Quantifying the Role of 3D Fault Geometry Complexities on Slow and Fast Earthquakes

Traditional models of slow slip events (SSEs) often oversimplify fault geometry, yet imaging studies show that real subduction faults are segmented and complex. We investigate how fault interactions influence slip behavior using 3D quasi-dynamic earthquake sequence simulations of two parallel faults with uniform rate-weakening friction, accelerated with hierarchical matrices. Our results identify four slip regimes-periodic earthquakes, coexisting SSEs and earthquakes, only SSEs, and complex sequences-while a single planar fault under the same conditions produces only earthquakes. We quantify fault interaction using the maximum Coulomb stress induced on a target fault by unit, spatially uniform stress drop on a neighboring fault. Because the source stress drop is normalized, the metric depends only on geometry and is independent of friction and nucleation length, and it can be extended to arbitrary fault configurations. The occurrence of SSEs is confined to an intermediate range of interaction strength. We also reproduce the observed moment-duration scaling and show that it depends on event detection thresholds. These results demonstrate that complex fault geometry can naturally generate both slow and fast earthquakes through evolving traction heterogeneities.

💡 Research Summary

The paper addresses a fundamental gap in earthquake modeling: most existing simulations of slow slip events (SSEs) treat faults as simple, planar surfaces, whereas geophysical imaging shows that subduction zones consist of segmented, non‑planar, and often parallel fault strands. To explore how such three‑dimensional (3‑D) geometry influences slip behavior, the authors perform quasi‑dynamic (QD) earthquake‑sequence simulations of two parallel faults that share identical rate‑weakening friction parameters and are driven by a uniform far‑field loading rate. The numerical framework is accelerated with hierarchical matrices (H‑matrices), enabling high‑resolution 3‑D calculations at reasonable computational cost.

A central contribution is the definition of a geometry‑only interaction metric, the “maximum Coulomb stress induced on a target fault by a unit, spatially uniform stress drop on a neighboring fault” (Δσ_C). By normalizing the source stress drop, Δσ_C depends solely on the relative positions, orientations, and depths of the two faults; it is independent of frictional parameters (a‑b), initial shear stress, or nucleation length. Consequently, Δσ_C can be evaluated for any arbitrary fault configuration and serves as a universal measure of fault‑to‑fault coupling.

Systematic variation of Δσ_C reveals four distinct slip regimes:

1. Negligible interaction (Δσ_C≈0): The faults behave independently, each producing only regular, fast earthquakes. This reproduces the behavior of a single planar fault under the same loading conditions.

2. Moderate interaction (Δσ_C≈0.1–0.3): A fast earthquake on one fault generates enough Coulomb stress on the neighbor to trigger a co‑existing SSE. The system exhibits mixed seismicity—fast events punctuate periods of slow slip.

3. Optimal interaction (Δσ_C≈0.3–0.5): Interaction is strong enough to suppress dynamic rupture entirely, leading to periodic, purely slow slip on both faults. The moment–duration scaling in this regime follows a near‑linear relationship (M∝T), consistent with many GPS‑observed SSEs.

4. Strong interaction (Δσ_C>0.5): Excessive stress transfer creates complex, cascading sequences where events of varying magnitude and duration overlap. Small earthquakes may be hidden within larger slow‑slip transients, and the classic moment–duration power law breaks down.

The authors further demonstrate that the apparent moment–duration scaling is highly sensitive to the detection threshold used in data analysis. Low thresholds that include small earthquakes produce a steep, quadratic scaling (M∝T²), while high thresholds that filter out fast events reveal the linear scaling characteristic of pure SSEs. This finding reconciles disparate scaling laws reported in seismological versus geodetic studies.

Overall, the study shows that 3‑D fault geometry alone can generate the full spectrum of observed slip phenomena—periodic earthquakes, co‑existing earthquakes and SSEs, pure SSEs, and complex mixed sequences—without invoking heterogeneous friction or external stress perturbations. The results underscore the importance of incorporating realistic fault architecture into seismic hazard models, especially in subduction settings where parallel fault strands are common. Future work should extend the interaction metric to networks of many faults, explore non‑uniform frictional properties, and test the robustness of the regimes under variable loading rates. Nonetheless, this paper provides a clear, physics‑based framework for linking fault geometry to the coexistence of slow and fast earthquakes.