Description
Saprock and corestone are common components of the landscape of southern California. Within this landscape, corestones represent ellipsoidal to spheroidal enclaves of crystalline basement rocks encased in their own weak and friable weathering products, i.e., saprock. Though saprock is commonly thought to have formed isovolumetrically, our preliminary studies in areas adjacent to the Elsinore fault revealed unusually high volumetric strains. As a result, one of us speculated that such unusual strains might be the result of ground shaking generated by earthquakes on the nearby Elsinore fault. In order to further evaluate this possibility, we undertook a study of saprock from corestone in the band of precariously balanced rocks lying approximately midway between the Elsinore and San Jacinto faults. Within the band of precariously balanced rocks, ground motion accelerations generated by ruptures on the Elsinore or San Jacinto faults are thought to reach minimum values while in adjacent areas closer to the faults they should increase progressively to higher values. Hence, if volumetric strains are related to ground shaking, then they should have minimum values within the band of precariously balanced rocks and progressively increase toward the Elsinore and San Jacinto faults. The results of our detailed studies of four sites located within the band of precariously balanced rocks indicate that volumetric strains in areas lying ~13 to 16 km from the Elsinore fault varies from 0 to 15%. In contrast, one location along with two additional sites reported in the literature, all lie within 3.5 km of the Elsinore fault, and have volumetric strains larger than ~20%. Because our petrologic and chemical data suggest that chemical weathering has played a minimal role in producing the saprock investigated during this study, the measured low bulk densities, a key parameter in the calculation of volumetric strain, must reflect largely increasing porosities. Significantly, our thin section studies indicated that Mode 1 dilational cracks are well developed at sites with large volumetric strains but are significantly less well developed at other sites. We explored the potential relationships between porosity and volumetric strain and orthogonal distance to the Elsinore fault by plotting our measured volumetric strains and mean saprock porosities derived from each of the four sites discussed here, along with two additional sites discussed in the literature, against perpendicular distance from the Elsinore fault. The resulting relationships fit linear trends, as indicated by high R-squared values. Specifically, 86% of the variation in mean porosity in saprock and 85% of the variation in volumetric strain is correlated with perpendicular distance from the Elsinore fault. Hence, saprock, a ubiquitous material exposed throughout southern California, may provide a means of mapping out the effects of ground shaking intensity where precariously balanced rocks do not occur.
