Aseismic growth of Durmid Hill, southeasternmost San Andreas Fault, California

Abstract

Our 2.27‐km leveling line across and normal to the San Andreas fault at Durmid Hill domed upward by 9 mm between September 1985 and December 1991. The flanks of the line tilted more than 1 μrad/yr, implying that the uplift extends beyond the ends of the line and that absolute uplift of the crest of the hill may greatly exceed the measured rate of 1.5 mm/yr. Trilateration measurements sampling 5 km of the fault zone within the hill from 1978 to 1992 yield a mean dextral slip rate of 2.4 ± 0.3 mm/yr and fault normal extension of ≈1 ± 0.3 mm/yr. The inferred uplift rate during the 5.5‐year leveling period (as well as from geomorphic samples at 330 years and 35,000 years and from erosion of sedimentary strata younger than 738,000 years in monoclines abutting the fault at Durmid Hill) is remarkably consistent at 1 to 4 mm/yr and implies that the current processes may have persisted for millennia. We investigate two mechanisms for the deformation that are defined by the symmetry of the observed doming: transpression on a vertical or near‐vertical subsurface San Andreas fault, or slip on a shallow flat fault or detachment that terminates beneath Durmid Hill. The resultant models suggest that uplift rates of 1 mm/yr will result from 6 mm/yr dextral shear along a vertical fault together with 1 mm/yr horizontal convergence across that fault, and this strain field is consistent with the paleoseismic record and our leveling data. We conclude that interseismic processes account for most of the growth of Durmid Hill, because the interseismic vertical deformation rates equal or exceed the long‐term vertical slip rate measured across the fault. Because dextral shear of 6 mm/yr at Durmid Hill represents a small fraction of the 25 mm/yr measured across the entire San Andreas fault zone in Coachella Valley and because the San Andreas fault at Durmid Hill may represent an earthquake nucleation zone, the observed deformation processes may be of significant importance for understanding future earthquake rupture initiation or termination.