5. How earthquakes work

Can we predict them?

Effects visible today from M 7.3 1959 Hebgen Lake, Montana earthquake

SAN FRANCISCO EARTHQUAKEApril 18, 1906

“The whole street was undulating as if the waves of the ocean were coming toward me.”

“I saw the whole city enveloped in a pile of dust caused by falling buildings.”

“Inside of twelve hours half the heart of the city was gone”

3000 deaths28,000 buildings destroyed

(most by fire)$10B damage

Motion along ~ 500 km of previously

unrecognized San Andreas Fault

~ 4 m of ground motion West side moved north

DD 6.2

Over many years, rocks on opposite sides of the fault move, but friction on fault "locks" it and prevents slip

Eventually strain accumulated overcomes friction, and fault slips in earthquake

ELASTIC REBOUND

Took 60 years to figure out why this happens!

DD 6.3

EARTH’S OUTER SHELL - PLATES

Plates move at few cm/yr (speed fingernails grow)

San Andreas fault: boundary between Pacific & North American plates

DD 9.7

1906 Commission insights for policySan Francisco’s city government and boosters stressed that most damage came from fire, because if businesses thought another earthquake would happen soon, they wouldn’t invest.

Elastic rebound showed that strain on the fault built up slowly over many years. Because the fault had just slipped in the earthquake, it wouldn’t slip again for a long time. Thus investors could buy bonds knowing that no similar earthquake would happen on a time scale that mattered to them.

Buildings needed to be built carefully, but there was no need to rush because it would be a long time before another large earthquake. Because buildings on weak soil or landfill suffered much greater damage than ones on solid rock, they recommended “studying carefully the site of proposed costly public buildings.”

1906 Commission insights for recurrence

Elastic rebound gave a method to tell whether and when another big earthquake would happen. The twelve feet that the fault had moved in a few seconds had accumulated over many years. Thus measuring the slow motion across the fault could tell how long it would be until another earthquake that big could happen.

The commission recommended that geodetic measurements be made to measure the motion across the fault over time as strain built up before a future earthquake. They suggested that

"we should build a line of piers, say a kilometer apart" across the fault so "careful determination from time to time of the directions of the lines joining successive piers ... would reveal any strains which might be developing."

Strain accumulated in rubber band applies force (shear stress) to block

Block slips when shear stress overcomes friction due to weight of block (normal stress)

GPS: GLOBAL POSITIONING SYSTEM

Satellites transmit radio signals

Receivers on ground record signals and find

their position from the time the

signals arrive

Find mm/yr motions from changes in

position over time

DD 12.1

GPS receiver finds position with radio

signals from different satellites

Conceptually like locating earthquakes

GPS positions 2-3 times more precise in

the horizontal than vertical, because

signals arrive only from above.

Davidson et al, 2002

Activity 5.1: GPS clock

To locate something to a meter, assess how accurate the clock must be

What’s the speed of light?

How long does it take to travel a meter ?

“Any sufficiently advanced technology is indistinguishable from magic”

Arthur C. Clarke

GPS uses very precise atomic clocks.

Synchronizing satellite clocks within nanoseconds (billionths of a second) lets a receiver find its

position on earth within a few meters.

Atomic clocks use the fact that atomic transitions have characteristic frequencies

(orange glow from sodium in table salt sprinkled on a flame).

Like all clocks, they make the same event happen over and over. For example, the pendulum in a grandfather clock swings back and forth at the same rate, and swings of the pendulum are

counted to keep time.

In a cesium clock, transitions of the cesium atom as it moves back and forth between two energy

levels are counted to keep time.

GPS CLOCK

Since 1967, the International System of Units (SI) defines the second as the

duration of 9 192 631 770 cycles

of the radiation produced by transition between two energy levels of the ground state of the cesium-

133 atom

Satellites transmit code - timing signals - on two microwave carrier frequencies synchronized to very precise on-board

atomic clocks.

Carrier has much higher frequency than code

METER PRECISION GPS

GPS receiver compares code signal arriving from satellite to one it

generates and finds time difference

Precision of several meters

Time difference x speed of light gives the distance

called pseudo range

$450 Handheld GPS unit with MP3 player

MILLIMETER PRECISION GPS

Carrier wavelengths are 19 and 24 cmPhase measurements resolve positions

to a fraction of these wavelengthsMeasuring phase to 1% gives 1-2 mm

precision

Higher precision obtained using phase of microwave carriers

Geodetic quality receivers cost about $10,000

SURVEY (EPISODIC) GPS

GPS antennas are set up over monuments for short periods, and

the sites are reoccupied later. Less expensive but less precise.

GPS = Great Places to Sleep

CONTINUOUS (PERMANENT) GPSContinuously recording GPS

receivers permanently installed. Give daily positions & can observe

transient effects

GPS VELOCITY ESTIMATESPrecision of velocity estimatesdepends on precision of site position & length of time

Velocity from a weighted least squares line fit to positions

Precision increases over time

Horizontal precision is better

Sella et al., 2002

Z.-K. Shen

Activity 5.2: GPS across the San Andreas

GPS site motions show deformation accumulating that will be released in future earthquakes

Like a deformed fence

How fast is the motion?

Activity 5.3: Longer term slip rate and earthquake recurrence on the

San Andreas Fault

San Andreas

Fault

Wallace Creek is offset by 130 m

This offset developed over 3700 years

What’s the average fault slip rate?

If this happens in large earthquakes with about 4 m slip, how often on average should they occur? DD 9.8

Activity 5.4: Time between earthquakes from paleoseismic record

What’s the mean and standard deviation of the time between large earthquakes? If the last was in 1857, when would you

expect the next one?

DD 9.8

Activity 5.5: Why no earthquake yet?

What might the problems be?What does this suggest about predicting

earthquakes?

“We are predicting another massive earthquake certainly within the next 30 years and most likely in the next decade or so.” W. Pecora, U.S. Geological Survey Director, 1969

1970’s optimism

Scientists will “be able to predict earthquakes in five years.”Louis Pakiser , U.S. Geological Survey, 1971

“We have the technology to develop a reliable prediction system already in hand.”Alan Cranston, U.S. senator, 1973

“The age of earthquake prediction is upon us”U.S. Geological Survey, 1975

Traditional skepticism

“Only fools and charlatans predict earthquakes” Charles Richter (1900-1985)

PARKFIELD, CALIFORNIA SEGMENT OF SAN ANDREAS

In 1985, expected next in 1988; U.S. Geological Survey predicted 95% confidence by 1993Occurred in 2004 (16 years late)

M 5-6 earthquakes about every 22 years: 1857, 1881, 1901, 1922, 1934, and 1966

Discounting misfit of

1934 quake predicted

higher confidence

Science, 10/8//04

"Parkfield is geophysics' Waterloo. If the earthquake comes without

warnings of any kind, earthquakes are unpredictable and science is

defeated. " (The Economist)

No precursors in seismicity (foreshocks), strainmeters,

magnetometers, GPS, creepmeter

$30 million spent on “Porkfield” project

So far, no clear evidence for consistent changes in physical properties (precursors) before earthquakes.

Maybe lots of tiny earthquakes happen frequently, but only a few grow by random process to large earthquakes

In chaos theory, small perturbations can have unpredictable large effects - flap of a butterfly's wings in Brazil might set off a tornado in

Texas

If there’s nothing special about the tiny earthquakes that happen togrow into large ones, the time between large earthquakes is highly

variable and nothing observable should occur before them.

If so, earthquake prediction is either impossible or nearly so.

WHY CAN’T WE PREDICT EARTHQUAKES?

Starting valuesSeries 1: 0.750Series 2: 0.749

A big earthquake can change stress on nearby faults, and make it easier or harder for them to move in a big earthquake. That's called "unclamping" or "clamping" a fault.

Think back to the soap bar and yoga mat. Increasing the tension in the rubber band or reducing the weight of the soap bar unclamps the soap and makes slip easier. Conversely, reducing tension in the rubber band or putting weight on the soap clamps it, making slip harder.

Deterministic contributor - stress changes

Stress transfer can cause earthquakes to migrate along a plate boundary. The best example is in Turkey, where since 1939 large earthquakes have occurred successively further to the west along the North Anatolian fault.

1906 San Francisco earthquake may have reduced failure stress on other faults in the area, causing a "stress shadow" and increasing the expected time until the next earthquake on these faults.

This is consistent with the observation that in 75 years before the 1906 earthquake, the area had 14 earthquakes with Mw > 6, whereas only three occurred since

USGS

SAF

Stress change

Status today

Meaningful prediction involves specifying the location, time, & size of an earthquake before it occurs

Long-term forecast: some success

- Use earthquake history to predict next one

- Use rate of motion accumulating across fault and amount of slip in past earthquakes

Short-term prediction: -Find precursors - changes in earth before earthquakes consistently resolvable from normal variability

Despite some claims, no reliable method yet…

Most claim of success to date have been postdictions

Texas sharp shooter Shoot at barn and then draw target around bullet holes