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).