11906 San Francisco earthquake

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What are Earthquakes?

• The shaking or trembling caused by the sudden release of energy

• Lithospheric plates move suddenly along faults in the crust

• Usually associated with faulting or breaking of rocks

• Continuing adjustment of position results in aftershocks

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What is the Elastic Rebound Theory?

• Explains how energy is stored in rocks– Rocks bend until

the strength of the rock is exceeded

– Rupture occurs and the rocks quickly rebound to an undeformed shape

– Energy is released in waves that radiate outward from the fault

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The Focus and Epicenter of an Earthquake

• The point on a fault with the greatest movement (where the ‘quake originates) is called the focus

• The point directly above the focus on the surface is the epicenter

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Earthquake Impacts

• Ground shaking depends on:– Distance to epicenter– Area’s type of geology– Size of the earthquake

• Liquefaction– Vibrations cause ground water to rise and

turn solid ground into liquid– Structures may sink into the ground

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• Ground Shaking– amplitude, duration, and damage increases

in poorly consolidated rocks

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Ocean Earthquakes-Tsunamis-

• Earthquakes can occur on faults in the ocean• If the ‘quake is large enough, the energy can

cause a tsunami to form• Tsunamis are fast moving waves that are no

higher than regular waves in deep water• However, when tsunamis approach the shore,

they are forced to slow down, causing the wave to build into a huge wave!

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• The Alaska earthquake of 1964 created a tsunami that affected the coast of BC, Washington, California, and Hawaii

• Port Alberni, on Vancouver Island received major damage

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It took the tsunami 4.1 hours to get to Port Alberni from Alaska

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The Economics and Societal Impacts of EQs

Damage in Oakland, CA, 1989• Building collapse

• Fire• Tsunami• Ground failure

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Where Do Earthquakes Occur and How Often?

• ~80% of all earthquakes occur in the Pacific Rim

• Most of these result from convergent boundaries (subduction)

• ~15% occur in the Mediterranean-Asiatic belt

• Remaining 5% occur in the interiors of plates and on spreading ridges

• Over 150,000 quakes strong enough to be felt are recorded each year

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What are Seismic Waves?

• Seismology is the study of earthquakes and their effects

• The energy released from a ‘quake travels in the form of waves

• Two types:– Body waves

• P and S– Surface waves

• L

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Body waves

• Body waves travel through ground• Faster than surface waves• Primary (P) waves

– 6 km/s– Move through any material (solids and liquids)– Cause ground to stretch and compress in direction of

travel• Secondary (S) waves

– 3.5 km/s– Cause ground to squeeze/stretch at right angles to

direction of motion– Travel through solids but NOT liquids!– Cause more damage than P waves

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Surface waves

• Surface waves aka Love (L) waves

• Travel less quickly than body waves

• Travel along surface in rolling action– Similar to ripples in a pond

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• P waves refract (bend) as they pass through different earth layers

• S waves disappear at bottom of mantle (can’t travel in liquid outer core)

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Earthquake waves

• P- waves (primary)– Fastest moving waves

– Compression wave

– Can travel through both solids and liquids

• S-waves (secondary)– Travel slower than p waves

– Shear wave

– Can only travel through solids (not liquids)

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Body Waves: P and S waves

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Surface Waves: R and L waves

• Surface Waves

– Travel just below or along the ground’s surface

– Slower than body waves; rolling and side-to-side movement

– Especially damaging to buildings

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Seismographs record earthquake events

•As the ground shakes during a ‘quake, the pen remains motionless

•Produces a seismogram

•Gives time, duration, magnitude (amount) of shaking

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How is an Earthquake’s Epicenter Located? Seismic wave behavior

– P waves arrive first, then S waves, then L and R

– Average speeds for all these waves is known

– After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter.

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How is an Earthquake’s Epicenter Located?

Time-distance graph showing the average travel times for P- and S-waves.

The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves

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How is an Earthquake’s Epicenter Located?

• Three seismograph stations are needed to locate the epicenter of an earthquake

• A circle where the radius equals the distance to the epicenter is drawn

• The intersection of the circles locates the epicenter

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How are the Size and Strength of an Earthquake Measured?

• Modified Mercalli Intensity Map

– 1994 Northridge, CA earthquake, magnitude 6.7

• Intensity

– subjective measure of the kind of damage done and people’s reactions to it

– lines identify areas of equal intensity

– Eg: Mercalli Scale

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How are the Size and Strength of an Earthquake Measured?

• Magnitude– Richter scale

measures total amount of energy released by an earthquake; independent of intensity

– Amplitude of the largest wave produced is corrected for distance

– Each step up on scale = 10x larger vibrations

– No upper limit

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Can Earthquakes be Predicted?

Earthquake Precursors – changes in elevation or tilting of land surface,

fluctuations in groundwater levels, magnetic field, electrical resistance of the ground

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Can Earthquakes be Predicted?Earthquake Prediction Programs

– include laboratory and field studies of rocks before, during, and after earthquakes

– monitor activity along major faults

– produce risk assessments

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Earthquake Risk in BCEarthquake Risk in BC

•Pacific Plate trying to slide past North American Pacific Plate trying to slide past North American Plate along Queen Charlotte FaultPlate along Queen Charlotte Fault

•Stress is buildingStress is building

•Juan de Fuca Plate subducting under North Juan de Fuca Plate subducting under North American PlateAmerican Plate

•Stress is buildingStress is building

•Major earthquakes in this area occur about every Major earthquakes in this area occur about every 150-300 yrs150-300 yrs

•The last one was in 1700The last one was in 1700

•The “Big One” is coming and there is nothing we The “Big One” is coming and there is nothing we can do about it!can do about it!