the earthquake cycle - appalachian state · pdf file• the earthquake cycle is split into...

The Earthquake CycleChapter :: n/a

Image courtesy of San Francisco Public Library

A German seismogram of the 1906 SF EQ

Stages of the Earthquake Cycle• The Earthquake cycle is split into several distinct phases /

stages based on the deformation observed:– Interseismic

• The time between large earthquakes

– Preseismic• The time just before an earthquake when anomalous things happen

– Coseismic• The time during an earthquake

– Postseismic• The time after a large earthquake when anomalous deformation occurs.

• The preseismic phase has proven elusive and inconsistent– It may not even exist!

• The other three phases are commonly observed– Postseismic involves complex math !

• We will only briefly discuss this stage

The Revolution :: Elastic Rebound• After the Mw7.9 1906 SF EQ, H.F. Reid proposed that

– Earthquakes represent rapid release of strain/stress built up over a long period of time (hundreds of years)

– Called elastic rebound theory

– Confirmed by• Geodetic measurements of surface

motion (triangulation)• Geologic measurements of offset

– 450 km long rupture (360 km on land)– Average slip 4.5 m

– Reid postulated: Pacific Ocean floor must be spreading, pushing the west side of the SAF to the NW.

– He recommended a monitoring program

– Not adopted until 60 yrs later

Reid’s Evidence for Elastic Rebound

Farallon Lighthouse

Duxbury Point, Bolinas Beach

Before the 1906 Earthquake…

Farallon Lighthouse


• Locations far from the fault were moving fast

• Locations near the fault were moving slow

• Same was true on other side of the fault, but motions were in the opposite direction

During the 1906 Earthquake…

Farallon Lighthouse


• Locations near the fault were displaced very far

• Locations far from the fault were displaced very little

• Same was true on other side of the fault, but motions were in the opposite direction

Reid’s Hypothesis :: Elastic Rebound Theory• Although plate tectonics theory was ~50+ years from being

developed, Reid’s hypothesis is consistent with plate tectonics

+ =

Interseismic Coseismic Long-Term Block Offset

Elastic Strain is localized near fault

Elastic strain is released

After the EQ, elastic strain has been released

• Elastic rebound is also consistentwith geologic observations!

The Earthquake Cycle: Graphical Form• Reid proposed:

– Interseismic strain accumulates slowly and is eventually released in an EQ

– The coseismic strain release = total accumulated interseismic strain

• The net result:– Block offsets over geologic

timescales

• He made the prediction that the next EQ would happen when the same amount of interseismic strain had accumulated– Called a time-predictable

model– Turned out to be unreliable

Interseismic

Coseismic

Long-Term / Geologic

What is Happening During the EQ Cycle?

• Interseismic– Deep, steady, &

slow aseismic slip (i.e. creep)

• Coseismic– Rapid shallow slip

xb

y

x

Conventional Interseismic Model• Semi-infinite vertical

dislocation embedded in an elastic earth.– Semi-infinite height– Infinite length– ux = displacement of

ground around the fault– x = distance from fault– b = fault slip rate– D = locking depth

This is an analytical model based on mathematics developed by the engineering community

Displacement• Displacement = u - uo

– Final position – initial position

– Measured anywhere in a medium• Applies to the motion of a single particle

– A vector quantity (has a magnitude and direction)

– Difficult to measure in the geologic record• Don’t know initial position, only know final position

Initial position = u0

final position = u

Slip• Slip = u+ + u-

– A.k.a: Offset / Displacement Discontinuity / Burgers Vector

– Displacements are discontinuous across a fault• This is why geophysicists refer to faults as discontinuities or dislocations

– Slip is the sum of the displacements on both sides of a fault• A vector quantity (recall that the slip vector has a “rake”)

– Applies to the relative motion across a fault• So it is only measured across faults!

Slip

• So, slip measures the distance along a fault surface between two points that used to be connected

Fault

Offset Feature

Slip vs. Displacement

• Can’t determine displacements unless you know the original position

– In geology, you almost never know the original position

– In geophysics, you sometimes know the original position (GPS)

u+ = 1

u- = 0

u+ = 0.5

u- = 0.5

slip = 1

slip = 1

• It is the sum of the displacements on both sides of a fault (i.e. the slip) that matters when considering earthquakes

• Coseismic ruptures commonly– Are longer than they are deep

• Can be approximated by a rectangle

• If surface ruptures…– Can be measured by geologists

• If no surface rupture…– Rupture can be mapped by

aftershocks

– Rupture can be estimated by surface deformation models

– Can also be determined by analyzing seismic wave patterns

Fault Trace

Offset road from the Mw7.1 1999 Hector Mine EQ

Coseismic Rupture Dimensions

Let’s Trench!

slip

The 1966 Parkfield EQ

brittle-ductile transition

Controls on Rupture Dimensions• Recall the two main layers of the Earth:

– Lithosphere: Brittle Rocks

– Asthenosphere: Ductile RocksBrittle-Ductile Transition

• Earthquakes only occur in the lithosphere

• Heat flow / geothermal gradient controls the level of the brittle ductile transition– Hot rocks: ductile– Cold rocks: brittle

• Subduction zones have greatest potential rupture width (depth)

• Mid ocean ridges have smallest potential rupture width

0

4

12

8

0 8006004002000

20

40

Dep

th (k

m)

Pres

sure

(Kba

r)

Temperature (oC)

Largest EQ’s: Subduction

• The 3 largest earthquakes recorded:

– MW9.2 1964 Good Friday EQ, Anchorage, Alaska

– MW9.1-9.3 2004 Great Sumatra EQ

• 20 m maximum slip!!

• 1200 km long rupture!

– MW9.5 1960 Chile EQ

Depth

Into theearth

Surface of the earth

Distance along the fault plane100 km (60 miles)

Slip on an earthquake fault

START

Slip on an earthquake faultSecond 2.0

Total Slip in the M7.3 Landers Earthquake

Rupture on a Fault

Quantifying Earthquake Size• There are two basic ways to quantify the size of an

earthquake.

– Intensity• Measures the amount of shaking at a given location• Depends on location

– i.e. a given earthquake will have lots of different intensities

– Magnitude• Measures the amount of energy released at the source• Does not depend on location

– A given earthquake will just have one magnitude (on each scale)

Haiti Photo Courtesy: UN Photo/Logan Abassi United Nations Development Programme

Intensity• Measured on the Modified Mercalli Scale (1931)

– Twelve categories

– Denoted by Roman numerals

– Plotted as isoseismals: zones of same intensity

– Intensity in general decreases away from epicenter, but local geology can completely control intensity in some cases…

(only measured by instruments)

• Magnitude of the earthquake• Distance from hypocenter• The nature of the substrate at location

– Stiff bedrock shakes less– Soft rock shakes a lot– Sedimentary basins can amplify waves

• E.g. 1985 Mexico city MW8.0 > 350 km away

• The frequency of the seismic waves– High frequency waves do most damage but do

not travel very far (i.e. they attenuate)• Car stereo analogy (bass)

– In general…– Long ruptures generate long wavelengths (low

frequencies)– Short ruptures generate short wavelengths

(high frequencies)

Severity of Shaking Depends On:

Mexico city

Buildings - Mexico City, 1985

Before

After

[TerraShake Animations]

• Thousands of buildings destroyed• Prompted Mexico to develop building

codes

Magnitude• Magnitude = A measure of the

amount of energy released at the source of the EQ.

• Richter Scale: A type of magnitude measurement coined by Charles Richter in 1935.– ML = log10 (max amplitude of S-waves in

units of 10-6 m)

– Used a logarithmic scale to make the wide range of measurements easy to deal with

• A change of one in Richter magnitude = 10x the ground motion and 30x the energy.

– Also called the “local magnitude”• Based on measurements of S-wave

amplitudes at 100 km from epicenter

• Can be effectively “corrected” for seismometers at different distances Photos of Charles Richter (1900-1985) courtesy of USGS

The Richter Nomogram• How seismogram

readings are made into ML

• Can have negative magnitude

• No mathematical upper limit on magnitude

– i.e. 10 is not max

Richter’s ups and Downs• Richter scale advantages:

– First quantitative measure of energy release– Can be computed minutes after an EQ– Good for nearby, shallow, and moderate EQ’s

• The Richter scale shortcomings:– At epicentral distances > 600 km, surface waves have greater amplitude

than S-waves• ML underestimates distant events• Instead, we use MS, “surface wave magnitude”, which is based on the amplitude of

surface waves (R-waves)

– Underestimates deep earthquakes (S-waves attenuate faster than P-waves)• Instead we use mb, “body wave magnitude”, for deep events.• Uses the maximum amplitude of either body wave.

• All of these underestimate very large EQ’s– We now use “Moment Magnitude”, MW = 2/3 log10 M0 - 10.7– M0 is the “Seismic Moment”

Background: seismogram from MW9.2 1964 Alaska EQ, courtesy USGS

Seismic Moment• Seismic moment, M0, is mathematically based on the

torque exerted by the shear stress couple (i.e. the deformation on both sides of a fault)

• M0 = μAd– μ = shear modulus– A = fault rupture area– d = average slip during earthquake

• μ does not greatly vary for different rock types at depth– Typically ~ 30 GPa

• So, A, and d are what matter– But what controls A and d?

Seismic Moment and the Sizes of Ruptures• Small EQ’s have small rupture areas and

small average slip

• Slip is much smaller than rupture length

• Due to finite fault width (brittle-ductile transition), small earthquakes follow different scaling

• Where would subduction EQ’s plot below?

Bigger (longer) Faults Make Bigger Earthquakes

1

10

100

1000

5.5 6 6.5 7 7.5Magnitude

Kilo

met

ers

8

Bigger Earthquakes Last Longer

1

10

100

5.5 6 6.5 7 7.5 8

Magnitude

Seco

nds

Earthquake Prediction (?)• Currently scientists can’t make short term predictions of earthquakes

– e.g. there will be an earthquake next Tuesday at 8:07 AM.

• We can make some long term predictions– There will very likely be a large earthquake on the San Andreas fault in the next

hundred years.– In the next hundred years it is unlikely that there will be a large earthquake in

central Canada

• Seismic Hazard– Is there a seismic source?

• Seismic Risk– What sort of risk does this source pose to civilization?– E.g. no people no risk

• Seismic Hazard Assessments are based on:– Locations of faults– Slip rates of faults– Recurrence intervals (time between events)– Local geology effects (liquefaction / basin fill)– Seismic gaps

Building Codes• In response to the 1971 M6.6 Sylmar EQ, the state of California passed new

laws prohibiting the building of public buildings within ¼ mile of an active fault zone (private houses within 50 feet)– Called “Alquist-Priolo Earthquake Fault Zones ”

• Since short term earthquake predictions may be impossible, building codes are the main way to save lives in future earthquake events

• Building codes (zoning laws) are based on seismic hazard assessments• Insurance companies also are very interested in seismic hazard maps

Seismic Gaps & The North Anatolian Fault, Turkey• Seismic Gaps: Areas where the fault has not moved in a long time

– These regions may be the next to go

• Stress Triggering: When an earthquake happens, the motion changes the stress on nearby faults, possibly making them more or less likely to fail.– The North Anatolian Fault is an excellent example of both of these phenomenon

(Ross Stein animations)

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