WHAT CAN STRIKE-SLIP FAULT SPACING TELL US ABOUT THE EVOLUTION OF THE WALKER LANE AND WESTERN NORTH AMERICA?

The spacing of parallel continental strike-slip faults can constrain the mechanical properties of the faults and fault-bounded crust. In the western US, evenly-spaced strike-slip fault domains are observed in the San Andreas and Walker Lane fault systems. In this study, we compared fault spacing (S) versus brittle-crust thickness (L) for each domain of parallel strike-slip faults. We constructed a topographic map of the base of the seismogenic zone (i.e., the brittle crust, or L) using published relocated earthquake data, which we compared with strike-slip fault locations and fault-spacing measurements (S). Our observations of the San Andreas and Walker Lane systems indicate that the San Andreas has a steeper S/L slope (~8 vs 1, respectively). If a stress-shadow mechanism guides parallel fault formation, the S/L slope should be controlled by fault or crustal strength. This predicts that the San Andreas-related strike-slip faults may be slightly weaker, and our analytical model predicts lower fault friction for the San Andreas: 0.15-0.18 versus 0.21 for the Walker Lane. Furthermore, our analysis suggests that as a large-scale fault system evolves with time, its S/L slope may steepen as observed with the relatively mature San Andreas fault system and newly forming Walker Lane incipient plate-boundary fault system.