Geology Reference
In-Depth Information
2. One theory for the prediction of earthquakes suggests that
new events will fill in gaps where earthquakes have not been
recently recorded. The gaps are presumed to be zones where
stress is accumulating, which will be released in a future
earthquake. In areas along faults with frequent earthquakes,
stress is released in each event.
Accelerographs,
which measure acceleration of
the ground, record data that is typically presented as a
percentage of the force of gravity (g). Buildings are
designed to withstand the vertical force of gravity
(1 g = 980 cm/sec/sec); that is, they are designed to
stand up under gravitational forces. They are not,
however, necessarily designed to be moved sideways,
and earthquakes generate strong sideways forces
against a building.
Figure 7.4 is a map showing the horizontal accel-
eration from the Loma Prieta earthquake as recorded
at many sites in western California.
a.
Describe the differences in locations of epicenters for
earthquakes that occurred before the Loma Prieta earth-
quake (Figure 7.2a) and the main shock and aftershocks
(Figure 7.2b and shaded zone in Figure 7.1).
QUESTIONS (7, PART A2)
1.
Describe the general relationship between the recorded
acceleration at different sites and the epicentral area, which
is indicated by shading.
b.
Do the Loma Prieta earthquake and its aftershocks fit
the pattern of new events filling gaps? Explain.
3. Figure 7.3 shows a map and an insert with a cross-section
view along line B-B'; of the Loma Prieta earthquake and its
aftershocks.
a.
Looking at the map view, would you infer that all of the
earthquakes occurred along the San Andreas Fault?
2.
a.
Draw a contour line around the sites with 20% g in
Figure 7.4. 20% g is a value for moderate damage.
b. What is the general shape of the contour?
b.
Compare the map view with the cross-section view.
What would you infer from the cross-section view about
the number of faults?
c.
How does its shape compare with the orientation of the
faults that are shown in Figure 7.1?
c.
What is the evidence for your inferences?
3. What geological factors contribute to the difficulties in
drawing the irregular 20% g isoseismal line?
Seismic Attenuation and Acceleration
(7, Part A2)
Attenuation is the decrease in the amplitude of seismic
waves as distance from the source increases. In gen-
eral, there is a geometric decrease in amplitude with
distance. Selected conditions, however, tend to cause
local variations in amplitude such that sites distant
from the epicenter will show an amplification of
ground motion.
Seismologists use sophisticated instruments to
measure many different components of the earthquake
waves that travel within and along the surface of the
Earth. These components include acceleration, veloc-
ity, and total displacement at a site. For example, in
automobiles acceleration is how fast the vehicle starts
from a stop (the faster it starts, the more you are
pushed back in your seat), velocity is how fast it is
going (has it reached 25 mph?), and displacement is
how far it has gone (has it moved a mile?). These are
practical measurements for an earthquake because the
damage a building sustains can be related to how
quickly it is hit by the shock waves, how fast it moves,
or how far it moves.
4. From the data presented in Figure 7.4, summarize the role
of distance from the epicenter in attenuating earthquake
waves.
5.
Compare the data shown in Figure 7.5, which is an isoseis-
mal map showing areas of similar damage to structures as
measured by the Modified Mercalli Intensity scale, with the
acceleration numbers shown in Figure 7.4. Describe the cor-
relation between acceleration and seismic impacts as
recorded on the Mercalli scale.
