Title: Displacements on the Imperial, Superstition Hills, and San Andreas Faults Triggered by the Borrego Mountain Earthquake
Abstract:The Borrego Mountain earthquake of April 9, 1968, triggered
small but consistent surface displacements on three
faults far outside the source area and zone of aftershock
activity. Right-lateral dis...The Borrego Mountain earthquake of April 9, 1968, triggered
small but consistent surface displacements on three
faults far outside the source area and zone of aftershock
activity. Right-lateral displacement of 1-2 1/2 cm occurred
along 22, 23, and 30 km of the Imperial, Superstition Hills,
and San Andreas (Banning-Mission Creek) faults, respectively,
at distances of 70, 45, and 50 km from the epicenter.
Although these displacements were not noticed until 4 days
after the earthquake, their association with the earthquake
is suggested by the freshness of the resultant en echelon
cracks at that time and by the absence of creep along most
of these faults during the year before or the year after the
event. Dynamic strain associated with the shaking is a
more likely cause of the distant displacements than is the
static strain associated with the faulting at Borrego Mountain
because (1) the dynamic strain was much larger and
(2) the static strain at the San Andreas fault was in the
wrong sense for the observed displacement. The principal
surface displacements on the Imperial fault took place within
4 days of the earthquake and may have occurred simultaneously
with the passage of the seismic waves, but the
possibility of delayed propagation to the surface is indicated
by a 1971 event on the Imperial fault in which the surface
displacement followed the triggering earthquake by 3-6 days.
All three of the distant faults are in that they show
evidence of repeated Quaternary movement, and surface displacements
occurred only along those segments where the
fault trace is well delineated in surface exposures, at least
in uncultivated areas. This is the first documented example
of fault displacement triggered by seismic shaking far from
the source area, although such displacement has probably
gone undetected many previous times here and in similar
tectonic environments. This phenomenon forces us to be
much more conservative in estimating the probabilities of
damage from surface displacements along active faults in
seismic regions.Read More
Publication Year: 1972
Publication Date: 1972-01-01
Language: en
Type: book-chapter
Access and Citation
Cited By Count: 78
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