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Visualizing Earth Science

By Z. Merali and B. F. Skinner

Chapter 8– Earthquakes & Earth’s Interior

• Earthquakes and Earthquake Hazards

• The Science of Seismology

Chapter Overview

gy

• Looking into Earth’s Interior

• How and Why Rock Breaks

Earthquakes and Seismology

• An earthquake– Suddenly releases energy– Which has slowly accumulated in crusted rock

• Seismology – the scientific study of earthquakesearthquakes– The waves in the Earth caused by earth quakes

• Earthquakes occur at tectonic plate margins– The largest occur at major subduction zones

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Earthquakes and Seismology

Earthquakes and Plate Motion• Slow sliding movement of two

tectonic plates– Stick due to friction– Cause deformation of rock in

the plates– Eventually the elastic pressureEventually the elastic pressure

overcomes friction– Plates slip – releasing energy

and causing an earthquake– Some plate movements result

in frequent small displacement – seismic creep

– Major earthquakes sometimes cause large displacements

Elastic Rebound Theory

• Rocks are somewhat elastic– Deformed rock tends to return to

original shape– Elastically deformed rock store

energyenergy• Deformation occurs at fault along

plate boundaries– Released energy becomes

vibration of rock– Vibrations travel through the

Earth as seismic waves

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Elastic Rebound Theory

Earthquake Hazards

• Ground motion causes collapse of structures• Crustal disturbance triggers slips or after shocks• Earthquakes cause secondary hazards

– Landslides– Fires– Ground liquefaction– tsunamis

Earthquake Hazards

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Earthquake Hazards

Earthquake Hazards

Earthquake Prediction• Short-term prediction

– Advance warning of date time and place of earth quake– Difficult and not accurate at present

• Long-term forecasting– Based on an understanding of tectonic disturbances– Hypothesis: earthquakes are recurring events– Paleo-seismology – study of ancient earthquakes

• Through geological evidence

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Earthquake Prediction

Safety and Hazard Prevention

• Even less intense quakes can cause significant damage– In populous areas– Poorly designed structures

• Tsunami damage can be lessened– By providing sufficient early warning– By mobilization to high ground

Safety and Hazard Prevention

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Safety and Hazard Prevention

Seismology: Seismographs

• Instruments to detect and record earthquake vibrations

• Seismograph records are seismograms

• Modern seismographs use sensitive optics and electronics

Seismology: Seismographs

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Seismic Waves

• Energy in earthquake travels outwards as waves– The originating point is where the slip occurs – the

focusfocus– Two main categories of earthquake waves

• Surface waves– Travel along the Earth’s surface– Are the last to reach a seismograph – slowest

• Body waves– Travel through the Earth’s interior

Seismic Waves• The two kinds of body waves:

– P-waves• Are compressional • Like sound waves

– Can travel through solids, liquids and gasesAre the fastest waves– Are the fastest waves

– The first waves recorded on a seismograph

– S-waves• Are shear waves• Like waves on a string

– Wave motion is perpendicular to travel direction

• Are slower than P-waves– Travel only through solid

Seismic Waves

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Locating Earthquakes• The epicenter

– Directly above the focus– Is located using seismograms

• Distance to epicenter is figured out:– From the seismogram– By using the arrival times of seismic waves

• The epicenter location– Can be located by triangulation– Needs three or more seismograph locations and records

Locating Earthquakes

Locating Earthquakes

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Measuring Earthquakes

• Early scale – Mercalli Scale– Based on descriptions and damage– Does not use or need seismograph records

• Modern scales– Richter magnitude scale

• Based on seismograph records• Each unit increase in Richter magnitude

– Is a 10x increase in seismic wave amplitude– Is a 32x increase in energy release

• Measurement takes distance to epicenter into account– Movement magnitude

• Update of Richter scale – based on geological measurements.

Measuring Earthquakes

Imaging Earth’s Interior

• Seismic body waves can be used to image Earth’s interior

• Seismic discontinuities are boundaries inside Earth where:– Interior composition changes– In response seismic wave behave differently– At seismic discontinuities: Seismic waves

• Can be bent – refracted• Can be reflected• Can be absorbed

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Imaging Earth’s Interior

Imaging Earth’s Interior

Imaging Seismic Discontinuities• Shadow zones

– P-wave• Due to bending of P-waves at

discontinuities– S-wave

• Due to absorption and reflection of S-waves at discontinuities

– Shadow zones indicated the presence of liquid in the metal core

• Sensitive computational methods– Seismic tomography

• Revealed details of compositional structure

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Imaging Seismic Discontinuities

• The Moho discontinuity– Density and composition difference between crust and mantle

rock– Body seismic wave speeds change

• Seismic discontinuities in the mantle• Seismic discontinuities in the mantle– The top of mantle: weak asthenosphere seismic wave velocity

reduced– Lower mesosphere extends to the metal core

• Has additional discontinuities important for plate tectonic convection

Imaging Seismic Discontinuities

Looking into the Earth’s Interior

• Drilling– Can only access the crust at

present– Magma

• Carries rocks form the mantle: xenolithsxenoliths

• Contains clues about mantle conditions and processes

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Looking into the Earth’s Interior

Deducing the Earth’s Interior• Magnetism– Earth has a measurable magnetic

field• Magnetic fields are produced by

moving charges• Flowing metals produce magnetic

fieldsfields• Hypothesis: Earth has liquid metal

in its interior– Observation: there is liquid in

the outer core– Astrophysical observations

• Earth’s mass and volume can be measured

• Calculated density gives clues to compositions

Deducing the Earth’s Interior

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Rock Strength and Deformation

• Directional forces:– Stress: force with a preferential direction

• Tension and compression: pull and push• Shear: twisting parallel forces

– Strain: response to stress• Changes shape or volume

• Deformation in response to stress and strain– Elastic: rocks snap back to original shape– Ductile and brittle: rock stretch or break

Rock Strength and Deformation

Faults

• Normal fault– Response to tension (pull) stress– Result in milder earthquakes

• Reverse and thrust faults– Response to compressional (push) stresses– Result in strong earthquakes

• Transform faults– Response to shear (parallel) stresses– Result in stronger, shallow earthquakes

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Faults

Chapter Summary

• Earthquakes and Earthquake Hazards– Seismology: Measuring and Locating Earthquakes– Elastic Rebound Theory– Earthquake Hazards, Prediction and Safety

• The Science of Seismologygy– Seismographs and Seismic waves– Locating Earthquakes: Epicenter and Triangulation

• Looking into Earth’s Interior– Imaging Seismic Discontinuities– Drilling and Magnetism

• How and Why Rock Breaks– Rock Strength and Deformation– Faults