Download 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.
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.
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.
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
Characteristic Effects
Number Annually
Usually not felt by people.
800,000
Felt indoors by some people.
30,000
IV
Felt by many people, windows rattle.
4,800
V
Felt by all; dishes break, doors swing.
1,400
VI-VII
Slight building damage, plaster cracks.
500
VII-IX
Much building damage, chimneys fall, houses
move on foundations.
Serious damage, bridges twisted, walls
fractured, many masonry buildings collapse.
Great damage, most buildings collapse.
100
7.0-7.3
X
7.4-7.9
>8.0
XI
XII
Total damage, waves seen on ground
surface, objects thrown into the air.
15
4
one every
5-10 yrs
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).
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..
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
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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.