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 M_w 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.