Ch8_EQ_students

CH. 8 – EARTHQUAKES

Earthquakes

Vibrations in crust caused by shifting rock masses

Focus- point w/in crust where EQ originates

Epicenter – point on Earth’s surface directly above focus

Earthquakes

Shallow-focus vs. deep-focus EQ’s

Causes of Earthquakes

1) Volcanic eruptions- used for predictions

2) Crustal Rebound - weak EQ’s- caused by melting of continental ice sheets

Causes of Earthquakes

3) Elastic Rebound- pressure builds up in rocks

(potential energy)- once rx break, stored energy

is released (kinetic energy)- rx return to original shape

Fault

Fracture in crust where rocks have shifted

Two categories of faults:1) Dip-slip faults (vertical motion)2) Strike-slip faults (horizontal

motion)

Dip-Slip Faults

Vocabulary (mining terms):1) Fault Plane – fracture

surface 2) Footwall3) Hanging wall

Mining along faults

Types of Dip-Slip Faults

1) Normal Fault- hanging wall moves down relative

to footwall- caused by tensional forces

Normal Fault

Types of Dip-Slip Faults

2) Reverse Fault- hanging wall moves up relative

to footwall- caused by compressional

forces

Reverse Fault

Fault-Block Mountains

Bounded by normal faultsMountain = horst; valley =

grabenEx: Basin & Range Province in

Nevada & UtahEx: Teton Mountains, Wyoming

Basin & Range Province

Teton Mountains, WY

Strike-slip faults

Motion is horizontalSurface features are displaced:

Fences, railroad tracks, tree rows, stream channels

Ex: San Andreas Fault, CA

Types of Seismic Waves

1) Body Waves - energy travels from focus

through Earth’s interior

Body Waves:

a) Primary Waves (P-waves)- fastest- energy travels in push-pull motion (linear)- travel through solids, liquids, and gases

P-wave w/slinky

Body Waves:

b) Secondary waves (S-waves)- approx. half as fast as P-waves- energy travels at right angles to

path of motion- only travels through solids

S-wave with rope

Types of Seismic Waves

2) Surface waves- energy travels across surface- causes the most structural damage

Locating EQ Epicenters

KEY: Difference in arrival times between P- and S-waves

Farther apart = seismic station is further from epicenter

Seismogram

Locating EQ Epicenters

Use multiple seismic stations to determine epicenter’s location

Measuring EQ’s

1) Intensity = damage-oriented

EQ Intensity Factors

1) Amount of energy released2) Distance from epicenter3) Type of surface material

- bedrock vs. sediment4) Building design5) Population Density

EQ Intensity

Measured using Mercalli Scale(Giuseppe Mercalli in 1902)- scale of I-XII based on damage

to location- not a measure of EQ’s actual

energy

Disadvantages of Mercalli Scale

1) Subjective2) Type of surface material

varies3) Building design varies4) No data for unpopulated areas

Measuring EQ’s

2) Magnitude – measures amount of energy released from focus

Richter Magnitude Scale

Measures largest amplitude of seismic waves on seismogram

Amplitude measures the ground shaking

Seismogram


Logarithmic scale (not linear!)Each higher number on Richter

Scale represents:a) 10 times more ground shakingb) 32 times more energy

released


2.0 Felt only by seismograph3.0 ____ x more energy than a 2.04.0 ____ x more energy than a 2.05.0 ____ x more energy than a 2.0


4.0-4.9 Felt by most people6.0-6.9 Destructive in populated

areas>8.0 Truly catastrophic EQ’s


Largest recorded U.S. EQ1964 Alaskan “Good Friday” EQ 9.2

Largest recorded EQ in world1960 Chile 9.6

Benefits of Richter Scale

1) Objective measurements from seismogram

2) Data available for unpopulated areas

Effects of EQ’s

1) Ground Shaking2) Ground Ruptures – cracks

occur in soil (not bedrock!)

Effects of EQ’s

3) Fault Scarp – cliff exposed by vertical movement along dip-slip fault

- used to measure growth rate of mountains

Effects of EQ’s

4) Liquefaction- occurs in sediment- vibrations release water

trapped between sediment grains“quick sand” or “quick clay”

Liquefaction

Foundations under buildings failEx: Marina District is San

Francisco

Effects of EQ’s

5) Landslides- more prevalent if pre-existing weakness exists in rxEx: EQ Lake in Montana 1959

Effects of EQ’s

6) Tsunami – “harbor wave”- seismic sea wave (not tidal wave)- caused by dip-slip fault movement in ocean crust

Tsumani diagram

Tsunami

Danger is from speed: between 500-950 km/hr

Ex: December 26, 2004- Indonesia: EQ of 9.0 offshore triggered tsunami- over 200,000 people killed

Tsunami Warning System

Effects of EQ’s

7) Fire – causes most property damage- liquefaction breaks buried gas & water pipelines Ex: Marina District in 1906 & 1989

EQ Prediction

Based on seismic “gaps” - look for areas on fault where no

EQ’s have occurred

EQ Prediction

EQ Prediction along San Andreas Fault

- best guess is 30-year estimate of likely EQ activity

- note Parkfield, CA

San Andreas EQ Probability

Earth’s Interior

Layers defined by physical properties & composition

Clues (indirect evidence): Seismic waves - variations in

speed & direction

Clues: seismic waves

1) P-waves speed up rapidly below 50 km depth within Earth

- marks a change in composition &/or physical state of the rocks

1909: Mohorovicic identified crust-mantle boundary


Within mantle:P-wave velocities slow back

down (100-660 km deep)- slower velocity zone is known

as the __________________

Asthenosphere

Weak layer in upper mantle- partially molten (~1-5%) which

slows P-waves down- not completely molten b/c S-

waves travel through it


Lithosphere – rigid rx of crust and uppermost mantle

Asthenosphere – weaker rocks (partially molten) in upper mantle

Asthenosphere


2) P-wave shadow zone- P-waves bend as they enter the

outer core3) S-wave shadow zoneS-waves disappear in outer core* Proof of liquid outer core

P-wave shadow zone


Outer core is moltenInner core is solid- pressure in center is too great

to allow minerals to expand & melt

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