Article
CROSSING FAULTS WITHIN THE SAN JACINTO ZONE, SOUTHERN CALIFORNIA
SCEC Final Reports
(2013)
Abstract
We are investigating the interaction of the SE tip of the large and very active Clark strand
of the San Jacinto fault and the crossing left-lateral faults SE of its tip at the surface (Fig. 1).
Janecke hypothesized that the Clark fault does not terminate NW of the crossing fault arrays of
the Extra and Elmore Ranch fault arrays (Fig. 2), but instead continues at depth beneath the
crossing faults, has a complex mutually interfering relationship at the surface and/or produces
enough distributed strain to rotate the crossing faults in a clockwise sense. Ten weeks of
fieldwork and ongoing, in-depth analysis of map, structural and published geophysical datasets,
supports our hypotheses. Although the Extra and Elmore Ranch fault arrays are more active,
preserve many Holocene scarps and are more continuous than dextral faults at the surface, there
is also strong evidence for a subsurface connection between the Clark fault zone and the
Superstition Hills fault in diverse data sets. The most impressive surface manifestation of the
Clark fault SE of the San Felipe Hills is the 40° to 60° reorientation of active left-lateral and left-normal
faults from northerly strikes to ENE strikes within a 12-15 km wide domain above the
buried trace of the Clark fault zone. A 70 km long by 15 km wide uplift of young basin fill is
centered on the Clark-Superstitions Hills fault zone, and has so much activity and relief that it
traps Holocene sediment on its upstream side. Aligned small earthquakes, InSAR data, and a
large step in the basement-cover contact further delineate the Clark fault at depth.
Keywords
- Active fault,
- San Jacinto fault zone,
- Extra fault array
Disciplines
Publication Date
2013
Citation Information
Janecke, S.U. and Thornock, S.J., 2013, Crossing faults within the San Jacinto zone: southern California: Final report for SCEC grant 2011; SCEC database. 15 p.
Janecke, S.U. and Thornock, S.J., 2011, Progress Report: Digital mapping of active interpenetrating faults within the San Jacinto fault zone, southern California: Final report for SCEC grant 2010; SCEC database. 11 p.
Janecke, S.U., 2010; Field mapping and dating of active interpenetrating faults within the San Jacinto fault zone, southern California: SCEC proposal unpublished and written in fall 2010.