Download Earthquakes

Earthquakes
Geology 101, Fall 2012
Stress and strain
• Stress is the stored energy built up due to
plate motion; strain is the volume change on a
rock due to stress
Faults
• At fracture point, the rock breaks; the
resulting offset between the two parts of the
rock is a fault. The energy released is an
earthquake
Types of faults
• The different types of plate boundaries lead to
different types of faults
Basic earthquake terminology
Earthquake energy
• Radiates away as body waves
Earthquake energy
• When body waves reach the surface, they
transfer into surface waves
Seismograph
Seismogram is a record of all waves
Measures of earthquakes
• Richter scale (Charles Richter
and Beno Gutenberg, Caltech,
1935)
• Based on movement of the
seismograph stylus
• Logarithmic scale, meaning
that each 1 point increase in
magnitude means a 10-fold
increase in movement of the
stylus
• Not accurate for big
earthquakes, or distant ones
Measures of earthquakes
• Moment magnitude scale is
commonly used today to
report earthquake
“magnitudes”.
• It is still logarithmic, but is
now based on a formula that
looks at other parts of the
seismogram than just the
largest movement.
• The formula calculates the
energy released by the quake,
which is more relevant to
engineers and planners than
the Richter scale magnitude.
Basic math of logarithmic scale
The point is, for each increase of 1 point on the moment magnitude scale, the
energy released by the earthquake goes up by a factor of 30 or so.
Measures of earthquakes
• Modified Mercalli
Intensity (MMI),
developed by Guiseppe
Mercalli, 1884 and 1906
• Measures effects on
humans and their
structures
• Often a better indicator
of potential earthquake
hazard than the moment
magnitude scale
Nisqually (2001) quake intensity
Aftershocks occur as stress transfers to
unrelieved parts of fault or rock
Earthquake prediction – which one is
true?
Interlude about resource extraction
When organic material dies...
• If oxygen can be excluded,
the decay process produces
a material called kerogen
from various fats and other
biological compounds.
• They are rich in carbon (C),
hydrogen (H) and oxygen
(O), which will burn well,
but also contain nitrogen
(N), sulfur (S) and
phosphorus (P) which
produce noxious gases (like
SO2 and NOx) when burned.
Petroleum needs three rocks
• Source rock, which contains
the organic material in the
first place (and allows the
organics to “cook”)
• Reservoir rock to which the
petroleum migrates and is
stored.
• Cap rock to prevent the
petroleum from rising
further.
Oil and natural gas can be trapped
Since oil is
less dense
than
water, it
will “float”
on groundwater. This
allows
trapping
by an
impermeable layer.
Using seismic studies profitably
Using vibrator trucks to shake the ground, and listening with an array of
geophones, petroleum engineers can determine oil-rich layers.
Interlude about wave behavior
• When a wave
passes through a
boundary, it can
either reflect off the
boundary or pass
through it in a
slightly different
direction.
• First shown for light,
the same physics
works for seismic
waves.
Probing the Earth’s interior
Changes in wave speed indicate deep Earth
structures
Specifically, the Mohorovicic
(Moho) discontinuity, where
seismic speeds suddenly
increase as they get deeper,
shows the change from
crust materials to mantle
within the lithosphere.
Note that the mantle does
get more solid toward the
core (speeds increase), but
that the outer core is liquid.
S-wave properties give us the size of
the outer core
P-wave properties give us the size of
the inner core