PRELIMINARY PALEOSEISMIC RESULTS FROM THE NEW ECHO PLAYA TRENCH SITE ON THE CENTRAL GARLOCK FAULT, CHINA LAKE NAWS, NEAR RIDGECREST, CA
KEMP, Christopher D.1, HARTLEB, Ross D.1, LUTZ, Andrew T.2, FROST, Erik K.1, LINDVALL, Scott C.1, ALM Jr, Steve3, TIEDEMAN, Andrew4, RITTASE, William M.5 and WALKER, J. Douglas6, (1)Fugro Consultants, 27220 Turnberry Lane #110, Valencia, CA 91355, (2)Fugro Consultants, 1777 Botelho Drive, Suite 262, Walnut Creek, CA 94596, (3)Department of Geology, University of Kansas, 2305 westdale rd, Lawrence, KS 66049, (4)Geothermal Program Office, U.S. Naval Air Weapons Station, China Lake, CA 93555, (5)Department of Geology, University of Kansas, 1475 Jayhawk Boulevard, Room 120, Lawrence, KS 66045, (6)Department of Geology, University of Kansas, Lawrence, KS 66045, c.kemp@fugro.com
Investigation of a new paleoseismic trench site along the central Garlock fault at Echo Playa on the China Lake Naval Air Weapons Station exposed evidence for at least four, and as many as six, surface-rupturing earthquakes. The Echo Playa site is located within a roughly 150-m-wide and 1.5-km-long transtensional step-over that has formed a closed basin fringed by alluvial fan and colluvial deposits that grade basinward into laterally continuous playa sediments. Numerous subdued north- and south-facing fault scarps are recognized between the playa edge and the ridges of the step-over. These faults generally deform bedrock or coarse alluvial fans, although two fault strands have formed low scarps along the playa margin. Two trenches opened at Echo Playa across these playa margin faults exposed roughly 5 m of generally conformable stratigraphy, recording several episodes of fan progradation and lacustrine/palustrine transgression along the playa margin.
Evidence for four surface-rupturing earthquakes is preserved within a roughly 30-m-wide zone of distributed deformation where correlative sets of faults and fissure fills terminate upward at the base of laterally continuous stratigraphic horizons. The distributed nature of deformation here has produced very small individual offsets that indicate different combinations of lateral and normal slip components. Age estimates for strata that bracket each rupture are pending radiocarbon analyses of detrital charcoal and optically stimulated luminescence analyses of fine-grained playa deposits. Initial age estimates based on an assumption of reasonable sedimentation rates and the observed lack of soil development suggests that these four to six surface-rupturing earthquakes occurred during the Holocene.
Echo Playa is the easternmost paleoseismic trench site along the Garlock fault, and data from this site are expected to provide a basis for developing an earthquake chronology for the eastern Garlock fault, which is currently not well characterized. These data also will provide an important opportunity to refine estimates of earthquake chronology, fault rupture length, and recurrence intervals along the central and western Garlock fault, contributing to an improved understanding of seismic hazard in southern California.
