Earthquake Information

September 28, 2004 Mw 6.0 Parkfield Earthquake
Summary of Surface Fault Rupture Mapping
(9/29/2004 – 11/4/2004)

Right-lateral strike-slip displacement occurred discontinuously for over 31 km along the San Andreas fault and the subparallel Southwest Fracture Zone. The location of the northern extent of rupture is not well-constrained due to property access issues and the presence of massive and extensive landslides. The 2004 rupture generally coincided with the 1966 rupture, although there were differences between the two earthquakes. The 1966 earthquake nucleated in the Middle Mountain area and propagated to the southeast. The 2004 earthquake nucleated near the southern end of Cholame Valley and propagated to the northwest. Maximum right-lateral offset of 44 mm occurred along the San Andreas fault about 7 ½ km northwest of Parkfield in the Middle Mountain area in 2004. The 1966 rupture was nearly twice the amount thus far documented in 2004. Locally a minor component of vertical slip was observed on both the San Andreas and Southwest Fracture Zone in 2004. Afterslip occurred in 1966 and has been documented in 2004. Monitoring of afterslip continues –nail quadrilaterals were installed at 10 sites across traces of the San Andreas, Southwest Fracture Zone, and one minor secondary fault. To date repeated measurements of these quadrilaterals indicate afterslip is occurring only on the San Andreas fault.

Field mapping following the 2004 Parkfield earthquake was a collaborative effort between CGS (J. Treiman, K. Clahan, A. Rosinski, D. Branum, W. Bryant, P. Irvine, J. Hernandez, and T. Jones), the U.S. Geological Survey (Michael Rymer, John Tinsley, Gary Fuis, Andy Snyder, G. Bawden, and S. Devlin), and Arizona State University (R. Arrowsmith, N. Toké).

The map below summarizes field observations between September 28 and November 4, 2004. There are two more-detailed maps in addition to the overview map. The detailed maps have accessible data tables for slip observations as well as photo links.