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.