GEODETIC CONSTRAINTS ON CASCADIA LOCKING

Surface velocities derived from GPS observations from 1993 to 2013 at several hundred sites across the deforming northwestern United States are used to further elucidate the region’s active tectonics and locking on the Cascadia subduction zone. The velocities reveal that the clockwise rotations, relative to North America, seen in Oregon and western Washington from earlier GPS observations, continue to the east to include the Snake River Plain of Idaho and south into the Basin and Range of northern Nevada. Elastic fault locking at the Cascadia subduction zone is evaluated using the GPS velocities and published uplift rates of Burgette et al. (JGR, 2009). The 20-year GPS and 80-year leveling data can both be matched with common locking models suggesting that the general locking pattern has been stable over many decades (notwithstanding variations due to slow-slip events). High locking is observed off the Olympic Peninsula of Washington (around 48N) and decreases to the south. The low uplift rates along coastal central Oregon (Newport area) do not indicate a lack of or relative decrease in locking but instead a more spread out locking distribution (as required by horizontal GPS observations). The modern rate of strain accumulation is consistent with hundreds of years between great subduction events.