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 – stress- increases 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 300-650 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)

http://upload.wikimedia.org/wikipedia/commons/0/09/Pacific_Ring_of_Fire.png

Seismic zones in India – zone V is the highest risk area for earthquakes of magnitude 8 or more.

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).

A major tremor is possible in the Middle East

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

http://images.google.ca/imgres?imgurl=http://www.lhup.edu/mkhalequ/Fieldtrip/pictures/Seismograph%40DGS.JPG&imgrefurl=http://www.lhup.edu/mkhalequ/Fieldtrip/GEOS253.htm&usg=__qdsYaAh1ftD_vHhYdjgCkq4LXXs=&h=1536&w=2048&sz=1415&hl=en&start=7&itbs=1&tbnid=FkjCcUQOr1w-aM:&tbnh=113&tbnw=150&prev=/images%3Fq%3Dseismograph%26hl%3Den%26gbv%3D2%26tbs%3Disch:1

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.

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.

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.

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)

Possible human causes for earthquakes