Seismicity and tectonics of the northeast Pacific Ocean

Abstract

The hypocenters of approximately 260 earthquakes that occurred during the 10-year period 1954–1963 in the northeast Pacific and in adjacent continental areas were relocated in order to study the relationship of seismicity to large-scale tectonic features in the region between the Mendocino fracture zone and central Alaska. The major seismic zones in this region are related to a series of growing oceanic ridges connected by transform faults. Nearly all the earthquakes on the Blanco fracture zone were concentrated between the Gorda and the Juan de Fuca ridges. Activity on the Mendocino fracture zone was restricted almost exclusively to a 400-km segment of the fracture zone between the coast of northern California and the Gorda ridge. Evidence from the distribution and mechanism of earthquakes suggests that the San Andreas fault bends abruptly near Cape Mendocino and follows the seismically active part of the Mendocino fracture zone. The oceanic block north of the Mendocino fracture zone and east of the Gorda ridge is being deformed internally as well as along its borders. Although the Gorda ridge, which possesses a prominent median rift, exhibited considerable activity during this period, only one event was found on the Juan de Fuca ridge, which lacks a prominent median rift. A requirement for the occurrence of seismic activity along a ridge crest seems to be the presence of a prominent rift valley. The distribution of epicenters suggests that the main zone of present-day seismic activity in southeast Alaska follows the Fairweather fault, passes out to sea near 58½°N, remains offshore, and continues as far south as the northern end of the Juan de Fuca ridge. This entire fault zone, which is called the Queen Charlotte Islands fault, is interpreted as a dextral transform fault that connects the Juan de Fuca ridge with the northeastern end of the Aleutian island arc system. Earthquake mechanism solutions for some of the larger events are in agreement with the above interpretations. No observable seismic activity was associated with the Rocky Mountain trench, the Tintina valley, or the Chatham Strait. Several areas in coastal Alaska that have been relatively aseismic during the past 60 years are discussed as possible sites of future earthquakes.