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Estimation of Diverse Tsunami Excitations with Ocean Bottom Arrays
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| Title: | Estimation of Diverse Tsunami Excitations with Ocean Bottom Arrays |
| Other Titles: | 海底観測網記録から推定される多様な津波励起源 |
| Authors: | 水谷, 歩 Browse this author |
| Issue Date: | 23-Mar-2023 |
| Publisher: | Hokkaido University |
| Abstract: | During the last decade, several dense and widespan ocean-bottom observation networks have been deployed offshore Japan. These networks enable us to observe the generation and propagation processes of tsunamis directly with a lot of stations. In this study, we developed three new methods to utilize these networks and applied them to actual records to confirm their effectiveness. In Chapter 2, we developed an estimation scheme for tsunami heights as well as ocean-bottom displacements with near-fault data on a real-time basis. The records inside a tsunami generation area are useful for a tsunami early warning, but they contain non-tsunami components such as sea-bottom acceleration. In comparisons of the records between collocated ocean-bottom pressure gauges and strong motion seismometers, we found that ocean-bottom pressure gauges punctually record sea-bottom acceleration in the frequency range of 0.05–0.15 Hz. Based on this result, we proposed a new extraction scheme, which can start with a record just 30 sec after an earthquake. In Chapter 3, we applied the back-projection anslysis, which has been used in seismic source studies, to tsunami records. It does not require any specific a priori information about a source to stabilize the result because its main procedure is a simple slant stacking of observed records. We found that the tsunami back-projection analysis can detect not only a source area but also an early stage of tsunami propagation. In addition, the absolute amplitude of the source can also be estimated. In the case of the 2016 Off-Fukushima earthquake with S-net data, the obtained spatial resolution was better than the one by conventional waveform inversion, indicating that the back-projection analysis can be useful for not only understanding a tsunami source mechanism but also tsunami early warning. In Chapter 4, we estimated the excitation process of large later tsunamis, which clearly observed in OBP records around Japan, associated with the 2022 Hunga Tonga-Hunga Ha’apai volcanic eruption. We applied the Vespa analysis to tsunami records to detect arrival times and incident angles of each signal. It revealed that the later phases as well as the first arrival tsunami were induced by the atmospheric waves. We also conducted numerical experiments with two methods of waveform modeling: the finite difference method and the normal mode theory. We found that both the atmospheric wave model and bathymetric effect were important to excite these large later tsunamis. A hybrid calculation approach combining these two methods successfully reproduced the major feature of the observed records, particularly in amplitude. The above three methods proposed in this study shall help us not only to investigate tsunami generation and propagation processes but also to improve the present system of tsunami early warning. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第15287号 |
| Degree Level: | 博士 |
| Degree Discipline: | 理学 |
| Examination Committee Members: | (主査) 特任教授 蓬田 清, 教授 古屋 正人, 教授 谷岡 勇市郎, 準教授 吉澤 和範 |
| Degree Affiliation: | 理学院（自然史科学専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/91561 |
| Appears in Collections: | 課程博士 (Doctorate by way of Advanced Course) > 理学院(Graduate School of Science)
学位論文 (Theses) > 博士 （理学）
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