Folds, Faults & EarthquakesRocks under stress can either bend or break

both are referred to as deformationAny stress can deform rocks

compressional (squeezing) = folded rocksanticlines and synclines

extensional (stretching) = “thinned” bedsWhether or not the rocks fold or break dependson the amount of stress, the strength of the rockand the duration and rate that the stress is applied to the rocks

Folded Rocks - synclineFolded Rocks - syncline = U-shaped fold

Anticline = A-shaped fold (in this case cut in half by a river

Large scale folding in the Appalachian Mountains

If the stress is applied too fast or if it greatly exceeds the rock strength… the rocks break => releasing the stored energyThe break is called a fault, the energy released is called an earthquake

Small earthquakes are cool- everyone feels them, no one gets hurt, minimal damageBig ones suck. They collapse buildings and other structures that kill people and create secondary events that do the same

Significant EarthquakesSignificant Earthquakes Year Location Magnitude Deaths 1964 USA (Alaska) 8.6 131 1970 Peru (Chimbote) 7.8 25,000 1971 USA (San Fernando, California) 6.6 65 1975 China (Haicheng) 7.3 ? 1976 Guatemala 7.5 23,000 1976 China (Tangshan) 8.0 242,000 1985 Mexico (Mexico City) 8.1 9,500 1988 Armenia 7.0 25,000 1989 USA (Loma Prieta, California) 7.1 63 1990 Iran 7.3 40,000 1992 Turkey 6.8 570 1992 USA (Landers, California) 7.5 1 1992 Egypt 5.9 500+ 1993 India 6.4 30,000+ 1994 USA (Northridge, California) 6.7 61 1994 Bolivia 8.2 ?

• 13 million+ lives have been lost in the past 4000years due to earthquakes and related phenomena.

• 13 million+ lives have been lost in the past 4000years due to earthquakes and related phenomena.

Earthquakes•

•

• Shaking caused by the sudden release of energy in the Earth, resulting from displacement of rock along a fault.

• Most are caused by faults associatedwith plate movement.

Fault- a break in the lithosphere that movesEpicenter- a projection of the location on the Earth’s SurfaceFocus- the location of the energy release (rupture) along the fault

Relationships between a fault plane, epicenter, focus, and energy waves

Head on Headwall

Feet on Footwall

HW is a normal fault

HW is a reverse fault

Opposite block moves rightmeans right lateral

Fault Plane

Vertical Faulting Lateral FaultingLook for relative motion-helps determine how force was applied

Fault type tells us something about motion, stress and forces needed to create them and thus gives hints as to plate tectonics setting

Normal fault = extensional forceReverse fault = compressional forceLateral fault = shearing force

What statement is most accurate concerning earthquakes?

A) The stronger the earthquake, the more people are killed.

B) All earthquakes occur close to a tectonic plate edge.

C) The poorer the country, the more deaths are likely to occur due to moderate to severe earthquakes

D) Earthquake epicenters are always associated with faults at Earth’s surface

Seismic Waves (1:3)Seismic Waves (1:3)

• cause shaking and destruction.

• Two types of seismic waves:• Body Waves travel through theEarth.

• Surface Waves travel only alongthe surface of the Earth

• cause shaking and destruction.

• Two types of seismic waves:• Body Waves travel through theEarth.

• Surface Waves travel only alongthe surface of the Earth

Lab exercise using P- and S-waves to locate an earthquake

Body WavesBody Waves• Primary Waves (P-waves)

• travel fastest• move through solid, liquid or gas.• P-waves are compressional.

• Secondary Waves (S-waves)• travels slower than P-waves • solids only!• S-waves

- shear waves that move materialperpendicular to their direction of travel.

• Primary Waves (P-waves) • travel fastest• move through solid, liquid or gas.• P-waves are compressional.

• Secondary Waves (S-waves)• travels slower than P-waves • solids only!• S-waves

- shear waves that move materialperpendicular to their direction of travel.

QuickTime™ and aSorenson Video decompressorare needed to see this picture.

QuickTime™ and aSorenson Video decompressorare needed to see this picture.

Surface WavesSurface Waves

• Rayleigh Waves (R-waves) • the slower of the surface waves.

• Love Waves (L-waves) • move similar to S-waves • displacement of material occurs in a

horizontal plane perpendicular to direction ofwave movement.

• Rayleigh Waves (R-waves) • the slower of the surface waves.

• Love Waves (L-waves) • move similar to S-waves • displacement of material occurs in a

horizontal plane perpendicular to direction ofwave movement.

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SeismographSeismograph• An instrument that measures and records earth movements produced by anearthquake.

Time-Distance GraphsTime-Distance Graphs• The time difference between P- and

S-waves arrivals allows calculationof the distance from the epicenter.

• The time difference between P- andS-waves arrivals allows calculationof the distance from the epicenter.

Dt = d {(Vp-Vs)/(VpVs)} solve for dwhere Dt = Time interval between arrival of P and S waves at a station measured from seismogramsD = distance of station from epicenterVp ; Vs = velocity in Km/sec of P waves and S waves

- generally Vp = 6.0 km/sec; Vs = 3.5 km/sec * making the (Vp-Vs)/(VpVs) term = (2.5/21) = 0.119

* Within 1000 km

Graphical version of the distance calculation.

• Time-distancegraphs only tellus how far weare from theepicenter.• Triangulationof three stationsallows us topinpoint theepicenter.

• Time-distancegraphs only tellus how far weare from theepicenter.• Triangulationof three stationsallows us topinpoint theepicenter.

Earthquake website

• Using three seismograph locationsthe location can be pinpointed on a

map• Triangulation

Using three seismograph locationsthe location can be pinpointed on a

map• Triangulation

Location of earthquakes

Earthquakes and Plate MarginsEarthquakes and Plate Margins

A seismic hazard map

New Madrid, MO• in 1811 and 1812 a seriesof strong intraplate

earthquakes occurred nearNew Madrid, MO.

• (imagine the toll if thisoccurred in the regiontoday), and the quake wasfelt in Boston, 1600 kmaway.

• in 1811 and 1812 a seriesof strong intraplate

earthquakes occurred nearNew Madrid, MO.

• Only twenty people died(imagine the toll if thisoccurred in the regiontoday), and the quake wasfelt in Boston, 1600 kmaway.

Exceptions toPlate Boundary

'quakes

Exceptions toPlate Boundary

earthquakes

Focal Depth & Plate Boundary TypeFocal Depth & Plate Boundary Type• Focal depth increases in a "Benioff Zone" adjacent to deep oceantrenches that define convergent boundaries.

Benioff zone

Categories-DepthCategories-Depth• Shallow-focus:

• focal depth less than 70 km.

• Intermediate-focus: • focal depth between 70 and 300 km.

• Deep-focus: • focal depth greater than 300 km.

• most earthquakes (~80%) occur atdepth of less than 100 km.

• Shallow-focus: • focal depth less than 70 km.

• Intermediate-focus: • focal depth between 70 and 300 km.

• Deep-focus: • focal depth greater than 300 km.

• most earthquakes (~80%) occur atdepth of less than 100 km.

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Earthquake Intensity & MagnitudeEarthquake Intensity & Magnitude• Intensity • • MMI - scale (Modified Mercalli Intensity)- ranges from 1 to 12- observation of damage

• Magnitude • a quantitative measure of the amount of energy released

by an earthquake.- measured on the Richter Scale- max energy in one instant-

•

• Intensity (qualitative data) • A measure of earthquake damage.• MMI - scale (Modified Mercalli Intensity)- ranges from 1 to 12, expressed as roman numerals (I, II, III, IV)- observation of damage, semi-independent of energy released

• Magnitude (quantitative data) • a quantitative measure of the amount of energy released

by an earthquake.- measured on the Richter Scale- max energy in one instant- each step is multiplied by a factor of 30

• Moment magnitude is energy released over the entire areaover the entire time

Earthquake PredictionEarthquake Prediction

• Short term predictions are still unsuccessful.• Long term forecast maps are more reliable

• offer a magnitude and probability ofoccurrence for a given interval of time• Damage maps are developed based on

intensity of past earthquakes and currentgeologic maps.

• Short term predictions are still unsuccessful.• Long term forecast maps are more reliable

• offer a magnitude and probability ofoccurrence for a given interval of time• Damage maps are developed based on

intensity of past earthquakes and currentgeologic maps.

Tsunamis and Ground failuresTsunamis and Ground failures• Seismic sea-waves

• produced by earthquakes on the sea floor.• Travel at very high speeds • 100's of km/hr in the open ocean

- Barely noticeable in open ocean• but may reach heights of 65 m as they near shore.

• Seismic sea-waves • produced by earthquakes on the sea floor.• Travel at very high speeds • 100's of km/hr in the open ocean

- Barely noticeable in open ocean• but may reach heights of 65 m as they near shore.

• Earthquake-triggeredlandslides cause muchdamage in mountainous

areas.• 1959 Madison County(Montana) earthquake

• Earthquake-triggeredlandslides cause muchdamage in mountainous

areas.• 1959 Madison County(Montana) earthquake

Mass movementsMass movements

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Notice the P- waves and S-waves traveling outward from the focus

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