Abstract

Parkfield is considered a transitional segment along the San Andreas fault (saf) between continuous fault creep to the northwest and segments to the southeast that last slipped in the great 1857 Fort Tejón earthquake. Historically, creep and recurring M 6.0 earthquakes have been observed at Parkfield, California, but the segment’s relevance in great saf earthquakes has remained uncertain. This paleoseismic study of the central Parkfield segment provides a >2000-year record of tectonically deformed fluvial and sag stratigraphy. Two fault-perpendicular excavations across a pressure ridge and a sag pond ∼200 m north of Carr Hill exposed five primary fault zones displaying apparent vertical offsets, upward splaying clay shear bands, and warped stratigraphy. Four of five fault zones extended into the uppermost stratigraphy, suggesting recent surface offset and fault creep. Several antithetic fault splays and one primary fault zone displayed upward terminations, but strong indicators of large-magnitude earthquakes with meter-scale surface offset and rupture such as filled fissures and colluvial scarp deposits were not observed. The absence of evidence for large-magnitude earthquakes does not preclude the possibility of 1857-style earthquakes extending into the Parkfield segment. However, all deformation exposed within these trenches is consistent with repeated small ground rupture and fault creep. The 2004 M 6.0 Parkfield earthquake ruptured through the site and activated at least three of the five fault zones exposed in our excavations. Comparison between 2004 vertical offset and vertical offsets within the exposed stratigraphy suggests a recurrence interval between 8 and 188 years for M 6.0 earthquakes at Parkfield.

Online material: Supplemental unit descriptions, unanalyzed radiocarbon samples, and trench logs and photos.

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