GEODETIC CONSTRAINTS ON FAULT COUPLING AND BLOCK MOTIONS IN CASCADIA

The degree of coupling on the Cascadia subduction zone and on upper plate faults and the motions of the surrounding plates, large and small, are inferred by simultaneous inversion of GPS velocities, surface tilt rates, surface uplift rates, surface horizontal strain rates, spreading rates, slip vectors, and transform fault azimuths. We use a GPS velocity field comprising over 300 sites derived from a decade of campaign data and regional continuous sites in Oregon and Washington. Angular velocities of the Pacific (Pac) and Juan de Fuca (JdF) plates relative to North America (NAm) are estimated from a combination of GPS, spreading rates, and transform fault azimuth data. In addition, we examine the possibility of independent motions of several smaller plates (micro-plates). Fault locking is estimated using 3D elastic halfspace dislocation models. Along the Cascadia subduction zone, the slip deficit rate (the rate of fault slip not taken up by steady slip), appears to be non-zero only offshore. In current models, the offshore coupled zone south of about 45ºN is only partially (50%) locked while in the north the offshore section of the thrust is fully locked. The Oregon crustal block, including most of the state of Oregon and SW Washington state, rotates clockwise at about 1º/Ma around a pole in NE Oregon. The GPS data show clearly that the southern Cascade volcanic arc does not localize slip or produce a forearc ‘sliver’, as often occurs in oblique subduction zones. Instead the rotation of most of Oregon as a large elastic block, probably driven by Basin and Range extension, removes most of the obliquity of the convergence at the Cascadia subduction zone. Tests also indicate that because the permanent strain rates expected from upper plate faults in the Puget Sound region can be matched equally well by elastic strain rates arising from the coupled subduction zone, it will be difficult to estimate the distribution of fault slip rates in the Puget region using local geodetic data. However, the angular velocity of the Oregon block estimated from GPS data in Oregon indicates an overall rate of convergence of the Oregon block with North America of 6.6 ± 0.7 mm/a near Seattle (at 47.6ºN, 237.7ºE).