Geology Reference
In-Depth Information
Geo-Focus
Paleoseismology
Paleoseismology
is the study of pre-
historic earthquakes. As more people
move into seismically active areas, it
is important to know how frequently
earthquakes in the area have oc-
curred in the past, and how strong
those earthquakes were. In this way,
prudent decisions can be made about
what precautions need to be taken
in developing an area and how strin-
gent the building codes for a region
need to be.
A typical technique in paleoseismol-
ogy is to excavate trenches across active
faults in an area to be studied and date
the sediments disturbed by prehistoric
earthquakes (
the sands yielded an age of 850 to
1250 years ago, well within the time
period during which the terraces
were uplifted.
Geologists also found evidence of
rock avalanches in the Olympic Moun-
tains that dammed streams, thereby
forming lakes. Drowned trees in the
lakes were dated as having died
between 1000 and 1300 years ago,
again fi tting in nicely with the date of
the ancient earthquake.
One of the fi nal pieces of evidence
is the deposits found on the bottom
of Lake Washington. The earthquake
apparently caused the bottom sedi-
ment of the lake to be resuspended
and to move downslope as a turbidity
current. Dating of these sediments
indicates that they were deposited
between 940 and 1280 years ago, con-
sistent with their being caused by an
earthquake.
All evidence points to a large (mag-
nitude 7 or greater) shallow-focus
earthquake occurring in the Seattle
area approximately 1000 years ago. If
history and the events of the geologic
past are any guide, it is very likely that
another large earthquake will hit the
Seattle area in the future. When this
will occur can't yet be predicted, but
it would be wise to plan for such an
eventuality. After all, metropolitan
Seattle has a population of more than
2.5 million people, and its entire port
area is built on fi ll that would prob-
ably be hard hit by an earthquake.
Furthermore, most of Seattle's schools,
hospitals, utilities, and fi re and police
stations are not built to withstand a
strong earthquake. What the future
holds for Seattle has yet to be deter-
mined. Geologists have provided a
window on what has happened seismi-
cally in the past, and it is up to today's
government and its various agencies
to decide how they want to use this
information.
Figure 1). By expos-
ing the upper few meters of material
along an active fault, geologists can
fi nd evidence of previous earthquakes
in the ancient soil layers. Furthermore,
by dating the paleosoils by carbon-14
or other dating techniques, geologists
can determine the frequency of past
earthquakes and when the last earth-
quake occurred, and thus have a basis
for estimating the probability of future
earthquakes.
Paleoseismic studies are currently
underway in many areas of North
America, particularly along the San
Andreas fault in California and in the
coastal regions of Washington. An
interesting case in point concerns an
ancient earthquake in what is now
Seattle, Washington.
Data from a variety of sources
have convinced many geologists that
a shallow-focus earthquake of at least
magnitude 7 occurred beneath Seattle,
Washington, less than 1100 years ago.
In a point not lost on offi cials, they
noted the catastrophic effects that a
similar-sized earthquake would have
if it occurred in the same area today.
The fi rst link in the chain of evi-
dence for a paleoearthquake came
from the discovery of a marine ter-
race that had been uplifted some
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Figure 1
Geologists examine a trench
across an active fault in California to determine
possible seismic hazards. Excavating trenches
is a common method used by geologists to
gather information about ancient earthquakes
in a region and to help assess the potential for
future earthquakes and the damage that they
might cause.
7 m at Restoration Point, 5 km west
of Seattle. Carbon-14 analysis of peat
within sediments of the terrace indi-
cates that uplift occurred between 500
and 1700 years ago. Carbon-14 dating
of other sites to the north, south, and
east also indicates a sudden uplift in
the area within the same time period.
The amount of uplift suggests to
geologists a magnitude-7 or greater
earthquake.
Because many earthquakes in and
around the Pacific Ocean basin can
cause tsunami, geologists looked for
evidence that a tsunami occurred,
and they found it in the form of
unusual sand layers in nearby tidal
marsh deposits. Carbon-14 dating
of organic matter associated with
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