Download Earthquakes

Earthquakes
6.1 Earthquakes
and Plate
Tectonics
6.2 Recording
Earthquakes
6.3 Earthquake
Damage
Earthquake images from Japan
Earthquakes
• Earthquakes can be caused by a variety
of events – volcanic eruptions, meteor
impacts, caverns collapsing, but most are
due to the movement of the Earth’s
tectonic plates
• About 3 million occur each year (about
one every 10 seconds) – but most are too
small to be noticed
Earthquakes
• They are part of the natural process of
rocks moving past each other as plates
move over the Earth’s surface
• It’s when the plates can’t move, that the
little ones don’t happen, and pressure
builds towards a large earthquake
Elastic Rebound Theory
• Earthquakes occur when rocks under stress
suddenly shift along a fault
– Rocks along a fault rub together as plates pass but when
they can’t they become “locked” and pressure – stressincreases until the point where the stress is so great they
fracture and slip past each other at their weakest point
– The “slippage” is what causes the ground to vibrate
– After slipping the rocks rebound or return to their
original shape (but at different locations than before)
Elastic Rebound Theory
• The energy that has built up in the stressed
rocks is quickly released in vibrations called
seismic waves
– These waves often increase stress in other areas
of the fault that then also fracture and rebound
in a series of aftershocks
• The point where slippage first occurs is the
focus of the earthquake, the point directly
above this on the surface is the epicentre
Note: the focus in some documents is also called the
hypocenter
Elastic Rebound Theory
• Seismic waves move out in all directions
from the epicentre
• 90% of earthquakes have a shallow focus of
70 km or less such as along mid ocean
ridges and transform boundaries
– These are the ones that cause the most damage
Elastic Rebound Theory
• Intermediate focus earthquakes occur
between 70-300 km
• Deep focus earthquakes occur between 300650 km
– For example, as found in subduction zones
Geology of Canada’s West Coast
Earthquake zones
• Most earthquakes are associated with the edges of
tectonic plates where the moving plates exert the
greatest strain on the rock
• The largest earthquake zone is the Pacific Ring of
Fire (mainly convergent boundaries)
• The next major zone is along mid ocean ridges (at
divergent boundaries) or along rifts
• The third zone is along the Eurasian Mountain belt
(which is a collision boundary)
Seismic zones in
India – zone V is
the highest risk area
for earthquakes of
magnitude 8 or
more.
A major tremor is possible in
the Middle East
Earthquakes in Lebanon
occur as a result of the
African Rift Valley, which
extends from Lebanon in the
north in Middle East to
Mozambique in the south in
Africa.
The Rift valley is the result of
normal fault in which two
tectonic plates moving away
from each other, forcing the
uplifting of one of the two
plates. As a result of
tensional forces beneath,
earthquakes can occur.
An earthquake struck
Lebanon in A.D. 551
destroying everything from
terrestrial to marine
creatures. The tremor was so
severe that it resulted in a
tsunami, drowning major
coastal cities from Tripoli to
Tyr to Berytus (Beirut).
Fault zones
• Groups of interconnected faults are often
found at plate boundaries
– The San Andreas fault is a good example of
this, with new faults being uncovered all the
time
• Sometimes ancient fault zones are the cause
of earthquakes even though they are no
longer part of a current plate boundary
Fault zones
• This explains why areas such as Eastern North
America sometimes experience earthquakes along
what were once the edges of ancient plates, or
where terranes were forced onto other continents
in the past
• Examples
– New Madrid 1812, Tennessee and Kentucky and along
the Appalachians
– Ontario and Quebec along the Laurentian and Gatinaeu
hills
Recoding Earthquakes
• Seismic waves are recorded using a
seismograph and are recorded on a seismogram
– Three motions are monitored – vertical movements
of the ground, motion in an east-west direction and
motion in north-south directions
• Each of these motions travel at different
speeds, mover differently through different
types of rock and have different effects on the
crust
Primary waves
• Also known as P waves or compression
waves and are the first to be recorded
– This means they move the rock particles together and
apart in the direction of the wave
• Move the fastest and are the first to be
recorded
• Travel through solids and liquids
– The more ridged the material the faster the waves move
through it
Secondary waves
• Also known as S waves or shear waves
– Cause rock particles to move are right angles to
the direction in which the wave is traveling
• These are the second waves to be recorded
• These can travel only through solid material
– Therefore these cannot be detected on the
opposite side of the Earth because they cannot
travel through the Earth’s liquid outer core
Surface waves
• These are the combined effects of the P and
S waves when they reach the surface
• These are the slowest moving waves, the
last to be recorded and the most destructive
• They move slowly over the surface like an
ocean wave (causing the surface to rise and
fall)
– These are particularly destructive in loose earth
S-P lag time allows the
calculation of distance
from your location to
the epicenter
Locating the Epicentre
• The difference in arrival times of the P and S
waves allows triangulation of the earthquake’s
epicentre
• P waves travel about 1.7 times faster than S waves
– The greater or smaller the time the S waves are
recorded the closer the epicentre is
– Recording from three stations at different locations
allows the epicentre to be located (triangulated)
– There are over 2500 stations in the United States alone
P waves travel faster and arrive
at the station first
S waves arrive later
Since the speed at which wave travels is known, the time
lag lets you calculate how far away the epicentre is
Each station knows the distance to the
epicentre but not it’s direction. When
the data from three stations are plotted,
the point where they intersect locates
the epicentre.
Measuring Earthquakes
• Magnitude is a measure of the energy
released but an earthquake
– This also describes the amount of ground motion
• The Richter scale is one type of moment
magnitude scale
– This is a logarithmic scale where each magnitude is 31
times more powerful than the previous one
– Because it only measures the intensity of ground
movements this scale has limitations, especially for
large earthquakes
Micro
1.0
Earthquakes this small happen below ground. You can't feel them although they can be detected by
seismometers.
Very
Minor
2.0
Locally, they are felt only by a few persons at rest, especially on upper floors of buildings; delicately
suspended objects may swing. Trees sway. Small ponds ripple. Doors swing slowly.
Minor
3.0
During the day, felt indoors by many, outdoors by few; at night, some awakened; dishes, windows,
doors disturbed; walls make creaking sound; sensation like heavy truck hitting building; standing
autos rock noticeably. Felt by most people; some breakage of dishes, windows, and plaster; unstable
objects overturned; disturbance of trees, poles, and other tall objects.
Light
4.0
Felt by all, many frightened and run outdoors; some heavy furniture may move; falling plaster and
chimneys, damage slight.
Moderate
5.0
Everyone runs outdoors; damage to buildings varies depending on quality of construction; noticed by
people driving autos. If you are in a car, it may rock. Glasses and dishes may rattle. Windows may
break.
Strong
6.0
Can be destructive in areas up to about 100 miles across in populated areas. Panel walls thrown out
of frames; walls, monuments, chimneys fall; sand and mud ejected; drivers of autos disturbed.
Buildings shifted off foundations, frame structures thrown out of plumb; ground cracked;
underground pipes broken.
Major
7.0
Can cause serious damage over larger areas. Most masonry and frame structures destroyed; ground
badly cracked, rails bent, landslides; sand and mud shift; water splashes over river banks. It is hard to
keep your balance.
Great
8.0
Can cause serious damage in areas several hundred miles across. Few structures remain standing;
bridges destroyed; broad fissures in ground, pipes broken, landslides, rails bent. Large rocks move.
Smaller objects are tossed into the air. Some objects are swallowed up by the earth.
Massive
9.0
Devastating in areas several thousand miles across. Damage total; waves seen on ground surface,
lines of sight and level distorted, objects thrown up into the air.
Meteoric
10.0
Never recorded; equivalent to a 20 km rocky meteorite impacting earth at 25 km/sec. Complete
devastation to the region.
Measuring Earthquakes
• Moment Magnitude scales
– This type of scale more accurately indicates the
total energy involved with an earthquake
• For example, comparing the scales:
1906 San Francisco Richter 8.3 / MM 7.9
1965 Alaska Richter 8.5 / MM 9.2 because it
was along a larger fault plane
Measuring Earthquakes
• The Mercalli scale expresses the
intensity of the quake based on the
amount of damage it causes
Earthquake damage
• Most injuries and damage are caused not by
the earth moving but by the collapse of
buildings/structures and falling objects
– Dams can be destroyed resulting in floods, gas
lines are cut resulting in fires
– Tsunamis often result from sea floor quakes
Dec 2004 Island of Phuket, source unknown
Sri Ekambareswarar and Sri Nilathunda Perumal Temple
The Hawaiian Islands are especially vulnerable to destructive tsunamis generated by
major earthquakes in the circum-Pacific Ring of Fire. Travel times (in hours) are shown
for the tsunamis produced by the 1960 Concepción, Chile, earthquake (purple curves)
and by the 1964 Good Friday, Valdez (Anchorage), Alaska earthquake (red curves). The
1960 tsunamis killed 61 people and caused about $24 million in damage.
PTWC
• Pacific Tsunami Warning Centre
– A network of seismograph stations around the
Pacific Ocean that locates and issues warnings
of magnitude of earthquakes and where
tsunamis may hit
Earthquake damage
• The time a quake lasts also affects the
amount of damage that occurs
• The type of ground through which the
waves pass also affects the damage
– Liquefaction can occur when loose soil takes on
some of the properties of a liquid
– A building on this type of soil will more easily
collapse
Kobe 1995
Predicting Earthquakes
• Scientists monitor movement in the Earth
constantly, especially in areas prone to
experiencing earthquakes
– Along the San Andreas fault there are hundred
of detectors
– Areas called seismic gaps (areas where the fault
is locked and can’t move) can be identified and
located as possible future sites of earthquakes
Predicting Earthquakes
• Other things to look for:
– Slight tilting of the ground
– Strains and cracks in rocks
– Changing magnetic and electrical properties in
rocks
– Water levels in wells changing
– Increased natural gas seepage
Earth’s Interior
• Studying earthquakes led us to understand the
structure of the Earth’s interior
– Scientists noticed that at about 2900 km at the
mantle-core interface that P waves greatly slowed
and S waves stopped, then the P waves increase
again after 5200 km
– Since S waves can’t pass through liquids and P
waves move more slowly in liquids, this led to the
conclusion that the outer core was a liquid and the
inner core was solid
Shadow zone
• This is a large zone on the opposite side of
the Earth from the focus that does not
receive any seismic waves
• This happens because the P waves are
refracted as they pass through the earth –
once as they enter the outer core and again
when they re-enter the mantle
The Moho
• The change in velocities of P and S waves
occurs at a boundary between the crust and
the mantle
• In 1909 Andrija Mohorovicic discovered
this fact while studying seismograms
• This boundary is now called the
Mohorovicic discontinuity or the Moho
The Transition Zone
• Between 400 and 670 km below the surface
seismic wave velocities change again
• This marks a region in the middle of the
mantle which separates the less dense
material of the upper mantle from the
denser, lower mantle
Saturday March 6,
2010 8:18 am
Magnitude 3.1
http://earthquakescanada.nrcan.gc.
Possible human causes for earthquakes
Prior to 1976, Gazli field had very
little earthquake activity, according to
a 1985 study in the Bulletin of the
Seisomological Society of America.
But then it was hit by three large
temblors in rapid succession, all of
which had magnitudes in excess of
6.8.
Such earthquakes can be triggered by
changes in fluid pressure in the pores
of rocks, said David Simpson,
president of the IRIS Consortium, a
seismological research group
headquartered in Washington, D.C.
A trio of major temblors between 1976 and 1984 is thought
to have resulted from natural gas extraction in the Gazli
natural gas field of Uzbekistan. Above, excess gas is flared
off at the Jonah natural gas field in Pinedale, Wyoming, in
August 2006. (National Geographic News Marc 6, 2010)
In a natural gas field, such changes
might come either from pumping gas
out or by a process called secondary
recovery, in which water or gas is
injected into the rocks to enhance
production, Simpson noted.