The binary data are available via anonymous FTP from the National Strong Ground Motion FTP-server:
https://ca.water.usgs.gov/nsmp/ in directory GEOS/PRK.
USGS Open-File Report 2004-1376: Borcherdt, Roger D., Malcolm J. S. Johnston, Christopher Dietel, Gary Glassmoyer, Doug Myren, and Christopher Stephens (2004). Acceleration and Volumetric Strain Generated by the Parkfield 2004 Earthquake on the GEOS Strong-Motion Array Near Parkfield, Calif.
Borcherdt, Roger D., Malcolm J. S. Johnston, Gary Glassmoyer, and Christopher Dietel (2006). Recordings of the 2004 Parkfield Earthquake on the General Earthquake Observation System Array: Implications for Earthquake Precursors, Fault Rupture, and Coseismic Strain Changes, Bull. Seism. Soc. Am. 96, 4B, S73-S89.
The GEOS Home Page is at /GEOS/geos.html
The URL of this page is /GEOS/PRK/parkfield.html
e-mail: glassmoyer@usgs.gov
or address:
Gary Glassmoyer
345 Middlefield Road M/S 977
Menlo Park, California 94025