Generation mechanism of large later phases of the 2011 Tohoku-oki tsunami causing damages in Hakodate, Hokkaido, Japan

Abstract

The 2011 Tohoku-oki earthquake generated a large tsunami that caused catastrophic damage along the Pacific coast of Japan. The major portion of the damage along the Pacific coast of Tohoku in Japan was mainly caused by the first few cycles of tsunami waves. However, the largest phase of the tsunami arriving surprisingly late in Hakodate in Hokkaido, Japan; that is, approximately 9 h after the origin time of the earthquake. It is important to understand the generation mechanism of this large later phase. The tsunami was numerically computed by solving both linear shallow water equations and non-linear shallow water equations with moving boundary conditions throughout the computational area. The later tsunami phases observed on southern Hokkaido can be much better explained by tsunami waveforms computed by solving the non-linear equations than by those computed by solving the linear equations. This suggests that the later tsunami waves arrived at the Hokkaido coast after propagating along the Pacific coast of the Tohoku region with repeated inundations far inland or reflecting from the coast of Tohoku after the inundation. The spectral analysis of the observed waveform at Hakodate tide gauge shows that the later tsunami that arrived between 7.5 and 9.5 h after the earthquake mainly contains a period of 45-50 min. The normal modes of Hakodate Bay were also computed to obtain the eigen periods, eigenfunctions, and spatial distribution of water heights. The computed tsunami height distributions near Hakodate and the fundamental mode of Hakodate Bay indicate that the large later phases are mainly caused by the resonance of the bay, which has a period of approximately 50 min. The results also indicate that the tsunami wave heights near the Hakodate port area, the most populated area in Hakodate, are the largest in the bay because of the resonance of the fundamental mode of the bay. The results of this study suggest that large future tsunamis might excite the fundamental mode of Hakodate Bay and cause large later phases near the Hakodate port.

Effects of Depth of Fault Slip and Continental Shelf Geometry on the Generation of Anomalously Long‐Period Tsunami by the July 2020 Mw 7.8 Shumagin (Alaska) Earthquake

“…Based on local tide gauge records, Tanioka et al. (2019) found that, irrespective of the bay resonance and edge waves influence, the 2011 Tohoku‐oki tsunami had a period of 25–28 min.…”

Ultrabroadband Seismic and Tsunami Wave Observation of High‐Sampling Ocean‐Bottom Pressure Gauge Covering Periods From Seconds to Hours

“…The main cause for this is its low sampling rate (1/60 Hz). However, the complex coastal site effect associated with local bathymetry might be another possible cause (e.g., Geist, 2018; Tanioka et al, 2019). The dispersive feature was also not clearly recognized in the tiltmeter spectrogram (Figures 4b and 4c).…”

“…PG spectra (Figure 11a) demonstrate an amplitude increase for the period range of ∼60-5,000 s, which are associated with tsunamis. Conversely, in the tide gauge spectra, the significant increase in the tsunami amplitude is limited within the period range of ∼2,000-4,000 s (∼0.25-0.5 mHz; Figure 11b), which is due to the coastal site effect (e.g., Geist, 2018;Tanioka et al, 2019). The broadband amplitude increase in the PG spectra demonstrates that PGs are capable of detecting the wider period range of tsunamis and are much less affected by the coastal site effect than coastal tide gauges.…”

Study on the 1906 Colombia‐Ecuador Megathrust Earthquake Based on Tsunami Waveforms Observed at Tide Gauges: Release Variation of Accumulated Slip Deficits in the Source Area

“…At Hakodate, the observed waveform fluctuated with a period of ∼1 h, and its periodic fluctuations appeared prior to the arrival of the tsunami. The primary resonance period along the longitudinal direction of Hakodate Bay, where the Hakodate tide gauge was located, was estimated to be ∼50 min (Tanioka et al., 2019). Thus, weather disturbance may have enhanced the bay‐scale resonance with a larger amplitude than the tsunami amplitude.…”