Saprock: A long term recorder of ground shaking, Peninsular Ranges, southern California Marck Maroun ABSTRACT Saprolization is commonly viewed... » More

Saprock: A long term recorder of ground shaking, Peninsular Ranges, southern California

Marck Maroun

ABSTRACT

Saprolization is commonly viewed as an isovolumetric process. However, it has not been studied extensively in seismogenic areas. Hence, we undertook a detailed study of five sites located at disparate distances to the seismically active Elsinore fault. Data obtained during this study along with the results of earlier investigation of five additional sites, support the idea of significant volume change during the conversion of corestone to saprock. Specifically, the results of these investigations reveal that within 0 to 4 km of major strands of the Elsinore fault, dilational strains range from 25 to 37%. In contrast, in areas located 13 to 20 km from the Elsinore volumetric strains range up to 15%. A positive correlation between the orthogonal distance to the nearest active strand and volumetric strains yields an R2 value of 0.833. In addition, porosity follows a similar trend. Preliminary results from a study of crack morphology suggest that porosity was produced by Mode 1 cracking. Layers of densely translocated clays line some cracks. The Chemical Index of Alteration (CIA) in areas within 0 to 4 km of the nearest active strand ranges from 53 to 63. In areas within 13 to 20 km of the nearest strand the CIA ranges from 53 to 57. The percent change in elemental mass is significantly negative for Ca, Na, Sr, and Ba due to the dissolution of plagioclase. In addition, the negative change in K, Fe, Mg, Mn, and Rb mass is due to the alteration of biotite. In summary, data presented in this paper is consistent with the idea that saprock derived from corestone is a faithful recorder of ground shaking events generated on strike-slip faults making of the San Andreas fault system.

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