Radiocarbon dates indicate rates of graben downfaulting, San Jacinto Valley, California

Recent radiocarbon dates for wood samples collected from three depths in San Jacinto Valley graben indicate active tectonic downfaulting during the past 42,000 yr. The flood plain of graded San Jacinto River, entering the valley from the southeast and leaving toward the west, serves as a reference datum across the graben. Depositional rates suggest that downfaulting averaged about 0.007 ft (2.1 mm)/yr from 42,000 to 15,270 yr B.P. and since 15,270 yr B.P. has increased to about 0.019 ft (5.8 mm)/yr.