Pacific‐North America Plate motions: New results from very long baseline interferometry

Abstract

Expansion and densification of the very long baseline interferometry (VLBI) network by the NASA Crustal Dynamics Project continues to clarify details of Pacific‐North America motions. In this paper I construct, for the first time, rigid plate tectonic models based purely on space geodetic data. Construction of VLBI plate models has the advantages of averaging the bias and errors of individual site velocities, making failures in the rigid plate assumption easy to document, and allowing the consistency of specified plate models to be quantified against the VLBI data set as a whole. VLBI‐based rigid plate models VM2 and VM3, obtained by fitting vector velocities of 10 sites in the stable interiors of the North America and Pacific plates, reveal a misfit of VLBI observations and RM2 (Minster and Jordan, 1978) predictions significant at the 99% confidence level. NUVEL l (DeMets et al., 1990) reproduces the velocities of these 10 sites far better than RM2 and offers a fit to the data which is statistically indistinguishable from VM2 and VM3. By including velocities of 16 additional stations in the deforming plate margin, VM2 and VMS can resolve all four terms (VPN, total Pacific‐North America motion, VSA San Andreas slip, VE,W, path integrated deformation east and west of the San Andreas) in the vector equation characterizing Pacific‐North America plate boundary deformation: VPN ‐ VSA = VE ‐ VW for several points along the San Andreas without reference to a global plate model, geological or local geodetic measurements, or restrictions on distributed shear. In most cases, instantaneous VLBI‐determined rates agree with geologically based estimates. Mapped to point C (36.0°N, 120.6°W) in central California, VM2 gives VPN(C) = 46.8±1.4 mm/yr (N36°±2°W), VSA(C) = 38.6±1.3 mm/yr (N36°±2°W), and VE(C) = 8.2±1.3 mm/yr (N34°±7°W). VLBI substantially eliminates VW(C), the component of the San Andreas Discrepancy west of C, although a few millimeters per year of right‐lateral strike slip on offshore faults can not be rejected. At point D (34.1°N, 117.0°W) in southern California, VM2 returns VSA(D) = 25.0±1.1 mm/yr (N52°±3° W), VE(D) = 8.8±1.1 mm/yr (N25°±8°W), and VW(D) = 13.3±1.6 mm/yr (N21°±5°W). VW(D) is accommodated west of the San Andreas to the Channel Islands. The azimuth of San Andreas slip suggests that the transverse ranges north of Los Angeles are absorbing 8±1 mm/yr of compression perpendicular to the trend of the fault.