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Incipient pulverization at shallow burial depths along the San Jacinto Fault, southern California

Joe Whearty, Thomas K. Rockwell, & Gary H. Girty

Under Review June 14, 2016, SCEC Contribution #6290

We studied the fault zone architecture and searched for evidence of shallow damage production in weakly consolidated sandstones of the Bautista Formation along the Clark strand of the San Jacinto Fault in Rock House Canyon, western Salton Trough, California. Here, the fault juxtaposes tonalite on the northeast against strata of the Bautista Formation on the southwest. The fault core and associated damage zone is visible at two separate fault exposures that have experienced total burial depths of ~70 m and ~120 m, respectively. Physical damage in the sandstone “wallrock” of the lower exposure resembles incipient pulverization, and exhibits a preferred crack orientation that is perpendicular to the fault, which in turn indicates the occurrence of significant in situ brittle deformation at this very shallow burial depth. The upper exposure at 70 m burial depth displays very little evidence of fragmentation of sand grains outside of the fault core; together, these observations suggest that the onset of pulverization occurs between about 70 and 120 m depth. Based on our observations and analysis of the data, we conclude that the observed physical damage at this shallow depth is likely the result of stresses produced during dynamic rupture.

Key Words
Pulverization, Dynamic Rupture, San Jacinto Fault, Shallow Burial, Bautista Formation

Whearty, J., Rockwell, T. K., & Girty, G. H. (2016). Incipient pulverization at shallow burial depths along the San Jacinto Fault, southern California, (under review).