Abstract

The Hayward fault, a major branch of the right-lateral San Andreas fault system, traverses the densely populated eastern San Francisco Bay region, California. We conducted a paleoseismic investigation to better understand the Hayward fault's past earthquake behavior. The site is near the south end of Tyson's Lagoon, a sag pond formed in a right step of the fault in Fremont. Because the Hayward fault creeps at the surface, we identified paleoseismic events using features that we judge to be unique to ground ruptures or the result of strong ground motion, such as the presence of fault-scarp colluvial deposits and liquefaction. We correlate the most recent event evidence (E1) to the historical 1868 M 6.9 earthquake that caused liquefaction in the pond and recognize three additional paleoruptures since A.D. 1470 ± 110 yr. Event ages were estimated by chronological modeling, which incorporated historical and stratigraphic information and radiocarbon and pollen data. Modeled, mean age and 95-percentile ranges of the three earlier events are A.D. 1730 (1650-1790) yr (E2), A.D. 1630 (1530-1740) yr (E3), and A.D. 1470 (1360-1580) (E4). The ages of these paleoearthquakes yield a mean recurrence of 130 ± 40 yr. Although the mean recurrence is well determined for the period A.D. 1470-1868, individual intervals are less well determined: E1-E2, 140 +80/-70 yr; E2-E3, 100 +90/-100 yr; and E3-E4, 150 +130/-110 yr.

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