The Santa Cruz and San Francisco areas were hit by a moderatesized
earthquake (magnitude 7.1) at 5:04 P.M. on Tuesday, October
17, 1989, during a World Series game played in San Francisco. As a
result, 67 people died, 3,757 people were injured, and 12,000 were
left homeless. A television audience of tens of millions of people
watched as the earthquake struck just before the beginning of
game three, and the news coverage that followed was unprecedented
for capturing an earthquake as it happened.
The earthquake was caused by a 26-mile (42-km) long rupture
in a segment of the San Andreas Fault near Loma Prieta peak in the
Santa Cruz Mountains south of San Francisco. The segment of the
fault that ruptured was the southern part of the same segment that
ruptured in the 1906 earthquake, but this rupture occurred at greater
depths than the earlier quake. The actual rupturing lasted only 11
seconds, during which time the western (Pacific) plate slid almost six
feet (1.9 meters) to the northwest, and parts of the Santa Cruz Mountains
were uplifted by up to four feet (1.3 meters). The rupture propagated
at 1.24 miles per second (2 km/sec) and was a relatively
short-duration earthquake for one of this magnitude. Had it been
much longer, the damage would have been much more extensive. As
it was, the damage totals amounted to more than $6 billion.
The actual fault plane did not rupture the surface, although
many cracks appeared and slumps formed along steep slopes. The
Loma Prieta earthquake had been predicted by seismologists
because the segment of the fault that slipped had a noticeable
paucity of seismic events since the 1906 earthquake and was identified
as a seismic gap with a high potential for slipping and causing
a significant earthquake. The magnitude 7.1 event and the numerous
aftershocks filled in this seismic gap, and the potential for large
earthquakes along this segment of the San Andreas Fault is now
significantly lower, since the built-up strain energy was released
during the quake. There are, however, other seismic gaps along the
San Andreas Fault in heavily populated areas, such as near Los
Angeles, that should be monitored closely.
The amount of ground motion associated with an earthquake
not only increases with the magnitude of the earthquake but also
depends on the nature of the substratum. In general, loose, unconsolidated
fill tends to shake more than solid bedrock. This was dramatically
illustrated by the Loma Prieta earthquake, where areas
built on solid rock near the source of the earthquake vibrated the
least (and saw the least destruction), and areas several tens of
miles away built on loose clays vibrated the most. Much of the Bay
area is built on loose clays and mud, including the Nimitz freeway,
which collapsed during the event. The area that saw the worst
destruction associated with ground shaking was the Marina district
of downtown San Francisco. Even though this area is located far
from the earthquake epicenter, it is built on loose unconsolidated
landfill, which shook severely during the earthquake, causing many
buildings to collapse, and gas lines to rupture, which initiated fires.
More than twice as much damage from ground shaking during the
Loma Prieta earthquake was reported from areas over loose fill or
mud than from areas built over solid bedrock. Similar effects were
reported from the 1985 earthquake in Mexico City, which is built
largely on old lakebed deposits.
through solids, liquids, and gases. The kind of movement
associated with passage of a P-wave is a back and forth type
of motion. Compressional (P) waves move with high velocity,
about four miles per second (6 km/sec), and are thus the first
to be recorded by seismographs. This is why they are called
primary (P) waves. P-waves cause a lot of damage because
they temporarily change the area and volume of ground that
humans built things on or modified in ways that require the
ground to keep its original shape, area, and volume. When
the ground suddenly changes its volume by expanding and
contracting, many of these constructions break. For instance,
if a gas pipeline is buried in the ground, it may rupture or
explode when a P-wave passes because of its inability to
change its shape along with the Earth. It is common for fires
and explosions originating from broken pipelines to accompany
earthquakes.
The second type of body waves are known as shear
waves (S) or secondary waves, because they change the
shape of a material but not its volume. Shear waves can
only be transmitted by solids. Shear waves move material at
right angles to the direction of wave travel, and thus they
consist of an alternating series of sideways motions. Holding
a jump rope at one end on the ground and moving it
rapidly back and forth can simulate this kind of motion.
Waves form at the end being held and move the rope sideways
as they move toward the loose end of the rope. A typical
shear-wave velocity is 2 miles per second (3.5 km/s).
These kinds of waves may be responsible for knocking
buildings off foundations when they pass, since their rapid
sideways or back and forth motion is often not met by
buildings. The effect is much like pulling a tablecloth out
from under a set table—if done rapidly, the building (as is
the case for the table setting) may be left relatively intact,
but detached from its foundation.
Surface waves can also be extremely destructive during
an earthquake. These have complicated types of twisting
and circular motions, much like the circular motions you
might feel while swimming in waves out past the surf zone
at the beach. Surface waves travel slower than either type of
body waves, but because of their complicated types of
motion they often cause the most damage. This is a good
thing to remember during an earthquake, because if you
realize that the body waves have just passed your location,
you may have a brief period of no shaking to get outside
before the very destructive surface waves hit and cause even
more destruction.
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