Numerical Tsunami Propagation of 1703 Kanto Earthquake

Transcription

1 Numerical Tsunami Propagation of 1703 Kanto Earthquake Wu Yifei Supervisor: Kenji Satake Introduction Near Tokyo Bay, the 1703 Genroku Kanto Earthquake is known to generate a tsunami which hit the nearby coast. Considering the strength of the earthquake and the fault is quite close to Tokyo, we use numerical method to simulate the tsunami generated by the 1703 Genroku Kanto earthquake and see if there are any hazards when such a tsunami recurs. In our procedure, we first obtain ground deformation by Okada s [1] program and then use the deformation as an initial condition to calculate the tsunami propagation. Properties of reverse fault Like the case of 1703 Genroku Kanto Earthquake, most big tsunamis that affect human being are caused by reverse fault. So we first examine the basic properties of reverse fault. Fig 1 and Fig 2 shows cross-section and map view Figure 1 Cross-section of deformation caused by reverse fault (W=15km, Slip=5m, Depth=0m) with different dips. Black line indicates the fault. The deformation is calculated by Okada s program.

2 Of the deformation caused by several reverse faults. Since in our procedure we consider vertical deformation as the cause of tsunami, we will just focus on that, or Uz. The most important thing we can gain is that for those reverse faults with dip not very large, we can observe uplift and subsidence. That makes the initial condition for tsunami propagation. Figure 2 Map view of the deformation caused by a reverse fault (L=85km W=55km Depth=5km Dip=30 Slip=6.7m) Yellow rectangle is the fault whose hanging wall is moving upward in the figure. Red line indicates uplift while blue one indicates subsidence. The contour interval is 0.5m. The deformation is calculated by Okada s program. Tsunami propagation formula [2] The equations for 2D tsunami propagation, including the advection terms, the Coriolis force and bottom friction can be written as follows. Vx Vx V V V V x h V x Vy fvy g C f t x y x d h 2 2 x x y

3 Vy Vy V V V V y h V x Vy fvx g C f t x y y d h h t x y V x h d Vy h d 2 2 y x y In our program, we use a liner model with Coriolis force, which means we do not consider advection and bottom friction terms in the first and second equation and h on the right side in the third equation. Area background The 1703 Genroku Earthquake s fault is near Sagami Trough where Philippine Sea Plate is thrusting under the North American Plate. Figure 3 shows this area. We use the fault model provided by Shishikura [3], although there is still argument about these models. Figure 4 shows the subfault location in Shishikura s model. Figure 3 The study area. Sagami Trough is where Philippine Sea Plate is thrusting under the North American Plate. From Wikipedia.

4 Figure 4 Subfault locations in Shishikura s model. Parameters As mentioned, we use Shishikura s fault model. The fault parameter is listed in table1. Besides, we use JTOPO 1 minute bathymetric data displayed in figure 5. We use a 1 minute grid in our program too. Subfault Longitude Latitude Depth Strike Dip Slip Length Width Slip km deg deg deg km km m A 139.8E 34.85N B 140.4E 34.98N C 141.3E 34.53N Table 1 Fault parameters in Shishikura s model.

5 Figure 5 JTOPO 1 minute bathymetric data. Results The ground deformation calculated by Okada s program using Shishikura s fault model is shown in figure 6. Figure 7 and 8 shows the snapshot of the Figure 6 Calculated ground deformation. Red line indicates uplift while blue line indicates subsidence. The contour interval is 0.5m.

6 tsunami wave and synthetic waveforms in two stations, respectively. From these two results, we can find that although the tsunami is quite large outside Tokyo Bay, it is rather safe inside it. So because of the topography, little tsunami comes into Tokyo Bay when such a tsunami occurs. This is good news for us. Figure 7 Snapshot of tsunami wave of 2 minutes, 10 minutes, 30 minutes and 1 hour after the earthquake. Further work Since we use a liner model with Coriolis force in this study, we can use a non-liner model in a further study and compare the difference between them.

7 Chiba Mera Figure 8 Synthetic wave forms in Mera and Chiba. Reference [1] Yoshimitsu Okada. (1985). Surface deformation due to shear and tensile faults in a half space, Bull. Seism. Soc. Am. 75, [2] Kenji Satake's lecture note. (Not published) [3] Kenji Satake, Masanobu Shishikura, Yuichi Namegaya, Ryotaro Fuji and Hitoshi Takeuchi. (2008). Fault models of the Genroku (1703) Kanto Earthquake and Tsunami along the eastern coast of Boso Peninsula, Historical Earthquakes 23, (in Japanese with English abstract). Acknowledgement I discussed some problems with Murotani san and Kutsumoto san during the study and they gave me some ideas. Murotani san also helped me with these programs in this study. Besides, all Satake lab s member helped me a lot. And UTRIP program gave me a chance to have such a wonderful experience.