lecture outlines physical geology, 15/esubduction.rocks/storage for lecture notes/300 spr...

Lecture Outlines

Physical Geology, 15/e

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Plummer, Carlson & Hammersley

Earthquakes

Physical Geology 15/e, Chapter 16


Earthquake – a trembling or shaking of the ground caused by thesudden release of energy stored in the rocks beneath Earth’s surface

• energy is released in the form of seismic waves

• released from a position along a break between two rock masses (fault)

Elastic rebound theory - earthquakes are a sudden release of strainprogressively stored in rocks that bend until they finally break and movealong a fault


Causes of Earthquakes

Focus – (or hypocenter) the point of initialbreakage and movement along a fault, whereseismic waves originate

Epicenter - point on Earth’s surfacedirectly above the focus

Two types of seismic waves:• body waves - travel outward from the

focus in all directions through Earth’sinterior

• surface waves - travel along Earth’ssurface away from the epicenter


Seismic Waves

Body Waves• P wave - compressional (longitudinal) body wave

in which rock vibrates back and forth parallel tothe direction of wave propagation

• fast (4 to 7 km/sec) wave that is the first orprimary wave to arrive at recording station

• pass through solids and fluids

• S wave - shearing (transverse) body wave inwhich rock vibrates back and forth perpendicularto the direction of wave propagation

• slower (2 to 5 km/sec) wave that is the secondarywave to arrive at recording station

• pass through solids only


Seismic Waves

Surface Waves

• Slowest type of seismicwaves produced byearthquakes

• Love waves – side-to-

side motion of the groundsurface, can’t travelthrough fluids

• Rayleigh waves –

ground moves in anelliptical path opposite thedirection of wave motion;extremely destructive tobuildings


Seismic Waves

Seismometers – used to measure seismic waves

Seismographs – recording devices used to produce a permanent

record of the motion detected by seismometers

Seismograms – permanent paper (or digital) records of the

earthquake vibrations

• used to measure the earthquake strengths


Locating and Measuring Earthquakes

Determining the Location ofan Earthquake

• Travel-time curve – used todetermine distance to focus

• based on time between first P- andS-wave arrivals

• P- and S-waves leave earthquakefocus at the same time

• P-wave gets farther and fartherahead of the S-wave with distanceand time from the earthquake



Determining the Location ofan Earthquake

• Plotting distances from 3 stationson a map, as circles with radiiequaling the distance from thequake, locates earthquakeepicenter

• Depth of focus beneath Earth’ssurface can also be determined



Measuring the “Size” of an Earthquake

Intensity – a measure of the effects an earthquake

produces (on both structures and people)

• Modified Mercalli Scale

• Range of I-XII

• Drawbacks

• Damage lessens with distance fromepicenter

• Damage also depends on severalother factors (type of geology,construction etc…)



Measuring the “Size” of Earthquakes

Magnitude – a measure of the amount of energyreleased by an earthquake by measuring theamplitude of seismic waves on a seismogram

•Richter scale

• Numerical scale of magnitudes

• Originally developed for shallowearthquakes in California

•Moment Magnitude Scale – moreobjective measure of energy released bya major earthquake

• uses rock strength, surface area of faultrupture, and amount of movement



Location and Size ofEarthquakes in the United States

• Most common in the western statesand Alaska

• Largest seismic risks or hazardsexist near the plate boundary alongthe Pacific coast and New Madrid,MO

• Seismic risk is determined based onthe assumption that large futureearthquakes will occur where theyhave occurred in the past



Ground motion – trembling and shakingof the land during an earthquake

Fire – from broken gas and water mains andfallen electrical wires

Ground Failure• Landslides – triggered by ground

shaking

• Liquefaction – occurs when water-saturated soil or sediment sloshes like aliquid during a quake

• Permanent displacement of theland surface – leaving fractures andscarps


Earthquake-Related Hazards

Aftershocks•Small earthquakes that follow the main earthquake•Can cause considerable damage to alreadydamaged buildingsTsunamiSeismic sea waves caused by sudden upward ordownward movement of the sea floor duringsubmarine earthquakes

• generally produced by magnitude 8+ earthquakes• may also be generated by large undersea landslides

or volcanic explosions• travel across open ocean at speeds of >700 km/hr.• reach great heights in coastal areas with gently sloping

seafloor and funnel-shaped bays


Earthquake-Related Hazards

Most earthquakes occur in narrow geographic belts which marktectonic plate boundaries

• important concentrations occur in the circum-Pacific andMediterranean-Himalayan belts

• shallow-focus earthquakes common along the crests of mid-oceanicridges

• nearly all intermediate- and deep-focus earthquakes occur in Benioffzones


World Distribution of Earthquakes

• Rock motion is determined by studying seismograms from manylocations

• Each seismogram can identify whether the first motion was a push(up deflection) or pull (down deflection) at that location

• There are two possible solutions for any pattern of first motionsthat can be solved by knowing the fault orientation


First-Motion Studies of Earthquakes

Earthquakes and Plate Boundaries• Plate boundaries are identified and defined by earthquakes

• Earthquakes occur at each of the three types of plate boundaries: divergent,transform, and convergent

Subduction Angle

• Determined by the horizontal and vertical distribution of earthquakes

• Controlled by plate density


Earthquakes and Plate Tectonics

Earthquake Precursors and Prediction

• Microseisms – small tremors caused by small breaks in therocks

• Rock properties changes - such as magnetism, electricalresistivity, seismic velocity, and porosity

• Water level in wells

• Radon emission increases

• Animal behavior

• Foreshocks

• These all appear to be very inconsistent, making short termprediction difficult


Earthquake Prediction and Forecasting

Earthquake Forecasting

•Paleoseismology – studies that determine where andwhen earthquakes have occurred and their size

•Study geologic record

•Digging trenches across fault zone

•Recurrence interval

•Seismic gaps – inactive segments along fault zone

•Slip rate along fault zones



Reducing Earthquake Damageand Risk

• Monitoring the amount ofshaking and relaying shake mapinformation rapidly to guideemergency personnel to themost damaged areas

• Develop early warning systems

• Shut down gas lines, subways,trains etc…



End of Chapter 16


lecture outlines physical geology, 15/esubduction.rocks/storage for lecture notes/300 spr...

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