Jon S. Galehouse

Variations in creep rate along the Hayward Fault, California, interpreted as changes in depth of creep

Variations in surface creep rate along the Hayward fault are modeled as changes in locking depth using 3D boundary elements. Model creep is driven by screw dislocations at 12 km depth under the Hayward and other regional faults. Inferred depth to locking varies along strike from 4–12 km. (12 km implies no locking.) Our models require locked patches under the central Hayward fault, consistent with a M6.8 earthquake in 1868, but the geometry and extent of locking under the north and south ends depend critically on assumptions regarding continuity and creep behavior of the fault at its ends. For the northern onshore part of the fault, our models contain 1.4–1.7 times more stored moment than the model of Burgmann et al. [2000]; 45–57% of this stored moment resides in creeping areas. It is important for seismic hazard estimation to know how much of this moment is released coseismically or as aseismic afterslip.

Inferences drawn from two decades of alinement array measurements of creep on faults in the San Francisco Bay Region

We summarize over 20 years of monitoring surface creep on faults of the San Andreas system in the San Francisco Bay region using alinement arrays. The San Andreas fault is fully locked at five sites northwest from San Juan Bautista, the southern end of the 1906 earthquake rupture, that is, no creep (<1 mm/yr) is observed. Likewise, the San Gregorio, Rodgers Creek, and West Napa faults show no creep. The measured creep rate on the Calaveras-Paicines fault from Hollister southward is either 6 or ∼10 mm/yr, depending on whether the arrays cross all of the creeping traces. Northward of Hollister, the central Calaveras creep rate reaches 14 ± 2 mm/yr but drops to ∼2 mm/yr near Calaveras Reservoir, where slip transfers to the southern Hayward fault at a maximum creep rate of 9 mm/yr at its south end. However, the Hayward fault averages only 4.6 mm/yr over most of its length. The Northern Calaveras fault, now creeping at 3-4 mm/yr, steps right to the Concord fault, which has a similar rate, 2.5-3.5 mm/yr, which is slightly slower than the 4.4 mm/yr rate on its northward continuation, the Green Valley fault. The Maacama fault creeps at 4.4 mm/yr near Ukiah and 6.5 mm/yr in Willits. The central and southern segments of the Calaveras fault are predominantly creeping, whereas the Hayward, Northern Calaveras, and Maacama faults are partly locked and, along with the Rodgers Creek and San Andreas, have high potential for major earthquakes. Manuscript received 6 November 2002.

Long‐term monitoring of creep rate along the Hayward Fault and evidence for a lasting creep response to 1989 Loma Prieta Earthquake

We present results from over 30 yr of precise surveys of creep along the Hayward fault. Along most of the fault, spatial variability in long-term creep rates is well determined by these data and can help constrain 3D-models of the depth of the creeping zone. However, creep at the south end of the fault stopped completely for more than 6 years after the M7 1989 Loma Prieta Earthquake (LPEQ), perhaps delayed by stress drop imposed by this event. With a decade of detailed data before LPEQ and a decade after it, we report that creep response to that event does indeed indicate the expected deficit in creep.

Provenance and Paleocurrents of the Paso Robles Formation, California

A provenance and paleocurrent study of the middle and upper Pliocene continental Paso Robles Formation in the Salinas Valley area, California, adds to the knowledge of the late Cenozoic geologic history of the California Coast Ranges. The geographical distribution of heavy minerals and pebbles in the Paso Robles Formation and in streams presently draining proposed source areas leads to the same conclusions as do measurements of foreset beds, pebble imbrication, and channels in the formation. Uplift in the Santa Lucia and La Panza ranges initiated Paso Robles deposition in early Pliocene time. The paleodrainage was southeastward from the Santa Lucia Range and northward from the La Panza Range, continuing across the present site of the Temblor Range and into a marine basin at the present site of the southern San Joaquin Valley. The Salinas River was a relatively unimportant stream during Paso Robles deposition. Near the end of the Pliocene, uplift in the Temblor Range and southwestward tilting of the Gabilan Mesa brought about the end of Paso Robles deposition by defeating the southeast-flowing drainage and creating the conditions for its capture by the modern Salinas River. Lithologic differences between juxtaposed beds of the Paso Robles Formation on either side of the San Andreas fault suggest that about 25 miles of right-lateral movement has occurred since deposition of the formation.

Anisotropy of Magnetic Susceptibility as a Paleocurrent Indicator: A Test of the Method

Axes of maximum magnetic susceptibility in oriented cores of the Paso Robles Formation are parallel to the paleocurrent directions determined by other criteria.