GROUND PENETRATING RADAR INVESTIGATIONS ALONG THE SAN ANDREAS FAULT AT THE WRIGHTWOOD, CA PALEOSEISMIC SITE

Ground Penetrating Radar (GPR) investigations of the Wrightwood, CA paleoseismic site on the San Andreas fault (SAF) were conducted to provide accurate information of subsurface stratigraphy and structure at this locality. Wrightwood, CA, located 70-km northeast of Los Angeles in the San Gabriel Mountains, is one of the most extensively studied localities on the SAF. In all, 37 trenches have been excavated over the last 15 years to produce a 30-event paleoseismic series to test recurrence models of seismic events along the SAF. This investigation complements previous paleoseismic trench work by using a non-invasive exploration technique as a means to produce results similar to trench log data and to provide information on subsurface structure and lateral stratigraphy changes (across the Wrightwood structural basin). GPR is a high resolution, shallow subsurface technique that is able to discriminate between the sediment types found at this site (gravel and peat) based on dielectric constants and is relatively time-efficient and inexpensive. This investigation focused on producing GPR images of the shallow subsurface to answer several questions: Do radar images show the same meter-scale (or finer) character as trench logs? Is GPR a reliable exploration technique for identifying areas of complex strata? Is GPR cheaper and more time efficient than digging a trench without any precursory exploration? Can GPR images show subsurface linkages between the main trace SAF and a major secondary fault forming the Wrightwood basin? Preliminary MatLab processing suggests that GPR data distinguishes gravel versus peat facies in at least meter-scale detail. The data express the same general character as trench logs and show areas of localized deformation, especially areas of titled blocks. Results also show low-angle features below trench depths that may represent a detachment fault linking secondary structures to the main trace of the SAF.