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Interpretation of S waves generated by near-surface chemical explosions at SAFOD

Fred F. Pollitz, William Ellsworth and Justin Rubinstein
Interpretation of S waves generated by near-surface chemical explosions at SAFOD
Bulletin of the Seismological Society of America (December 2015) 105 (6): 2835-2851

Abstract

A series of near-surface chemical explosions conducted at the San Andreas Fault Observatory at Depth (SAFOD) were recorded by high-frequency downhole receiver arrays in separate experiments in November 2003 and May 2005. The 2003 experiment involved approximately 100 kg shots detonated along a 46-km-long line (Hole-Ryberg line) centered on SAFOD and recorded by 32 three-component geophones in the pilot hole between 0.8 and 2.0 km depth. The 2005 experiment involved approximately 36 kg shots detonated at Parkfield Area Seismic Observatory (PASO) stations (at approximately 1-8 km offset) recorded by 80 three-component geophones in the main hole between the surface and 2.4 km depth. These data sample the downgoing seismic wavefield and constrain the shallow velocity and attenuation structure, as well as the first-order characteristics of the source. Using forward modeling on a velocity structure designed for the near field, both observed P- and S-wave energy for the PASO shots are identified with the travel times expected for direct and/or reflected phases. Larger-offset recordings from shots along the Hole-Ryberg line reveal substantial SV and SH energy, especially southwest of SAFOD from the source as indicated by P-to-S amplitude ratios. The generated SV energy is interpreted to arise chiefly from P-to-S conversions at subhorizontal discontinuities. This provides a simple mechanism for often-observed low P-to-S amplitude ratios from nuclear explosions in the far field, as originating from strong near-field wave conversions.


ISSN: 0037-1106
EISSN: 1943-3573
Coden: BSSAAP
Serial Title: Bulletin of the Seismological Society of America
Serial Volume: 105
Serial Issue: 6
Title: Interpretation of S waves generated by near-surface chemical explosions at SAFOD
Affiliation: U. S. Geological Survey, Menlo Park, CA, United States
Pages: 2835-2851
Published: 201512
Text Language: English
Publisher: Seismological Society of America, Berkeley, CA, United States
References: 35
Accession Number: 2016-007464
Categories: Seismology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. geol. sketch maps
N35°47'60" - N36°12'00", W120°47'60" - W120°24'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 201604
Program Name: USGSOPNon-USGS publications with USGS authors
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