Dekameter-scale geologic structure validation of shallow seismic properties along the San Jacinto fault at Sage Brush Flat, Anza, California

Adam Wade, Ramon Arrowsmith, Andrea Donnellan, Frank L. Vernon, & Yehuda Ben-Zion

Published August 15, 2017, SCEC Contribution #7738, 2017 SCEC Annual Meeting Poster #132

In support of observations and interpretations from the dense seismic array across the San Jacinto Fault Zone (SJFZ), we performed detailed (1:1000-1:2500 scale) mapping, excavation and unmanned aerial vehicle (UAV) data collection at Sage Brush Flat (SGB) near Anza, California. Recent studies here include the dense array, lidar analysis and Sharp’s geologic map of the SJFZ from 1967, to interpret 3D seismic structure at 10-100 m scale, fault rupture slip distribution, and regional structure respectively. UAV data was processed for high-resolution topography using structure from motion (SFM) techniques and compared to B4 airborne lidar resulting in absolute difference mean values between 0.4 - 0.5 m with larger differences (>0.3 m) attributed primarily to vegetation growth. SFM derived elevation models were used to measure 6.9 m of dextral channel offset using LaDiCaoz, matching previous offset clusters of the Clark Fault mapped from B4 data in the north neighboring Burnt Valley. Site mapping illustrates differential landform distribution and morphology (steep slopes with small fans vs shallow slopes and broader fans on the east and west sides of the fault zone respectively). Our map work also indicates a network of faults with varying senses of slip, juxtaposing basement metamorphic rock against Quaternary sediments (Bautista beds). We adopted a damage classification scheme to our mapping effort which demonstrates disproportionately more mechanical damage to gneiss, compared to adjacent tonalite within the local basin. Our roughly 50 m long fault-perpendicular trench, excavated across a prominent shutter ridge, exposed two bounding fault zones with west-vergent slip displacing damaged basement rock over the Bautista. In addition to collecting aerial imagery and topography, we flew UAV surveys of fault core outcrops to aid in constructing a three dimensional model of fault zone. To define material properties of fault damage, our mapping efforts included collection of pulverized rock samples which will be analyzed for particle size distribution and mineralogical characterization. Our preliminary results indicate that the SJFZ at SGB consists of multiple faults with varying sense of slip. These results will be further evaluated with laboratory work to better define and characterize the lithologic structural framework of the SJFZ at SGB.

Key Words
San Jacinto Fault, UAV, high resolution topography, structure, Sage Brush Flat, Structure From Motion,

Wade, A., Arrowsmith, R., Donnellan, A., Vernon, F. L., & Ben-Zion, Y. (2017, 08). Dekameter-scale geologic structure validation of shallow seismic properties along the San Jacinto fault at Sage Brush Flat, Anza, California. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology