Earthquake: Vibration of Earth produced
by the rapid release of elastic energy
accumulated in rocks.
Earthquakes
• Tectonic forces that move crustal blocks cause
fractures in the lithosphere called faults.
• Crustal blocks on either side of fault continue to
move, resulting friction causes fault to “seize up”.
• Elastic energy is stored in rocks adjacent to the
jammed up fault, causing them to buckle and
deform.
• Finally, elastic energy accumulated in the rock
exceeds the friction that holds rock along an
existing fault line.
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.
Scarp: vertical offset caused by
faulting ( in this diagram it’s
actually the mountain range).
Earthquake energy
• Interior forces
(convective flow)
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 open fissures
(not the same as faults, but dangerous!
Seismology: Study of Wave Energy
in the Earth
Seismic – relating to earthquakes or
earth vibrations
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
Primary wave
Secondary
Wave
Seismic Wave Motion
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
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 – correct!
c. Less than 6.2
Magnitude Scales
• measure energy released (objective,
rather than intensity (subjective)
• Logarithmic – each number on the
scale increases by a factor of 10, allows
wider range
Table 14.2
Richter Magnitude (ML)
• introduced by Richter and Gutenberg in 1935 for
comparison of California earthquakes.
• Based on the amplitude (height) of the largest seismic
wave recorded during earthquake.
• Accounts for the decrease in wave amplitude with
increased distance from epicenter
• Each unit corresponds to a tenfold increase in wave
amplitude and a 32-fold energy increase
• Originally for use in California only with a WoodAnderson torsion seismograph only up to 6.5
• Modern Richter Scales (Ms , Mb ) have been modified
and work on all seismographs, measuring up to 8.
Figure 14.11
The table at the right is
my original slide for the
amount of energy
released (in terms of
TNT needed) for
Richter Scale
measurements. A
metric ton = 1000 kg
This is before our text
included fig. 14.15 (p.
345).
There is considerable
disagreement!
Here is the source of the information in our text,
look at the difference!
http://www.iris.edu/hq/files/publications/brochures_onepagers/doc/EN_OnePager3.pdf
http://earthquake.usgs.gov/learn/faq/?faqID=33
Magnitude
Es (from Me)
Es (from Ms or Mw)
Tons of TNT
Nuclear Bomb
Equivalence (# of
bombs)
4
5
6
7
8
9
0.22E+11
0.71E+12
0.22E+14
0.71E+15
0.22E+17
0.71E+18
0.63E+11
0.20E+13
0.63E+14
0.20E+16
0.63E+17
0.20E+19
15.
475.
15023.
475063.
15022833.
475063712.
0.00
0.02
0.79
25.0
790.6
25,003.3
Other sources
http://www.themeter.net/sism_e.htm
http://en.wikipedia.org/wiki/Richter_magnitude_scale
http://hvo.wr.usgs.gov/volcanowatch/archive/2008/08_02_21.html
Moment Magnitude (Mw)
• developed because none of the “Richterlike” magnitude scales adequately
estimates the size of very large
earthquakes. Wave amplitude is limited
on seismograph.
• Derived from the amount of displacement
that occurs along a fault
• Gained wide acceptance
– Measures very large earthquakes more
accurately
– Can be derived mathematically
– Can be verified by field studies and
seismographic methods
Map showing major tectonic boundaries and
earthquake distribution (in yellow)
Circum-Pacific Belt aka Ring of Fire
Table 14.3
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
Earthquakes and plate boundaries
• 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.
Fig. 7.20, p.165
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.
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.
Chapter 14 Opening Figure
Tsunami – deep and shallow
water
Tsunami
Tsunami advancing after initial retreat
Tsunami Travel Times to Hawaii
Laguna Salada Earthquake
April 4, 2010
ftp://hazards.cr.usgs.gov/maps/sigeqs/20100404/20100404.jpg
This site has some good information about the recent “Easter Quake”
near Mexicali. Click on any part of the site to enlarge it
Haiti Earthquake – Jan 2010
Magnitude 7.0
Energy released is
equal to approximately
32 Megatons TNT
Why do earthquakes
happen in Haiti?
The answer is Plate
Tectonics!
Plate Tectonic Map
A close up showing the Caribbean Plate; it’s
northern boundary runs through Haiti
the Caribbean Plate moves eastward while the North
American Plate moves westward. The yellow lines are
faults which define a small microplate:
The red dots show historic earthquake activity. The plates
try to move past each other and get stuck, building up a
lot of energy in the process
The energy release when the plate gets
“unstuck” causes the earthquake and
resultant damage
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.
Plate tectonics of Southeast Asia
Much of Indonesia is
actually located on a
separate plate called
the Sunda Plate
(“Sonde” here).
•
•The Sunda plate is
bounded on the north
by the Burma
Microplate
(“Birmanie”), site of
the 2004
quake/tsunami.
•Note the large subduction zone,
as the Australian plate pushes
underneath these plates.
•Several microplates bound the
the Sunda on the SE corner
Plate tectonics of Southeast Asia
Here is another view
of the Sunda Plate.
The yellow line is a
fault zone which has
been ruptured by
large earthquakes in
the last decade
Plate tectonics of Southeast Asia
This shows the
sliver of Sunda
Plate between the
subduction zone
and the Sumatran
Fault, which is a
right lateral
transform fault,
similar to the San
Andreas.
Ercis Earthquake, October 2011
Anatolian Plate and Bitlis-Zagros Thrust, showing
Van Lake, Epicenter Location
•Anatolian
microplate is
squeezed in
between Arabian,
Eurasian and
African plates.
•This area is part
of the AlpineHimalayan Belt
Earthquake Activity in Turkey
•Along the North
Anatolian Fault, a
transform fault boundary
much like the San
Andreas Fault
•At the Bitlis-Zagros
Thrust Zone, where the
Arabian Plate and
Anatolian Plate
experience continental
collision.
•Note location of Van
Lake, epicenter of most
recent earthquake, just
north of the collision
zone.
Comparison of Ercis quake (2011) earthquake
with Izmit quake (1999)
Comparison of San Andreas and North
Anatolian Faults
USGS Site
http://earthquake.usgs.gov/earthqu
akes/map/