DEFORMATION OF THE GORDA PLATE
[c5, p145]

As the small, youthful Gorda plate is subducted obliquely northeastward beneath the North American plate, it is being subjected to north-south compression in response to a component of convergence between the larger, older Juan de Fuca plate to the north and the Pacific plate to the south (Jachens and Griscom, 1983; Wilson, 1986). Distorted marine magnetic anomalies within the Gorda plate indicate that it has undergone progressive internal deformation over the past 5 Ma in response to this compression (Silver, 1971), and current seismicity within the plate (Figure 5.4) indicates that this deformation continues to the present.

The 1980 Eureka M=7.2 earthquake emphasizes that part of this deformation occurs with left-lateral slip on northeast-striking faults within the plate. The seismicity map and cross sections (Figure 5.4) demonstrate that deformation associated with the Gorda plate terminates abruptly against the Pacific plate in a steeply north-dipping zone of interaction along the MFZ, which can be followed on shore beneath the North American plate as a gently east-dipping, subhorizontal zone of widely scattered small to moderate earthquakes. Thus, convergence between the Gorda and Pacific plates across the MFZ apparently occurs by crushing and thickening of the southern margin of the Gorda plate as it is jammed against the anvil-like mass formed by the thicker and colder Pacific plate. Diminished east-west stress in the Gorda plate resulting from the subducting limb of the plate farther east serves to increase the difference between the maximum (north-south) and minimum (east-west) compressive stresses within the plate, leading to left-lateral strike-slip displacements along northeast- d striking faults, as in the M=7.2 Eureka earthquake. This process accommodates the convergent component of Gorda-Pacific plate motion along the east end of the MFZ p at the expense of fragmentation and eastward expansion of the Gorda plate north of the MFZ.