Research Projects

What Controls the Delayed Dynamic Triggering?

Dynamic earthquake triggering often involves a time delay relative to the peak stress perturbation. In this study, we investigate the physical mechanisms responsible for delayed triggering.

We compute detailed spatiotemporal changes in dynamic and static Coulomb stresses at the 2019 Mw 7.1 Ridgecrest mainshock hypocenter, induced by the Mw 5.4 foreshock, using 3D dynamic rupture models (using SeisSol). The computed stress changes are used to perturb 2D quasi-dynamic models of seismic cycles on the mainshock fault governed by rate-and-state friction (using Tandem).

Our results imply that the Ridgecrest mainshock fault was already on the verge of runaway rupture prior to the Mw 5.4 foreshock. These results highlight the contribution of both foreshocks and aseismic deformation to earthquake triggering and emphasize the importance of considering the physics of fault-system-wide processes when assessing triggering potential.

For more information, check out our preprint!

Yun et al., submitted

Effects of Stress and Friction Heterogeneity on Slip Complexity

Numerical and laboratory models of earthquake cycles on faults governed by rate-and-state friction often show cycle-invariant behavior, while natural faults exhibit considerable variability in slip history. Possible explanations include heterogeneities in fault stress and frictional properties. We investigate how various types of heterogeneity in simulations of quasi-dynamic sequences of seismic and aseismic slip affect rupture complexity, hypocenter location, and slow slip events (SSEs).

We model a 2D vertical strike-slip fault and study the roles of self-affine fractal heterogeneities in normal stress, rate-and-state parameter (a-b), and characteristic slip-weakening distance, as well as the effects of a low-rigidity fault zone. We find that only a combination of heterogeneous parameters introduces variability in the modeled rupture extent, hypocenter depth, and recurrence interval. In particular, variable hypocenter depths require velocity-strengthening patches within the velocity-weakening seismogenic zone. A low-rigidity fault zone can encourage pulse-like ruptures but adds little to slip complexity. Slip law simulations produce fewer partial ruptures, smaller stress drops, and lower peak slip rates compared to aging law simulations. We also show that the ratio of the seismogenic zone thickness to nucleation size does not entirely predict slip complexity.

For more information, check out our preprint!

Spatial Variation of Lg wave Q value in Central California

Lg is a seismic phase that can only propagate on the continental path, making it a powerful tool for imaging continental seismic structures. Lg wave is also useful for constraining the quality factor Q because it can be easily identified even for small-magnitude events owing to its large amplitude. In this study, we investigate lateral variation and frequency dependence of crustal Lg Q along a great circle profile in the westernmost part of the US.

We compute relative site responses at each station using the reverse two-station method. The computed site term is removed to obtain a site-response-corrected Lg Q model based on the two-station. Traditional methods have limitations in the depth resolution in that they can only image crustal average properties. We found that the removal of the site response in Q models allows the models to have a depth sensitivity.

Our modeled Q = (81 ± 8) f (0.62 ± 0.11) reflects the active tectonic setting and the presence of fluids in the region. A change in lithology correlates well with the trend of the Lg Q values. Site responses mostly correlated with surficial lithology along the profile rather than the thickness of the sediments, and exhibit a strong negative correlation to the VS30 data.

For more information, check out Yun et al. (2022)

Yun et al. (2022)

Page updated

Google Sites

Report abuse