STRAIN ACCUMULATION IN THE SHUMAGIN ISLANDS - RESULTS OF INITIAL GPS MEASUREMENTS

Deformation in the Shumagin seismic gap has been monitored with repeat ed trilateration (EDM) in the 1980-1987 interval and with the Global P ositioning System (GPS) in the 1987-1991 interval. The geodetic networ k extends for 100-km across the Shumagin Islands to the Alaska Peninsu la. Results from the GPS surveys are consistent with those previously reported for the EDM surveys: we failed to detect significant strain a ccumulation in the N30-degrees-W direction of plate convergence. Using the method of simultaneous reduction for position and strain rates, w e found the average rate of extension in the direction of plate conver gence to be -25+/-25 nanostrain/yr (nstrain/yr) during the 1987-1991 i nterval of GPS surveys compared with -20+/-15 nstrain/yr during the 19 81-1987 interval of complete EDM surveys. We found a marginally signif icant -26+/-12 nstrain/yr extension rate in the 1981-1991 interval cov ered by the combined EDM and GPS surveys. Strain rates are higher, but not significantly so, in the part of the network closest to the trenc h. Spatial variation in the deformation is observed in the 1980-1991 a verage station velocities, where three of the four stations closest to the trench have an arcward velocity of a few mm/yr. The observed stra in rates are an order of magnitude lower than the -200 nstrain/yr rate predicted by dislocation models.