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Abstract Shallow slow-slip events (SSEs) contribute to strain release near the shallow portions of subduction interfaces and may contribute to promoting shallow subduction earthquakes. Recent efforts in offshore monitoring of shallow SSEs have provided evidence of possible interactions between shallow SSEs and megathrust earthquakes. In this study, we use a dynamic earthquake simulator that captures both quasi-static (for SSEs) and dynamic (for megathrust earthquakes) slip to explore their interactions and implications for seismic and tsunami hazards. We model slip behaviors of a shallow-dipping subduction interface on which two locally locked patches (asperities) with different strengths are embedded within a conditionally stable zone. We find that both SSEs and earthquakes can occur, and they interact over multiple earthquake cycles in the model. Dynamic ruptures can nucleate on the asperities and propagate into the surrounding conditionally stable zone at slow speeds, generating tsunami earthquakes. A clear correlation emerges between the size of an earthquake and SSE activities preceding it. Small earthquakes rupture only the low-strength asperity, whereas large earthquakes rupture both. Before a large earthquake, periodic SSEs occur around the high-strength asperity, gradually loading stress into its interior. The critically stressed high-strength asperity can be ruptured together with the low-strength one in the large earthquake, followed by a relatively quiet interseismic period with very few SSEs and then a small earthquake. An SSE may or may not directly lead to nucleation of an earthquake, depending on whether a nearby asperity is ready for spontaneously dynamic failure. In addition, because of different SSE activities, the coupling degree may change dramatically between different interseismic periods, suggesting that its estimate based on a short period of observation may be biased. 

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.