Abstract

Application of cone penetrometer testing (CPT) is a promising method for studying subsurface fault zones in stratified, unconsolidated sediment where trenching is not feasible. Analysis of data from 72 CPTs, spaced 7.5 to 30.0 m apart, and 9 borings indicates that the North Branch fault, the active strand of the Newport-Inglewood fault zone (NIFZ) in Huntington Beach, has generated at least three and most likely five recognizable surface ruptures in the past 11.7 ± 0.7 ka. Additional smaller earthquakes similar to the Mw 6.4 1933 Long Beach earthquake may also have occurred but would not be recognizable with this method. The minimum right-lateral Holocene slip rate of the NIFZ in the study area is estimated to be 0.34 to 0.55 mm/yr. The actual slip rate may be significantly higher.

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