Environmental Engineering Reference
In-Depth Information
substantially and does not occur in uniform amounts along the fault, and some sections
may not displace at all. In general, displacement is related to the magnitude of an event.
The maximum displacement (MD) along a fault rupture length for the western United
States can be estimated (NRC, 1997) by the relationships developed by Wells and
Coppersmith (1994):
log (MD)
a
bM
w
(11.16)
where
strike-slip:
a
7.03m,
b
1.03m,
s
0.34
normal:
a
5.90m,
b
0.89m,
s
0.39
all:
a
5.46m
with
M
w
being the moment magnitude,and
s
the standard deviation.
The work of Wells and Coppersmith (1994) is widely referenced in the literature, includ-
ing the NRC Regulatory Guide 1.165 (NRC, 1997). They studied 167 events and developed
regressions of rupture length, rupture width, rupture area, and displacement, in terms of
moment magnitude.
Creep (Slip Rate)
Before or after an earthquake, slow movement can occur along a fault (tectonic creep),
which can range from a few millimeters to a centimeter or more every year. This fault slip-
page apparently occurs in faults filled with gouge from previous rupture as strain energy
accumulates in the rock below the gouge zone. Creep does not usually occur along an
extensive line, but rather is limited to certain areas.
In recent years, increased attention is being paid to slip rates as an expression of the
long-term activity of a fault. They reflect the rate of strain energy release on a fault, which
can be expressed as the seismic moment. Because of this they are being used to estimate
earthquake recurrence, especially in probabilistic seismic hazard analysis (Schwartz and
Coppersmith, 1986).
The Southern California Earthquake Data Center (Internet 2004) gives some slip rates
for California faults as follows: San Andreas, 20-35 mm/year; San Jacinto, 7-17 mm/year;
Garlock, 2-11 mm/year; Elismore, 4 mm/year; and Owens Valley, 0.1-2.0 mm/year.
Strike-Slip Displacement
One of the largest movements on record is the 20 ft of horizontal displacement that
occurred during the San Francisco quake of 1906; vertical movement did not exceed 3 ft.
Horizontal movement of the Imperial Valley event (El Centro) of 1940 reached 10 ft, East
of El Centro, at a location along Highway 40, displacement across the roadway was 18 in.
By 1966, displacement was 25 in. because of fault creep and slips over the 26 year interval.
Dip-Slip Displacement
Surface tilting and warping often result in dip-slip displacement. California events nor-
mally have about a few feet of displacement. The largest recorded vertical displacement
appears to be the 30 ft that may have occurred in Assam in 1897, or possibly the 45 ft that
may have occurred in the Yakutat Bay, Alaska, quake of 1899. The problem in determining
displacements on old scarps is that of erosional changes as discussed by Wallace (1980).
Strong ground motions are strongly influenced by fault geometry. Now referred to as
directivity
, it should be considered in selection of the design earthquake. Two sites located
at similar distances but on opposite sides of a fault may experience significantly different
ground motions; larger, short-period motions may occur on the hanging wall side. For
