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portion of the fault. The Garlock fault has been active throughout the Cenozoic and
exhibits abundant surficial evidence of late Pleistocene and Holocene-age displacement

(Clark, 1973; Clark and Lajoie, 1974; Carter, 1980; LaViolette and others, 1980; and McGill
and Sieh, 1991) and folding (Smith, 1991). Roquemore and others (1982) indicated as many
as six surface rupture events may have occurred on this fault during the Holocene. While
historical seismic activity has been relatively low on the Garlock fault compared to other
major faults, an average of three local magnitude (MI) > 2.5 earthquakes per year occurred

on the fault during the period of 1932 to 1981. The largest of these events was ML 4.3
(Astiz and Allen, 1983). A preferred slip rate of 7 to 11 mm/yr on the central portion of
the fault has been reported (Clark and others, 1984), although more recent data suggest the
slip rate may be 4 to 8 mrn/yr (McGill and Sieh, 1991). A conservative slip rate of 9 mm/yr
was used for assessing the seismic potential of the Garlock fault. Based on the slip
per surface rupture event indicated by offset geomorphic features and estimated slip rates,

McGill and Sieh (1991) estimated recurrence intervals of between 200 and 3,000 years for
large earthquakes which generate surface faulting on the various segments of the Garlock
fault.

7.2.3 an Gabri I Fa l

The San Gabriel fault is a northwest trending, right-lateral strike-slip fault which dips steeply
(70-80‘) to the north. South of the Honor Rancho Oil Field, the fault exhibits a significant
reverse component of slip (Stitt, 1986). The portion of the fault exhibiting late Pleistocene
and Holocene displacement extends at least 50 miles (80 km) from near the San Andreas
fault on the north to the Sierra Madre fault on the south (Weber, 1982, 1986; Ziony and

Jones, 1989; Jennings, 1992). The surface trace of the San Gabriel fault is located
approximately two miles (3.2 km) northeast of the project property at its closest point.
Holocene displacement has been documented only along a 5-mile (8-km) long segment of
the fault in the Saugus area (Jennings, 1992). The most recent displacement documented
at this location occurred between 3,500 i 250 years before present (ybp) and 1,500 1' 190

ybp (Cotton, 1986). Some microseisrnicity may be associated with the San Gabriel fault, but
the data are inconclusive. There are no reliable slip rate data for this fault (Cotton, 1986),
by which to judge its degree of activity. Given the geomorphic expression of this fault
relative to other strike-slip faults in southern California, a 1 mm/yr slip rate was assumed
in assessing the seismic potential of the San Gabriel fault.

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