Download Chapter 16 - Heritage Collegiate

Chapter 16
Earthquakes
Notes
(3 classes notes + 1 class lab + 1 class Xword & review)
Objectives:
1. Define earthquake. p. 441
2. Explain the causes of earthquakes. pp. 441-443
3. Define foreshock and aftershock. p. 443
4. Explain how a seismograph works. p. 444-445
5. Compare the properties of earthquake waves. pp. 445-447
6. Compare the Richter and Mercalli scales. pp. 451-453
7. Locate the epicenter of an earthquake given appropriate
seismographic data. pp. 447-449 and Core Lab #5
An earthquake is the vibration of the earth produced by the rapid
release of energy. They are most often caused by slippage along large
fractures in the earth's crust called faults. The energy radiates in the
form of waves in all directions from its source, called the focus of the
earthquake (see Figure 16.3 p. 442 text). This focus is located under
the earth's crust. The location on the earth's surface directly above the
focus is called the epicenter (see Figure 16.3 p. 442 text). In North
America, the most famous earthquake region is along the San
Andreas Fault. See animations here & here
How Earthquakes Form (see Figure 16.5 p. 443 text)
Tectonic forces (the forces that move the roughly 20 plates around)
slowly deform the crust on both sides of a fault (see Figure 16.5B p.
443 text). This causes the rocks to bend and gain elastic energy.
Eventually, enough force will build up to overcome the forces which
hold the rocks together and slippage (see Figure 16.5C p. 443 text)
occurs at the weakest point along the fault (focus). This movement
will cause other points along the fault to come under stress and
additional slippage will occur until all the built up elastic energy is
1
released (see Figure 16.5D p. 443 text). This slippage will allow the
rock to return to its original shape and it is the vibration that occurs as
the rock returns to its original shape that we call an earthquake. The
returning of the rock to its undeformed state is called elastic
rebound, as the rock behaves much like an elastic band which is
stretched and then returns to its original shape when released. See
animations here, here, here, and here
Sample Exam Question
1. After an earthquake, what happens to the rocks on either side of the
fault?
a) They return to their unstrained positions and retain their original
shapes.
b) They are metamorphosed by the extreme heat and pressure.
c) They are melted to form new igneous rocks at the fault boundary.
d) They remained stretched to their breaking point.
2. Earthquakes commonly occur at plate boundaries. With reference
to elastic rebound, what causes an earthquake?
Foreshocks – smaller earthquakes that occur before a major
earthquake. These can help predict when a major earthquake will
occur.
Aftershocks – smaller earthquakes that occur after a major
earthquake.
The Location of Earthquakes
Nearly all earthquakes (95%) are found in a small number of
earthquake belts around the globe (see Figure 16.13 p. 449). The
largest of these runs around the outer edge of the Pacific Ocean.
Another belt runs north of the Mediterranean Sea through the
Himalayas. A third runs down the middle of the Atlantic Ocean.
2
Note that most earthquakes occur at divergent, convergent and
transform plate boundaries. Also note that not all quakes occur at
these boundaries (see Figure 16.A, p. 450).
Earthquake Depths
Earthquakes are classified by how far below the surface the
earthquake occurs (see Figure 16.14, p. 451).
i. Those that occur less than 70km under the surface are called
shallow focus earthquakes. These usually cause the most
damage.
ii. Those that occur between 70km and 300km under the
surface are called intermediate focus earthquakes.
iii. Those that occur more than 300km under the surface are
called deep focus earthquakes.
It was discovered that in subduction zones, the depth of the
earthquake increased as you moved farther from the trench. These
regions were named Wadati-Benioff zones after the scientists
who first studied them extensively.
The depth of the focus of an earthquake depends on the type of
plate boundary at which it occurs. Divergent and transform
boundaries are typically the location of shallow focus earthquakes
whereas shallow, intermediate, and deep focus earthquakes can
occur at convergent boundaries.
How a Seismograph Works
Seismology is the study of earthquake waves. Seismographs are
instruments that recode earthquake waves (see Figure 16.7 and Figure
16.8 p. 445 text). A seismograph consists of a weight that is freely
suspended from a support that is attached to the bedrock. When an
earthquake occurs, the bedrock and the support move but the weight
3
remains stationary because of its inertia (Newton's First Law of
Motion or the Law of Inertia). The movement of the bedrock in
relation to the stationary weight is recorded on a rotating drum which
contains the paper. This record of the ground's motion is called a
seismogram (see Figure 16.10 p. 447 text). See animations here,
here, and here
Seismic waves – Waves that are created by an earthquake and
that carry the energy created by the earthquake.
Types of Seismic (Earthquake) Waves
Seismograms reveal that there are two main types of seismic waves.
1. Surface Waves (Long (L) Waves) - those that travel along the
earth's solid outer layer. These are the slowest type of seismic waves.
Surface waves cause the ground either to move from side to side
horizontally, perpendicular to the path of the wave itself (see Figure
16.9C, p. 446) or to move in an elliptical motion (see Figure 16.9D, p.
446).
2. Body Waves - those that travel through the earth's interior. Body
waves are further divided into two types, depending on how they
move through the earth.
i. Primary Waves (Push-Pull (P) Waves) - these are
compression waves which transmit energy much like air transmits
sound wave energy. In a compression wave, the particles of the
medium move horizontally and parallel to the direction in which the
wave moves (see Figure 16.9A p. 446 text). P waves can travel
through solids, liquids and gases. P waves travel fastest through the
earth, up to 6km/s. See animations here, here, here, and here
ii. Secondary Waves (Shear (S) Waves) - these are transverse
waves which transmit energy much like a rope transmits energy using
a wave. In a transverse wave, the particles of the medium move
vertically or horizontally perpendicular to the direction in which the
wave moves (see Figure 16.9B p. 446 text). S waves can travel
through solids only and not through liquids and gases. S waves are
4
slower (3.6km/s) than P waves but faster than surface waves. See
animations here, here, here, and here
Sample Exam Question
Explain how S waves differ from P waves.
Activity: Complete the table below.
Name
Other Name
Surface
Wave
(L Wave)
Long Wave
Medium
that Wave
will Travel
Through
Particle
Motion
Slowest
Horizontally
parallel to
the direction
of the wave.
Primary
Wave
(P Wave)
Secondary
Wave
(S Wave)
Relative
Speed
Solid
The Mercalli Intensity Scale and the Richter Magnitude
Scale
The Mercalli Intensity Scale (see Table 16.1 p. 452 text) assesses the
intensity (damage, destruction) caused by an earthquake at a specific
location. Note that the damage caused by an earthquake depends on
several variables.
i.
The strength of the earthquake
ii. The distance from the epicenter
iii. The type of material on which objects sit.
5
iv.
The design of buildings
The Richter Scale (see Table 16.2 p. 454 text) measures the
magnitude (amount of energy released) of an earthquake by
measuring the amplitude of the largest wave recorded on the
seismogram (see Figure 16.16 p. 453 text). To ensure that all seismic
stations record the same Richter magnitude for a given earthquake,
adjustments are made for the weakening of the seismic waves as the
distance from the epicenter increases and for the variations in
sensitivity of the seismograph. The scale is logarithmic with an
increase of 1 in magnitude meaning a wave recorded on the
seismogram has an amplitude which is 10 times larger. As well, each
increase of 1 in magnitude means that about 32 times more energy is
released by the earthquake. So an earthquake measuring 6 on the
Richter scale releases 32 times as much energy as an earthquake
measuring 5 on the Richter scale and 32x32 ≈ 1000 times as much
energy as a magnitude 4 earthquake (see Table 16.3 p. 454 text).
The two scales are different because a modest earthquake on the
Richter scale could occur in a populated area with weak bedrock and
poorly constructed buildings and cause enormous damage. This
would cause the earthquake to receive a relatively high rating on the
Mercalli scale. As well, an earthquake with a high Richter magnitude
may occur far from a populated area and be felt by no one and cause
no damage. This would cause it to have a very low rating on the
Mercalli Scale.
Activity: Complete the following table.
Type of
Scale
Richter
What is
measured
How is it
measured
Magnitude Seismographs
(energy
released)
6
Type of
numbers
used
Type of
scale
Modified
Mercalli
Roman
Numerals
Closed (I –
XII)
Sample Exam Question
1. Distinguish between an earthquake’s magnitude and intensity.
2. What information do you need to calculate an earthquake's
intensity?
a) The difference in arrival times of P and S waves.
b) The time that the ground was shaking.
c) Records of 3 seismographs at different distances from the source.
d) Records of observations and damages produced by the event.
How to Locate the Epicenter of an Earthquake
The difference in the speeds of P and S waves allows the distance to
the epicenter to be determined. In general, since P waves travel faster
than S waves, the farther the away the epicenter is located, the greater
the time difference between when these waves are detected by the
seismograph.
First, several seismograms similar to the one in Figure 16.10 on p 447
of the text are used to create a travel-time graph (see Figure 16.11 p.
448 text). If you examine the seismogram on page 447, you should be
able to determine that approximately 5 minutes passed between the
arrival of the first P wave and the first S wave. Looking at the traveltime graph in Figure 16.11, we can then determine where the travel
time difference between the S and P waves is 5 minutes. Extending a
vertical line down to the distance axis gives the approximate distance
to the epicenter, which is approximately 3800km in this example.
You will follow this procedure in Parts 1 and 2 of Lab #7.
Knowing the distance to the epicenter alone will not allow you to
determine the exact location of the epicenter. You also need to know
the direction. The exact location of the epicenter can be located using
7
the distance from three seismic stations. Once the distance to the
epicenter is determined for the three stations, a circle of the correct
radius is drawn around each station. The point of intersection of the
three circles gives the exact location of the epicenter (see Figure
16.12 p. 449 text). You will do this in Lab #5.
Sample Exam Question
Why is a seismic record from 3 locations required to find the location
of the epicenter?
Core Lab #5: Locating an Earthquake Epicentre
Do #'s 1, 2, 4, 6, 7, 8, 9, 10, 13, 14, 15, 17, 22 p. 469 text.
Read Chapter Summary items 1-4, 6, 7 p. 468 text.
Read pp. 475-477, 483-485 for next day
8