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Presentation
Abs. The width of dextral fault zones and shallow decollements of the San Jacinto fault zone, southern California
Annual meeting of the Southern California Earthquake Center (2007)
  • Susanne U. Janecke, Utah State University
  • B. E. Belgarde
Abstract

Cross-sectional, structural, geomorphic and map analysis of recently relocated earthquakes (Shearer et al., 2005) reveals steep NE dips and transpression across much of the San Jacinto fault zone in accord with growing evidence for widespread transpression across the southern San Andreas fault (Fuis et al., 2007). The seismically defined San Jacinto fault zone in the Peninsular Ranges is typically 9-10 km wide perpendicular to microseismic alignments, 12-15 km wide in map view, and consists of several identifiable steeply NE-dipping alignments of seismicity in the middle to lower seismogenic crust. The geologically defined fault zone is narrower in the same area. The width of the Clark fault zone in the Salton basin, on the other hand, is up to 13 km wider at the surface than the seismically active central part of the fault zone in the subsurface between 3 and 12 km depth (Belgarde, 2007). Mismatching shallow structures and deeper structures require flake tectonics and shallow decollements in some areas. These shallow decollements must have limited lateral extents because many parts of the adjacent fault zone persist as steep NE-dipping faults to 10-12 km depth from previously and newly mapped dextral faults at the surface. We correlate some NE-striking left-lateral faults with subsurface alignments of microseismicity; there are more right-lateral faults highlighted by microseismicity then left-lateral ones. A secondary concentration of earthquakes between 3-5 km depth may be the locus of decollements within the Salton basin or might be a zone of greater fluid pressures. In some places there are excellent correlations between active surface structures and seismicity at depth. In other areas active fault zones produced no earthquakes in the last 25 years. We interpret those seismically quiet areas as locked or inactive during the period of study. Examples include many segments of the Coyote Creek fault, the Extra fault, the San Felipe Hills fault, and the northern third of the Clark fault zone in the Arroyo Salada segment. Another such area is a broad band along the SW margin of the Coachella Valley where numerous E and NE-facing fault scarps show Quaternary activity within a newly hypothesized fault zone that we here name the Torres Martinez fault zone. If further work confirms this fault zone it would have significant hazards implications because it projects northward to the most populated areas of the Coachella valley. We propose that deep Quaternary sedimentary basins SW of the dextral reverse parts of the San Jacinto and San Andreas fault zones are the result of the component of contraction across the strike-slip faults that likely dates back only 1-2 m.y., and are not as transtensional as previously thought. Deep Quaternary sedimentary basins SW of the dextral reverse parts of the San Jacinto and San Andreas fault zones are the result of the component of contraction across the strike-slip faults and are not as transtensional as previously thought. Belgarde, Benjamin, 2007, Structural characterization of the three southeast segments of the Clark fault, Salton Trough, California [M.S thesis]: Utah State University: 4 plates, map scale 1:24,000. 216 p. This abstract was a poster presentation at the 2007 SCEC Annual Meeting.

Keywords
  • Dextral fault zone,
  • decollement,
  • Fault zone
Publication Date
January 1, 2007
Citation Information
Janecke, S. U., Belgarde, B.E., 2007, The width of dextral fault zones and shallow decollements of the San Jacinto fault zone, southern California: Annual meeting of the Southern California Earthquake Center, Palm Springs, CA, v. 17. http://www.scec.org/meetings/2007am/index.html