Geology Reference
In-Depth Information
EXERCISE 7
The Loma Prieta Earthquake
of 1989
INTRODUCTION
As many people prepared to watch World Series
baseball on October 17, 1989, the earth in the San
Francisco Bay region shook from a major earth-
quake. The U.S. Geological Survey has named this
earthquake after the small community of Loma Prieta
near the epicenter. Although San Francisco and
Oakland received the most media coverage for their
damage, it is important to note that these large
communities were about 100 km away from the
epicenter. Closer communities, such as Watsonville
and Santa Cruz, also suffered great damage and
fatalities in the earthquake. This exercise looks at
the regional geology of the San Andreas Fault and
the geology of areas in San Francisco and Oakland
that were damaged.
Damage that results from an earthquake is a
result of many factors. Among these are
b.
soil saturation: saturated soils will typically
perform less well than dry soils
c.
soil grain size and sorting: well-sorted, fine-grained
sands and coarse silts that are geologically young
have the highest potential to liquefy (flow) if satu-
rated; soils that liquify lose their ability to hold up
structures such as buildings and bridges
d.
types and properties of the bedrock at the site:
unweathered igneous and other massive rocks
typically perform better than weak and fractured or
jointed rocks
3.
The nature of the building
a.
architectural simplicity of the building: seismic
response of simple buildings is easier to predict
and control than the response of complex buildings
b.
size and use of the building: a single family
home will behave differently than a large commercial
or industrial building
c.
type of construction: small wood structures will
typically flex more and therefore perform better in
earthquakes than brick or concrete block buildings
d.
type of seismic design considerations: is the
building new or old, is it built to appropriate seismic
standards or not?
1.
The geologic nature of the earthquake and the
rocks between the focus and a site
a.
the type of fault movement: different earth-
quakes generate different frequencies
b.
the size of the earthquake: larger earthquakes
will release more energy, and therefore cause more
acceleration and have longer duration
c.
the distance from the source of the earthquake
to a site: in general, sites closer to the focus fare
worse
d.
characteristics of the rocks through which the
seismic waves travel from the source to the site:
some rocks transmit seismic energy better than
others
The emphasis in this exercise is on the geology
of the earthquake and the geologic reasons for building
failures in different areas. The actual damage that
occurred to specific buildings and other structures is
not covered.
This exercise is divided into several parts. We
first explore the nature of the San Andreas Fault, gen-
eral seismic activity in the area, the specific sequence
of Loma Prieta earthquakes, and seismic attenuation in
this area. We will investigate the geology and hazards
in the San Francisco Bay region in general, and focus
in particular on the Marina district.
2.
The types and properties of the materials at the
site determine the ground motions at the site
a.
soil thickness: shallow soils may not perform as
well as deep, well-consolidated soils
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