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The three lithospheric plates that dominate the modern tectonics of coastal California meet at the Mendocino triple junction, which is marked by a dense cluster of epicenters just off Cape Mendocino ( Figure 5.5A; see chap. 3). North of this triple junction, oblique subduction dominates, with the eastern margin of the Gorda plate (the southernmost section of the Juan de Fuca plate) slipping beneath the North American plate north-northeastward at a rate of 30 to 50 mm/yr (Wilson, 1986). South of the triple junction, the Pacific plate is moving past the North American plate along the San Andreas transform boundary on a heading of 35�-38� W. of N. at a rate of approximately 50 mm/yr (DeMets and others, 1987).

The dense lineation of epicenters that trends west-northwest from Cape Mendocino corresponds closely to the Mendocino Fracture Zone (MFZ) and the Pacific-Gorda plate boundary but follows a slightly more northerly trend. Details of how the San Andreas fault ties into the triple junction, however, are unclear. The trace of the San Andreas fault lies off shore north of Point Arena, and the broad area of seismic quiescence south of the triple junction offers few clues to the kinematics of this northernmost segment of the fault.

The conspicuous linear zone of epicenters marking a northeast-trending slice across the southwest corner of the Gorda plate is the aftershock zone of the M=7.2 Eureka earthquake of November 8, 1980. This was the largest earthquake to occur in California during the interval 1980-86. The N. 53� E. trend of its aftershock zone agrees closely with the strike of the left-lateral slip plane inferred from the focal mechanism of the main shock, which was located about a fourth of the way downstrike from the northeastern end of the aftershock zone (Eaton, 1989). The aftershocks from this earthquake died off within a few months after the main shock - a notably short aftershock sequence for an earthquake of this size.

A third, more diffuse group of earthquakes in the vicinity of the triple junction shows little tendency to concentrate in linear zones. These epicenters form an irregular zone with the greatest concentration in the vicinity of Cape Mendocino, gradually dying off with distance to the north, east, and west. To the south, the seismicity dies off abruptly across the MFZ and its landward extension.

The three-dimensional aspects of this triple junction seismicity are illustrated by four cross sections (Figure 5.5B and Figure 5.5C). The two cross sections perpendicular to the N. 75� W. trend of the MFZ ( Figure 5.5B) compare the distribution of focal depths within the submarine section of the Gorda plate adjacent to the triple junction (A-A') with that within the adjacent, subducted section of the Gorda plate and the overlying North American plate (B-B'). Earthquake hypocenters along the southern margin of the submarine Gorda plate define a dense, vertically elongate zone beneath the MFZ that dips 70�-75� N. and extends to depths of nearly 35 km. Earthquakes north of this zone cluster in a somewhat less dense, wedge-shaped core outlined by a northward shallowing of maximum focal depths accompanied by a northward deepening of minimum focal depths that converge at a depth of about 20 km (Eaton, 1989). Overlying this relatively dense wedge is a more diffuse distribution of hypocenters, the northernmost of which represent the 1980 M=7.2 event and adjacent after-shocks. The absence of seismicity south of the MFZ indicates that the northeast corner of the Pacific plate behaves as a rigid block in its interaction with the younger, thinner, and internally deforming Gorda plate (Wilson, 1986; Eaton, 1989).

In the onshore cross section (B-B', Figure 5.5B), the intense seismicity associated with the MFZ loses most of its expression. Here, the southern margin of the subducted Gorda plate is marked only by isolated clusters of hypocenters at depths of 10 and 25 km. To the north, however, a band of hypocenters concentrated at about a depth of 20 km corresponds closely to the base of the wedge-shaped distribution beneath the submarine section of the Gorda plate, including the downward flexing of this band as it approaches the southern margin of the Gorda plate. South of the landward extension of the MFZ, the seismicity shallows rather abruptly, reflecting the edge of the subducted Gorda plate beneath the North American plate and the rather thin seismogenic crust associated with the San Andreas fault system to the south.

The cross section parallel to and including the MFZ and its landward extension (D-D', Figure 5.5C) reveals that the dense seismicity cluster along the MFZ near the triple junction tapers westward along the MFZ with a wedge-like geometry to a 20-km-deep band of hypocenters, much the same as it does to the north (cross sec. A-A', Figure 5.5B). (The pronounced linear concentration of hypocenters at 15-km-depth beneath the submarine Gorda plate in cross sections A-A' and D-D' represents the default focal depth for the more poorly located earthquakes beyond the perimeter of the onshore seismic network.) The landward extension of the MFZ shows up only weakly as a diffuse scattering of hypocenters extending to a small, isolated cluster of 25-km-deep hypocenters some 50 km east of the triple junction (A = 190 km, cross sec. D-D') and, possibly, as far as a handful of 30-to 50-km-deep hypocenters 100 km east of the triple junction (A=260 km, cross sec. D-D'). Focal depths of the shallow seismicity in the northern Coast Ranges are confined to the upper 10 km of the crust. Farther east, however, focal depths increase again to depths of 35 to 40 km beneath the north end of the Great Valley. An east-dipping quiescent band, about 10 km thick, appears to separate the seismicity associated with the MFZ from that beneath the northern Coast Ranges and the Great Valley to the east. The geometry of this band suggests that it may somehow be related to the geometry of the subducted Gorda plate beneath the western margin of the North American plate.

The parallel cross section immediately to the north (C-C', Figure 5.5C) reinforces the impression that the distribution of hypocenters carries an image of subducted-Gorda-plate geometry. Maximum focal depths increase systematically from 25 to 30 km beneath the submarine Gorda plate to nearly 80 km beneath the southern Cascade volcanoes. Although the easternmost of these deep earthquakes are small and few, their locations are well constrained (Cockerham, 1984; Walter, 1986). As in the section of the fault to the south, a seismically quiescent, east-dipping band appears to separate earthquakes within the Gorda plate from those In the overlying North American plate.