Dynamics, Radiation, and Overall Energy Budget of Earthquake Rupture With Coseismic Off‐Fault Damage

Abstract

Earthquake ruptures dynamically activate coseismic off-fault damage around fault cores.Systematic field observation efforts have shown the distribution of off-fault damage around main faults, while numerical modeling using elastic-plastic off-fault material models have demonstrated the evolution of coseismic off-fault damage during earthquake ruptures. Laboratory-scale microearthquake experiments have pointed out the enhanced high-frequency radiation due to the coseismic off-fault damage. However, the detailed off-fault fracturing mechanisms, subsequent radiation, and its contribution to the overall energy budget remain to be fully understood because of limitations of current observational techniques and model formulations. Here, we constructed a new physics-based dynamic earthquake rupture modeling framework, based on the combined finite-discrete element method, to investigate the fundamental mechanisms of coseismic off-fault damage, and its effect on the rupture dynamics, the radiation and the overall energy budget. We conducted a 2-D systematic case study with depth and showed the mechanisms of dynamic activation of the coseismic off-fault damage. We found the decrease in rupture velocity and the enhanced high-frequency radiation in near field due to the coseismic off-fault damage. We then evaluated the overall energy budget, which shows a significant contribution of the coseismic off-fault damage to the overall energy budget even at depth, where the damage zone width becomes narrower. The present numerical framework for the dynamic earthquake rupture modeling thus provides new insights into earthquake rupture dynamics with the coseismic off-fault damage. Plain Language SummaryThe medium surrounding fault cores can be damaged due to stress concentration caused by the dynamic earthquake ruptures propagating on the faults, which is called coseismic off-fault damage. Systematic field observation efforts have shown the distribution of off-fault damage around main faults, while numerical modeling has demonstrated the evolution of off-fault damage during earthquake ruptures. Laboratory-scale microearthquake experiments have pointed out the enhanced high-frequency radiation due to the off-fault damage. However, the detailed off-fault fracturing mechanisms, subsequent seismic wave radiation and its contribution to the overall energy budget remain to be fully understood. Here, we constructed a new physics-based dynamic earthquake rupture modeling framework to investigate the fundamental mechanisms of coseismic off-fault damage and its effect on the rupture dynamics, the radiation, and the overall energy budget. We found the enhanced high-frequency radiation in near field due to the coseismic off-fault damage. We then evaluated the overall energy budget, which shows a significant contribution of the coseismic off-fault damage to the overall energy budget even at depth. The present numerical framework for the dynamic earthquake rupture modeling thus provides the insight into the earthquake rupture dynamics with the cosei...

Origin of High‐Frequency Radiation During Laboratory Earthquakes

“…This is consistent with seismological observations of mega-thrust subduction earthquakes where zones of highfrequency energy release correspond to deeper portions of the fault (Ishii, 2011). First, the increase of stress concentrations in the process zone with stress conditions and rupture velocity would likely enhance physical processes as off-fault damage (Okubo et al, 2018;Thomas et al, 2017;Thomas & Bhat, 2018) taking place in the vicinity of the rupture front leading to more radiated high-frequency energy. First, the increase of stress concentrations in the process zone with stress conditions and rupture velocity would likely enhance physical processes as off-fault damage (Okubo et al, 2018;Thomas et al, 2017;Thomas & Bhat, 2018) taking place in the vicinity of the rupture front leading to more radiated high-frequency energy.…”

“…We carefully ensured that the back-projection results are reliable and are not manifestations of system noise (see supporting information for details). Recent numerical studies (Okubo et al, 2018;Thomas et al, 2017;Thomas & Bhat, 2018) also demonstrated that high-frequency radiation was highly enhanced when coseismic damage was implemented in their rupture propagation models. The correlation between the spatial and temporal evolution of high-frequency sources and the propagation of the rupture front provides concrete experimental evidence that highfrequency waves are concurrent with the propagation phase of the rupture front and that high-frequency radiation is emitted close to or behind the rupture tip.…”

“…This result is in agreement with most of the studies that addressed the issue of high-frequency radiation which proposed that high-frequency radiation is related to changes in rupture velocity due to fault stress or frictional heterogeneity, and predict high-frequency waves to be mainly generated in the vicinity of the rupture front (Aki, 1967;Haskell, 1964;Madariaga, 1977Madariaga, , 1983Spudich & Frazer, 1984). Recent numerical studies (Okubo et al, 2018;Thomas et al, 2017;Thomas & Bhat, 2018) also demonstrated that high-frequency radiation was highly enhanced when coseismic damage was implemented in their rupture propagation models. This is supported by microscopic analysis of the fault surface after stick-slip experiments under Scanning Electron Microscopy (see supporting information), which revealed the presence of microcracks at the grain scale.…”

“…There are different ways to interpret these results. First, the increase of stress concentrations in the process zone with stress conditions and rupture velocity would likely enhance physical processes as off-fault damage (Okubo et al, 2018;Thomas et al, 2017;Thomas & Bhat, 2018) taking place in the vicinity of the rupture front leading to more radiated high-frequency energy. Also, as the rupture velocity increases, more abrupt acceleration/deceleration phases of the rupture front develop, leading to local slip accelerations which would enhance high-frequency radiation (Hartzell & Heaton, 1983;Olson & Apsel, 1982).…”

The Effects of Earthquakes and Fluids on the Metamorphism of the Lower Continental Crust

“…Field observations (Austrheim et al, ; Petley‐Ragan et al, ; Rempe et al, ) and laboratory measurements (Incel, ; Yuan et al, ) demonstrate that the width of wall rock damage and pulverization (fragmentation with negligible strain) zones become increasingly narrow with increasing depth/pressure. This observation has also been confirmed by models showing the dependence of coseismic off‐fault damage with depth (Okubo et al, ). Near the Earth's surface (e.g., across the San Andreas fault; Rempe et al, ) the damage zone may reach exceed 100 m, with a pulverization threshold at around 50 m from the fault core.…”