Download Earthquakes

Earthquakes
Global Earthquake Locations
Earthquakes
• Shaking of earth due to movement of rocks along a fault.
• Rocks under stress accumulate strain energy over time.
• When stress exceeds strength of rocks, rock breaks.
• Strain energy is released as seismic waves. The longer that
energy is stored up and is maintained without release, the
more likely that a strong earthquake will occur.
Types of Stress
• Tension
• Compression
• Shearing
Kinds of Faults
• Normal Fault
– caused by a tension or pulling apart force
• Reverse Fault
– caused by a compression or pushing force
• Strike-slip Fault
– caused by a shearing force when plates move past each
other
What is the Elastic Rebound Theory?
• Explains how energy
is stored in rocks
– Rocks bend until
the strength of
the rock is
exceeded
– Rupture occurs
and the rocks
quickly rebound to
an undeformed
shape
What is the Elastic Rebound Theory?
• Explains how energy
is stored in rocks
– Energy is released
in waves that
radiate outward
from the fault
Earthquakes
Types of seismic waves
1. Body waves -- travel through interior
2. Surface waves -- travel on surface of earth
Primary or
“P” Wave
Secondary
or “S”
Wave
Body Waves: P and S waves
– P or primary waves
• fastest waves
• travel through solids,
liquids, or gases
• compressional wave,
material movement is in
the same direction as
wave movement
– S or secondary waves
• slower than P waves
• travel through solids
only
• shear waves - move
material perpendicular
to wave movement
Surface Waves: R and L waves
•
Surface Waves
– Travel just below or along the ground’s surface
– Slower than body waves; rolling and side-to-side
movement
– Especially damaging to buildings
Types of Seismographs
Seismogram Printout
How is an Earthquake’s Epicenter Located?
– P waves arrive first, then S waves, then L and R
– Average speeds for all these waves is known
– After an earthquake, the difference in arrival times at a
seismograph station can be used to calculate the distance
from the seismograph to the epicenter.
Determining the location of an earthquake
1. First, distance to earthquake is determined.
2. Seismographs record seismic waves
3. From seismograph record called the seismogram,
measure time delay between P & S wave arrival
4. Use travel time curve to determine distance to
earthquake as function of P-S time delay
Determining the location of an earthquake
1.
Now we know distance waves traveled, but we don't know the
direction from which they came.
2.
We must repeat the activity for each of at least three (3)
stations to triangulate a point (epicenter of quake).
3.
Plot a circle around seismograph location; radius of circle is
the distance to the quake.
4.
Quake occurred somewhere along that circle.
5.
Do the same thing for at least 3 seismograph stations; circles
intersect at epicenter. Thus, point is triangulated and
epicenter is located.
Focus and Epicenter of Earthquake
Time-Travel Curve
Triangulation
of 3 stations
to locate
earthquake
epicenter
Determining the magnitude of an earthquake
• Magnitude -- measure of energy released during
earthquake.
• There are several different ways to measure
magnitude.
• Most common magnitude measure is Richter
Magnitude, named for the renowned seismologist,
Charles Richter.
Richter Magnitude
• Measure amplitude of largest S wave on
seismograph record.
Richter Magnitude
• Measure amplitude of largest S wave on seismograph record.
• Take into account distance between seismograph &
epicenter.
Richter Scale
• Logarithmic numerical (NOT a physical) scale
• Increasing one whole unit on Richter Scale represents 10
times greater magnitude.
• Going up one whole unit on Richter Scale represents about a
30 times greater release of energy.
• Mercalli Scale is used to express damage
Intensity
• Intensity refers to the amount of damage done in an
earthquake
• Mercalli Scale is used to express damage
Hazards associated with Quakes
• Shaking:
• Frequency of shaking differs for different seismic waves.
• High frequency body waves shake low buildings more.
• Low frequency surface waves shake high buildings more.
• Intensity of shaking also depends on type of subsurface
material.
• Unconsolidated materials amplify shaking more than rocks do.
• Fine-grained, sensitive materials can lose strength when shaken.
They lose strength by liquefaction.
• Buildings respond differently to shaking depending on
construction styles, materials
• Wood -- more flexible, holds up well
• Earthen materials -- very vulnerable to shaking.
Hazards associated with Quakes
• Ground displacement:
Ground surface may shift during an earthquake (esp. if focus
is shallow).
• Tsunamis (NOT called tidal waves)
Tsunamis are huge waves generated by earthquakes
undersea or below coastal areas.
If earthquake displaces sea surface, wave is generated that
can grow as it moves over sea surface.
• Fires
Usually occurs from shifting of subsurface utilities (gas
lines)
Tsunami Movement
Tsunami Movement: ~600 mph in deep water
~250 mph in medium depth water
~35 mph in shallow water
Earthquake Prediction (?)
How can scientists predict an earthquake?
Currently, that is not possible.
Future technology will monitor subsurface seismic
waves and periodic shifting indicative of future
slippage.
Tracking organic movement is also a source of future
study.
Earthquake Hazard Potential Map
Parkfield, CA
“Earthquake Capital of the World”
World’s Largest Earthquake: 1964 Anchorage, Alaska
Registered 8.6 on Richter Scale
Key Terminology
Seismic waves
Body waves
Surface waves
Primary (“P) waves
Secondary (“S”) waves
Love (“L”) waves
Seismograph
Seismogram
Focus
Epicenter
Time-travel curve
Magnitude
Intensity
Richter Scale
Mercalli Scale
Logarithmic
Liquefaction
Tsunami