CRUSTAL DEFORMATION NEAR THE MENDOCINO TRIPLE JUNCTION

We use geodetic measurements to constrain how compressional deformation associated with the Cascadia subduction zone is replaced by strike-slip deformation across the San Andreas fault system as the Mendocino triple junction migrates northward along the California margin. Geodolite trilateration measurements from 1981 to 1989, and GPS measurements since 1989 have measured interseismic deformation near Cape Mendocino and along the northern San Andreas fault system, and coseismic deformation caused by several earthquakes, including the 1992 M=7 Cape Mendocino earthquake, which caused a meter-level uplift of the coast revealed by the die-off of intertidal marine organisms.

The trilateration data show a complex transition from right-lateral shear distributed across the San Andreas fault system accommodating 25 mm/yr (about 50% of the total predicted Pacific-North America relative plate motion), to about 15 mm/yr of uniaxial contraction across the Eel River and Mad River fault systems in the onshore accretionary wedge, in general agreement with geologic studies. The GPS measurements span a broader region, including stations distributed globally and on the Pacific and North America plates, which allows the interseismic crustal deformation to be modeled as a combination of plate tectonic motions and interseismic elastic strain accumulation on the megathrust and crustal faults. The northern San Andreas fault system accommodates about 40 mm/yr of right-lateral motion on 3 major faults, about half on the San Andreas fault itself. Interseismic deformation near Cape Mendocino is more difficult to resolve due to coseismic and possible postseismic earthquake effects. We will investigate models incorporating the megathrust, Mendocino fault, and San Andreas fault systems, and assess whether strain on other crustal faults can be resolved by the observations.