Geology lecture 11

Chapter 10

EarthquakesEarthquakes

Chapter 10

Outline

• Earthquakes and seismicity-Basics

• Faulting and earthquakes-Hypocenter and epicenter-Fault motion and initiation-Fault types

• Seismic waves-Body waves (P & S), surface waves (Love & Raleigh)-Seismology, seismographs

• Earthquake further details-Locating them, size, frequency, depths -Tectonic settings and occurrences-Damage and prediction

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Chapter 10

What is an Earthquake?• Earth shaking caused by rapid energy release.

• Tectonic/other stresses cause rocks to break. • Energy moves outward as waves. • Waves can be measured.

• Earthquakes (EQs) are destructive.• ~3.5 million deaths in the last 2,000 years.

• VERY common.

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Seismicity• Seismicity (earthquake activity) occurs due to…

• Motion along a new fracture (fault) in the crust• Motion on existing fault• Sudden change in mineral structure• Magma movement at depth• Volcanic eruption• Giant landslides• Nuclear detonations

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Earthquake Concepts

• Hypocenter (focus) - Spot where earthquake waves originate

• Usually occurs on a fault plane

waves expand outward from hypocenter• Epicenter – Land surface spot above hypocenter

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Faults and Earthquakes• Most earthquakes (EQs) occur along faults.

• Faults are fractures along which rocks move• Movement termed displacement, offset, or slip• Markers may reveal amount/direction of offset

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Faults and Fault Motion• Faults are common.

• Active faults – ongoing stresses producing motion• Inactive faults – motion occurred in the geologic past

• Displacement can be visible.• Fault trace – a surface tear• Fault scarp – a small cliff

• Blind faults don’t reach the surface (no trace/scarp)]

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Fault Motion• Faults move in jumps.• Once motion starts, it quickly stops due to friction• Builds up again, finally causing failure• Behavior is termed “stick-slip”.

• Stick – friction prevents motion• Slip – friction briefly exceeded by motion

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• Most faults slope (although some are vertical)• On sloping fault, crustal blocks are classified as:

• Footwall (block below the fault)• Hanging wall (block above the fault)

Faults and Fault Motion

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Fault Types• Fault type based on relative block motion.

Normal fault:

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Fault Types• Fault type based on relative block motion.

Reverse fault:

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Fault Types• Fault type based on relative block motion.

Thrust fault • Low angle reverse fault:

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Fault Types• Fault type based on relative block motion.

Strike-slip fault:

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Fault Types• Fault type based on relative block motion.

Oblique fault • A combo of vertical (dip) slip and horizontal (strike) slip.

Chapter 10

Outline

• Earthquakes and seismicity-Basics

• Faulting and earthquakes-Hypocenter and epicenter-Fault motion and initiation-Fault types

• Seismic waves-Body waves (P & S), surface waves (Love & Raleigh)-Seismology, seismographs

• Earthquake further details-Locating them, size, frequency, depths -Tectonic settings and occurrences-Damage and prediction

Chapter 10

Chapter 10

Seismic Waves

• Body waves – travel through Earth’s interior• Compressional or Primary (P) waves:

Push-pull (compress and expand- volume change) motion• Travel through solids, liquids, gases• Tend to travel very fast

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Seismic Waves• Body waves- pass through Earth’s interior

• Shear or Secondary (S) waves:• Change in position, not volume• “shaking motion”• Only travel through solids; not liquids• Slower than P waves

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Seismic Waves• Surface waves- travel along Earth’s surface

1. Love waves – S-waves that intersect the surface

Back and forth motion

2. Rayleigh waves – P-waves at the surface

move like ripples on a pond

These waves- slowest and more destructive

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Seismology Instruments• Seismographs – instruments that record seismicity

• Detect EQs anywhere on Earth

• Reveals size and location of EQs

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Seismograph Operation• Waves always arrive in sequence.

• P-waves 1st

• S-waves 2nd

• Surface waves last

• Arrivals captured by • seismograph

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How a Seismograph Works

Seismologists use two basic configurations of seismographs, one for measuring horizontal ground motion, like the one shown in this animation, and the other for measuring vertical ground motion. Both work on the principle of inertia as described by Newton’s law, which states that an object at rest tends to remain at rest unless acted on by an outside force. Thus, during an earthquake, vibrations cause the frame of the seismograph to move. The pendulum apparatus remains fixed as the paper cylinder moves back and forth beneath it. For more information, see “Seismographs and the Record of an Earthquake” starting on p. 315 and Figure 10.13 in your textbook.

How a Seismograph Works

Chapter 10

Outline

• Earthquakes and seismicity-Basics

• Faulting and earthquakes-Hypocenter and epicenter-Fault motion and initiation-Fault types

• Seismic waves-Body waves (P & S), surface waves (Love & Raleigh)-Seismology, seismographs

• Earthquake further details-Locating them, size, frequency, depths -Tectonic settings and occurrences-Damage and prediction

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Chapter 10

Locating an Epicenter• P- & S-waves travel at different velocities.• 1st arrivals of P- and S-wave varies with distance• Travel-time graph plots distance of each station to the

epicenter

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Locating an Epicenter• 3 stations can pinpoint epicenter.

• A circle is drawn around each station• radius= distance to epicenter• Circles around 3 (or more) station will intersect• Intersection> epicenter

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Earthquake Size• Two means of describing earthquake size:

1. Intensity.

2. Magnitude.

1. Mercalli Intensity Scale.• Intensity – degree of shaking

based on damage.• Roman numerals assigned to

different damage levels. • Damage occurs in zones.• Intensity decreases with

distance.

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Earthquake Size

• Magnitude – amount of energy released. • Max amplitude of motion from• a seismograph • Value normalized for• seismogrpahic distance

• Several magnitude scales: • Richter• Moment

• Scales are logarithmic.• 1 unit increase= 10x increase• in size

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Measuring Earthquake Size

• Energy released can be calculated.• M 6.0= energy of the

Hiroshima bomb• M 8.9= annual energy

released y all other earthquakes

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Earthquake Size & FrequencySmall EQs are frequent

~100,000 M 3 EQs/yearLarge EQs are rare

~32 M 7 EQs/year

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Earthquake Occurrence• EQs linked to tectonic plate boundaries• Shallow depth- convergent/transform boundaries• Intermediate/deep- convergent boundaries

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Earthquake Depths• Shallow EQs – 0-20 km.

• Along mid-ocean ridges• Transform boundaries• Shallow part of trenches• Continental crust

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Earthquake Depths• Intermediate/deep EQs – along subduction trace

(Wadati- Benioff Zones)• Intermediate- 20-300 km- down-going plate still brittle• Deep- 300-670 km- mineral transformations

• Earthquakes rare below 670 km (mantle is ductile)

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Convergent Boundaries

• Cities near subduction zones have to contend with frequent & occasionally large EQs.

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Continental Earthquakes• EQs in continental crust.

• Continental transform faults (San Andreas)

Continental rifts (Basin and range, East African Rift)

Collision zones (Himalayas, Andes, Alps)

Intraplate settings (ancient crustal weaknesses)

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San Andreas Fault• Pacific plate passes by North American plate.• San Andreas is an active strike-slip fault.

• Very dangerous; 100s of EQs/ year

Examples:

San Francisco- 1906 (burnt down)

Loma Prietà- 1989 (world series)

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Intraplate Earthquakes• 5% of EQs not near plate boundaries.• “Intraplate” EQs poorly understood

• Remnant crustal weakness in failed rifts or shear zones?• Stress transmitted inboard? Transmitted far into the plate• Isostatic adjustments?

• Gravitational balance

• Clusters• New Madrid, Mo• Charleston, S.C.• VA seismic zone!

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Earthquake Damage• Ground shaking and displacement.

• EQ waves arrive in distinct sequence• Different waves cause different motion

• P-waves are 1st to arrive• Produce rapid up and down motion

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Earthquake Damage• S-waves arrive next.

• Produce back and forth motion• Motion usually much stronger than P-waves• S-waves cause extensive damage

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Earthquake Damage• Surface waves lag behind S-waves.

• Love waves are the first to follow• Ground moves like a snake

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Earthquake Damage• Raleigh waves are last to arrive.

• Land surface- ripples in a pond• May last longer than others• Causes extensive damage

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Earthquake Damage• Severity of shaking & damage depends on…

• Magnitude (energy) of EQ• Ditance from hypoenter• Intensity/duration of vibrations• Subsurface material

• Bedrock transmit waves quickly= less damage• Sediments bounce waves= amplified damage

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Earthquake Damage

• Landslides & avalanches.• Shaking causes slope failures• Rockslides/snow avalanches

follow EQs in uplands• An EQ stated the landslide the

uncorked Mt. St. Helens on May 18, 1980

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Earthquake Damage

• Liquefaction – waves liquefy H2O-filled sediments. • High pore pressure force grains apart reducing friction• Liquefied sediments flow as a slurry• Sand becomes “quicksand” from solid

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Earthquake Damage• Tsunamis or seismic sea waves (not tidal waves).

• Result of EQ displacing seafloor• Instantly displaces overlying water• May be enormous (up to 10,000 mi2 area)• Occur ~1/year

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Tsunami Behavior• Move at jetliner speed across ocean.• May be imperceptible in deep water.

• Low wave high (amplitude)• Long wavelength (frequency)

• As water shallows, waves • slow from frictional drag

• Waves grow in height,• reaching 10-15 or more

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Tsunami Reality• Indian Ocean Tsunami

• Dec 26, 2004, strong trust EQ (M 9.0+) originated in trench near Sumatra

• Largest EQ in 40 years• Slip exceeded 15 m; fault rapture > 1100 km long• Killed ~283,000 people (10 countries around Indian Ocean)

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The Indian Ocean Tsunami• Destroyed coastlines around the Indian Ocean. • Death tolls in:

• Northern Sumatra• Thailand• Malaysia• Sri Lanka

Banda Aceh

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Tsunami Prediction• Scientific modeling predicts tsunami behavior. • Tsunami detection:

• Detectors placed on seafloor• Sense pressure changes due to sea thickness change

• Prediction/detection can save lives

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Earthquake Prediction• Prediction would help reduce catastrophic losses. • Can we predict earthquakes? Yes and No

• CAN be estimated long-term (10-100s of years)• CANNOT be predicted short-term (hours-months)

• Seismic hazards are mapped to assess risk

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Earthquake Prediction• Long-term:

• Probability of a certain magnitude EQ occurring on a time scale of ~30 to 100 years

• Based on idea that EQs are repetitive (tend to move multiple times over a long period of time)

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Earthquake Prediction• Long-term:

• Require determination of seismic zones by:• Mapping historical epicenters (after ~1950)• Evidence of ancient EQs (before seismographs)

• Evidence of seismicity- fault scarps, sand volcanoes, etc.

• Historical records

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Earthquake Prediction• Long-term:

• Estimate recurrence interval- average time between EQs

• Historical Records • Geologic evidence- requires dating of events

• Sand volcanoes• Offset strata• Drowned forests

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Earthquake Prediction• Long-term:

• Seismic gaps: places that haven’t slipped in a while• More likely candidates to slip next

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• Short-term: • Goal: location and magnitude of a large EQ• No reliable short-range predictions• BUT, EQs do have precursors

• Clustered foreshocks• Stress triggering• And, possibly…

• Water level changes in wells

• Gases (Rn, He) in wells

• Unusual animal behavior

Earthquake Prediction