in the single and multiple time window inversions. Our results show that ocean density stratification accounts for travel time delay up to 2 minutes. In addition, replacing the great circle distance and 4 km reference depth by the non-great-circle ray path length and corresponding depth account for up to 0.8 minutes in travel time.
The inversion result of only far-field data shows a long wavelength sea surface displacement in the source area with a crest of ~6.1 meters located at the midpoint of the epicenter and the trench. Inversion with only near-field data leads to a detailed result which locates a peak displacement of ~6.4 meters near the trench. In the result of both near- and far-field data, a compact ~6.9-meter-high peak displacement appears at the midpoint of the epicenter and the trench. An improved detailed result is obtained by combining near-field stations with far-field stations. The accurate calculation of far-field stations is essential for utilizing them in tsunami source studies and trans-ocean tsunami warnings.
In this study, we attempt to improve the phase correction method by including the effect of ocean density stratification and calculating the realistic tsunami ray path, not along the great circle, which minimizes the tsunami travel time from the source to the station. The characteristic ocean depth 4 km is replaced by the corresponding depths along each path. We apply the improved phase correction method to waveforms of near-field stations (travel time < 3 hours, including wave gauges, GPS stations, and OBP-type stations) and far-field stations (only DART stations) in the single and multiple time window inversions. Our results show that ocean density stratification accounts for travel time delay up to 2 minutes. In addition, replacing the great circle distance and 4 km reference depth by the non-great-circle ray path length and corresponding depth account for up to 0.8 minutes in travel time.
The inversion result of only far-field data shows a long wavelength sea surface displacement in the source area with a crest of ~6.1 meters located at the midpoint of the epicenter and the trench. Inversion with only near-field data leads to a detailed result which locates a peak displacement of ~6.4 meters near the trench. In the result of both near- and far-field data, a compact ~6.9-meter-high peak displacement appears at the midpoint of the epicenter and the trench. An improved detailed result is obtained by combining near-field stations with far-field stations. The accurate calculation of far-field stations is essential for utilizing them in tsunami source studies and trans-ocean tsunami warnings.