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Seismicity of the mendocino escarpment and the aftershock sequence of June 26, 1968: Ocean bottom seismic measurements

LAMONT-DOHERTY GEOLOGICAL OBSERVATORY OF COLUMBIA UNIVERSITY, PALISADES, NEW YORK 10964

Abstract

Since May 1966, OBS 3, an ocean-bottom geophysical station located 220 km south of the Mendocino fracture zone at a depth of 3903 meters and PTA, a supporting land station at Point Arena, California, have been operating continuously. During a 5-year period OBS 3 recorded over 650 local earthquakes with suboceanic epicenters, about 200 of which were also recorded by PTA. Earthquakes recorded by OBS 3 have clear P, S, and T phases; those recorded by PTA have clear P and S phases but generally do not have clear T phases. The 200 earthquakes recorded by both stations were located on the basis of S —P time intervals at OBS 3 and at PTA. Readings from the Berkeley network of seismographic stations were also used when possible. Most of the submarine earthquakes located are related to the Mendocino and Gorda escarpments. Earthquakes do not occur beyond the intersection of the Mendocino fracture zone and the Gorda Ridge. The entire Gorda Basin is seismically active, implying that it is presently undergoing internal deformation.

In an attempt to confirm a possible subduction zone in the vicinity of the coast and north of the Mendocino fracture zone, five earthquakes were relocated, two of which were probably subcrustal. One earthquake (August 23, 1962) had a focal depth of 59.4 km, which is confirmed by the time intervals of the pP—P and sP—P phases. The second (September 4, 1962) had a depth of 45 km, but because of the earthquake's small magnitude, its depth could not be confirmed. These subcrustal earthquakes do indicate a possible active subduction zone close to the coast north of the Mendocino fracture zone. The submarine aftershock sequence of June 26, 1968 indicates that (1) b = 0.5 ± 0.1 (This is slightly smaller than other b values given for the adjacent continental region.); (2) the magnitude versus the length of the fault for this sequence is approximately similar to that of other recent sequences in California; (3) the composite fault-plane solution for all of the aftershocks is similar to that of the main shock and gives a fault trend of N41°E.