Earthquakes
Chapter 6
February 2012
This topic will shake you up
Stress
Stress- a force (energy) that acts on rock to change
its shape or volume
Types of Stress
Tension- pulls or stretches rock so that it becomes
thinner in the middle. This happens at divergent
plate boundaries.
Compression- squeezes rock until it folds or breaks.
This happens at convergent plate boundaries.
Stress continued
• Shearing- stress that pushes a mass of rock in
opposite directions. This occurs at transform
boundaries.
Faults
Most faults occur along plate boundaries.
Normal faults- fault is at an angle so one block
lies above the fault and the other is below.
Reverse fault- Rock pushed together. Blocks
move in opposite directions
from a normal fault.
To correctly identify a fault, you must first figure out which block
Normal Faults
is the footwall and which is the hanging wall. Then you
determine the relative motion between the hanging wall and
footwall. Every fault tilted from the vertical has a hanging wall
and footwall. To determine which is which, visualize yourself
creating a mine in along the fault plane. The block below your
feet is the footwall, and the one upon which you would hang
your miner's lamp is the hanging wall. It is that simple. Strikeslip faults are vertical and thus do not have hanging walls or
footwalls. If the hanging wall drops relative to the footwall, you
have a normal fault. Normal faults occur in areas undergoing
extension (stretching). If you imagine undoing the motion of a
normal fault, you will undo the stretching and thus shorten the
horizontal distance between two points on either side of the
fault.
Faults continued
Strike slip faults- rocks slip past each other
sideways.
Changing the Earth’s Surface
Plate movement can cause the crust to fold and bend.
Anticline- a fold that bends upward
Syncline- a fold that bends downward
Changing the E’s surface cont.
The collision of two plates can cause compression
and folding over a wide area (mountain ranges).
- Himalayas in Asia
- Alps in Europe
Stretching the Earth’s Crust
Fault Block Mountains pg 167
Uplifting- a plateau is a large area of flat land
pushed up high above sea level.
The kaibab Plateau
Earthquakes
Section 2
Earthquakes are the shaking and trembling that
results from the movement of rock beneath Earth’s
surface.
Stress builds up in rock along a fault until the
rock breaks releasing enormous amounts of energy.
Most EQ’s begin in the lithosphere.
Earthquakes continued
Focus- the area beneath the surface where rock
that is under stress breaks.
Epicenter- the point on the surface directly
above the focus.
Types of Seismic Waves
Seismic waves- carry energy away from the focus,
through the Earth’s interior and across the surface.
There are three categories of seismic waves:
1. P- waves (primary waves)
2. S- waves (secondary waves)
3. Surface waves
Types of Seismic Waves cont.
P- waves- primary waves. First to arrive. Compress
and expand the ground like an accordion.
S- waves- Secondary waves vibrate side to side and
up and down. S waves cannot travel through a
liquid. These are the waves that shake structures.
Surface waves- when S and P waves meet the
surface. They move slower than P and S waves.
Produce severe ground movements.
Measuring EQ’s
Mercalli scale- rates EQ’s according to level of
damage. 12 steps (Low, moderate, severe, etc..)
Measuring EQ’s cont.
Richter scale- a number assigned based on EQ
size. Measures seismic waves using a
seismograph.
Moment magnitude scale- measures total
energy released.
Locating the Epicenter
Scientists use the speed of the waves to
determine the epicenter. Scientists use the
arrival time of the different waves. The further
away the earthquake, the greater the arrival
time.
Monitoring Earthquakes
The seismograph- an instrument that uses a rotating
drum and a weighted pen to record vibrations as
zigzagging lines. Records the P waves first, then the
S waves.
Measuring Faults
Tiltmeters- measures the tilt or raising of the
ground. Similar to a carpenters level.
Measuring Faults cont.
Creep meter- uses a laser to measure horizontal
fault movements. (Distance)
GPS- measures changes in elevation as well as
horizontal movements.
Using Seismograph Data
page 182
Seismic waves are reflected at faults. This enables
scientists to map the faults in an area by studying
the wave patterns.
So with all this technology can scientists predict
earthquakes? Nope!
Earthquake Safety
Scientists can determine risk by locating where faults
are active.
How EQ’s cause damage
Shaking- can trigger landslides, destroy buildings etc.
Liquefaction- turns soft loose soil into mud.
Aftershocks- EQ that occurs after a larger EQ hours or
days later. May collapse already weakened buildings
Tsunamis- when an EQ occurs under water it can
displace water causing a large wave
Steps to EQ safety
The best way to protect yourself is to drop cover and
hold.
Designing safer Buildings
Buildings can be made stronger and more flexible by
using:
-springs
-dampers
-shear walls
-ties
-reinforcement
-flex pipes
Tuned mass damper
730 tons