Production of Brief Extreme Ground Acceleration Pulses by Nonlinear Mechanisms in the Shallow Subsurface

Norman H. Sleep, & Shuo Ma

Published March 8, 2008, SCEC Contribution #1144

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

Citation
Sleep, N. H., & Ma, S. (2008). Production of Brief Extreme Ground Acceleration Pulses by Nonlinear Mechanisms in the Shallow Subsurface. Geochemistry, Geophysics, Geosystems, 9, Q03008. doi: 10.1029/2007GC001863.