A geologic and structural overview of the precariously balanced rocks of Grass Valley site, San Bernardino Mountains, California

Larger imageSCEC Field Notes: Tyanna Schlom Photo 1

Precariously balanced rocks (PBRs) are known natural seismic indicators common to semi-arid regions, including those of Southern California. Typically found in granitic bodies, the soil surrounding the rocks weathers away, allowing them to sit in unstable positions. These rocks can be shaken down at a ground acceleration of 0.3g or less, and thus provide constraints on ground motion from previous earthquakes in a given area. By indicating the strength of ground motions, PBRs can also provide significant data for seismic hazard analysis (Brune, 2002).

Larger imageSCEC Field Notes: Tyanna Schlom Photo 2

An investigation of one particular location of PBRs in the San Bernardino Mountains—called the Pilot Rock Road site in this study—aims to characterize, map, as well as provide ground motion constraints. Many faults surround this structurally complex area, including those of the North Frontal Thrust System of the San Bernardino Mountains, the San Andreas Fault zone, and the southwest edge of the Eastern California shear zone. Some of these faults have had earthquakes in the recent historic and prehistoric past, and/or have high reported (> 1mm/yr) slip rates.

Larger imageTyanna Schlom

This summer’s study of the site focuses on an overview of the geologic and structural information provided by previous research, combining it with field documentation of the precariously balanced rocks, and including samples that will provide exposure ages of the rocks using cosmogenically-produced isotopes. By knowing the age of the PBRs, the slip rates and the most recent offset for nearby faults, it will be possible to understand the recent history of ground motions within this area.

View field notes from other SURE Interns:   Select an intern... Alex Wilk Alexandra Millar Christina Velasquez Hannah Waterhouse Joshua Soble Lauren Steinberg Melissa Nunley Nathan Williams Patrick Dooling Trevor Thomas