Variation of seismic slip in the Gulf of California and the possible effect on geodetic measurements of Pacific‐North American Plate motion

Abstract

The Gulf of California is a unique tectonic transition zone which progresses from an oceanic ridge‐transform system at the mouth of the gulf to a predominantly continental transform system in southern California. Slip rates estimated from seismic moment data amount to about 17 to 30% of the accumulated tectonic slip in the gulf predicted by the NUVEL‐1 plate motion model, even after accounting for the transition in crustal structure. The seismic to tectonic slip ratios also appear to increase slightly from south to north. This trend in the seismic moment release may reflect changes in the accommodation of Pacific‐North American plate motion along the gulf which can be examined geodetically. On the other hand, geodetic measurements of the relative plate motion can be disturbed by short‐term effects associated with episodic seismicity. To assess these effects, the surface displacements due to typical transform events in the gulf are estimated using a simple dislocation model. The results of this numerical calculation suggest that if a large transform event (M0 ∼ 1.5 × 1026 dyne cm) were to occur within 100 to 200 km of a geodetic baseline, for example one comprised in the GEOMEX Global Positioning System network across the southern gulf, the relative distance measurements could be affected by up to 15 mm. This is marginally at the error level of a few millimeters plus 2 parts in 108 of baseline length for GEOMEX measurements, which thus are sensitive only to the far‐field displacement along the plate boundary (over baselines of a few hundred kilometers in length).