Realistic Model:  We found previously that the energy of a rupture growing through a region of high strength (a barrier) becomes intensely focused within the obstacle. For the simple model of a shear crack growing with an initially straight front, the energy concentration is sufficient to push the rupture front ahead of its unperturbed position at velocities greater than the shear wave speed. To test whether or not this could occur during a real earthquake, we simulated rupture on a finite rectangular fault dropping vertically about 15km below the surface of the earth. The length of the fault is about 40km, making this a magnitude 6-7 earthquake.

Our results are shown in the two movies below. The first gives two views of the entire earthquake process from the same perspective. You can see the earth's surface with the rectangular fault cutting beneath it. The earth's surface shakes in response to the seismic waves generated by the rupture. What we show is the vertical motion of the surface, obviously magnified in scale. In the top view, the earth's surface is darkly shaded to emphasize ground motion; in the bottom view, it is transparent to emphasize the rupture process. The color scale on the fault denotes how fast the two sides of the fault are sliding past each other (hot colors for fast sliding, cold for slow). Most of the ground motion comes from the rupture front. After the rupture breaks through the barrier, it is moving at a supershear velocity and strong seismic wave pulses can be seen traveling along the fault surface.

The second movie zooms in close to explore the rupture process. After initially being delay by the strong barrier, the rupture front encircles it and breaks it from all sides. Energy concentration is reflected in the large sliding velocities between the two sides of the fault (the bright flash when it breaks). Afterwards, it is easy to see the distinctive seismic wave pulses traveling radially outward from the barrier; these create further sliding between the two sides of the fault and allow for energy to be efficiently transported to the surface. Furthermore, the rupture front shoots forward when some of the energy catches up to it, making it move faster than the shear wave speed.

Questions? E-mail Eric Dunham