THE 1906 BREAK AND THE NORTHERN COAST RANGES [c5, p124]

Aside from a light scattering of epicenters about the trace of the San Andreas fault through the San Francisco peninsula and the Santa Cruz Mountains to the south, the rupture zone of the 1906 earthquake is nearly aseismic. This pattern has persisted not only through the period 1980-86 shown in Figure 5.6 but also at least since the mid-1930's, when instrumental data became available for reliable earthquake locations in the area (see Bolt and Miller, 1975; Hill and others, in press). It was interrupted, however, by the M=7.1 Loma Prieta earthquake on October 17, 1989, which ruptured the southernmost 45 km of the 1906 break (cross sec. L-L', Figure 5.7; see chap. 6). The cluster of epicenters along the fault just west of San Francisco coincides closely with Boore's (1977) estimate of the epicentral location for the 1906 main shock. We note that the greatest offsets along the 1906 rupture occurred north of the epicenter along the stretch of the fault that now shows the lowest seismicity (see Thatcher and Lisowski, 1987).

The pair of subparallel epicenter lineations through the northern Coast Ranges east of the 1906 break closely follow the Rodgers Creek-Healdsburg-Maacama fault zone (west) and the Green Valley-Bartlett Springs fault zone (east). In both lineations, the epicenters tend to cluster along the eastern margins of these 2- to 3-km-wide fault zones, which are characterized by multiple strands of subparallel, curvilinear fault traces (see maps at front of book; Dehlinger and Bolt, 1984). These fault zones are essentially colinear with the Hayward and Calaveras faults to the south, although an aseIsmIc interval coincident with the eastern arm of the San Francisco Bay (San Pablo and Suisun Bays) obscures the connection between these branches of the fault system. Most of the earthquakes defining the pair of subparallel lineations through the northern Coast Ranges are small (M3), and, indeed, these fault zones were not recognized as seismically active branches of the San Andreas system until after the northern Coast Ranges section of the telemetered network ( Figure 5.2) became operational in late 1979.

Dense clusters of epicenters just south of Clear Lake define a northeast trending pattern transverse to and midway along these northern two branches of the San Andreas fault system. The southwesternmost of these clusters represents microearthquake activity associated with the Geysers geothermal field (Eberhart-Phillips and Oppenheimer, 1984; Oppenheimer, 1986). The cluster just to the northeast underlies the Clear Lake volcanic field, which last erupted about 10 ka (Donnelly-Nolan and others, 1981). Scattered clusters farther to the northeast suggest a tenuous link between this Geysers-Clear Lake trend and the north-south-trending lineation of epicenters along the axis of the north end of the Great Valley.

In longitudinal cross section (H-H', Figure 5.6), earthquakes occurring along the two northern branches of the San Andreas fault system are moderate in number and rather evenly distributed except for the dense, shallow cluster beneath the Geysers-Clear Lake area. The depth to the base of the continuously seismogenic crust shows considerable relief, deepening to between 12 and 13 km south of the Geysers-Clear Lake area and shallowing to only 5 km beneath and just north of the geothermal field. Farther north, the base of the seismogenic crust deepens gradually to a maximum of about 10 km. Note the isolated cluster of small but well-located events at depths of 13 to 18 km beneath Clear Lake, just north of the shallowest depths to the base of the continuously seismogenic crust. Another isolated cluster of deep earthquakes located beneath Suisun Bay have focal depths as great as 15 to 25 km. The more numerous earthquakes along branches of the fault system south of Suisun Bay paint in a dense distribution of hypocenters throughout the upper 10 to 15 km of the crust.

Although the two transverse cross sections across the northern Coast Ranges (E-E', F-F', Figure 5.8A) show the concentration of hypocenters around the Rodgers Creek and Green Valley branches of the San Andreas fault, they provide no hint of the location of the main branch of the San Andreas fault that ruptured in 1906. The southern of these two cross sections (F-F') includes the dense, shallow cluster of events associated with the Geysers-Clear Lake activity, as well as the cluster of deeper events beneath the eastern margin of the Coast Ranges. Focal depths in the latter cluster, which falls at the northeast end of the Geysers-Clear Lake lineation and at the south end of the north-south-trending lineation beneath the Great Valley, range from 10 to 25 km, a depth range common to earthquakes near the north end of this Great Valley seismicity lineation (cross sec. D-D', Figure 5.5C). Maximum focal depths increase by several kilometers from west to east in both cross sections. Farther east, they increase abruptly to depths of 25 km or so along the Great Valley lineation.