Elastic Impact Consequences for High-Frequency Earthquake Ground Motion

Abstract

A fundamental question of earthquake science is what produces damaging high-frequency ground motion, with the classic Brune-Haskell model postulating that abrupt fault slip causes it. However, even when amended with heterogeneous rupture, the model fails to explain observations of different sized repeating earthquakes and has challenges explaining high-frequency radiation patterns. We propose an additional cause for high-frequency earthquake spectra from elastic collisions of structures within a rupturing fault zone. The collision spectrum is set by an impact contact time proportional to the size of colliding structures so that spectra depend on fundamentally different physical parameters compared with slip models. When added to standard models, collisions can reconcile the discrepant observations since the size, shape, and orientation of structures vary between different fault zones but remain constant within a fault segment. High-frequency earthquake ground motions and damage may therefore be an outgrowth of fault-zone structure rather than sudden initiation of slip.