Figure 10. Total slip distribution normalized to Scenario 1: (a)
Scenario 2, (b) Scenario 3, (c) Scenario 4, (d) Scenario 5. The contour
interval is 0.2.
In subduction zone earthquakes where largest coseismic slip concentrated
near the trench, such as in the 2011 Mw 9.0 Tohoku-Oki earthquake (Ide
et al., 2011), the friction and the rigidity may be close to our
Scenario 5 (homogeneous friction and heterogeneous velocity structure),
though some thin layer of velocity strengthening may exist, as proposed
by Kozdon and Dunham (2013) and Lotto et al. (2017). While some
subduction zones exhibit a rupture propagation barrier near the trench
such as the 2010 Mw 8.8 Maule earthquake (Lin et al.,
2013), we expect that depth-varying friction plays a dominant role,
which is similar to our Scenarios 3 and 4 (heterogeneous friction and
homogeneous velocity structure), or considering a realistic upper-plate
rigidity (e.g., Sallares & Ranero, 2019), closer to our Scenario 2
(heterogeneous friction and heterogeneous velocity structure).
We address the effects of heterogeneous velocity structure, in
particular the low-velocity layer in the shallow portion in Scenarios 2
and 5. An updip low-velocity zone is equivalent to a compliant
accretionary prism, which yields a larger slip near the trench. This
observation is consistent with the results reported by Lotto et al.
(2017). Although we do not focus on varying a – b in the
unstable regime, we agree with Lotto et al. (2017) that a more
velocity-weakening friction enhances final overall slip, in that a more
velocity-weakening prism induces a larger stress drop (equation (1)) and
results in a larger total slip. In addition, the wall rock in our
numerical simulations is elastic. We remark that plastic yielding in a
compliant accretionary prism can slow down rupture propagation and
enhance seafloor displacement, as reported by Ma (2012) and Ma and
Hirakawa (2013).
This study examines and compares roles of depth-varying fault friction
and heterogeneous upper-plate material properties in depth-dependent
rupture characteristics of megathrust earthquakes that rupture the
entire seismogenic zone. In a separate study, Meng and Duan (2022)
explore roles of heterogeneous fault friction and heterogenous
upper-plate material properties in rupture characteristics of tsunami
earthquakes that occur on shallow portions of subduction planes and
generate abnormally large tsunami waves. In their heterogeneous fault
friction models, they introduce asperities (unstable patches) with
strongly velocity-weakening friction properties embedded in a weakly
velocity-weakening conditionally stable zone. Their findings corroborate
our results obtained in this study, including (1) the dominant roles of
fault friction in slow rupture speed (and thus long rupture duration)
and high-frequency depletion at shallow depth and (2) heterogeneous
upper-plate material properties mainly contributing to large slip near
the trench.
5 Conclusions
We design five rupture scenarios to quantify the effects of
depth-varying fault friction and heterogeneous upper-plate rigidity on
dynamics of megathrust earthquakes. Our numerical simulations on rupture
scenarios reveal that the updip transition from velocity-strengthening
behavior near the trench to velocity-weakening behavior downdip
suppresses rupture propagation toward the trench and a thicker
velocity-strengthening layer results in a more confined total slip at
depth. With employment of a conditionally stable layer, total slip and
rupture velocity significantly decreases, resulting in a longer rupture
duration as the thickness of the conditionally stable layer increases.
As the low-velocity zone leads to a more compliant medium near the
trench, total slip is significantly higher in the scenarios with
low-velocity upper-plate layers. Slip rate history and its frequency
content show that depth-varying fault friction dominates high-frequency
depletion at shallow depth, whereas depth-varying rigidity enhances
high-frequency radiation. We conclude that fault friction plays more
important roles than wall-rock properties in depth-dependent rupture
characteristics of megathrust earthquakes.