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.