Earthquakes

• More than 1,000,000 occur annually around the world
• That figures to about one every 30 seconds!
• But, most are too small to be noticeable by people
• About 3,000 are strong enough to move a section of a plate…they are "feelable"
• Several hundred quakes can be detected that make major movements of the crust (lithosphere)
• About 20 earthquakes per year cause major damage to different areas of the earth

An Earthquake is a shaking of Earth's crust caused by a release of energy.

• An earthquake is similar to a volcano because each is caused by motions of the lithosphere, however the area affected is much greater
• A quake in a given area can cause buildings to collapse, pipes of gas or water to break, fires, pollution, electrical outages, food shortages, etc.
• Coastal locations a long way from the earthquake can flood due to waves created by the shaking
• Ultimately, the earthquake may cause much more widespread effects to a greater area of population

What causes an Earthquake?

• The ground can shake from a volcanic eruption
• The collapse of a cavern
• The impact of a meteor from space
• The stress built up at plate boundaries

• Stress that builds at the plate boundary can build up causing the plates to actually change shape
• The stresses build up so much that finally a release of stress occurs - this is the earthquake
• The plates will then return back to original shapes
• This is called the Elastic-Rebound Theory

Physical make-up of an Earthquake

• All earthquakes occur at the point on the plate boundary that is withholding the movement of the plates
• The depth of the stress built up at the boundary varies by the type of boundary

1. Spreading centers and Sliding boundaries produce rather shallow earthquakes
2. Converging boundaries produce very deep, powerful earthquakes because one plate lies under another

• The point on the fault plane at which the first movement occurs is called the Focus of the earthquake
• The point on Earth's surface directly above the focus is the Epicenter - often referenced in a news report - not the actual point of the quake

Earthquake Waves

• Earthquakes produce three basic types of vibration wave motions

1. Compressional or 'P-wave' moves the crust in a back-and-forth motion. P-waves can travel through any material - rock, magma, water, and even air
2. Shear or 'S-wave' moves the crust in a side-to-side motion. S-waves can only travel through solids
3. Surface or 'L-wave' moves the crust in a combination of movements combining motions of the P- and S- waves. The L-wave will only move across the surface of the land

• The speed of these three waves is distinctly different as well - P-waves travel twice as fast as S-waves and the L-wave (the last to form) will be the slowest of the three waves.

How do we measure earthquake activity?

• The instrument used to measure earthquake activity is called a Seismograph
• Because of different motions in the shaking or quaking of the earth's plates, seismographs are built in a variety of styles
• ALL seismographs however have similar components

1. Solid base mounted or attached to the ground
2. A Seismogram (a record sheet - roll of paper - that records the vibrations of earth)
3. The arm and pendulum mechanism called the Seismometer which holds the marking device for writing on the seismogram
4. Difference occurs in the way the arm of the seismograph swings - either vertically or horizontally
5. Vertical vibrations - recorded by vert. arm; Horizontal vibrations - recorded by horiz. Arm

• Reminder that the P-wave will arrive on the seismograph first and then the S- and L-waves later

Locating the Epicenter

• Drawn on the board map….

Earthquake Magnitude

• Seismographs often determine how powerful the earthquake is by measuring the quakes magnitude
• The most widely used scale is the Richter Scale from the 1940's
• Each magnitude number represents an earthquake 32 times stronger than the next lower number
• Ex. - Quake with a magnitude of 6 is 32x stronger than a quake of 5; more than 1000x stronger than a magnitude of 4!
• A Seismic Moment scale has recently been developed
• It factors in magnitude of strength AND the area affected by the quake
• Ex. - San Francisco of 1906 had the same Richter scale as the Alaskan quake of 1964 (Mag. of 8.3), but they were very different in the amount of area affected!
• The Seismic Moment scale factored in that the S.F. quake is a 7.9 and the Alaskan was 9.2 because it covered much more area!
• This new method shows that the Alaskan quake was over 100x more powerful

Earthquake Prediction

1. Where will it occur?
2. When will it occur?
3. What magnitude will it be?

(Thought is also given to the prediction that it is "predicable" when it will occur based on a time frame - every 100 years, or 200 years, or 350 years, etc.)

Earthquakes in History

• Alaskan earthquake of 1964
• Largest magnitude in 20^th century
• Over 5 minutes of rolling ground!
• Movement occurred along a subduction boundary
• Over 800 km (700 miles) of fault plane was moved!
• Seismographs around the world recorded the quake!!!!!
• Over 10,000 aftershocks were detected!
• Multiple tsunamis were created and flooded numerous fishing villages on the northern Pacific Ocean - even hit Hawaii and Japan!
• Only 115 were killed…

• San Andreas Fault quakes
• Recorded up to an impressive 7.5
• Seldom any major damage because area close to the fault is rather rural
• Averages a major shift about every 35-40 years
• Devastation-style quake every 160 years
• Occurs along a sliding boundary

• New Madrid quakes of 1811-1812
• Three of the largest quakes in history!
• Occurred in a two-year period
• Mystery is that the quakes occurred in the middle of the North American plate at New Madrid, Missouri???
• They were 8.6, 8.4, and 8.7 respectively
• It claimed to ring church bells in Boston, MA
• Scientists have found that three faults occur deep under Mississippi River sediment
• These faults are not thought to be actual plate boundaries - not broken all the way through
• Shifting at these faults has caused the quakes
• They have been classified as a Midplate Earthquake