Dynamic stress evolution during earthquake rupture contains information of fault frictional behavior that governs dynamic rupture propagation. Most of earthquake stress drop and evolution studies are based on kinematic slip inversions. Several dynamic inversion methods in the literature require dynamic rupture modeling that makes them cumbersome with limited applicability. In this study, we develop a fault-stress model of earthquake sources in the framework of the representation theorem. We then propose a dynamic stress inversion method based on the fault-stress model to directly invert for dynamic stress evolution process on the fault plane by fitting seismic data. In this inversion method, we calculate numerical Green’s function once only, using an explicit finite element method EQdyna with a unit change of shear or normal stress on each subfault patch. A linear least-squares procedure is used to invert for stress evolution history on the fault. To stabilize the inversion process, we apply several constraints including zero normal slip (no separation or penetration of the fault), non-negative shear slip, and moment constraint. The method performs well and reliably on a synthetic model, a checkerboard model and the 2016 Mw 5.0 Cushing (Oklahoma) earthquake. The proposed fault-stress model of earthquake sources with inversion techniques such as one presented in this study provides a new paradigm for earthquake source studies using seismic data, with a potential of deciphering more physics from seismic recordings of earthquakes.

Megathrust earthquakes with a wide along-dip rupture extent show clearly depth-dependent variations in rupture characteristics such as rupture velocity, frequency contents of seismic radiation and slip distribution. Some recent studies propose that heterogeneous upper-plate rigidity determines this phenomenon, though along-dip variations in fault friction have long been thought to play a dominant role. In this study, we use dynamic rupture modeling to explore and compare roles of these two factors in depth-dependent rupture characteristics of megathrust earthquakes along a shallow-dipping subduction plane that is governed by the rate- and state- dependent friction. We find that an updip transition from velocity-weakening behavior downdip to velocity-strengthening behavior near the trench suppresses rupture propagation toward the trench and a thicker transition zone results in a more confined slip at depth. The updip transition in velocity-dependent frictional property also dominates high-frequency depletion in seismic radiation at shallow depth. With an addition of a conditionally stable zone at shallow depth, rupture velocity significantly decreases, resulting in longer rupture duration as the thickness of the conditionally stable zone increases. The low-velocity layers in the upper plate at shallow depth lead to a more compliant prism and thus significantly higher total slip near the trench. Although they place some limits to rupture velocity at shallow depth, they enhance high-frequency radiation and thus do not contribute to high-frequency depletion observed in recent megathrust earthquakes. We conclude that fault friction plays more important roles than upper-plate rigidity in determining depth-dependent rupture characteristics of megathrust earthquakes.

Tsunami earthquakes are a type of shallow subduction zone events that rupture slowly (<1.5 km/s) with exceptionally long duration and depleted high frequency radiation, resulting in a large discrepancy of Mw and Ms magnitudes and abnormally large tsunami along coastal areas. Heterogeneous fault frictional properties at shallow depth have been thought to dominate tsunami earthquake generation. Some recent studies propose heterogeneous upper-plate material properties determine rupture behavior of megathrust earthquakes, including characteristics of tsunami earthquakes. In this study, we use a recently developed dynamic earthquake simulator to explore tsunami earthquake generation and systematically examine roles of upper-plate material properties and fault frictional properties in tsunami earthquake characteristics in a physics-based framework. For heterogeneous fault friction, we consider isolated asperities with strongly velocity-weakening properties embedded in a conditionally stable zone with weakly velocity-weakening properties. For heterogeneous upper-plate properties, we consider a generic depth profile of seismic velocity and rigidity constrained from seismic surveys. We design a set of models to explore their effects on tsunami earthquake generation and characteristics. We find that the conditionally stable zone can significantly slow down rupture speeds of earthquakes that nucleate on asperities to be < 1.5 km/s over a large depth range (1-20 km), while heterogeneous upper-plate properties can only reduce rupture speeds to be ~1.5-2.0 km/s over a narrow depth range (1-3km). Nevertheless, heterogeneous upper-plate properties promote cascading rupture over multiple isolated asperities on the shallow subduction plane, contributing to large tsunami earthquake generation. We also find that heterogeneous friction dominates normalized duration and high-frequency depletion in tsunami earthquakes. In addition, the effective normal stress on the subduction plane, which affects fault frictional strength, significantly influences the characteristics of tsunami earthquakes, including long normalized duration and low stress drop.

An M5 earthquake occurred on November 7th, 2016, near the city of Cushing in Oklahoma, the largest crude oil storage site in the USA, after nearby disposal wells had been shut-in responding to three M4+ earthquakes in 2015. In this study, we investigated the rupture process of these M4+ events with finite fault model (FFM) inversions and computed Coulomb stress changes during this Cushing sequence. We found that the rupture processes of the four M4+ earthquakes are very complex, and they appeared to trigger one another, as evidenced by the inverted finite fault slip distribution and the calculated Coulomb stress change after each event. The foreshocks of the first M4 earthquake are probably triggered by Coulomb stress changes from previous earthquakes during 2014 and 2015 on unmapped faults several kilometers to the south. Fluid diffusion likely drives the bilateral seismic migration of the Cushing earthquake sequence after the foreshocks were triggered. In addition, fluid injection from the northwest of Cushing fault might have gradually increased the pore pressure on the Cushing fault, making the shallow part of the fault critically stressed.