Full-resolution photomosaics of trench walls from the 2006 paleoseismic study of the San Andreas Fault at Coachella, California

The Coachella paleoseismic site is located on the San Andreas Fault along the northeast edge of the Coachella Valley in southern California, southwest of the intersection of Dillon Road and Avenue 44. In 2006, three benched trenches were excavated across the fault zone as part of an Alquist-Priolo fault investigation study by a private geotechnical firm (see APSI_003361 at https://maps.conservation.ca.gov/cgs/informationwarehouse/apreports/). These trenches exposed a thick section of latest Holocene lacustrine, alluvial, and shoreline deposits. Only the central and eastern trenches exposed faulting so paleoseismic investigations were confined to those two trenches.
In the central trench (T1), we photographed and logged in detail both walls of about 70 m of the trench where it spanned several zones of complex faulting which form a 15-m-wide depression. After carefully cleaning the trench walls, we put up a 1- by 0.5-m string- and nail-grid. We photographed each 1- by 0.5-m panel individually and then logged features directly on these unrectified photos. The photos were digitally rectified later to remove distortion caused by irregularities in the trench walls and to correct the slight distortion introduced by the camera lens. The rectified photos were spliced together to make photomosaics of the trench walls.  The assembled interpreted mosaics were previously published in an Open File report (Philibosian et al., 2009) and the data were fully analyzed, interpreted, and discussed in a journal article (Philibosian et al., 2011).  However, due to file size constraints the resolution of the photomosaic images in the Open File report was reduced significantly from the original photographs.  Given recent renewed interest in the coupled earthquake and lake chronology in the region, we publish herein the full-resolution trench wall photomosaics.  Each bench riser is broken into a series of files with each file typically showing a 14-m-long section of wall.  Also included are additional images showing alternative views of a few areas where the wall was cut back or with faults accentuated by carving grooves into the wall with a tool.  These alternative views were used along with the others to produce the interpretations, but the images were not incorporated into the published photomosaics.  Some of these additional images remain unrectified.  Finally, a few overview photos showing sections of wall extending a few meters beyond the ends of the logged sections are provided to give further context to the logged sections.
For the eastern trench (T2), we took a set of overview photographs of the full length (about 200 m) of each wall. These photographs were taken from the top of the trench towards the opposite wall. Because the photographs were taken at a downward angle, there is significant distortion. We logged directly on these photos in the field, recording significant contacts, primarily between lacustrine and subaerial deposits, along with descriptions. The Open File report (Philibosian et al., 2009) included low-resolution photomosaics made by splicing together these unrectified overview photos, overlain with our interpretations.  Herein we publish the original full-resolution overview photographs (each covering a section of wall 5–10 m long), as well as composite photomosaics of both walls that are slightly lower resolution than the original photos but significantly higher resolution than the previously published photomosaic.

Tags: california, coachella-valley, earthquakes, faulting-geologic, geoscientificinformation, hazards, lake-or-marine-sediment, paleoseismology, sedimentation, trenching, usgs-64aca930d34e70357a291c1e
Organization: Department of the Interior