Ages of late Holocene earthquakes on the central Garlock fault near El Paso Peaks, California

A trench across the central Garlock fault within a small playa adjacent to El Paso Mountains revealed buried fissures and fault scarps that provide strong evidence for five surface-rupturing earthquakes within the past 5 kyr. Weaker evidence suggested the possibility of three additional events within this same time period. These observations indicate an average recurrence interval of 700 to 1200 years. Nearly all 28 radiocarbon dates on samples of detrital charcoal, wood and shell are in correct stratigraphic order. These dates constrain the ages of the individual faulting events and indicate that the recurrence interval is not periodic but is quite irregular. Individual recurrence intervals range from 190 to 1545 years (if all eight events an actual earthquakes) or from 190 to 3405 pears (if only the five well-documented events represent earthquakes). The two most recent earthquakes occurred within the past 550 years, with preferred ages of about A.D. 1790 and A.D. 1600. The A.D. 1790 event ruptured a narrower width of the fault zone, and may have been a smaller earthquake than the others. The radiocarbon dates from the trench also indicate that the sedimentation rate on the playa has fluctuated within the past 5 kyr. Low sedimentation rates (0.1 to 0.35 mm/yr) and infrequent floods (less than or equal to 1 per century) prevailed from about 2000 B.C. to A.D. 100 and from A.D. 500 to 1300. During the remaining intervals within the past 5 kyr, the sedimentation rate averaged 1.0 to 1.7 mm/yr and major floods occurred two to four times per century. The changes in sedimentation rate and flood frequency may reflect climatic changes, or they may simply reflect changes in drainage patterns.