3D eight-order elastic finite-difference modeling of refraction and strong-motion data from the Coyote Lake region, California

Abstract

Abstract 3D elastic modeling of (vertical component, explosive source) refraction data is performed to constrain the shallow part of a velocity model, followed by 3D modeling of three-component strong-motion data from a (spatially and temporally extended) earthquake source in that model. Modeling is by eighth-order finite differencing on a staggered grid, which automatically generates all body and surface waves, including multiples and conversions. Application is to the 1981 USGS refraction survey and the 6 August 1979 Coyote Lake event in central California. 3D modeling was able to reproduce the main observed features in both the refraction and earthquake data. For the refraction data, the synthetics explain local variations in first-arrival travel times, reverberations in the near-surface sediments, and surface waves. For the strong-motion data, the initial arrival times and amplitudes are determined by the position and orientation of the initial rupture; the shapes of the trailing edges of the pulses are modified by the actual source distribution. Changes of source position of 1 to 2 km are easily resolvable through changes in the position of nodal planes. Discontinuities in material properties (e.g., layering) produce a coda of multiples and converted waves. Lateral variations in velocities, especially in the upper few kilometers, change the energy distribution among the trace components, by 3D refraction, and so shift the apparent source directivity and position, as well as contributing to the coda. Remaining differences between the observed and synthetic data are attributed to local seismometer coupling effects, very local site structures, and phenomena such as anelasticity and anisotropy, which are not explicitly modeled.