4-9 Large-scale 3D simulation of seismic wave propagation and strong ground motions using the Earth Simulator

 

The Earth Simulator at JAMSTEC Yokohama Institute is currently the fastest machine in the world. The massive vector-parallel computer of 5120 vector processors connecting to a fast crossbar network achieves peak performance of 40Tflops. With the use of the Earth Simulator, it is now possible to conduct large-scale 3D simulation of seismic wave propagation in heterogeneous crust and upper-mantle structure.

 

4-9-1 Numerical Simulation for the Nankai-Trough Megathrust Earthquakes

 

Figure 1 illustrates the snapshot of simulated ground motions for the 1944 Tonankai (M8) and the 1946 Nankai (M8) earthquakes (the purple zones correspond to the fault-rupture areas) derived from the parallel FDM simulation using the Earth Simulator. The simulation model assumes that both earthquakes occur simultaneously from Shiono-misaki, and the fault ruptures run bilaterally along the Nankai trough. The large seismic energy at ground velocity over 10cm/s is clearly found in the snapshots, which cause larger seismic intensities of over 5 around the fault-rupture area. For the simulation, the subsurface structure model for western Japan was divided into 2.6 million grid points with variable grid interval of 0.6 to 2.4 km. The computation took a memory of 64Gbyte and a wall-clock time of 1 hour by parallel computing using 128 processors of the Earth Simulator.

Fig.1 Snapshots of horizontal ground velocity motion during the Nankai Trough scenario earthquake.　

 

4-9-2 Numerical Simulation for the 1993 Kushiro Earthquake and Intensity Anomalies

 

Figure 2 displays the pattern of ground motion intensity (seven point JMA scale) from the 1993 Kushiro M7.8 Earthquake. The intensity pattern for the earthquake shows abnormal extension of contours from Hokkaido to Tohoku along the eastern coast of the Pacific Sea. The abnormal isoseismic pattern is usually found during deeper plate earthquakes, because the seismic waves can propagate longer distances in the high-Q (low attenuation) plate than in the surrounding low-Q mantle. The result of the 3D simulation using a high-resolution structural model including high-velocity/high-Q Pacific Plate and lower-velocity/low-Q mantle wedge in northeastern Japan and a high-frequency (2 Hz) double-couple point source clearly demonstrates the pattern of extended seismic intensities for the earthquake. The 3D simulation of 3.2 billion grid-points model required computer memory of 386Gbyte and a wall-clock time of 2 hours by parallel computing using 1024 processors of the Earth Simulator.

Fig.2 (a) Intensity pattern of the 1993 Kushiro Earthquake and (b) simulation result. The structural model is shown bottom.