Abstract

Comparison of fault slip throughout most of the central, creeping section of the San Andreas fault zone indicates that near-fault (about 100-m aperture) and intermediate-scale (about 1- to 2-km aperture) geodetic measurements of right-lateral slip are in good agreement. An exception is present near Monarch Peak, near the center of the creeping section. Here, near-fault measuring techniques yield no more than two-thirds of the slip rate determined by intermediate-scale measurements. Anomalously low near-fault creep at this site has continued during 10 yr of measurement. Detailed geologic mapping shows the presence of geologically youthful subsidiary faults along an area about 7 km long and parallel with the main trace, centered on Monarch Peak. We infer that the subsidiary faults are active, taking up the difference in horizontal offset. If the subsidiary faults are in fact active, it makes the structural setting near Monarch Peak anomalous in the creeping section of the fault zone.

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