 |  |  | 9.2.2 Role of Fracture Energy |
9.2.2 Role of Fracture Energy
We switch the friction model from the slip-weakening friction model to
the two-phase friction model, while applying the same initial
tractions. This coincides with decreasing the characteristic slip
distance to zero in the slip-weakening friction model. However, the
discrete nature of the finite-element model does provide some inherent
or effective fracture energy. Upon initiation of sliding the
coefficient of friction immediately drops to its minimum value, and no
energy is required for fracture.
Figure 9.6 shows the slip time
histories at a depth of 8.0 km at the left and right quarter
points of the fault (labeled LQP and RQP in
figure 9.1); the left quarter point lies at the
center of the asperity used to initiate the rupture. As expected with
the instantaneous drop in friction, the slip quickly accelerates
inside the asperity. With no fracture energy, the rupture propagates
with an approximate rupture speed in the direction of slip of
4.9 km/sec compared to the shear wave speed of
3.3 km/sec, or nearly three times faster than the rupture with
slip-weakening friction. At the right quarter point, the slip rate
exhibits only a minor increase when we remove the fracture energy. At
both locations, slip occurs in closely spaced, multiple events due to
the introduction of numerical noise associated with the sharp
initiation of slip and the inability of the model to accurately handle
frequencies above 0.5 Hz. Thus, we confirm our intuition
outlined in the discussion of the fracture energy and rupture speed in
section 8.3.2; the fracture energy displays a
strong influence on the speed of the rupture.
Brad Aagaard, October 5, 2000
| | | 9.2.2 Role of Fracture Energy |
