Figure 7. Slip rate and the corresponding frequency content for
Scenario 3 (a and b) and Scenario 4 (c and d). Frequency contents at 8.3
km (e) and at 22.0 km (f) are compared among these two scenarios and
Scenario 1.
3.3 Roles of depth-varying upper-plate rigidity
We examine the role of depth-varying upper-plate rigidity in Scenario 5
(heterogeneous velocity structure and homogeneous friction with Lof 0 and d of 0). Similar to Scenario 1, Scenario 5 exhibits a
large stress drop up to the trench (Figure 8a) and the large slip is
concentrated near the trench (Figure 8b). In particular, this scenario
produces a maximum slip > 50 m at the trench (Figure 8b),
which is much larger than all other scenarios, including Scenario 1.
This result is intuitive because as the wall rock becomes less rigid,
the trenchward portion becomes more compliant. Thus, more slip is
generated under the same amount of stress drop. Comparing with the other
scenarios, this scenario suggests that low-velocity rock layers in the
upper plate dominates total amount of shallow slip, if the shallow
portion of a subduction plane is velocity-weakening.
The rupture propagation features show that, except for the initial
increase in rupture velocity at the deep part of the subduction plane
and the trench portion, rupture velocity generally ranges from 2-3 km/s
(Figure 8d and 8e). The near-trench rupture velocity significantly
exceeds Vs in both hanging wall and footwall
(i.e., supershear rupture), indicating that an employment of low-
velocity upper plate layers do not fully cap the rupture velocity, due
to the effects of free surface and shallow-dipping fault geometry.
Comparing with other scenarios (e.g., rupture velocity along the central
profile in all scenarios), we can find that the upper-plate low-velocity
layers contribute significantly to slow rupture at a narrow range of
shallow depth (e.g., 1-3 km depth).