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SUMMARY Observations of historical tsunami earthquakes reveal that ruptures of these earthquakes propagate slowly at shallow depth with longer duration, depletion in high-frequency radiation and larger discrepancy of Mw–Ms than ordinary megathrust earthquakes. They can effectively generate tsunami and lead to huge damage to regional populated areas near the coast. In this study, we use a recently developed dynamic earthquake simulator to explore tsunami earthquake generation from a physics-based modelling point of view. We build a shallow-dipping subduction zone model in which locally locked, unstable patches (asperities) are distributed on a conditionally stable subduction interface at shallow depth. The dynamic earthquake simulator captures both quasi-static and dynamic processes of earthquake cycles. We find that earthquakes can nucleate on these asperities and propagate into the surrounding conditionally stable zone at slow speeds, generating tsunami earthquakes. A high normal stress asperity, representing a subducted seamount, can act as an asperity in some events but as a barrier in other events over multiple earthquake cycles. Low normal stress asperities typically act as asperities in tsunami earthquakes. The degree of velocity-weakening in the conditionally stable zone, which may sustain rupture at different speeds or stop rupture, is critical for tsunami earthquake generation and affects its recurrence interval. Distributed asperities may rupture in isolated events separated by tens of years, or in a sequence of events separated by hours to days, or in one large event in a cascade fashion, demonstrating complex interactions among them. The recurrence interval on a high normal stress asperity is much larger than that on low normal stress asperities. These modelling results shed lights on the observations from historical tsunami earthquakes, including the 1994 and 2006 Java tsunami earthquakes and 2010 Mentawai tsunami earthquake. 

Abstract Understanding mechanical conditions that lead to complexity in earthquakes is important to seismic hazard analysis. In this study, we simulate physics‐based multicycle dynamic models of the San Andreas fault (Carrizo through San Bernardino sections) and the San Jacinto fault (Claremont and Clark strands). We focus on a complex fault geometry based on the Southern California Earthquake Center Community Fault Model and its effect over multiple earthquake cycles. Using geodetically derived strain rates, we validate the models against geologic slip rates and recurrence intervals at various paleoseismic sites. We find that the interactions among fault geometry, dynamic rupture and interseismic stress accumulation produce stress heterogeneities, leading to rupture segmentation and variability in earthquake recurrence. Our models produce earthquakes with rupture extents similar to a recent comprehensive paleoseismic catalog. The “earthquake gates” of the Big Bend and the Cajon Pass occasionally impede dynamic ruptures. The angle of compression, which is the subtraction of the maximum shear strain rate direction from the local fault strike, can better determine the likelihood of the impedance of restraining bends to dynamic ruptures. Because the Big Bend has an angle of compression of ∼20°, ruptures that traverse the Big Bend, like the 1857 Fort Tejon earthquake, are more frequent than expected based on empirical relations which predict the ∼40° restraining bend to terminate most ruptures. Our models indicate that large ruptures tend to initiate north of the Big Bend and propagate southwards, similar to the 1857 earthquake, providing critical information for ground shaking assessment in the region. 

ABSTRACT Large earthquakes on strike-slip faults often rupture multiple fault segments by jumping over stepovers. Previous studies, based on field observations or numerical modeling with a homogeneous initial stress field, have suggested that stepovers more than ∼5  km wide would stop the propagation of rupture, but many exceptions have been observed in recent years. Here, we integrate a dynamic rupture model with a long-term fault stress model to explore the effects of background stress perturbation on rupture propagation across stepovers along strike-slip faults. Our long-term fault models simulate steady-state stress perturbation around stepovers. Considering such stress perturbation in dynamic rupture models leads to prediction of larger distance a dynamic rupture can jump over stepovers: over 15 km for a releasing stepover or 7 km for a restraining stepover, comparing with the 5 km limit in models with the same fault geometry and frictional property but assuming a homogeneous initial stress. The effect of steady-state stress perturbations is stronger in an overlapping stepover than in an underlapping stepover. The maximum jumping distance can reach 20 km in an overlapping releasing stepover with low-static frictional coefficients. These results are useful for estimating the maximum length of potential fault ruptures and assessing seismic hazard.