Abstract

Trenches excavated at two sites on scarps of the Pyramid Lake fault zone revealed evidence of recurrent Holocene strike-slip faulting. Control for estimating the ages of faulting events is provided by soil stratigraphy, C-14 ages on organic sediments, and the Mazama ash (7000–6700 yr), which is present at both sites.

The first site displayed evidence of at least three, probably four, and possibly five displacements within the past 7500 yr. Evidence of at least three and probably four post–Mazama-ash faulting events was found at the second trench site. The juxtaposition of lithologically contrasting deposits and units with different thicknesses on opposite sides of the faults indicates strike-slip faulting. The estimated average recurrence of surface faulting events (Ms ⩾ 6.5) is about 1800 yr assuming four post–Mazama-ash events and about 2400 yr for three events. The most recent event occurred within the past few hundred years. The rate of recurrence of surface faulting events on the Pyramid Lake fault zone is three to five times higher than for most faults in the western Basin and Range province.

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