STRAIN ACCUMULATION RATES IN THE WESTERN UNITED-STATES BETWEEN 1970 AND 1978

The rate of dilatation and the rate and directio of shear have been determined from trilateration data for 23 geodolite networks in the western US. Sixteen nets are located along the San Andreas fault system betweeen Point Reyes, California, and the US-Mexico border. Implicit in the treatment are the assumptions that the strain was accumulating at a constant rate over the time period (within the interval 1970-1978) and over the local areas (usually about 50km diameter) covered by the surveys. Of the nets located away from the San Andreas fault, only Ogden and Hebgen show significant strain accumulation. At Ogden the deformation is principally an east-west compression of 0.23 +- 0.05 mu-strain/yr and at Hebgen Lake a northeast-southwest extension of 0.17 +- 0.03 mu-strain/yr. Along the San Andreas fault system the rate of shear agrees very well with the surface strike of nearby faults.-Authors