THE GEODETIC SIGNATURE OF MODERN DEFORMATION WITHIN THE SOUTHERN CASCADIA MARGIN, NORTHWESTERN CALIFORNIA

Humboldt State University and University of Alaska-Fairbanks are using GPS observations to study the Mendocino Triple Junction and the southern Cascadia margin. We carried out a GPS measurement campaign in May 2001, and combined this data with several years of previous data collected by various groups. This region includes the transition zone between the northwestward propagating Pacific-North American plate boundary that is the San Andreas fault (SAF) system, and the northeast converging Juan de Fuca/Gorda-North American plate boundary. We present GPS survey results for 68 sites in northwestern California for the time period 1993-2002. The average uncertainty for the horizontal velocity components is 2.1 mm/yr (easting) and 1.2 mm/yr (northing). This gives us for the first time a velocity field that spans the entire triple junction region as well as the areas inland to the east. Furthermore, GPS site velocity estimates of strain accumulation immediately south of this region related to the faults of the SAF system demonstrate a substantial amount of contemporary translational strain entering this region from the south at 39.6 +1.5/-0.6 mm/yr (68% confidence; see Freymueller et. al., 1999). We depict azimuths and velocities along two profiles, one parallel to the Gorda/Juan de Fuca margin, and the other perpendicular to the margin at the latitude of the triple junction. The first demonstrates that the transition from northwest directed strain to northeast directed strain occurs within the zone of upper plate contractional faults in the Humboldt Bay region. The other profile demonstrates that right lateral displacements associated with the SAF system increase from its inception at the trace of the Lake Mountain fault zone westward to the triple junction region. Dislocation modeling of Gorda-North America plate convergence allows us to quantify the magnitude and direction of SAF system displacements influencing the southern Cascadia margin, as well as estimate the contribution of upper plate contractional structures to these interseismic observations. We will for the first time provide estimates of the relative influences of the competing tectonic processes contributing to the total observed interseismic measurements in this region.