Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Tsunami Wave Height by Bandpass Filter along the Southern Kyushu States Considering Offshore Islands
Akio Nagayama
Department of Ocean Civil Engineering, Kagoshima University
Korimoto, Kagoshima, Japan

Takahashi(1998) focused on Hokkaido’s Iburi Hidaka coast and Tokachi Kushiro coastal area to study tsunami water level of time fluctuations and coastal topography. He has performed Fast Fourier Transformanalysis(FFT) based on the water level records at the tide station on thatcoast. And he investigated the occurrence of tsunami edge waves. Thestudy applied a bandpass filter to the water level. Time-series data of
the water level of a single component is calculated from the fluctuation,and the time when the water level reaches its maximum is selected.
Next, by reconstructing the spatial water level distribution, He pointed
out that it generated six stationary edge waves from the recorded waterlevel fluctuations.

ABSTRACTThis study focuses on the water level fluctuations when a wide range ofactive faults move and the primary appearance spectrum calculated
from the changes filtered bandpass filter. Time fluctuations of the tsunami waves include reflected waves along with Miyazaki and Kagoshima prefecture shore areas, which have more extended shallow terrain until offshore than other areas. Previous studies do not sufficientlyinvestigate.

KEY WORDS: Tsunami wave; Bandpass filter; Southern Kyushu
states; Reflected waves; Osumi Peninsula; Shibushi Bay; TanegashimaislandThis study focuses on the water level fluctuations when a wide range ofactive faults moves and the primary appearance spectrum calculated
from the changes filtered bandpass filter. After that, the study examined
the relationship between the maximum wave height arrival time and theshore topography. Finally, the study considered the relationship between the tsunami and the islands offshore due to the reflected waves
from the Osumi Peninsula.

INTRODUCTION
Japan’s Local government has released an estimate of the wave heightand inundation range of earthquake-tsunami originating from the Nankai Trough, a 30-year probability of occurrence exceeding 60%. Thebasis for the values is the second report by the Nankai Trough GiantEarthquake Model Study Group of the Japanese Cabinet Office. Miyazaki Prefecture compared the estimated deals of Case 4 and Case 11,
where the epicenter is located offshore of Miyazaki prefecture in thetsunami model proposed by the study group, and the estimated valuesof the Miyazaki prefecture original model, and determined the maximum value in these values.
Kagoshima Prefecture uses the results of Case 5 and Case 11, where theepicenter location is close to Kagoshima prefecture in the tsunami
model of the study group. It announced the inundation range of thetsunami in the coastal area. Furthermore, the site has the Nansei Islands.
It also investigates the tsunami waves with the epicenter located in theNansei islands trench. Miyazaki and Kagoshima prefectures disclosed
the maximum water level of the tsunami, the tsunami arrival time, andthe estimated inundation range with the Nankai Trough Trench as theepicenter and the entire coastal areas. However, the relationship between the time variation of tsunami water level and coastal topography
in these areas has not been sufficiently investigated.

GEOLOGICAL CONDITIONS
Fig.1 shows the seafloor topography in the calculation area. This figure
uses the Japan Hydrographic Association seafloor topography digital
data M7008 (Minamisatsu), and the water depth interval is 100 m. The
target calculation areas were the coast from the southern coast of Miyazaki Prefecture in Miyazaki City, Miyazaki Prefecture to the eastern
coast of Kagoshima Bay, Kagoshima Prefecture, including the Osumi
Peninsula, and the eastern and western shores of Tanegashima island
off the coast of the Osumi Peninsula. The characteristic of this area isthat the Hinata Basin, with a maximum depth of 700 m, exists 50 km
offshore in a part of the Nankai Trough, and the terrain is shallow witha depth of 200 m from the coastline to just before this trench.

NUMERICAL ANALYSIS OF TSUNAMI WAVE
Gravamen Equations

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