Production of brief extreme ground acceleration pulses by nonlinear mechanisms in the shallow subsurface

摘要: [1] A brief pulse of extreme acceleration ∼20 m s−2 was recorded at Station FZ16 during the 2004 Mw 6.0 Parkfield, California, earthquake. The sustained dominant frequency is a factor ∼2 below maximum. Here we show that pulses might originate in shallow subsurface, rather than on main fault plane. Two nonlinear mechanisms can feasibly convert low-frequency energy incident wave into high-frequency with acceleration: (1) Dynamic stress carried by strong seismic waves causes small cracks to slip secondary earthquakes. (2) elasticity subsurface nonlinear. relationship between dynamic shear and strain linear strains. At large strains associated ground motion, deformation locks compliant bringing their asperities contact. locking increases instantaneous modulus cracked rock. Computations indicate both produce high accelerations later part motion. These testable hypotheses imply are likely not property hence harbingers prolonged acceleration.