PALEOSEISMOLOGY ON THE OWENS VALLEY FAULT AND HOLOCENE STRATIGRAPHY OF PLUVIAL OWENS LAKE NEAR LONE PINE, EASTERN CALIFORNIA

Two different paleoseismic sites (elev. 1115-1128 m) on the Owens Valley fault (OVF), located 4 and 7 km north of Lone Pine, California, having a total of 7 trenches and 5 exploration pits, expose direct stratigraphic evidence for only two earthquakes since ~12 cal. ka; the historic 1872 earthquake (MRE) and the penultimate event (PE).

Our principal results rely on the structural and geochronologic relations of fluvio-deltaic and lacustrine stratigraphy associated with oscillating pluvial lake-levels in the lower Owens Lake basin near Lone Pine. Stratigraphic analyses of these sediments exposed in trenches, pits, and along 1.5 km of Owens River bluffs reveal a relatively complete record of ~12-7 cal. ka sequence stratigraphy. On the basis of tephra chronology, one Holocene highstand occurred after ~8 cal. ka and reached an elevation of ~1135 m, ~10 m below the latest overflow sill of Owens Lake basin. This highstand modified and reset the landscape and evidence of pre-1872 fault scarps near and below this elevation.

The paleoseismic results provide average vertical offsets for the PE and MRE and a confident estimate of the age of the PE. Measured in 3 trenches from two sites, the cumulative vertical offset from the last two earthquakes (the PE and MRE) is 2.3 ± 0.3 m (2s). In 5 trenches, the average vertical offset for the MRE is 0.9 ± 0.3 m (2s). In each trench, the vertical offset of the MRE is subtracted from the cumulative vertical offset to derive an average vertical offset for the PE of 1.4 +0.3/-0.4 m (2s). Analyses of ¹⁴C in charcoal and tufa materials that bound the PE event horizon indicate the PE occurred between 10,210 ± 60 cal. yr B.P. and 8,790 ± 210 cal. yr B.P. The age of the PE is estimated at 9,300 ± 300 cal. yr B.P. on the basis of the PE horizon position with respect to the location of the ¹⁴C ages and other stratigraphic relations. Accounting for the elapsed time between 1872 and 1950, the interseismic interval between the two earthquakes is ca. 9,500 to 8,900 yrs.

The scaling ratio of 6:1 (horiz:vert; Beanland and Clark, 1994) is used with the average vertical offsets of the PE and MRE to estimate the average horizontal displacements. Pending further ¹⁴C analysis, the average horizontal displacements divided by the single interseismic interval results in a single-event lateral slip rate estimate of 0.8 +0.5/-0.4 m/k.y. (2s).