Short-term variations in slip rate and size of prehistoric earthquakes during the past 2000 years on the northern San Jacinto fault zone, a major plate-boundary structure in southern California

Most of the displacement across the North American-Pacific plate boundary in southern California is accommodated by the San Jacinto and the southern San Andreas fault zones. If and how the rate of displacement across these fault zones varies along strike and through time are still being resolved. Here, we present four calculations of late Holocene slip rate and average slip per event from the Claremont fault of the northern San Jacinto fault zone that show variations in strain distribution over the past 2000 yr and illustrate how plate-boundary displacement is distributed between the San Jacinto and southern San Andreas fault zones. We calculate a slip rate of 12.8-18.3 mm/yr and an average slip per event of 2.5 m from two measurements of streams offset by 9-11 earthquakes in the past 1500-2000 yr. Faster slip rates of 21-30 mm/yr and an average slip per event of 2.7-3 m were determined from measurements of a stream and a buried channel that were offset by three earthquakes in the past 400-500 yr. The 2000 yr slip rate is similar to the range in slip rates reported for the adjacent San Bernardino section of the San Andreas fault zone, suggesting that the northern San Jacinto accommodates a similar amount of displacement as the San Andreas fault zone at the same latitude. The rate is also slightly faster (by similar to 2-3 mm/yr) than reported slip rates from the central San Jacinto fault zone to the southeast. A slip rate of 15 +/- 2 mm/yr is within the range of uncertainty for almost all the geologic and geodetic data for the entire length of the San Jacinto fault zone and may be the best approximation for long-term average slip rate of the fault zone. Alternatively, 2-3 mm/yr of slip along the northern San Jacinto fault zone may be accommodated to the south along the lesser-studied Hot Springs, Thomas Mountain, Buck Ridge, and Santa Rosa faults, the lateral slip rates of which are not well known nor included in typical estimates of slip rate along the central San Jacinto fault zone. We infer that the faster slip rate over the past 500 yr is due to a cluster of earthquakes along the Claremont fault between A.D. 1400 and A.D. 1850 and larger-than-average surface displacement of 3 m or more during the third event back. The 3 m or more measurement of displacement in this event corresponds to rupture lengths that are slightly longer than the total length of the Claremont fault, and previously published paleoseismic data indicate that this event occurred coincident in time with an event on the adjacent Clark fault. We propose that this combination of slip per event data and paleoseismic data from adjacent fault strands is strong evidence for rupture through the releasing step over that separates these two segments of the San Jacinto fault zone.