The Hayward fault (H1 and H2 in Figure
2 ) had at least one major historical earthquake
in 1868. Lawson [1908] stated that the surface rupture in
1868 extended at least from Agua Caliente Creek (AC in Figure
5) northerly to San Leandro (SL), and less
certainly to Mills College (MC). Recent trenching investigation
[Lienkaemper and others, 1995] and analysis of 19th-century
triangulation data [Yu and Segall, 1996] suggest that sizable
slip occurred as far north as northern Oakland (MT, Montclair trench;
BT, BART tunnel). We assume that the location of the southern termination
of the 1868 subsurface rupture, at the base of the seismically active
crust, coincides with the point where most slip is transferred from
the Calaveras to Hayward fault. High creep rate (9-10 mm/yr) occurs
on the Calaveras fault north of Halls Valley (HV). North of Calaveras
Reservoir (CR) creep on the Calaveras fault is 3-6 mm/yr. We deduce
that the high creep rates (9 mm/yr) that occur on the Hayward fault
from Agua Caliente Creek (AC) northward [Lienkaemper and others,
1991] indicate that the main subsurface connection of creep between
the Calaveras and Hayward faults lies north of the line between
Halls Valley and Agua Caliente Creek. Because the microearthquakes
(Figure
5) that connect the southern Hayward fault
to the Calaveras fault have dominantly strike-slip focal mechanisms,
the connection of deep slip probably occurs along a path associated
with these small earthquakes [Ellsworth and others, 1982;
Wong and Hemphill-Haley, 1993].
The Rodgers Creek fault (H3; Figure
2a and Figure
8) is believed to be entirely locked (i.e.,
no recognized creep, <2 mm/yr; Galehouse, 1995; USGS trilateration
data, 1978-88) and has not had a major historical earthquake. Segmentation
is fairly straightforward because the fault terminates both northward
at H4 and southward at H2 in distinct right stepovers of a few kilometers
width. In consultation with CDMG we chose to simplify segmentation
by eliminating the overlaps of H3 with H4 and H2, so that seismic
moment would not be duplicated in the hazard analysis. Actual ruptures
would be expected to taper into the stepover overlap region in some
complex way. We have adopted the 230 ± 130 yr paleoseismic
recurrence time based on 3 events and characteristic coseismic slip
of ~2 m from Schwartz and others [1993]. This slip yields
Mw of 7.0. Slip rate of 8.4 ± 2 mm/yr by Schwartz
and others [1993] suggests that the 9 mm/yr used on the Hayward
fault be adopted for the Hayward fault subsystem north of San Pablo
Bay as well.
U.S. Geological Survey, Earthquake Hazards Program
URL http://quake.wr.usgs.gov/prepare/ncep/hayward.html
Contact:webmaster@ehznorth.wr.usgs.gov
Last modification: December 29, 2000