Analysis and Modeling of the Wavefield Generated by Explosions at the San Andreas Fault Observatory at Depth
Abstract
Surface array and deep borehole recordings of chemical explosions in the near-source (0-20 km) region are studied to better understand the radiated wavefield, and in particular the generation of S-waves by explosions. More than 120 explosions have been conducted in the immediate vicinity of the San Andreas Fault Observatory at Depth (SAFOD) between 2002 and 2006 as part of a comprehensive program to determine the crustal structure and precise location of shallow microearthquakes near the deep drill holes at SAFOD. The data set features more than 70 high-quality digital surface and shallow borehole stations within 10 km of SAFOD; extensive observation of the downgoing wavefield by up to 80 3-component geophones deployed at depths of up to 2.7 km in the SAFOD boreholes; and 86 shot points, including 17 shot points located within a few tens of meters of seismic stations creating an extensive set of reciprocity observations. Shallow crustal earthquakes have also been extensively observed by the recording networks. We compare these observations with synthetic seismograms generated using published 3-D crustal models for SAFOD to test prevailing hypotheses of S-wave generation. The methods include a combination of basic source characterization (i.e., point sources represented by a moment tensor), imaging of scattering sources, non-linear wave generation associated with the initial shock wave, and non-linear free surface effects. We compute high-frequency synthetic seismograms in 3-D laterally heterogeneous structure using a new approach based on coupled normal modes. Applying the pure path approximation to a suitably smooth representation of the 3D structure and appropriately revising the eigenfrequencies of the normal modes, amounting essentially to a phase shift of each seismic arrival, reduces the non-linearity of the seismogram with respect to structural parameters.
Document Details
- Document Type
- Technical Report
- Publication Date
- Sep 01, 2010
- Accession Number
- ADA569489
Entities
People
- Fred Pollitz
- Justin L. Rubinstein
- William L. Ellsworth