Earthquakes Mar2010

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3/17/20

Geology 11 - Earthquakes

What is an Earthquake?

An earthquake is vibration of the Earth caused

by a rapid release of energy.

– This energy is released when tectonic forces cause

rocks to break.

– The energy radiates as waves in all directions from

its source, the focus.or hypocenter

– This energy can be measured around the globe.

Earthquake mechanismLike energy stored and released by a spring

Before stress

Elastic strain accumulating

Rupture!

Strain relieved; new equilibrium

Sound of breaking spring is

exactly analogous to earthquake waves

Elastic Rebound

– Rocks on either side of an existing fault are deformed by

tectonic forces.

– Rocks bend and store elastic energy.

– When the frictional resistance holding the rocks together

is overcome….

Earthquake !!!

Seismology

• Seismology is the study of earthquake waves.

• Seismographs are instruments that record

seismic waves.

– Record the movement of Earth in relation to a

stationary mass on a rotating drum.

Foreshocks and aftershocks

– Smaller earthquakes that follow a major

earthquake are called aftershocks.

– Small earthquakes, called foreshocks, often

precede a major earthquake by days or, in some

cases, by as much as several years.



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Epicenter vs. Hypocenter

July 1990 earthquake in Luzon, Philippines

EpicenterLat 16.05NLong 119.69E

Ground rupture

Fault surface extends at least 50 km below the surface

Hypocenteror focus

Rupture surface determined from hypocenters of aftershocks

CircumPacificMediterranean --Trans-HimalayanMid-Oceanic Ridge system

Zones outline boundaries of Earth’s

7 major tectonic plates…

Antarctic

Pacific

… and several smaller plates.

Philippine

Scotia

Other quakes occur within plates,and are harder to explain.

Global Distribution of Quakes:

Most occur in three beltsFaults & Plate Tectonics

Tectonic plate boundaries are characterizedby specific fault types.

Transform Divergent Convergent

Recognizing Faults

Topographic featuresFault scarpsOffset streams, ridgesFacetted or triangular spursParallel valleys, ridges (differential erosion)

WARNINGS: •Faults are not simple lines, usually have several branches! •Many quakes occur on “rogue” or “blind” previously unrecognized) faults!

Hydrologic featuresSag ponds, spring lines, tree lines

Stratigraphic featuresOffset formations and strata, drag folds

Fault plane featuresGouge, breccia, slickensides

(Right lateral) What is an earthquake?

fault scarp

- cliff from vertical slip on fault



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1906 San Francisco EarthquakeTypes of Seismic Waves

• Surface waves - Travel along surface of Earth

•

Body Waves–

Travel through the earth. – Primary (P) waves

• Push-pull (compress and expand) motion.

• Travel through solids, liquids, and gases.

• Faster

– Secondary (S) waves

• Shaking" motion .

• Travel only through solids.

• Slower

Seismic Wave Motions

P waves, 6.12 km/s. Motions back and forthin direction of propagation.

Propagation direction

compressions

dilatations

S waves, 4.09 km/s. Motions at right anglesto propagation direction.

First, body waves (P and S)Propagation direction

Love waves

Very damaging to structures!

Rayleigh waves

Surface Wave Motions

Seismographs record earthquakes

Horizontal seismograph

Vertical seismograph

Two horizontal instruments (N-S, and E-W)and a vertical one record motion in 3D

Ground and recorder move

SupportWire

Rotating recording drum

Heavy weight

Pen

Ground andrecorder move

Springflexes Heavy weight and

pen don’t move

doesn’tmove (inertia)

movesflexes

A Typical

Seismogram

First P (primary) wave. Travels

fastest; 6.12 km/s or 13,680 mph

First S (secondary) wave.Travels 4.09 km/s or 9144 mph

Surface (Love and Raleigh)Waves. Velocity of first arrival3.15 km/s or 7043 mph.

P and S waves are body waves (travel through Earth). Used to locate quake epicenters...

…from lag time between arrivalof first P and first S

Over a few hundred kilometers,the ratio of lag time to distanceis virtually constant.P wave travels 100 km in 16.39 sec,S wave travels 100 km in 24.45 sec,so P-S lag time is 8.06 sec/100 km.



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P and S Wave Velocities on a Global Scale

increase with travel distance because the wavespenetrate more deeply, into denser, more rigid material.

Surface-wave velocities do not change.

New York4.6 min S-P lag

Stuttgart, Germany4.0 min = 3220 km

Locating Earthquake Epicenter

Rio de Janeiro, Brazil8.2 min = 7080 km

= 4180 km

Intensity v. Magnitude

Intensity (and damage): determined partlyby duration and number of "jerks", butmainly by maximum rate of change ofground movement, or acceleration .

If acceleration >9.8 m/sec2, or that ofgravity, objects are thrown into the air.

Intensity is very subjective; based onperceptions of people and effects onstructures. What if earthquake occursin wilderness?

Magnitude generally reported by the mediaas a number on the “Richter scale”, afterseismologist Conrad Richter of Cal Tech.

based on maximum motion at a station 100km from epicenter.

Magnitude scales measure energy,independent of perception and damage.

Richter magnitude ML (“local”) technicallyonly refers to southern Calif. earthquakes.

Magnitude of largest surface wave now mostcommonly used (MS).

How Quake Energy Dissipates with Distance

Earthquake Magnitude

To handle great size range of quakes, magnitude defined as “Thelogarithm to the base ten of the maximum seismic-wave

amplitude in microns (1/1,000,000 meter), recorded on a standardseismograph 100 kilometers away from the epicenter.”

Microns

100

101

102

103

104

105

106

107

108

109

ML

0123456789

Joules

2.75x1010

7.59x1013

2.09x10155.75x1016

1.58x1018

“Hiroshima”

= MS 5.3= 7.45x1012J

0.410

2807,720

212,000

Megaton

0.0060.25.2

1444000

MS 8 energy > total annual U.S. electricity

Between succeeding magnitude numbers, amount of motion is 10 times greater.But amount of energy is about 30 times greater! Difference of 2 wholenumbers = 1,000x more energy.

Calculating Richter magnitude ML

seismogram

P S

Procedure:A. S-P lag time yields distance to focus.

A

24 seconds

24 seconds = 215 km

B. Maximum vertical amplitude:

23 mm

23 mm

BA

C. Straight line between points A & B yields magnitude: ML = 5.0

C



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Earthquake PredictionBased mainly on dilatancy : Under ½ of stressneeded to crush rocks, they crack and swell,causing various effects in sequence.

1.Rocks

start todeform

2.Cracks,

poresform

3.Faultzonedeforms

rapidly,fluidsenter

Majorquake

Successful Predictions?

Successes rare, record spotty:

Hiacheng and Yongkow, China evacuated Feb 4, 1975.Quake predicted in next 24 hours. People kept outdoors inwinter cold. 7:36 PM: Ms 7.3 quake destroyed 90% of all

houses.July 28, 1976:Tang Shan MS 7.8 earthquake killed 655,000.Predicted within time frame of 2 months, but noprecursors.

Another quake predicted for August, 1976 did not occur.

Earthquake Hazard

Five principal causes:

1. Ground shaking•Direct structural damage to buildings, water mains, dams

•Liquefaction

•Landsliding, mudslides

2. Epicentral faulting – Direct structural damage

3. Tsunami

4. Rupture of artificial and natural dams from ground-shaking,faulting

5. Fire

Effects of Surface Waves on Buildings

Love waves rapidly and repeatedly pull the ground out fromunder structures.

Passages of Rayleigh-wave crests bend adjacent buildingsaway from each other…

Buildings designed to withstand verticalstress of gravity, not lateral stresses.

Passage of troughs bend them towards each other.

Building DamageVaries with characters of both foundation

and structure. Mexico City, 1985:

Buildings of different heights vibrate At different frequencies

Soft clay, 30 m thick Firm sedimentary bedrock2 km thick

Building on rollers slides back and forth

Buildings bumped each other; smaller one collapsed

Unevenly distributed stresses twisted asymmetrical buildings.Structural failure, collapse. Little

damage



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Baguio Hyatt Hotel, Philippines, 1990

Consequence of lateral motion

l important factors: Intensity & duration of shaking

Soil type (soft? hard rock?)

Building design

l other effects:

Liquifaction

Tsunamis

Mexico City, 1985

Earthquake Destruction

Liquefaction

Affects porous rock or sediment saturatedwith water. On a microscopic scale:

Except during quakes,sand and rock particlesrest on each other, bearthe vertical overburdenload , transmit itvertically downward.

Water sits passively in pores,bearing no load.

During a quake:Rapid, random seismicmotions break grain-to-grain contacts,

Particles temporarilysuspended in water,which bears the load.During quake, water-sediment mix is a slurry,behaves as a liquid with no strength.

Liquefaction Effects

Unevenly loaded building tilted and fell.Mexico City, 1985.

Liquified subsurfacesands, squeezed by

weight of overlyingsoils, emerge as“sand boils”.

l important factors: Intensity & duration of shaking

Soil type (soft? hard rock?) Building design

l other effects:

Liquefaction

Tsunamis

Earthquake Destruction Tsunami

Commonly and mistakenly called a “tidal wave”.

Fault

Initial wave crest,5 km deep, 800 km/h

Entering shallow water, friction with bottomincreases. Wave slows down, shortens andincreases its height.

800 m deep 300 km/h 20 m deep

60 km/h

Depending on relative motion, first arrival at coast could be acrest or a trough.



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Tsunami warning

Easy to predict arrival times at different coasts,fromepicenter and time of quake and from relationship betweenocean depth and wave velocity:

3.13gD D m/s.v =

Tsunami damage in Hawaii :

From 1960 Chile earthquake,

15 hours later


U.S. Geological Survey

Worst natural disasters: EarthquakesThe world’s twelve worst:

Date AD Location Casualties 1201 Egypt-Syria 1,100,0001556 Shanxi, China 830,0001737 Calcutta, India 300,0001976 Tangshan, China 242,0002006 Indian Ocean 200,000893 India 180,0001923 Tokyo, Japan 143,0001730 Hokkaido, Japan 137,0001138 Syria 100,0001920 Gansu, China 100,0001908 Messina, Italy 83,0001755 Lisbon, Portugal 70,0001970 Chimbote, Peru 66,000Salient points:

•Disaster magnitudes are functions of geography and population density.•As world population increases, so will magnitude and frequency of d isasters.

Earthquake / Seismic

Hazards

• Defined as the risk of injury or death toman and damage of his works by groundmotion caused byearthquakes



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Earthquake Generators

Surrounding Baguio

City

Manila Trench

East Luzon Trough –

Philippine Trench0 50 100 km

S E A

CENTRAL PLAIN

CARBALLO MOUNTAINS

Baguio C O A S T A L

Tuguegarao

AparriLaoag

1200 1210 1220

180

17 0

16 0

BaguioAn upland plateau

characterized by

relatively low angle

hill-slopes withingenerally rugged and

elevated region

Elevations range from

1,000 to 1,500 masl

Earthquake Generators

Surrounding Baguio City

4 Northwest segments of

the Philippine Fault Zone

(PFZ):

- Digdig Fault

- San Manuel Fault

-Tebbo Fault-Tuba Fault

0 50 100 km

N

M ani l aT r e n c h

South

China

Sea

Pacific

OceanA b r aRi v e r

Baguio City

2 Seismic Generators near

Baguio City

Tuba Fault

- west of Baguio City, approximately

5 km away with shortest 50 km

long, NW trending

- could generate an earthquake of

maximum of Ms 7.25

0 50 100 km

N

M ani l aT r e n c h

South

China

Sea

Pacific

OceanA b r aRi v e r

Baguio City

Tebbo Fault-located approximately 10 km

southeast of Baguio City- 70 km long

- could generate an earthquakemagnitude of maximum Ms 7.4

Epicenter vs. Hypocenter

July 1990 earthquake in Luzon, Philippines

EpicenterLat 16.05NLong 119.69E

Ground rupture

Fault surface extends at least 50 km below the surface

Hypocenteror focus

Rupture surface determined from hypocenters of aftershocks

Earthquake hazard: ground shaking



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Earthquake

hazard:

Ground rupture

Imugan River

near Digdig, N. Vizcaya

Earthquake hazard: liquefaction

Residential Bldg. in Umingan, Pangasinan


• Amount of structural damage attributable to

earthquake vibrations depends on…

– Intensity and duration of the vibrations.

– Nature of the material upon which the structure

rests.

– Design of the structure.

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Earthquakes Mar2010