1 Seismology 4

7/29/2019 1 Seismology 4

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Earthquake

seismology

The San Andreas fault in the

Carrizo plain, California

Offset drainage along the San

Andreas fault, Wallace Creek,

California

Fence offset by the 1906 SanFrancisco earthquake


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Elastic strain

accumulation

(Most) faults are locked

between earthquakes

The area around faults

accumulates elastic strainGPS-derived velocities in Southern California (1992-2000). Velocities are shown with respect to North

America. The active faults of California are shown in

orange.


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The seismic

cycle

Between earthquakes:

Faults are locked

Area around faults accumulate deformation

During an earthquake: A fault slips suddenly

The deformation accumulated around the fault is

released

After an earthquake:

Stresses around the fault are modified

Readjustments on the fault plane = aftershocks


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The seismic cycle

During an earthquake:

A fault slips suddenly

The deformation accumulated around the

fault is released

Stresses around the fault are modified

Between earthquakes:

Faults are locked

Area around faults accumulate deformation

Animation: R. Stein, USGS

Click for earthquake cycle animation


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Earthquake seismology Location of the earthquake (hypocenter)?

Frequency of similar earthquakes?

Focal mechanism? Rupture mechanism?

Size?

Intensity

Magnitude

Moment

Energy release

Earthquake triggering?


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Locating earthquakes

Difference in travel time

for P and S waves

increases with increasingepicentral distance:

tS=

D

VS

tP=

D

VP

" tS# t

P= D 1

VS

# 1V

P

$

%& '

()

VP= 5.85 km /s V

S= 3.38 km /s

" D = tS# t

P( )* 8.0


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Locating earthquakes

Errors:

Picking arrivals

Actual travel times are slightlydifferent from theoretical location

is dependent on the Earth model used

(global or local).

With at least 3 stations:

Calculate S-P time difference

Convert to distance

Draw circles centered on stations

Location = intersection of circles


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Earthquake focal

mechanisms

Earthquake = release ofaccumulated elastic energy bydisplacement on a fault

Problem: what type of fault motion? Case of a strike-slip fault: particle

motion due to fault slip:

Blue quadrants: particles pushedaway from the focus compressional first motion = UP

Red quadrants: particles pulled

towards the focus dilatationalfirst motion = DOWN

As a result, we obtain 4 quadrants:

2 compressional quadrants: firstmotion down

2 extensional quadrants: firstmotion up

dilatationalfirst motion

compressionalfirst motion

fault plane

auxiliary plane

compressional

quadrant

compressional

quadrant

extensional

quadrant

extensional

quadrant

tensionaxis

compressionaxis


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Earthquake focal

mechanisms

Earthquake = release of accumulated

elastic energy by displacement on a

fault

Problem: what type of fault motion?

Lets assume an earthquake on a

reverse fault:

Compressional / tensional quadrants

Compressional quadrant: surface ispulled down first motion DOWN

Tensional quadrant: surface is pushedup first motion UP

If we map first motion, we can find:

2 focal planes

P- and T-axis


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Earthquake focal mechanisms

Seismic rays travel away from the focus

Each ray samples a dilatational or compressional quadrant around the

focus

Seismic stations at different distances record up or down first motions Rays along nodal planes?

in cross-section

focal mechanism

(stereonet proj.)


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Earthquake focal mechanisms The focal sphere:

Center = earthquake hypocenter

In each quadrant: first motion identical

Seismic stations are at the surface,(usually) not underground

Rays bend upward and eventually

reaches a seismic station at the surface

The important parameter is the initial

take-off angle

Take-off angle can be calculated

knowing the earths structure =>

accuracy of focal mechanisms depend

on our knowledge of the Earth structure

(local, regional, global)


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Earthquake focalmechanisms

Strike-slip faulting:

Vertical focal planes Horizontal P-axis and T-axis

Other types of faulting:

Focal planes will have a dip

P-axis and T-axis will have a

dip

For representation: focal sphere

+ stereographic projection of

focal planes and P-T-axis

Beach balls

In the horizontal plane:

The focal sphere:


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Focal mechanisms define

the type of faulting that

occurred during the

earthquake.

The actual fault plane is

ambiguous

Focal mechanisms can

combine these types of

faulting.

Focal mechanisms in an

actively deforming area

contain information about

the strain regime

reverse

normal

strike-slip


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Earthquake focal

mechanisms

Eastern Mediterranean

Earthquake focalmechanism illustrate:

Strike-slip faulting

Reverse faulting

Extensional faulting

Compare with GPSvelocities

(McClusky et al., JGR, 2000)


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Earthquake rupture An earthquake usually breaks a segment of a fault

The rupture does not always reach the surface

The earthquake is followed by aftershocks:

Readjustments on the rupture plane

Help define the rupture plane

Animation http://www.scecdc.scec.org/northreq.html

Northridge earthquake, January 1994, M=7.2


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Earthquake rupture

Time and space history of a rupture, example of the Northridge earthquake

Slip on the rupture plane is not homogeneous

Asperities and barriers

Animation D. Wald, http://www.scecdc.scec.org/northrup.html


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Earthquake size

Shear forces on a faults

moment

Hookes law relates stress

and strain for elasticsolids: for shear,

proportionality factor isrigidity

MO= 2bF

"shear

= #$shear

with $shear

=

d

2band "

shear=

F

A=

F

L #W

% MO= Ad

Moment = rigidity x displacement x rupture area

F

-F

b

Rupture area:A = L x W


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Earthquake magnitude

1935: Richter worked on rankingearthquakes as a function of theirsize

First definition:

Maximum amplitude recordedat 100 km from the epicenter:

For local earthquakes: S-waveshave the largest amplitude

Correction for distance: (=angular epicentral distance indegrees)

Richter magnitude scale:

Open scale

Largest magnitude recorded =Chile, 1960, MW=9.6 (MS=8.3)

Negative magnitudes arepossible

ML=log10(Amax) + 3 log10- 2.92

nomogram used to compute magnitude quickly by eye


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ML = local magnitudes (~ 600 km from earthquake)

At larger distances: Using surface waves (they have the largest amplitude)

A=max. amplitude of vertical component in microns, T = period inseconds, D = angular distance in degrees.

Using body-waves (P-waves)

Ms mb relationship:

MS= log10(Amax/T)+1.66 log10+ 3.3

mb = log10(Amax/T)+0.01 + 5.9

mb = 0.56 MS+ 2.9


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2/TR 2/TD

Spectrum of seismogram gives spectral

amplitude at all frequencies

Static moment = amplitude at low

frequencies

Corner frequency depends on duration

of rupture time TD and and rise time TR

Above corner frequency

there is destructive interference

Shaking cannot get higher amplitude but

continues in time longer

As a result:

Ms saturates at 8.3

mb saturates at 6.2

Use of moment magnitude:

MW= (2/3) log10MO 10.7


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Less than 3.5: Generally not felt, but recorded.

3.5-5.4: Often felt, but rarely causes damage.

Under 6.0: At most slight damage to well-designedbuildings. Can cause major damage to poorlyconstructed buildings over small regions.

6.1-6.9: Can be destructive in areas up to about 100kilometers across where people live.

7.0-7.9: Major earthquake. Can cause serious damageover larger areas.

8 or greater: Great earthquake. Can cause seriousdamage in areas several hundred kilometers across.


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Earthquake size Logarithmic relationship between magnitude and:

Coseismic displacement: M5=1 cm, M8=10m

Rupture length: M5=1 km, M8=400 km

Large earthquakes have a MUCH LARGER rupture displacement andlength than smaller ones


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Energy release

Energy release:

Increase of one level of

magnitude corresponds to:

Amplitude increase: 101 = 10

Energy increase: 101.5

30 Energy release increases very

rapidly with magnitude

log10 E= 4.4 +1.5MS


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Largest Earthquakes in the World Since 1900

1. Chile - 1960 05 22 - 9.5 (Ms = 8.5)

2. Prince William Sound, Alaska - 1964 03 28 - 9.2 (Ms = 8.3)

3. Off the West Coast of Northern Sumatra - 2004 12 26 - 9.0

4. Kamchatka - 1952 11 04 - 9.05. Off the Coast of Ecuador - 1906 01 31 - 8.8

6. Northern Sumatra, Indonesia - 2005 03 28 - 8.7

7. Rat Islands, Alaska - 1965 02 04 - 8.7

8. Andreanof Islands, Alaska - 1957 03 09 - 8.6

9. Assam - Tibet - 1950 08 15 - 8.6

10. Kuril Islands - 1963 10 13 - 8.5

11. Banda Sea, Indonesia - 1938 02 01 - 8.5

12. Chile-Argentina Border - 1922 11 11 - 8.5

Visit: http://neic.usgs.gov/


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Earthquake

frequency

There are far more small earthquakesthan large ones

Many small earthquakes are not

detected Gutenberg-Richter law:

Linear relationship betweenlog[number of earthquakes] andmagnitude:

LogN = a b x M

Slope = b-value

Worldwide average is 1.0 May vary regionally

Lab. experiments show: High stress low b (less small eqs)

Low stress high b (more small eqs)

Empirical tool for seismic hazardstudies


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Earthquake

frequency

The USGS estimates that several

million earthquakes occur in the

world each year. Many goundetected because they hit

remote areas or have very small

magnitudes. The NEIC now

locates about 50 earthquakes

each day, or about 20,000 a year.

Earthquake information:

http://neic.usgs.gov/

Descriptor Magnitude Average Annually

Great 8 and higher 1

Major 7 - 7.9 18

Strong 6 - 6.9 120

Moderate 5 - 5.9 800

Light 4 - 4.9 6,200 (estimated)

Minor 3 - 3.9 49,000 (estimated)

Very Minor < 3.0Magnitude 2 - 3: about 1,000 per day

Magnitude 1 - 2: about 8,000 per day

Frequency of occurrence of earthquakes based on observations since 1900


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Intensity Qualitative description of earthquake size Based on damage assessment Mercalli scale

Can be severely biased

Area with local amplification of seismic waves or secondary effects such as liquefaction

Subjective reports from people

Depends on vulnerability

Often the only information available for historical earthquakes


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Seismic hazard Earthquake damage:

Ground acceleration, ing(up to 2g)

Secondary effects: liquefaction, landslides, fires, etc

Seismic risk= seismic hazard vulnerability

Seismic hazard = seismic potential (When? Where? Whatsize?) propagation of seismic waves Seismic potential = probability for an earthquake of a given size

to occur

Propagation = attenuation of seismic waves, site response

Seismic hazard = probability to exceed a givenacceleration for a given time period


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Seismic potential

Derived from Gutemberg-Richter law, tailored forthe are under study

Requires earthquakecatalogb value

Can be complemented byinformation on activefault: geometry, slip rate

Can be complemented bygeodetic information:strain rate


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Site response Ground acceleration decreases

with distance, but can vary by a

factor of 10 for 2 sites at the

same distance to an earthquake site response

Site response depends on

geological factors:

Softness of soil or rocks near

the surface: ground motion

amplified by soft rocks

Sediment thickness above

bedrock: ground motion

amplified by thick sediments

Snapshots of simulated wave propagation in the LA area for the

hyp othetical SAF earthquake (K. Olsen, UCSB)


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Final result: seismic hazard maps

Peak acceleration that has a 2% probability to be exceeded in 50 years

Compare New Madrid and California!


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What have we learned? Active faults are (usually) locked between earthquakes, while the area

around them is accumulating elastic strain.

An earthquake is the sudden release of the elastic strain accumulatedover decades.

The earthquake results in:

A rupture, that may sometimes reach the surface Seismic waves, that propagate away from the rupture area

Using seismic wave, one can figure out:

The location of the earthquake

The type of fault motion (focal mechanism)

The magnitude of the event (energy released)

The slip distribution on the rupture plane

Magnitude scale:

Is not linear but power law

Gutemberg-Richter law:N = a b M

Earthquake hazard depends on source, attenuation, and site response


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What have we learned?

One can use seismic waves generated artificially to image

deep structures:

Seismic reflection:

Receiver and source close

Arrivals describe hyperbolas

Seismic refraction:

Receiver and source far apart

Arrivals describe straight lines

Data collection, processing (increase SNR and remove

artefacts), interpretation

Applications: oil exploration, sequence stratigraphy, etc.

Documents

1 Seismology 4