Chapter 16

Earthquakes

What is an earthquake?What is an earthquake?

• Shaking of the ground caused by the Shaking of the ground caused by the sudden release of energy stored in sudden release of energy stored in the rocks.the rocks.

• When rocks are stressed, potential When rocks are stressed, potential energy is stored in them. energy is stored in them.

• Rocks can behave plastically upto a Rocks can behave plastically upto a certain limit (elastic limit), after which certain limit (elastic limit), after which they fail and release the energy.they fail and release the energy.

• Energy waves produced by an Energy waves produced by an earthquake are called Seismic Waves. earthquake are called Seismic Waves.

• The generation of an earthquake is The generation of an earthquake is explained by Elastic Rebound Theory.explained by Elastic Rebound Theory.

• According to this theory, the sudden According to this theory, the sudden release of progressively stored strain in release of progressively stored strain in rocks causes movement along a fault rocks causes movement along a fault and an earthquake is generated.and an earthquake is generated.

Causes of earthquakesCauses of earthquakes

• Movement along a faultMovement along a fault

• Volcanic eruptionsVolcanic eruptions

• Plate movementPlate movement

• Subsidence Subsidence

Earthquake terminologyEarthquake terminology

• Focus: the point inside the earth Focus: the point inside the earth where seismic waves first generate.where seismic waves first generate.

• Epicenter: the point on ground Epicenter: the point on ground surface directly above the focus. It surface directly above the focus. It may be any geographic locality.may be any geographic locality.

Seismic wavesSeismic waves

• 2 types2 types

–Body waves: traveling inside the Body waves: traveling inside the earthearth

–Surface waves: travel on the Surface waves: travel on the earth’s surface.earth’s surface.

• Body waves:Body waves:

–P-waves (primary)P-waves (primary)

–S-waves (secondary)S-waves (secondary)

• P-wave: is the fastest (4-7 km/sec) P-wave: is the fastest (4-7 km/sec) and is the first to arrive at a and is the first to arrive at a recording station.recording station.

• P-wave: compressional or P-wave: compressional or longitudinal wave in which rocks longitudinal wave in which rocks vibrate back and forth parallel to vibrate back and forth parallel to the direction of wave propagation.the direction of wave propagation.

• S-wave: slower (2-5 km/sec) and S-wave: slower (2-5 km/sec) and arrives after the P-wave at a arrives after the P-wave at a recording station.recording station.

• It’s a transverse wave in which the It’s a transverse wave in which the rock vibrates perpendicular to the rock vibrates perpendicular to the direction of wave propagation.direction of wave propagation.

• Surface waves: Surface waves:

–slowest waves generated by an slowest waves generated by an earthquake. earthquake.

–They take longer to pass and They take longer to pass and cause more property damage.cause more property damage.

EQ measurementEQ measurement

• The instrument used to measure The instrument used to measure an EQ is called a seismometer.an EQ is called a seismometer.

• Seismograph: paper or electronic record of an EQ.

World seismograph stations

Locating EQ EpicenterLocating EQ Epicenter

• Travel time of the seismic waves from Travel time of the seismic waves from the focus to the seismograph station the focus to the seismograph station is used to determine the EQ epicenter.is used to determine the EQ epicenter.

• P and S waves gradually separate P and S waves gradually separate because they travel at different because they travel at different velocities.velocities.

• The P-S interval is compared with a standard travel-time curve.

• Data from one station can be used Data from one station can be used to determine only distance to the to determine only distance to the station and not the direction.station and not the direction.

• At least three stations are required At least three stations are required to determine the location of the to determine the location of the epicenter.epicenter.

Measuring the size of an EQMeasuring the size of an EQ

• Two ways of determining the EQ Two ways of determining the EQ size;size;

– IntensityIntensity

–MagnitudeMagnitude

Measuring the size of an EQMeasuring the size of an EQ

• EQ Intensity: it is a measure of how EQ Intensity: it is a measure of how an area has been damaged by the an area has been damaged by the EQ.EQ.

• Modified Mercalli Scale is used to Modified Mercalli Scale is used to measure intensity.measure intensity.

• EQ magnitude: it’s a measure of EQ magnitude: it’s a measure of the amount of energy released by the amount of energy released by an EQ.an EQ.

• EQ magnitude is reported on the EQ magnitude is reported on the Richter scale.Richter scale.

• It is done by measuring the height It is done by measuring the height (amplitude) of a specific wave.(amplitude) of a specific wave.

• MML = log10L = log10A(mm)A(mm) + + (Distance (Distance correction factor)correction factor)

Magnitude (log Scale) Possible Effects

1-2 Normally only detected by instruments

3 Only faintly felt

4 Faint tremor causing little damage

5 Structural damage

6 Distinct shaking, less well-constructed buildings collapse

7 Most buildings destroyed

8 Major structures destroyed

9 Ground seems to shake-catastrophic

Effects of EQEffects of EQ

• Primary effects: occur immediately Primary effects: occur immediately from ground shakingfrom ground shaking

• Secondary effects: damage caused Secondary effects: damage caused by;by;

Effects of EQEffects of EQ–FloodsFloods–FireFire–LandslidesLandslides–Liquefaction: wet, saturated soil Liquefaction: wet, saturated soil

changes from solid to liquid as a changes from solid to liquid as a result of shaking.result of shaking.

–TsunamisTsunamis

Global distribution of EQGlobal distribution of EQ

• 3 belts;3 belts;

–Circum-Pacific beltCircum-Pacific belt

–Mediterranean-Himalayan beltMediterranean-Himalayan belt

–Continental interiorContinental interior

Mitigating EQ effects

• Avoiding EQ prone zones

• Foundation design

• Structural modifications

• Building codes

Predicting EQ

• Rock monitoring: rock properties begin to change before its failure.

• Water levels: increase or decrease.

• Radon emission: increases before an EQ.

Predicting EQ

• Seismic gap: regular pattern of EQ occurrence.

• Animal behavior: snakes, dogs.

Assignment 2Assignment 2

Rivers, groundwater, lakesRivers, groundwater, lakes

Ohio EPAOhio EPA

Great lakes, lake erieGreat lakes, lake erie

Relative proportionsRelative proportions

Maximum Contaminant LevelMaximum Contaminant Level

Primary contaminantsPrimary contaminants

Secondary Secondary

Water resources of Ohio: are they contaminated?Water resources of Ohio: are they contaminated?

3-4 page typed report3-4 page typed report

Due: Friday, Nov. 19Due: Friday, Nov. 19