Download Earthquakes Terminology of Earthquakes Elastic Rebound Theory

Geol 104: Earthquakes
Earthquakes
• What are they?
– An earthquake is a trembling or shaking of the
ground.
• What causes them?
– Earthquakes are caused by the sudden release
of stored [elastic] strain energy by brittle
fracture.
Terminology of Earthquakes
• Focus or Hypocenter - the point within the
Earth where the rocks rupture.
• Epicenter - the point on the Earth’s surface
directly above the focus.
• Focal Depth - distance of focus from
Earth’s surface.
Elastic Rebound Theory
• Tectonic forces slowly build up with time,
initially the rocks deform elastically.
• Eventually the build up of stored energy
exceeds the strength of the rock and it
fractures.
• The stored elastic strain energy is rapidly
released and propagates outward as
vibration waves called seismic waves.
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Geol 104: Earthquakes
Seismic Waves
• Body waves
– Seismic waves that travel through the interior
of the Earth. As the waves propagate, the rocks
are elastically deformed by change in either
volume or shape.
Types of Body Waves
– Compression (P) waves - deform rocks
through change in volume.
• P-waves have greatest velocity (4-7 km/s in crust
and ~8 km/s in mantle). As such, they are the first
waves to arrive at a distant point.
– Shear (S) waves - deform rocks through
change in shape.
• S-waves travel ~2 km/s slower than P-waves, and
their velocity depends on the density and resistance
to shearing of the material. Fluids do not have shear
strength and thus cannot transmit S-waves.
Seismic Waves
• Surface waves
– Seismic waves that travel along the surface of
the Earth. Slowest moving waves collectively
referred to as L or Long waves.
• Love waves - transverse side-to-side wave motion
in a horizontal plane parallel to Earth’s surface.
• Rayleigh waves - backward rotating, circular
motion similar to water molecule in ocean waves.
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Geol 104: Earthquakes
Measuring Seismic Waves
• Seismographs - detect and record
vibrations/motion of the Earth caused by an
earthquake.
– Inertial seismographs
– Strain seismographs
Locating an Earthquake
• Travel time curves
– Use the time lag between P and S seismic
waves to determine distance from the epicenter
to the seismograph (but not direction).
• Three-point construction
– Intersection point of time lag distance circles
from three different seismographs yields
location of epicenter.
Earthquake Energy
• Intensity
– Mercalli Intensity Scale
• Qualitative measure of intensity based upon amount
of vibration people feel and extent of damage.
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Geol 104: Earthquakes
Earthquake Energy
• Magnitude
– Richter Magnitude Scale
• Quantitative scheme based upon maximum
amplitude of strongest body wave.
• The Richter scale is logarithmic; each increase in
magnitude corresponds to a 10-fold increase in the
amplitude of the wave.
• More importantly, each increment of magnitude
corresponds to a 30-fold increase in energy
released.
Earthquake Energy
• Seismic Moment Scale
– Based upon the shear strength of the rock,
rupture area of the fault and the average
displacement (slip) on the fault.
– The seismic moment scale provides a better
measure of energy released by large (>6 on
Richter scale) earthquakes.
Earthquake Magnitudes, Characteristic
Effects and Frequencies
Richter
Mercalli
Magnitude Intensity
<3.4
I
3.5-4.2
4.3-4.8
4.9-5.4
5.5-6.1
6.2-6.9
II-III
IV
V
VI-VII
VII-IX
7.0-7.3
X
7.4-7.9
>8.0
XI
XII
Characteristic Effects
Number Annually
Usually not felt by people.
800,000
Felt indoors by some people.
Felt by many people, windows rattle.
30,000
4,800
Felt by all; dishes break, doors swing.
Slight building damage, plaster cracks.
Much building damage, chimneys fall, houses
move on foundations.
1,400
500
100
Serious damage, bridges twisted, walls
fractured, many masonry buildings collapse.
Great damage, most buildings collapse.
Total damage, waves seen on ground
surface, objects thrown into the air.
15
4
one every
5-10 yrs
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Geol 104: Earthquakes
So how much elastic energy can
rocks accumulate before failure?
• ~100 joules of energy can be accumulated in 1 m3 of
elastically deformed rock.
• 100 joules is equivalent to ~25 calories of heat
energy (about two peanut M&Ms).
• But, when billions to trillions of m3 of rock are
strained the results are impressive...
• The 1989 Loma Prieta quake released ~1015 joules
and the 1906 San Francisco quake released ~1017
joules (about the same amount of energy as a
hydrogen bomb blast).
Distribution of Earthquakes
• ~80% originate in the circum-Pacific
seismic belt that corresponds to the volcanic
belt known as the Ring of Fire.
• ~15% originate in the MediterraneanHimalayan belt that extends from Gibraltar
to Southeast Asia.
• Lesser seismic belts follow mid-ocean ridge
systems.
Tectonic Settings of Earthquakes
• Divergent Boundaries
– Tensional stress
– Low magnitude and shallow foci (often <20
km).
– Rocks are weaker in tension and temperature is
higher near the surface (brittle-ductile transition
nearer surface).
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Geol 104: Earthquakes
Tectonic Settings of Earthquakes
• Transform Boundaries
– Shear stress
– High magnitude and shallow foci (<100 km).
– Shear stress can build at “locked” segments
(Stick-slip behavior).
– Temperature follows normal geothermal
gradient.
– Water may act to lower strength.
Tectonic Settings of Earthquakes
• Convergent Boundaries
– Subduction zones
• Bending within down-going slab can result in
tensional stress and small magnitude earthquakes at
very shallow depth.
• Sliding of down-going slab in the rigid lithosphere
yields large magnitude earthquakes at depths to
~100 km.
• Sinking through ductile asthenosphere, earthquakes
are within the down-going slab down to 670 km.
Tectonic Settings of Earthquakes
• Convergent Boundaries
• Bennioff zone - zone of earthquake foci dipping
into the mantle away from a trench due to
subduction.
– Collision zones
•
•
•
•
Where two continental lithosphere plates converge.
Very strong compressive forces.
Rocks are much stronger in compression
High magnitude earthquakes at depths to 300 km..
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Geol 104: Earthquakes
Tectonic Settings of Earthquakes
• Intraplate (Bowling Green)
– Most often associated with failed rift zones and
reactivation of ancient faults.
– Shallow foci but can have high magnitude due
to high strength of continental basement rocks.
Review - Earthquakes
– Elastic rebound theory
– Types/characteristics of seismic waves
– Locating earthquakes
– Intensity and magnitude of earthquakes
– Energy released by earthquakes.
– Geographic distribution of earthquakes
– Tectonic settings and depth/magnitude of
associated earthquakes.
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