Geodetic Deformation Measurements Research Articles

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

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).

Geodetic measurement of deformation in Owens Valley, California

Repeated (1934, 1956, and 1974) geodetic surveys across Owens Valley near the site of the 1872 earthquake indicate right-lateral deformation at the rate of 4 +- 1 mm/yr and possible extension across the valley at the rate of 1 +- 1 mm/yr. Repeated level surveys across the valley indicate a down-to-the-east tilt of about 0.22 +- 0.04 ..mu..radians/yr for the western block with a hinge line near the rupture trace of the 1872 earthquake. This tilt is equivalent to an uplift of the west edge of the valley of 2.2 +- 0.4 mm/yr (i.e., base of the Sierra Nevada scarp) relative to the center of the valley. Although the measured deformations are scarcely above survey noise, they all indicate an accumulation of strain which would be consistent with a repeat of the 1872 earthquake (normal, right-lateral oblique slip).