San Andreas Fault Characterization at the LADWP Elizabeth Tunnel

Scott C. Lindvall, Scott Kerwin, James P. Evans, Jeffrey Tyson, James Chestnut, Chris Heron, Kevin Mass, Katherine M. Scharer, Devin McPhillips, Diane Moore, Michael Farr, Christopher Ballard, Randolph T. Williams, Kelly K. Bradbury, Christie D. Rowe, & Heather M. Savage

Published August 14, 2017, SCEC Contribution #7668, 2017 SCEC Annual Meeting Poster #115 (PDF)

Newly acquired subsurface data across the San Andreas Fault (SAF) provides insight into the geometry, structure, and composition of the upper part of the SAF zone in the northern Sierra Pelona Mountains, CA. The purpose of the investigation was to characterize the Los Angeles Aqueduct crossing of the SAF in the Elizabeth Tunnel, an 8-km-long tunnel section of the aqueduct that delivers water from Owens Valley to the City of Los Angeles. Several transects of CPT soundings and geotechnical boreholes define a 40-m-thick section of unconsolidated Holocene and Pleistocene alluvial deposits above a faulted and deformed buried bedrock surface. Some of the abrupt separations in the buried bedrock surface correlate with scarps and tonal lineaments observed at the ground surface in historical air photos and lidar. Seven 55°- 65° northeast plunging boreholes acquired a total of ~ 750 m of rock core to a maximum vertical depth of 140 m, across the ~150 m wide fault zone. Core recovery was ~95%, and the core samples consist of a range of indurated fault-related rocks including zones of foliated cataclasite and a few local intervals of clay-rich gouge. The cataclasite zones have an estimated local thickness up to 2 - 3 m and are developed within granodiorite to granitoid gneiss. Outside of the primary cataclasite/gouge zones, the damage zones are defined by more discrete deformation such as cataclastic shear bands. In most cases, cataclasite and gouge zones correlate with prominent resistivity lows observed in wireline logs. A tentative correlation of slip surfaces observed across the boreholes indicates that the principal fault surfaces dip steeply south, flower and flatten near the upper parts of the holes, and may connect to the active surface traces of the SAF as interpreted from the offset buried bedrock surface and topography. Fault-related rocks throughout the boreholes exhibit evidence for fluid-fault interactions in the form of variably developed clay-rich shear bands/zones and hydrothermal alteration products. The degree of induration and alteration indicate that the fault zone here consists of numerous slip surfaces in a hydrothermally altered and “cemented” sequence. Future analyses of core samples will provide insight into the combined mechanical and chemical processes responsible for fault-zone development in this upper part of the SAF, with a particular focus on how these processes operate throughout the seismic cycle.

Key Words
San Andreas fault, Elizabeth Tunnel

Citation
Lindvall, S. C., Kerwin, S., Evans, J. P., Tyson, J., Chestnut, J., Heron, C., Mass, K., Scharer, K. M., McPhillips, D., Moore, D., Farr, M., Ballard, C., Williams, R. T., Bradbury, K. K., Rowe, C. D., & Savage, H. M. (2017, 08). San Andreas Fault Characterization at the LADWP Elizabeth Tunnel. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology