North Coast
The North Coast for the following discussion includes the Hayward
and Calaveras fault subsystems that lie north of the metropolitan
San Francisco Bay region (Figure 8: segments: H4, H5, H6, H7, C5,
C6, C7, and C8). In general, the Holocene fault traces in this region
are rather poorly known, hence the continuity of microseismicity
and locations of ongoing fault creep are especially important in
locating and characterizing active traces. No major historical earthquakes
have been associated with specific North Coast fault segments, nor
has paleoseismologic evidence yet clarified any characteristic behaviors
of these faults.
Hayward fault subsystem, North Coast
The Maacama fault (H4, H5, H6) to the north has a sizable creep
rate, ~7 mm/yr at Willits (on H5) and Ukiah (on H6) (see Table
A-1 and Figure 8, below). This large creep rate makes plausible
our assumption that the 9 mm/yr long-term slip rate adopted for
the Hayward and Rodgers Creek fault continues northward along the
Maacama fault zone. As we discuss later, the observed geodetic strain
rate and current global plate rate analysis support extending such
a rate into the Garberville-Briceland segment (H7). The slip per
event, ~2 m, that we adopted for the Hayward and Rodgers Creek,
based on historic and paleoseismic data, is larger than the values
expected for the lengths of these North Coast segments using our
standard methodology. Hence, we continue to assume ~2 m slip per
event for the North Coast segments: H4, H5, H6, and H7. Although
the segment boundary from H5 to H6 is rather subtle, we felt that
short segments were more comparable to the creeping Hayward fault
(H1 and H2) which appears to show some segmentation despite being
an unusually straight and simple fault.
Click for high-resolution
image
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| Figure 8. North
Coast seismicity (upper map, USGS catalog, 1968-1985) and creep
localities (lower map). Rectangular area near Geysers (near
L08) has seismicity deleted. Creep localities and geodetic sites
marked by large triangles for alinement arrays and by small
triangles with vectors for USGS trilateration stations and velocity
analysis of M. H. Murray (unpub. data, 1996). Velocity vectors
(arrows) indicate net right-lateral from P (Poonkinney) to Covelo
of 8.3 mm/yr (1985-1989). See Figure 2a for location map of
segments: C5, C6, C7, C8, H4, H5, H6 and H7 |
Calaveras fault subsystem, North Coast
The map location of the Holocene fault traces is especially uncertain
for this fault subsystem in the North Coast region. The terrain is
steep, landslides and vegetational cover are extensive, thus making
difficult the usual procedures for delineating active faults, especially
the geomorphic interpretation of aerial photography. Segmentation
relied on weighted judgment based on the ages of various mapped Quaternary
fault traces of Jennings [1992] and the density of microseismic activity.
Creep is recognized on all segments of the Calaveras fault subsystem
in the San Francisco Bay area, so we expect it may continue into the
North Coast but few data are available to test this assumption. On
Figure 8 (lower), two velocity vectors straddle the Round Valley fault
(C7), indicating creep of ~8 mm/yr, 1985-1989. Although the 5-6 yr
duration of the survey is too brief to be highly exact, this demonstration
of a sizable creep rate so near its north end may have important implications
for the Calaveras fault subsystem as a whole. At least we may reasonably
extend further northward our assumption of 6 mm/yr long-term slip
rate with some greater assurance, but with some additional concern
that it might be a low estimate. |
