Geodetic measurements of deformation associated with the Oroville, California, earthquake

Abstract

Comparison of preearthquake vertical geodetic surveys with postearthquake surveys in the region affected by the magnitude 5.7 Oroville earthquake of August 1, 1975, indicates appreciable elevation changes. The data are consistent with 0.36 m of normal slip on a 12×10 km2 rectangular fault dipping 60° to the west. Comparison of level surveys run 1 and 6 months after the earthquake suggests an additional 0.08 m of postseismic slip on the fault. Horizontal geodetic data do not show any significant changes, but the horizontal data are noisier than the vertical data and allow as much as 0.7 m of coseismic left lateral slip within a 95% confidence limit. There is no evidence for postearthquake dilatancy recovery, but the data are not adequate to exclude small amounts (10 mm) of relaxation, or even larger amounts if the dilatant region were very broad (80 km in diameter).

P wave velocity variations in the Coso Region, California, derived from local earthquake travel times

“…Weaver and Hill [1979] suggest that the Coso volcanics represent a local spreading center and is in an extensional state, which would allow magma to rise near the surface. Geodetic data [Savage et al, 1975] support current modest extension for the Coso region [Roquemore, 1981] in accord with the Weaver and Hill [1979] hypothesis. The presence of regional arcuate structures sparked a controversy about Coso's fundamental structural style: Austin et al [1971] and Duffield [1975] favor a calderalike structure, while Roquemore [1981] maintains that the arcuate features result from stresses induced by strike-slip faulting.…”

The Oroville Earthquakes: A study of source characteristics and site effects

“…Both lines actually continue to the west from their common point in the town of Oroville, but observed and theoretically predicted displacements are small on the western extensions, so we have chosen to concentrate on those segments that will best constrain the fault slip at depth. Savage et al [1977] achieve a satisfactory fit to the displacements observed on the southern level line ( Figure 2c), but we feel that greater constraint may be placed on the extent of fault slip if both level lines are modeled simultaneously. Clark et al [1976] plot the change in elevation for the points along each of the lines between the surveys encompassing the Oroville earthquake.…”

“…Figure 2 compares these calculations with the observed deformation. In Figure 2c we compare the observed displacements between preearthquake and postearthquake leveling along the southern level line, corrected for the predicted subsidence due to lake loading, with predicted displacement calculated for a fault model similar to the model presented by Savage et al [1977]. In this model the fault plane runs 12 km down a 60 ø dip from its upper edge at 1.8-km depth, and it extends 10 km along its north-south strike.…”

“…from the filling of Lake Oroville in 1968 and 1969, as this occurred between the preearthquake and postearthquake surveys for both lines. Savage et al [1977], using data and the formulation given by Beck [1976], calculated theoretical subsidence due to the loading of the reservoir. We have taken their values, which they presented as contours on a regional map, to remove this contribution from the values of Clark et al [1976].…”

“…The fault plane, if extended up dip would intersect the surface fractures. This model corresponds to Figure 1 by Savage et al [1977] rather than to the dimensions given in their text, which were misprinted and should read, '... 10 km along strike .... ' (J. C. Savage, verbal communication, 1980). The fit between this model and the observations for the southern level line is not encouraging.…”