Northern California Seismicity:
1980–1986
The distribution of earthquakes in northern California for the seven years 1980–1986 is shown in figure 5. Only earthquakes of magnitude 1.3 and larger with data from seven or more stations in their hypocentral determinations are included. This time period was chosen because network coverage
Figure 4
Low-gain vertical (HQRZ) and north-south (HQRN) component records of a 3-km-deep
magnitude-3 earthquake 22 km from station HQR. Top traces are without filters. Second
through seventh traces were played back through 24 dB/ octave bandpass filters. Filter
corner frequencies and relative playback gains are indicated on the individual traces.
has not changed substantially since 1980. Even for these years, however, the catalog is believed to approach completeness at magnitude 1.3 only in the core of the network between Cholame and Laytonville. In the northern and northeastern parts of the net, the catalog is incomplete below magnitude 2.0.
Several broad zones of seismicity dominate the map. The most prominent coincides with the Coast Ranges between Cholame and Laytonville. Another prominent zone is associated with the Mendocino fracture zone and triple junction, within about 100 km of Cape Mendocino. A third, somewhat less well-defined zone runs along the east side of the Sierra Nevada from near
Figure 5
Northern California seismicity: 1980–1986. Symbol sizes are scaled according
to magnitudes. Only events with magnitudes greater than or equal to 1.3 and
with seven or more stations in the hypocentral solution were included in the
plot. Abbreviations: SAF = San Andreas fault, NFZ = Nacimiento fault zone,
OF = Ortigalita fault, CF = Calaveras fault, HF = Hayward fault, GF =
Greenville fault, GVF = Green Valley fault, BSF = Bartlett Springs fault,
HBF = Healdsburg fault, MF = Maacama fault, MFZ = Mendocino fracture
zone, COA/KET = Coalinga/ Kettleman aftershocks region.
Mount Shasta on the northwest to Mono Lake on the southeast. A fourth zone appears to run along the western foothills of the Sierra Nevada from Shasta reservoir to Oroville, with a branch that deflects southward to the center of the valley north of the Sutter Buttes. Other small concentrations of earthquakes are scattered beneath the western Sierra Nevada and beneath the Great Valley, but these do not form a continuous zone like those described above.
We shall explore the distribution of earthquakes around Cape Mendocino and in the Coast Ranges in more detail in search of an explanation for the contrasting styles of seismicity in these principal regions of northern California.
Mendocino Seismic Zone
An expanded map of the Mendocino seismic zone is shown in figure 6, which also outlines the subregions for which cross sections are presented below. The Gorda/Juan de Fuca, Pacific, and North American plates meet at a common point, the Mendocino Triple Junction, where the Mendocino fracture zone meets the coastline. Global analysis of relative plate motions (Atwater, 1970) suggests that the relative motion between the Gorda/Juan de Fuca plate and the Pacific plate can be resolved into about 5.1 cm/year right-lateral displacement along the Mendocino fracture zone and about 2.7 cm/ year convergence across it (fig. 7). The same analysis suggests that the Gorda/Juan de Fuca plate is subducting obliquely (in about a N52°E direction) beneath the North American plate at about 2.5 cm/year if the motion between the Pacific and North American plates is 5.8 cm/year right-lateral strike-slip along the San Andreas. For somewhat smaller San Andreas slip rates, the direction of subduction is more easterly, but the rate of subduction is almost unchanged.
Just how the San Andreas ties into the triple junction is not clear. The trace of the San Andreas is very poorly defined between Cape Mendocino and Point Arena, a distance of about 150 km. Moreover, the region between the end of the Coast Ranges seismic zone and the Mendocino seismic zone appears to be almost aseismic. These problems, as well as the basic framework of plate motions (fig. 7), should be kept in mind as we examine the pattern of seismicity around Cape Mendocino in more detail.
The highest concentration of epicenters in the Mendocino seismic zone (fig. 6) is in a 50-km-long band extending about N75°W from the shoreline at Punta Gorda, just south Cape Mendocino, to about longitude 125°W. This zone lies beneath the north-facing Gorda escarpment at the eastern end of the Mendocino fracture zone (Bolt et al., 1968). A more diffuse zone of epicenters extends this band 80 km farther west to about 126°W longitude. The overall trend of this zone is 10º to 15° more northerly than the near-west trend of the Mendocino fracture zone west of 126°W longitude.
Figure 6
Map of the Mendocino seismic zone showing regions (A–Á through E–É) for
which cross sections were prepared. Abbreviations: SAF = San Andreas fault,
MF = Maacama fault, BSF = Bartlett Springs fault, MFZ = Mendocino fracture
zone, 1980 M. S. = epicenter of the November 8, 1980, Eureka earthquake.
The second prominent linear zone of epicenters was entirely defined by aftershocks within the first month after the November 8, 1980, Eureka earthquake. This zone extends from about 40.5°N, 126°W, where it joins the west end of the zone of epicenters described above, for a distance of about 140 km along a N53°E trend to a point about 30 km northwest of Trinidad Head. The trend of this zone agrees with the strike of the fault deduced from the fault-plane solution of the November 8 earthquake. It appears to mark the principal zone of faulting (left-lateral strike-slip) associated with that earthquake (Eaton, 1981).
A third group of earthquakes in the Mendocino region shows little tendency to concentrate in linear zones. Events of this group spread over a sub-
Figure 7
Plate tectonic setting of the Mendocino seismic zone. Relative plate motions
are from Atwater (1970). G/JF = Gorda/Juan de Fuca, P= Pacific, and
NA = North American plates. The G/JF-P motion is resolved into its components
parallel to (G/JF
fracture zone. The location (if not the definition) of the Mendocino triple
junction, the subduction zone north of Cape Mendocino, and the San Andreas
fault north of Point Arena are uncertain.
rectangular zone defined by the shoreward projection, parallel to a line trending S75°E, of the 1980 aftershock zone. These events are most concentrated near Cape Mendocino and die off gradually with increasing distance from that point. Their concentration also diminishes abruptly about 100 km inland from the coastline.
To explore the three-dimensional aspects of the distribution of Mendocino earthquakes we have plotted a sequence of five cross sections in which vertical projection planes are perpendicular to the N75°W trend of earthquakes associated with the Mendocino fracture zone and approximately parallel to the trend of the coastline between Cape Mendocino and Trinidad Head. The sections A–Á through E–É depict earthquakes in five contiguous 40 by 200-km rectangles that span the range from 60 km offshore to 140 km onshore. Vertical lines on the sections at 75 km show the position of the southern edge of the band of earthquakes west of Cape Mendocino or its landward projection. Just offshore, this line (fig. 6) coincides with the inferred fault that Jennings (1975) shows branching southeastward from the Mendocino fault zone. Horizontal lines at 20- and 30-km depths are shown to ease comparison of the sections. Sections west of A–Á were not included because focal depths are unreliable. All five sections are shown in figure 8, with A–Á (westernmost) at the top and E–É (easternmost) at the bottom. This sequence of sections should be viewed like sequential transverse sections of a biological specimen, which can be used to trace longitudinal variations in complex structures. The principal mapped features on figure 6 that we wish to trace and compare are: (1) events on the Mendocino fracture zone, (2) aftershocks of the 1980 Eureka earthquake, (3) events in the diffuse swarm centered on the triple junction and lying north of the fracture zone and its landward projection, and (4) events in the linear zones of epicenters in the northern Coast Ranges that terminate south of the Mendocino seismic zone.
Events along the Mendocino fracture zone are very prominent in sections A–Á and B–B́, where they are concentrated in a dense, vertically elongated zone of hypocenters at depths of 10 to 35 km that dips 70° to 75° toward the north (that is, to the right in fig. 8). Seismicity appears to terminate abruptly south of the fracture zone. The concentration of hypocenters at depths of 10 to 25 km and at distances of 80 to 85 km on section B–B́ corresponds to the cluster of events on figure 6 where the Mendocino fault zone (Jennings, 1975) approaches the shoreline at Cape Mendocino. On section C–Ć, 20 to 60 km inland from the shoreline, the trend of the fracture zone is represented
Figure 8
Transverse cross sections of the Mendocino seismic zone. Regions
corresponding to sections A–Á through E–É are outlined in figure 6. Only
earthquakes with magnitudes greater than or equal to 1.5 ( M ³ 1.5) and with
seven or more stations in the hypocentral solution ( NS > 6) were included.
only by modest clusters of events at 10- and 25-km depths, and farther east is unmarked by earthquakes.
The aftershocks of the 1980 Eureka earthquake are prominent on section A–Á at depths from the surface to about 20 km and at distances of 120 km to 180 km on the profile. The 1980 aftershock zone runs diagonally across region A–Á and passes out of it to the west at about 120 km on the profile. On section B–B́ the 1980 aftershocks are represented by the shallow cluster at 170 km. This cluster occurred about 20 km southeast of the eastern end of the principal 1980 fault break during the 1980 aftershock sequence (fig. 6).
The character of the scattered events north of the fracture zone and east of the 1980 aftershock zone appears simplest in sections C–Ć and D–D́, from 20 to 100 km east of the shoreline. On C–Ć these earthquakes appear to occur largely in a 100-km-thick zone between 20 and 30 km deep. This zone is horizontal between about 130 and 190 km on the profile. It appears to bow upward between 95 and 130 km and to bend downward sharply south of 95 km just before an abrupt cutoff at 85 km. In section D–D́ the zone is slightly thicker and more uniform in depth, although it does bend downward at its south end between 90 and 100 km on the profile. In section E–É events of this group are sparser than farther west and appear primarily in three patches: 80 to 100 km, 120 to 150 km, and 170 to 190 km. The patch on the south, at about 90 km, descends to a depth of about 50 km. A west-northwest to east-southeast profile through the zone of scattered earthquakes suggests that the zone dips very gently eastward for about 70 km from the coastline and then more steeply farther east. However, the number of events defining the steeply dipping part of the zone is very small (Cockerham, 1984).
The group of earthquakes discussed above is more difficult to isolate on sections A–Á and B–B́ because it merges with events along the fracture zone on the south and along the 1980 aftershocks zone on the north. On section A–Á these events appear in a horizontal band 10 to 20 km deep between 110 and 180 km on the profile. This band appears to thicken south of 110 km and to merge with events along the fracture zone at depths of 15 km to at least 30 km. On section B–B́ these events define a band 15–25 km deep between 110 and about 180 km. South of 110 km the zone thickens and appears to blend into the concentrated zone of hypocenters just north of the fracture zone. Heavier concentrations of events just north of the fracture zone suggest that the earthquakes may be related to disrupted seismogenic slab fragments. The scattered events on C–Ć at distances less than about 75 km mostly lie offshore, west of the inferred trace of the San Andreas fault. They suggest that earthquakes occur as deep as 20 km, but the network coverage is poor, and hypocentral locations are uncertain in this region.
The northern ends of the Maacama and Bartlett Springs faults appear as the shallow zones of earthquakes (0–10 km deep) between 20 and 50 km on section D–D́ and o and 65 km on section E–É. The sharp change in max-
imum depth of earthquakes marks the boundary between the Mendocino and Coast Ranges seismic zones.
Coast Ranges Seismic Zone
An expanded map of the Coast Ranges seismic zone, with outlines of regions for which cross sections were constructed, is shown in figure 9. The pattern of seismicity and its relationship to the San Andreas fault, as well as the position of the fault within the Coast Ranges, vary greatly from southeast to northwest. The most extensive feature of the seismicity is a nearly continuous band of earthquakes near the midline of the Coast Ranges, extending from Cholame on the southeast to Laytonville on the northwest. This line coincides with the creeping section of the San Andreas fault where it runs diagonally across the Coast Ranges between Cholame and Corralitos. Seismicity along the San Andreas is weak where it approaches the Coast northwest of the actively creeping section, between Corralitos and San Francisco along the southern end of the 1906 earthquake rupture zone. Northwest of San Francisco, where the 1906 offsets were largest, the San Andreas lies along the western edge of the Coast Ranges and is almost aseismic. Southeast of Cholame, in the region of the 1857 earthquake rupture zone, the San Andreas lies along the eastern edge of the Coast Ranges and, as northwest of San Francisco, is virtually aseismic.
In the region of complex faulting southeast of Hollister, adjacent to the section where creep dies out along the San Andreas, both the dense line of earthquakes and fault creep branch northward off the San Andreas fault onto the Calaveras fault farther east. The line of earthquakes follows the Calaveras, Hayward, Healdsburg, and Maacama faults, in order, past the east side of San Francisco Bay and on northwestward to Laytonville. A second, less continuous line of earthquakes branches eastward from the Calaveras fault south of Livermore. The second line can be followed northwestward from Livermore along the Greenville, Concord, Green Valley, and Bartlett Springs faults as far as Covelo. This line lies 30 to 40 km east of the principal Calaveras/Maacama line, and along its southern half it lies near the eastern edge of the Coast Ranges.
An apparent southeastward continuation of the line of earthquakes near the east edge of the Coast Ranges follows the Ortigalita fault from San Luis Reservoir to New Idria, east of the northern part of the creeping section of the San Andreas fault. An ill-defined linear zone of earthquakes west of the Coalinga/Kettleman aftershock zone suggests that the Ortigalita trend may extend even southeast of New Idria.
Additional, more prominent features of the southern part of the Coast Ranges seismic zone, however, are the broad bands of scattered earthquakes that lie along the flanks of the Coast Ranges, particularly on the eastern flank southeast of New Idria. There, the 20-km-wide by 50-km-long aftershock
Figure 9
Map of the Coast Ranges seismic zone showing regions (I–Í, A–Á, etc.)
for which cross sections were prepared. Abbreviations: SAF = San Andreas
fault, NFZ = Nacimiento fault zone, OF = Ortigalita fault, CF = Calaveras
fault, HF = Hayward fault, GF = Greenville fault, GVF = Green Valley fault,
BSF = Bartlett Springs fault, HBF = Healdsburg fault, MF = Maacama fault,
COA/ KET = Coalinga/Kettleman aftershock region.
Figure 10.
Longitudinal cross sections along the Maacama/Calaveras fault zone (I–Í)
and the San Andreas fault (II–IÍ). Only earthquakes with magnitudes
greater than, or equal to 1.5 (M ³ 1.5) and with seven or more stations
in the hypocentral solutions (NS > 6) were included. The dashed lines
mark the apparent lower limit of the zone of continuous seismicity along
the faults. The widths of the zones of earthquakes included on the plots
are 60 km for I–Í and 30 km for II–IÍ. Vertical exaggeration of these
sections is two times. Abbreviations: LV = Laytonville, CL = Clear Lake,
SB = Suisun Bay, LVM = Livermore, SF = San Francisco, COR =
Corralitos, BV = Bear Valley, PKF = Parkfield, CH = Cholame.
zone of the 1983 Coalinga and 1985 Kettleman Hills earthquakes, is the dominant feature on the map for 1980–1986. Both these large earthquakes, as well as almost all of their larger aftershocks, had thrust or reverse fault sources on planes with strikes nearly parallel to the San Andreas (Eaton, in press). The aftershock zone shows the extent of the zone of crustal shortening near Coalinga. Its large width contrasts sharply with the narrow lines of epicenters that mark creeping sections of the major near-vertical strike-slip faults and of the aftershocks of the large earthquakes that occur along them (Eaton et al., 1970; Cockerham and Eaton, 1987).
To examine the Coast Ranges seismicity in more detail, we have constructed cross sections for each of the boxes outlined on figure 9. Events within each box were projected onto a vertical plane parallel to the long axis of the box. Sections I–Í and II–IÍ (fig. 10) are longitudinal sections parallel to the dense lines of earthquakes in the northern Coast Ranges (Maacama, Bartlett Springs to Calaveras faults) and the southern Coast Ranges (San Andreas fault), respectively. The northern box (I–Í) is 60 km wide in order to include both the central and eastern lines of earthquakes, while the south-
ern box (II–IÍ) is only 30 km wide in order to separate activity on the San Andreas from that on its major branches north of Hollister.
The longitudinal profiles illustrate two principal aspects of seismicity along the faults: (1) the intensity of seismic activity along the fault and (2) the depth to the bottom of the continuous seismogenic zone along the fault. Two reference lines are drawn on the longitudinal sections. The first is a straight line at 15-km depth. The second (dashed) line marks the depth of the abrupt decrease in the abundance of earthquakes at the apparent base of the continuous seismogenic zone. Some of the scattered hypocenters below this line are reliably located and deserve special note. Others are possibly poorly located events that occurred at shallower depths.
Earthquakes on profile I–Í are moderate in number and rather evenly distributed along the fault from Laytonville to Suisun Bay (0 to 250 km) except for the dense shallow cluster of events in the Geysers/Clear Lake region (135 to 155 km). The depth to the bottom of the continuous seismogenic zone averages about 10 km over this region. It descends to about 12–13 km south of the Geysers/Clear Lake region and rises to only 5 km beneath, and just north of, that region. A small deeper cluster of well-located events, however, lies at 13–18 km beneath the shallow continuous seismogenic zone near Clear Lake. Near Suisun Bay the eastern line of seismicity lies at the eastern edge of the Coast Ranges, and earthquakes beneath Suisun Bay occur as deep as 15–25 km. Farther southeast along profile I–Í the bottom of the seismogenic zone ranges between 10 and 15 km and averages 12 to 13 km deep. The heavy concentration of events along this southern section of the profile reflects both an increased level of background seismicity and aftershocks of several large earthquakes.
The most prominent feature on section II–IÍ is concentration of earthquakes along the creeping section of the San Andreas fault, between Corralitos (120 km) and Parkfield (280 km). The base of the continuous seismogenic zone ranges from 10 to 15 km and averages 12 to 13 km deep along this section. Northwest of Corralitos, between 90 and 120 km on the profile, earthquakes shallower than 10 km are virtually absent, although weak seismicity between 10 and 15 km deep continues beneath the quiet zone. Farther northwest, patches of small, infrequent earthquakes occur between the surface and about 12 km deep in the distance ranges 55 to 90 km and 5 to 35 km. The northernmost of these earthquakes are on the San Andreas fault adjacent to San Francisco.
Seismicity along most of the southern half of the actively creeping section of the San Andreas, between about 210 and 256 km on the profile, is less intense than along the northern half (120 to 210 km) and at the extreme southern end just north of Parkfield. Most events in this section of less intense seismicity are between 2 and 12 km deep. The section of the fault that
broke in the 1966 Parkfield earthquake, between about 280 and 320 km on the profile, is less seismic than the section northwest of Parkfield, and earthquakes along this section die out southeastward. Southeast of Cholame (320 km) there is a 10-km gap in seismicity followed by weak seismicity between 330 and 360 km. This southeasternmost patch of earthquakes on the profile is mostly between 8 and 15 km deep.
The transverse sections A–Á to D–D́ in figure 11 were positioned to illustrate how the pattern of seismicity transverse to the Coast Ranges varies along their length. Each section is 200 km long and shows earthquakes from an 80-km wide box across the Coast Ranges projected onto a vertical plane that is perpendicular to the dense line of earthquakes traversing the box. On these sections, strike-slip faults perpendicular to the boxes appear as narrow, near-vertical linear concentrations of events. On all four sections the vertical line of events near 100 km on the profile represents the most prominent line of epicenters (fig. 9) near the midline of the Coast Ranges. It represents the San Andreas fault on D–D́, the Hayward fault on C–Ć, the Healdsburg fault on B–B́, and the Maacama fault on A–Á. The vertical lines of events at 130 to 140 km on profiles A–Á, B–B́, and C–Ć represent the line of epicenters, about 35 km east of the midline, along the eastern edge of the Coast Ranges. In section D–D́ the great mass of events between about 125 and 145 km represents the aftershocks of the 1983 Coalinga and 1985 Kettleman Hills earthquakes athwart the boundary between the Coast Ranges and the Great Valley.
South-to-north variations in seismicity of the San Andreas are demonstrated dramatically by the cross sections. Earthquakes along the southern 80 km of the creeping section of the San Andreas are densely clustered in a vertical line from the surface to a depth of about 13 km (D–D́). Earthquakes on the San Andreas near San Francisco are few in number and scattered over a somewhat broader zone than farther south (C–Ć). Section B–B́ north of San Francisco shows the San Andreas to be virtually aseismic. Section A–Á from Point Arena northward shows only minor seismicity along the San Andreas.
Most of the deeper earthquakes on the cross sections occur beneath the Great Valley or near the eastern boundary of the Coast Ranges. On section B–B́ the events 10 to 20 km deep near 190 km on the profile occurred about 10 km east of the Coast Ranges boundary, 20 km southwest of the Sutter Buttes. On section C–Ć the sparse, vertically elongated zone of earthquakes 15 to 25 km deep between 130 and 140 km on the profile is beneath Suisun Bay where the Valley indents the eastern edge of the Coast Ranges. On section D–D́ the earthquakes deeper than 15 km mostly lie beneath and east of the Coalinga/Kettleman aftershock zone. Although relatively few in number (about one percent as many as the Coalinga/Kettleman aftershocks), these
Figure 11
Tranverse cross sections of the Coast Ranges seismic zone. The regions
projected onto profiles A–Á through D–D́ are outlined in figure 9. Each
profile is centered on, and perpendicular to, the continuous line of
earthquakes near the Coast Ranges midline. Dashed lines indicate the
approximate bottom of the seismogenic zone beneath the Coast Ranges.
The events deeper than 15 km on the right end of the profiles are beneath
the eastern edge of the Coast Ranges or the adjacent Great Valley. The
dotted line indicates the approximate bottom of the seismogenic zone
beneath the Coast Ranges/Great Valley boundary region. Two times
vertical exaggeration. Abbreviations as for figure 9.
events cannot be ascribed to poor locations, and they reveal a broad zone of mild seismicity in the lower crust beneath the eastern edge of the Coast Ranges and adjacent Great Valley.
On sections A–Á to C–Ć the San Andreas fault lies near the coastline and the continental margin. On D–D́ it is about 70 km inland, and seismicity in the Coast Ranges west of the fault is remarkably different from that west of the fault, offshore on the northern cross sections. A landward-thickening zone of diffuse seismicity extends inland from the west end of the profile to near 60 km on the profile. The zone of modest concentration of events near 50 km corresponds approximately to the Nacimiento fault. Except for a concentrated cluster of events 10 to 13 km deep near 80 km on the profile (San Ardo), there is little seismicity between about 65 and about 100 km (San Andreas fault). The lower limit of the seismic zone is very sharp from about 10 km, where it is about 5 km deep, to the San Andreas, where it is about 13 km deep. Crustal seismicity is weak for about 15 km northeast of the San Andreas. Farther east, but still west of the Coalinga/Kettleman aftershocks, the density of earthquakes in the upper crust increases moderately, and earthquakes appear in the lower crust. Focal mechanisms of large earthquakes in the bands of seismicity along the coast, as well as in the Coalinga/ Kettleman region, show that both flanks of the Coast Ranges in box D–D́ are under compression normal to the San Andreas fault, and that the predominant mode of failure is along thrust and reverse faults on planes with strikes parallel to the San Andreas (Eaton, 1985; Eaton and Rymer, in press).