Seismological evidence on the tectonics of central and northern California and the Mendocino escarpment

ABSTRACT Some 300 regional earthquakes have been located for 1962 through June 1965 using readings from local stations. The three greatest concentrations occur close to the north-facing Gorda escarpment off Cape Mendocino (latitude 40.°4 N); in the Coast Ranges west of the San Andreas fault (latitude 36.°8N) between Watsonville and Salinas; and 40 km southwest, in the Gabilan Range west of the San Andreas fault near the Pinnacles (latitude 36.°5 N). For 280 km northward of a small cluster which straddles the San Andreas off San Francisco, earthquakes were not detected near to the San Andreas but eastward in the Coast Ranges. The bulk of earthquakes with magnitudes in excess of 2 1 2 in the Coast Ranges have focal depths less than 5 km; no reliable depths were estimated greater than 20 km. Seismicity on the Gorda scarp is high; it decreases westward along the Mendocino escarpment. Submarine earthquakes were detected westward to the northerly-trending (Escanaba) trough which traverses the Gorda ridge and then northward along the Gorda ridge. Earthquakes also were located within the Gorda basin between the California coast and the Gorda ridge. The first motions of P were read for 29 earthquakes using mainly the high-resolution near-stations. The patterns are consistent with dominant right-lateral strike-slip along the San Andreas-Calaveras fault system from Parkfield to San Francisco Bay; some exceptional motion occurs near the San Andreas fault in the vicinity of the San Francisco Peninsula. In the Coast Ranges north of San Francisco Bay, right-lateral strike-slip appears again to be the rule. Along the Gorda wall, the P polarities are consistent with the local escarpment trend of about N70°W. Earthquakes on the continental shelf north of Cape Mendocino and in the Gorda basin have fault-plane solutions which again indicate right-lateral strike-slip with strikes of about N40°W. The motions are consistent with a broad system of branching faults whose trend at the northern extremity becomes aligned with the Blanco fracture zone. On the Gorda ridge, the shock of 18 April 1965 shows normal fault characteristics. Mechanisms for four earthquakes on the Blanco fracture zone are not uniform; right-lateral strike-slip motion is again found for the earthquakes of 8 May 1968 at 128° west and of 14 June 1965 at nearly 130° west, while an earthquake nearly midway between them has normal faulting. Overall, from the Gulf of California to the Juan de Fuca ridge, seismicity and earthquake mechanism agree well with a mechanical model based on the theory of J. Tuzo Wilson. Some motion is transferred by transcurrent faulting from the San Andreas fault across the Gorda basin to the Blanco transform-fault system.