EARTHQUAKE PROCESSES

Pray To Understand What Man Has Forgotten,

Lumbee

The sudden release of energy accumulated in

deformed rocks.

Seismology -the study of earthquakes.

What is an Earthquake?Danny M. Vaughn, Ph.D., CMS

Anatomy of an Earthquake

• Earthquake – a sudden release of strain energy stored along stressed boundaries. • All energy moves in the form of wavelengths. • Some elastic behavior in shallowdepths w/ cooler, more brittle rocks.• Failure occurs (yield point) w/ fracturing & faulting.• Plates move releasing energy as seismic (shaking) waves.

(Hypocenter)

Body waves• P-

waves• S-wavesSurface

waves• Compression forces contract & expand (dilate) the rocks.• First to arrive from the focus.• Crustal velocity: ~ 4 mi/sec. (6-7 km/sec.).• Wavelengths travel in all directions.

Seismic Body Waves

Primary (P) Waves

Seismic (Body) WavesSecondary (S) Waves

Body waves• P-waves• S-

wavesSurface

waves• Crustal velocity: ~ 2 mi/sec. (3.5 km/sec.).• Rocks move perpendicular (up & down) to the direction waves travel.• Vertical drag occurs as the wave passes, sheering rocks.

Shape is changed due to sheering.Volume remains unchanged.

Body waves• P-waves• S-wavesSurface

waves• Surface vibrations are initiated by body waves.• Crustal velocity: ~ 1.5 mi/sec (2.5 km/sec.).• Results in most damage to structures.

Seismic (Surface) Waves

Measuring Earthquakes

Mercalli Intensity Scale

• Intensity (amount of energy released), compared to destructiveness.• Fails to accurately locate an epicenter.• Inconsistent in comparing different rock types, building structure damage from poor construction. • Not useful in no population regions.

Measuring EarthquakesSeismographs & the Richter

Scale

Measures amplitude of largest peak.Logarithmic scale.Magnitude 1 to 2 = 10x increase in wave Amplitude.~33x increase in energy released.

e.g. 33x energy released from a 6 to 7;33x33 = 1,089x energy released from a6 to 8 magnitude earthquake.

therefore,

It would take >1,000 magnitude 6 quakesto release the accumulated energy of onemagnitude 8 quake.

Calibrated for California rocks, ineffective >7.0.


Moment-magnitude scale

Seismic moment (total energy released) =(Total length of fault rupture) x

(Depth of fault rupture)x (Total amount of slip along rupture)

x (Strength of rock)• Longer fault; greater potential for a large quake.• Stronger rock, greater energy storage potential, larger quakes.• Present reporting system.• Chilean quake (1960) magnitude 9.5 (8.5 Richter).


Energy released by earthquakes

Distribution and Frequency of Earthquakes

Significant earthquakes of the last 100 years

Figure 10-3

Measuring EarthquakesLocating an Earthquake’s Epicenter

Effects of Earthquakes

• Ground displacements

• Landslides• Liquefaction• Seiches• Tsunami• Fires

• What type of fault is this? • What is the direction of displacement?

(Strike-slip fault)

(Right lateral)



• Landslides

• Liquefaction

• Seiches• Tsunami• Fires



• Landslides• Liquefact

ion• Seiches• Tsunami• Fires• Unconsolidated sediment.

• Silts & sands, some clay as best candidates.• Weakly cohesive (particle attractiveness).• Sheer strength (resistance to stress) weakened by vibration.• Water is disassociated from particles.• Cohesiveness & sheer strength are diminished.• Flowage ensues.


Damage by Liquefaction



• Landslides• Liquefacti

on• Seiches

(saych)• Tsunami• Fires

The rising and falling of water as it sloshes back and forth in an enclosed body of water, such as a lake or bay.

Standing waves w/ up & down movement.

Formation of Tsunami

Caused by –

• Vertical displacement on the seafloor due to faulting.• Submarine landslides.


Tsunami


• Landslides

• Liquefaction


(December, 2004): Maximum wave height: ~90 ft.



• Landslides

• Liquefaction


Homes surviving the 1906 San Francisco earthquake.

Earthquakes and Plate Boundaries

Now can I have that new car,dear?

Earthquakes and Plate Boundaries

• Earthquakes occur where energy builds up in cooler, brittle rocks.• Depth is variable dependent on plate boundary.• Major quakes occur along convergent (subduction) & transform boundaries.• Most stored energy.• Maximum depth ~450 miles. What controls the depth?

Intraplate EarthquakesNew Madrid, Missouri, 1811-12

Reelfoot Lake, Tennessee

Three major 8.5 shocks, 1,500 after shocks over 53 days.

Causes:• Unloading over old faults. • Isostatic rebound (ice removal).• Rainwater acting as a lubricant along faults.• Intraplate seismic activity. Rising softened mantle.• Plate motion creates frictional stress w/in the plate.

Coping with Earthquakes

Limit Quakes Caused by Humans

Relationship of injection of nerve gas waste to earthquake frequency.

Coping with EarthquakesAssessing Local Seismic History & Risk

What is not apparent about earthquakes on this map?


Assessing Local Seismic History & Risk

What does the principle of cross-cuttingrelationships tell us about relative age?


Careful Land-use Planning

Seismic waves shake more violently in unconsolidated deposits then in bedrock.


Building Quake-resistant Structures

What questions need to be asked with respect to building structures in earthquake prone regions?


Research on Earthquake Prediction

Long-range

forecastsSeismic gaps –

• Plates are temporarily locked.• Pressure builds.• Strain energy is released & activity along the fault dies down.

Anti-gap model –• Stress is not completelyreleased, rather increasesalong other faults.

Tectonic creep –• Gradual, continuous release of energy.

San Andreas Fault Observatory at Depth Project.

Understanding & Controlling Earthquakes

Releasing strain energy



Long-range forecasts:Seismic gaps

Regions overdue a major seismic event.



Short-range predictions

• Stress yields small cracks before complete failure.• Rocks expand in volume as cracks widen (dilate).• Dialancy produces side effects that aid in earthquake prediction.

Tectonics & Earthquake Activity

What Can You Do to Prepare?

What Can You Do During an Earthquake?

Moonquakes

• No tectonic motion.• Some quakes from meteorite impacts.• Deep quakes by gravitational attractionw/ Earth.

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EARTHQUAKE PROCESSES