Fluid Pressure‐Triggered Foreshock Sequence of the 2008 Mogul Earthquake Sequence: Insights From Stress Inversion and Numerical Modeling

Abstract

In Mogul west of Reno, Nevada, USA, in late February 2008 an earthquake sequence occurred that culminated in a magnitude 4.9 mainshock after a foreshock-rich period lasting approximately 2 months on previously unidentified fault structures. In this article, we show that the foreshock sequence may have been driven by a fluid pressure intrusion. We use 1,082 previously calculated earthquake focal mechanisms to infer the local stress field as well as 1,408 relocated foreshock events to determine the required excess fluid pressure field in the source region of the Mogul earthquake sequence to trigger these events. A model of nonlinear pore pressure diffusion is used to model the fluid flow in a highly fractured subsurface. We find a strong correlation between high fluid pressure fronts and foreshock hypocenters, suggesting a natural fluid-driven earthquake sequence.

Evidence of Aseismic and Fluid‐Driven Processes in a Small Complex Seismic Swarm Near Virginia City, Nevada

“…Nearby, the previously described 2008 Mogul swarm showed evidence of both fluid migration and aseismic slip (Bell et al, ; Ruhl, Abercrombie, et al, ). While the Virginia City 2014 sequence did not involve any injected fluids, there is no reason that natural geothermal fluids from depth could not lead to the same manifestation of diffusivity and aseismic slip (e.g., Jansen et al, ). Poroelastic triggering, seen in induced seismicity studies (Deng et al, ), is another possible mechanism that could explain the sequence development.…”

Fault valving and pore pressure evolution in simulations of earthquake sequences and aseismic slip

“…ault shear strength τ = f × (σ − p) is controlled by both friction coefficient f and effective normal stress σ − p, the difference between compressive total normal stress σ and pore pressure p. Much attention in the earthquake modeling community has been placed on friction over the past decades, with specific focus on rate-and state-dependent effects that control the stability of sliding, as well as additional dynamic weakening processes that are likely relevant at coseismic slip velocities. With some exceptions [1][2][3][4][5][6][7][8][9] , less attention has been placed on pore pressure dynamics, and most earthquake simulations use pore pressure (or really effective stress) as a tuning parameter chosen to produce reasonable stress drops and slip per event 10,11 .…”

Partitioned Fault Movement and Aftershock Triggering: Evidence for Fault Interactions During the 2017 Mw 5.4 Pohang Earthquake, South Korea

“…Direct evidence of the hypothesized fluid reservoir at depth is still lacking. The aftershock triggering observed in the northern source area (i.e., the fault intersection of F N and F M ) requires detailed studies such as stress drop estimation (e.g., Chen et al, 2012; Ruhl et al, 2017) and critical fluid pressure analysis (e.g., Jansen et al, 2019; Yeo et al, 2020) from the perspective of fluid‐driven earthquake sequences. This would be a valuable theme for future studies on the Pohang earthquake sequence, as well as on the general mechanism of fluid‐faulting interactions.…”