Liam Reidy

A Record of Large Earthquakes on the Southern Hayward Fault for the Past 500 Years

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 fault9s past earthquake behavior. The site is near the south end of Tyson9s 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.

The Most Recent Large Earthquake on the Rodgers Creek Fault, San Francisco Bay Area

The Rodgers Creek fault (rcf) is a principal component of the San Andreas fault system north of San Francisco. No evidence appears in the historical record of a large earthquake on the rcf, implying that the most recent earthquake (mre) occurred before 1824, when a Franciscan mission was built near the fault at Sonoma, and probably before 1776, when a mission and presidio were built in San Francisco. The first appearance of nonnative pollen in the stratigraphic record at the Triangle G Ranch study site on the south-central reach of the rcf confirms that the mre occurred before local settlement and the beginning of livestock grazing. Chronological modeling of earthquake age using radiocarbon-dated charcoal from near the top of a faulted alluvial sequence at the site indicates that the mre occurred no earlier than a.d. 1690 and most likely occurred after a.d. 1715. With these age constraints, we know that the elapsed time since the mre on the rcf is more than 181 years and less than 315 years and is probably between 229 and 290 years. This elapsed time is similar to published recurrence-interval estimates of 131 to 370 years (preferred value of 230 years) and 136 to 345 years (mean of 205 years), calculated from geologic data and a regional earthquake model, respectively. Importantly, then, the elapsed time may have reached or exceeded the average recurrence time for the fault. The age of the mre on the rcf is similar to the age of prehistoric surface rupture on the northern and southern sections of the Hayward fault to the south. This suggests possible rupture scenarios that involve simultaneous rupture of the Rodgers Creek and Hayward faults. A buried channel is offset 2.2 (+1.2, −0.8) m along one side of a pressure ridge at the Triangle G Ranch site. This provides a minimum estimate of right-lateral slip during the mre at this location. Total slip at the site may be similar to, but is probably greater than, the 2 (+0.3, −0.2) m measured previously at the nearby Beebe Ranch site.