Download Chapter 7 earthquakes

Earthquakes
Earthquake
An earthquake is the vibration of Earth
produced by the rapid release of elastic
energy accumulated in rocks.
Earthquakes occur when:
• Elastic energy exceeds rock strength
and the rock breaks forming a fault.
• Elastic energy accumulated in the rock
exceeds the friction that holds rock
along an existing fault line.
Earthquakes and Faults
• Fault: large
fracture in earth’s
crust caused by
plate motion.
Faults are often,
but not always
associated with
plate boundaries.
• Scarp: vertical
offset caused by
faulting ( in this
diagram it’s
actually the
mountain range).
Earthquakes and Faults
• The earthquake begins
at the focus, which is
the initial point of
rupture along the fault,
at depth.
• The epicenter is the
location on the earth’s
surface directly above
the focus.
• Vibrational energy
radiates from the focus
in all directions, in the
form of waves.
Earthquake energy
• Interior forces (from
heat, etc) push
tectonic plates.
• at plate boundaries,
frictional forces hold
plates stationary.
While rocks along
plate boundary are
“stuck”, elastic
energy is stored and
causes elastic
deformation.
Energy release – Elastic
rebound
• Frictional resistance
holding the rocks
together is overcome.
• Resulting movement
releases stored energy
as rocks return to
original shape.
Energy Storage and Release
Released energy is in
the form of waves,
which cause
movement on the
earth’s surface and
interior
Energy storage and release
• Energy release may also result in fractures in
the earth’s crust.
Seismology: Study of Wave
Energy in the Earth
• Types of seismic waves
• Body waves travel entirely through
earth’s interior
– Primary (P) waves
» Push-pull (compress and expand)
» Travel through solids, liquids, and gases
– Secondary (S) waves
» Slower velocity than P waves
» Slightly greater amplitude than P waves
» Travel though solids only
Seismology
• Types of seismic waves (cont’d)
• Surface waves
– Travel along surface of Earth
– Cause greatest destruction
– Waves exhibit greatest amplitude and
slowest velocity
Seismic Wave Motion
Earthquake Waves
Seismograph
• Instrument used to record surface and
body waves passing through the earth
• More than one type of seismograph is
needed to record both vertical and
horizontal ground motion
• Records obtained are called
seismograms
Seismographs
Figure 14.6
Figure 14.8
Figure 14.9
How Big
• Modern measurement
scales (Richter, moment
magnitude) measure the
amount of energy
released by a quake.
• This value is the same no
matter how far away from
the epicenter the
measurement is taken
How Big
An earthquake occurs in
Pakistan. Pakistanis
report a magnitude
of 6.2. Scientists in
California would
probably measure a
magnitude of:
a. Greater than 6.2
b. 6.2
c. Less than 6.2
Magnitude Scales
• measure energy released (objective,
rather than intensity (subjective)
• Logarithmic type scale – the energy
released by the earthquake increases
by a factor of about 30 for each
increment on the scale (i.e. a
magnitude 6 earthqake releases
roughly 30 times as much energy as a
magnitude-5 earthquake.
Measurement of Energy Released during an
Earthquake
Richter Magnitude (ML)
• introduced by Charles
Richter in 1935
• Based on the amplitude of
the largest seismic wave
recorded during earthquake
Moment Magnitude (Mm)
• Measures the amount of
movement and surface area
of a fault that moved during
earthquake
• More accurate than ML,
especially for very large
earthquakes
The table at the right
shows the amount of
energy released (in
terms of TNT needed)
for Richter Scale
measurements
(corrected to account
for saturation)
Most earthquakes occur at tectonic
plate boundaries
Transform boundaries, e.g. San
Andreas Fault Zone
• Strike-slip fault;
movement along fault is
mainly horizontal
• Fault creep – small, slow
movements along fault
• Stick-slip movement –
fault moves in a series of
jolts with no movement in
between. Significant
energy buildup possible,
resulting in is largemagnitude damaging
earthquake
Figure 14.21
Convergent boundaries – one plate
sliding under another
• Benioff zone – upper
part of sinking plate,
where it scrapes past
opposing plate,
causing earthquake
activity along the
down-plunging
contact zone
• Pacific NW evidence
• India-Pakistan border,
2005
Tsunami
• When an earthquake occurs
beneath the sea, the sea
floor rises and falls, due to
rupture and elastic rebound.
• Resulting water
displacement forms a fastmoving wave.
• In the deep water of the
open ocean, tsunami are
barely detectable.
• In shallow water near shore,
the wave speed decreases
as it drags against the
bottom.
• The water “stacks up”,
causing a large wall of water
to make landfall.
December 2004 Sumatra-Andaman Tsunami
• This shows a portion of the
convergent boundary between
the Eurasian plate and the
Indo-Australian plate. The
latter is moving northward,
pushing against the former
• Green star shows epicenter of
earthquake that caused
tsunami.
• Red arrows show plate motion
• Red dots show earthquakes >
magnitude 5.0, from 1965 to
2004.
Sea floor moves due to slippage;
radiating energy results in tsunami
Tsunami
Often the water retreats before the
wave hits
Figure 14.17
Earthquakes along divergent boundaries
Friction along sliding blocks (transform faults)
Earthquakes in plate interiors
• 1811-12, New Madrid, MO, site of aborted
divergent boundary
• This area is still seeing active movement
along the fault.
Table 14.2
Table 14.3
Figure 14.21