EARTHQUAKES Chapter 9
A. Hazards
1.
Historic Examples: in historic time, earthquakes
(EQ) have been responsible for millions of deaths, more than any other geologic
hazard (volcanoes, for example) and second only to floods in terms of natural
disasters p. 188,
Table 9.1
2.
Potential for fatalities depends on:
a.
Magnitude of the EQ
b.
Population density of the affected area
c.
Distance from the epicenter of the EQ
d.
Time of day (rush hour in a metropolitan area is
the worst case scenario)
e.
Local Geology
f.
Type of
construction---for a severe EQ, building codes may mean the difference between
tens of deaths and tens of thousands of deaths-----this is extremely important.
i.
To avoid collapsing during a severe EQ, all
buildings (large and small) should have adequate lateral (=horizontal)
reinforcement such that the building acts as a single unit and does have parts
moving in different directions at the same time
ii.
All concrete used in construction should be
reinforced with steel rods or mesh
iii.
EQ engineering of foundations can reduce the
size of the vibrations that the building experiences during an EQ (“shock
absorber” approach)
iv.
For single family dwellings, in general wood
frame structures survive severe EQ better that brick or (unreinforced) masonry
structures
3.
Specific threats posed by severe EQ pp. 198-199 & 202-203
a.
Shaking = Ground Motion—the shaking itself is
not directly lethal to humans, but the flying debris, falling objects and
collapsing buildings, bridges and freeways are major causes of EQ deaths
b.
Landslides---significant EQ almost always
trigger landslides, often with fatal consequences
c.
Liquefaction---a major EQ will trigger
liquefaction if, and only if, there is sand underlying the surface and the
ground water table is a few feet within the surface. Many (most?) EQ do not trigger liquefaction
because the subsurface conditions are not favorable, however when it does
occur, the surface can approach a state similar to quick sand with deadly
results
d.
Tsunamis---Only EQ that affect the ocean floor
can cause tsunamis. Most EQ do not, but
on occasions where one does, the results may be catastrophic---For example, the
Tsunami caused by the December 26, 2004 Sumatran EQ
e.
Fault Rupture---occurs when the fault breaks
through to the land surface
Secondary
threats associated with severe EQ
f.
Fire---broken gas lines and water lines on a
massive scale lead to fires. Most of the
damage to
g. Disease---today with instantaneous world wide communication and
rapid transportation, this is much less of a threat than before when an EQ that
disrupted municipal water and sewer systems would lead to contaminated water
supplies causing outbreaks of local epidemics of various diseases (typhoid
fever, for example) a few weeks later.
1.
Definition---event caused by a sudden release of
energy below the surface. This is a bit
vague & includes:
a.
Quarry blasts
b.
Underground nuclear explosions
c.
Movement of magma beneath a volcano
d.
MOVEMENT ALONG FAULTS—this is cause of what we
normally consider to be EQ
2.
Terminology---see p. 190
& Figure 9.4, p. 191
a.
Focus—“point” within earth where the energy
release occurs.
b.
Epicenter---point on earth’s surface directly
above the focus
c.
Depth---distance from the epicenter to the focus
i.
Shallow EQ----includes vast majority---depth is
less than 60 miles
ii.
Deep Seated EQ----few, mostly in western
Pacific—where depth is more than 60 miles (and can be up to several hundreds of
miles)
Note: the text considers 3 categories
(shallow, intermediate and deep) but I prefer to use only 2
3.
Elastic Rebound Theory---used to explain how EQ
occur—analogous to slowly stretching a rubber band to the breaking point. See p.189 and Fig. 9.2,
p. 189
4.
Distribution of EQ.: Most, but not all, EQ lie along plate
boundaries. See text, p. 191-192 & Fig. 9.5 (map),
p. 191
a.
Notable belts include:
i.
Circum-Pacific Belt—all margins of the
ii.
An irregular belt that runs from the
Mediterranean to the
iii.
The Mid-Atlantic Ridge (no severe EQ and mostly
underwater)
b.
Non-Plate Boundary EQ in the
i.
ii.
New Madrid, MO. (1811-1812)—series of closely spaced
EQ in late Dec./early Jan.---the greatest EQ experienced in North America is
historic time. At that time the area was
not heavily populated so deaths were minimal; the same EQ occurring today would
kill many, particular in the
iii.
Ohio: only
small EQ occur in Ohio---one belt around east-central Ohio (Anna) and another
in NE Ohio along Lake Erie---possibly due to isostatic rebound (from the last
Ice Age)
5.
Seismic Risk Maps---show greatest potential EQ to be expected; probability is
NOT factored in.
1.
Types of Seismic Waves see
page 193 and Fig 9.8, page 194
a.
Body Waves
i.
P-Waves (Primary Waves----travel faster than S
or Surface waves and can travel thru a liquid)
ii.
S-Waves (Secondary Waves---travel slower that P
waves and can NOT travel thru a liquid)
b.
Surface Waves—no need to distinguish between Rayleigh & Love waves.
As the name implies, these travel along the surface of the earth. They travel slower than either P or S
waves. Assume that all damage in an EQ
is due to the Surface Waves (this is a bit over simplified, but OK)
2.
The Earth’s Interior
a. Crust
b. Mantle
b. Outer Core (liquid iron-nickel)
c. Inner Core (solid iron-nickel)
3.
Detection and Location of EQ
a.
Seismograph---requires a reference point that
does not move during an EQ and another point that experiences the full effect
of the shaking
b.
Seismic Record :
Note: 1st arrivals time for P and S; See
Fig. 9.9 a & b, p.195
c.
Distance from an EQ: The difference in arrival times of P &S
at a given seismic station is used to calculate how far away the EQ is from
that station; it is exactly the same idea as determining your distance from a
lightning strike by the time delay required to hear the thunder. p 194; fig 9.9 p 195;
fig 9.10 p 196
d.
Location of the Epicenter: Triangulation using data from a minimum of 3
widely spaced seismic stations. See Fig. 9.10, page 196.
1.
Modified Mercalli
Scale Look at, but do NOT memorize Table 9.2 (p. 197); see p. 196 (“Intensity”)
i.
Qualitative scale based on extent of destruction
and human response
ii.
Scaled in Roman numerals form I (not generally
felt) to XII (total destruction)
iii.
Data presented in the form of a contoured
map
iv.
Takes a while to compile, so it is not
used by the news media and therefore not well known
2.
Richter Scale See text p. 196-197 (“Magnitude’)
i.
Quantitative
scale based on displacement measured on (standardized) seismic record: 1,2,3...7,8...
ii.
Said to be
“open ended”, but about a “9” is as high as is ever reached in nature; a break
around the entire earth with movement which is entirely impossible, would cause
a 13-15 Richter reading.
iii.
The scale is
exponential (or logarithmic) such an increase of one unit corresponds to a
10-fold increase in the displacement measured on the seismic record and about a
30-fold increase in the energy released
iv.
The Richter
value determined at all stations are adjusted to what the value should be at a
(hypothetical) seismic station exactly 100 km from the epicenter
v.
Unlike the Mercalli value, the Richter value can be determined
immediately and therefore it is the value usually cited by the news media.
3.
Moment Magnitude---now
gaining favor---for the great EQ, the scale is usually bit higher than the
Richter
4.
Frequency of
EQ------the vast majority of EQ are not felt by humans; the greater the
magnitude of an earthquake, the less frequently it will occur because to be
larger, it usually has to acquire strain energy over a longer period of time. See Table 9.3
p.198
1.
Any acceptable method
for predicting EQ must:
a.
Be 100% reliable---no
major EQ missed (not predicted) and no false predictions (EQ must occur if
predicted)
b.
Have a useful “window”
within the predicted interval; the window must be days or weeks, not months or years
2.
Parameters that can be
measured or observed in order to make predictions
a.
Strain accumulation in
rocks
b.
Sudden changes in
ground water table
c.
Increase in radon gas
content of ground water
d.
Tilting of rocks
(related to “a”)
e.
Animal behavior
f.
Using the geology and
C dating to establish periodicity and regularity of major EQ in seismically
active area
3.
Control---- (not covered on test)