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Fault complexity has been linked to high‐frequency earthquake radiation, although the underlying physical mechanisms are not well understood. Fault complexity is commonly modeled with rough single faults; however, real‐world faults are additionally complex, existing within networks of other faults. In this study, we introduce two new ways of defining fault complexity using mapped fault traces, characterizing fault networks in terms of their degree of alignment and density. We find that both misalignment and density correlate with enhanced high‐frequency seismic radiation across Southern California, with misalignment showing a stronger correlation. This robust correlation suggests that high‐frequency radiation may arise in part from fault‐fault interactions within networks of misaligned faults. Fault‐fault interactions may therefore have important consequences for earthquake rupture dynamics, energetics and earthquake hazards and should not be overlooked. Plain Language Summary: The faults on which earthquakes occur sometimes form complex interconnected patterns. The level of this complexity may promote higher frequency ground motions coming from earthquakes occurring on such faults. We describe ways of quantifying the complexity of groups of faults based on how they are aligned and how densely they are spaced. We find that high‐frequency ground motions in Southern California tend to correlate with misaligned faults, suggesting that structural interactions between different parts of the fault system may play a role in generating the ground motions felt during earthquakes. Future work involving rupture simulations, ground motion modeling, and hazards assessments in complex fault geometries should consider the effects of these structural interactions. Key Points: We introduce two new metrics quantifying the complexity of interacting faultsIn Southern California, enhanced high‐frequency seismic radiation is associated with earthquakes on misaligned faultsHigh‐frequency seismic radiation may thus be produced in part by fault‐fault interactions [ABSTRACT FROM AUTHOR]

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 Summary: The 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 coseismic off‐fault damage. Key Points: Earthquake ruptures dynamically activate coseismic off‐fault damage, whose feedback plays an important role in rupture dynamicsWe show the mechanism of dynamically activated off‐fault fractures and its effect on rupture velocity and enhanced high‐frequency radiationThe contribution of off‐fault damage to the overall energy budget associated with earthquakes is nonnegligible even at depth [ABSTRACT FROM AUTHOR]

We monitor dynamic rupture propagation during laboratory stick‐slip experiments performed on saw‐cut Westerly granite under upper crustal conditions (10–90 MPa). Spectral analysis of high‐frequency acoustic waveforms provided evidence that energy radiation is enhanced with stress conditions and rupture velocity. Using acoustic recordings band‐pass filtered to 400–800 kHz (7–14 mm wavelength) and high‐pass filtered above 800 kHz, we back projected high‐frequency energy generated during rupture propagation. Our results show that the high‐frequency radiation originates behind the rupture front during propagation and propagates at a speed close to that obtained by our rupture velocity inversion. From scaling arguments, we suggest that the origin of high‐frequency radiation lies in the fast dynamic stress‐drop in the breakdown zone together with off‐fault coseismic damage propagating behind the rupture tip. The application of the back‐projection method at the laboratory scale provides new ways to locally investigate physical mechanisms that control high‐frequency radiation. Plain Language Summary: Over geological time scales, partially or fully locked tectonic plates accumulate stress and strain. The stress and the strain build up on discontinuities that we call "faults." Natural faults exist either inside a tectonic plate or at the boundary between two distinct tectonic plates. When the stress accumulated on a fault exceeds the strength of the fault, the accumulated stress and strain, which can be interpreted in term of accumulated energy, are suddenly released. This natural phenomenon is called an "earthquake." During an earthquake, part of the energy is released in the form of seismic waves. Those seismic waves are responsible for the ground shaking. High‐frequency waves usually cause most of the damage. To better understand the physical parameters that influence the generation of high‐frequency waves, we experimentally reproduced microearthquakes and used them as a proxy to study real earthquakes. Our results showed that the higher the pressure acting on the fault when an earthquake is generated, the higher the amount of high‐frequency radiations. Moreover, our observations underlined that, during an earthquake, high‐frequency waves are released in specific areas on the fault. Thus, these results might be of relevance to improve seismic hazard assessment. Key Points: High‐frequency radiation is enhanced with both confining pressure and rupture velocityAcoustic sensors can be used as an array to track high‐frequency sources during rupture propagationHigh‐frequency radiation sources propagate consistently with the rupture front and are located behind it [ABSTRACT FROM AUTHOR]