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Seismic Creep |
Seismic creep is the constant or periodic movement on a fault as contrasted
with the sudden erupture associated with an earthquake. It is a usually slow deformation
of rock resulting from constant stress being applied over a period of time. Sometimes
a seismic slip is observed at the ground surface along a ruptured fault that has produced
a substantial earthquake. Examples are from the Hollister and Hayward, California,
region. Several of the slides are split images of a location, comparing fault movement
over the years. Seismic creep is the constant or periodic movement on a fault as contrasted
with the sudden rupture associated with an earthquake. It is a usually slow deformation
of rock resulting from constant stress being applied over a period of time. Active
faults exhibit a variety of deformational phenomena; the most obvious is earthquake
slip. In addition, faults may continue to move after an earthquake occurs (after slip),
move slowly over time in the absence of earthquake activity (fault creep), or move
slowly as pressure builds before an earthquake (pre-seismic slip). Fault creep may
be constant over time, or the motion may alternate between periods of slow and relatively
rapid movement. Usually, such episodes of fault slip are a seismic-that is, they are
not accompanied by local earthquakes. The characteristics of the fault itself and
the total mechanical system that produces the fault movement are the major factors
determining the type of creep. As more information is acquired about the fault itself
and about the stresses upon it, interpretations are made about movement along that
fault, including creep. Fault creep (horizontal fault slippage) has been identified
on a number of faults around the world including the north Anatolian fault at Ismetpasa
in Turkey, along the Jordan Valley rift in Israel, and the Hayward, San Andreas, and
Calaveras faults in California. Sometimes a seismic slip is observed at the ground
surface along a ruptured fault that has produced a substantial earthquake. For example,
along the San Andreas fault near Parkfield, California, offsets of road pavement increased
by a few centimeters in the days following the June 27, 1966, earthquake. Such continued
adjustment of the crustal rock after the initial major offset may be caused partly
by the minor slips that produce aftershocks. They may also result from the yielding
of the weaker surface rocks and gouge in the fault zone as they accommodate to theadjusted
tectonic forces in the region. Seismic creep, when it occurs in developed areas, may
result in structural damage. Creep has damaged the huge concrete-lined water tunnel
from the Sierra Nevada that brings water to cities on the east of San Francisco Bay.
It has damaged Berkeley Memorial Stadium at the University of California and the Almaden
Cienega Winery near Hollister. The tunnel and the stadium straddle the active Hayward
fault which is undergoing right-lateral slip at the rate of 2 to 5 millimeters a year.
When the tunnel was drained in 1966, cracks several centimeters across were discovered.
These encircled the concrete lining where the tunnel and fault zone intersect. A concrete
drainage culvert under the stadium shows considerable cracking where it crosses the
Hayward fault trace; instruments that were placed across the cracks in 1966 show that
the right-lateral slipcontinues to occur. Obviously, faults should be avoided when
considering locations for man-made structures. When utility lines, roads, and railroads
must be laid across active faults, they should have jointed or flexible sections within
the fault zone. Slides for this set were provided by Joe Dellinger, AMOCO TTC 1F10,
4502 E. 41st, Tulsa, OK 74135, and by Dr. Sue Ellen Hirschfeld, Department of Geological
Sciences, California State University, Hayward, CA 94542-3088. Their slides are in
the public domain, copyright-free. Dr. Eduard Berg asks that you contact him personally
if you intend to reproduce his slides. He may be reached at: University of Hawaii,
Department of Geology and Geophysics, School of Ocean and Earth Science and Technology,
2525 Correa Road, Honolulu, HI, 96822.
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