IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS002 Poster presentation 1828
Numerical analysis of tsunami caused by seabed deformation
Dr. Toshinori Ogasawara
Department of Civile and ENvironment
Shigeki Sakai, Akifumi Wakamatsu
The seismic intensity scale of the Meiji Great Sanriku Tsunami of 1896 was small scale but the tsunami
height was very large. The maximum wave height was 32 m. Such an earthquake is called a tsunami
earthquake or a slow earthquake. Note that tsunami heights are not necessarily proportional to
earthquake magnitude. This is one of reasons that the tsunami heights in coast lines cannot be forecast
easily. In the present forecast of tsunami, wave heights are estimated by water surface disturbances
due to the seismic fault model. The model is based on the fault parameters calculated by the observed
seismic wave, which are the length, the depth, and the dip angle etc. The information of the initial wave
profile generated by a seabed deformation is more important for the accuracy of tsunami forecast. This
study makes clear how the physical quantities related to the seabed deformation exert an influence on
the tsunami generation. In particular, the relationships between the physical quantities and the
maximum wave height are described. Here, the physical quantities represent the vertical velocity, the
width, and the shape of the seabed deformation. Numerical analysis on the tsunami generation was
done for various seabed deformations. This numerical method defined a two-dimensional water tank of
fluid domain. The water depth was set to four types from 1,000 m to 4,000 m. Tsunami sources used
the vertical displacement field of sea bottom directly computed by using a time-domain solution
involving the boundary element method. The width of seabed deformation was changed from 20 km to
100 km. The speed of seabed deformation was changed at a constant velocity and the maximum
displacement was set to 3.0 m. The waves generated by the seabed deformation permeate the open
boundary on the both side of tank. As a result, when the seabed deformation was low-speed at 0.01
m/s, the maximum wave heights were proportional to the width of seabed deformation. However, when
the seabed deformation became 100 m, the wave heights were found to almost reach as high as the
seabed deformation. On the other hand, when the seabed deformation was high-speed at 0.3 m/s, if
the seabed deformation width exceeded more than 20 km the wave heights agreed with the seabed
deformation. It is concluded from the result that the width of seabed deformation is strongly required at
the determination of the maximum wave height at tsunami generation. Further, the results show that
the initial wave heights will almost agree with the seabed displacement when the width of the seabed
deformation becomes long.
Keywords: tsunami, seabed deformation, simulation