Download Earthquakes: How do we measure their impact?

Earthquakes: How do we
measure their impact?
Review: Forces and Faults
• What are 3 types of
forces responsible for
earthquakes?
– Compression
– Tension
– Shear
• What faults are
associated with each type
of force? Describe.
– Compression – reverse
fault
– Tension – normal fault
– Shear – strike – slip fault
Reverse
Energy and Seismic Waves
• When rocks move and break along a fault,
energy is released and this energy creates
damage
– Energy is released from the
focus, located under the
surface of the Earth
– This energy travels away from
the focus as seismic waves
– “Seismic” is from the Greek
word for shaking
– Click to see an animation of the seismic waves
Seismic Waves
• Primary waves (P – waves)
– Make rocks vibrate back and forth
– Fastest waves at about 6km/second
• Secondary waves (S – waves)
– Shake rocks from side to side
– Half the speed of P waves about 3 km/second
• Energy that reaches the surface is in the form of waves
called surface waves
– Cause the most damage but are the slowest…why?
Surface Waves
• The point on the Earth’s
surface directly above
an earthquake’s focus
is the epicenter
• Surface waves travel outward
from the epicenter
and move up, down
and side to side
• Can cause buildings to move in
different directions and fall
apart
How do we measure the strength of an earthquake?
• Seismologists use the different speeds of the
seismic waves to calculate the distance to the
epicenter of the earthquake
• A seismograph measures the energy released, or
magnitude of the earthquakes
• Which types of waves
arrive first to a station?
1)Primary
2)Secondary
3)Surface
How do we measure the strength of an earthquake?
• Earthquake magnitude is usually measured
using the Richter scale
– 1.0 – 9.0
– Each 1 step increase =
10 x stronger
Ex: From 6.0 to 8.0 = 100x
Earthquake Occurrences
Richter Magnitude
Number expected/year
1.0 – 3.9
> 949 000
4.0 – 4.9
6200
5.0 – 5.9
800
6.0 – 6.9
120
7.0 – 7.9
18
8.0 – 8.9
<2