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Chapter 4

Earthquakes

Map is from the

United States

Geological Surveyand shows

earthquake hazard

for the fifty United

States.

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Fig. 4.02

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Basic Theory

Earthquakes (seismic slip) Friction between rocks against slipping

generates elastic deformation and builds up

energy before failure

When the stress exceeds the friction (orrupture strength of the rock), a sudden

movement occurs to release the stress

Elastic Rebound: rocks snap back elasticallyto their previous dimensions after the sudden

displacement and associated stress release

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Fig. 4.3 Theelastic rebound

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Earthquakeor Seismic Slip

Energy releases from a dynamic earthoccur along faults

Earths crust moves very slow

over time enough stress builds up and a brittlerelease occurs - an Earthquake

Stress is released and transferred

Elastic Rebound

Actual site of the first movement along afault is the focus (or hypocenter)

Actual point on the earths surface directly

above the focus is the epicenter

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7/37Figure 4.4 Simplified diagram of a normal fault

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Earthquake Locations

Generally occur in linear belts Intraplate earthquakes also occur and quite severe

Linear belts correspond to plate boundaries most earthquakes occur at plate boundaries

Earthquake focal depths are Deep

Intermediate

Shallow

The deep-focus earthquakes are

concentrated in subduction zones

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9/37Figure 4.5 World seismicity (1979-1995)

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Seismic Waves

Energy released by an earthquake willsend seismic wavesout from the focus

Body Wavetravels through the interior ofthe earth

P wavesare compression waves S wavesare shear waves but pass only

through solid rock (not magma)

Surface wavetravels along the surface Larger ground displacement than body waves Results in most earthquake damage

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Fig. 4.06

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Locating the Epicenter Energy released from an earthquake must travel

through the earth Density of rock will affect the travel time for seismicwaves

Waves move fast through high density rocks

Waves move slow through low density rocks

Seismographdetects ground movement and canbe useful in calculating the location of anepicenter Records arrival of different seismic waves

Interval of time between the first arrivals of P wavesand S waves is a function of distance to the epicenter

Requires at least 3 seismographs to locate anearthquake

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Figures 4.7 c, a, and b

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Earthquake Energy

Energy is released during an earthquake

As the waves of energy are transmitted through

the rock, this energy with be felt by people at

the surface

Magnitudethe amount of ground motionrelated to an earthquake

Intensityeffect on humans, and their

structures, caused by the energy releasedby an earthquake

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Figures 4.8 and 4.9

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Earthquake Magnitude

Measured by a seismograph Richter magnitude scale most common

Richter scale is logarithmic

An earthquake of magnitude 4 causes 10times more ground movement as one of

magnitude 3

The energy released by an earthquake of

magnitude 4 releases about 30 times more

energy than an earthquake of magnitude 3

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Earthquake Intensity

Measures the impact of an earthquake

event on humans and surface features

Many local factors are considered such as

local geology, construction practices, and

distance to the epicenter

Modified Mercalli Intensity Scale is widely

used in the United States

Intensities are reported as roman

numerals ranging from I (for not felt) to

XII, for (damage nearly total)

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Figures 4.10 a, b, and c

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Figure 4.11

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Figure 4.12 and Figure 4.13

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Figure 4.14 and Figure 4.15

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Figure 4.16 and Figure 4.17

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Tsunami and Coastal Effects

Tsunamis are seismic sea waves. Whenan undersea or near-shore earthquakeoccurs, sudden movement of the sea floormay set up waves traveling away from thatspot, like ripples in a pond caused by adropped pebble

Other Coastal Effects

Fireis caused by broken gas lines andinfrastructure

Downed power lines

Fi 4 19

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Figure 4.19

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Predication and Forecasting Seismic gaps: quiescent (dormant) sections of

fault zones with little or no seismic activity alongmajor faults

Seismic gaps are locked segments of faultsalong which friction is preventing slip

Seismic gaps may be sites of future seriousearthquakes. Large earthquakes may beexpected in the future

Precursors: uplifted and tilted ground surface,

change of seismic-wave velocities, change ofelectrical resistivity, change of water table, andchange of radon

Prediction: problematic, unreliable

Forecasting: who knows?

Fi 4 25 4 21 d 4 23

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Figures 4.25, 4.21, and 4.23

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Earthquake Control

Fluid injection: fluids in fault zones mayfacilitate movement along a fault.

Fluid injection: might be used along locked

sections of major faults to allow the releaseof built-up stress through small earthquakes

However, could lead to the release of all the

stress at once in a major earthquake Better to plan away from earthquake zones

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Figure 4.26 Correlation between waste disposal and the

frequency of earthquakes

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Earthquake Awareness

When we turn routine earthquake predictioninto reality, we are going to face some social

and legal complications, such as logistics,

vandals/looters, prediction accuracy, and

legal issues

Increase public awareness of earthquakes

as a hazard

Improve public understanding and response

to earthquake hazards

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Figures 4.27 property damage from the 1964

earthquake in Anchorage, Alaska

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Fig. 4.28 The southern locked segment of the San Andreas Fault

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Fig. 4.32

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Figure 4.29 U.S. Seismic Hazard Map

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Fi 4 30 Th id ti t i t l t th k