Characteristic and Uncharacteristic Earthquakes as Possible Artifacts: Application to the New Madrid and Wabash Seismic Zones

1
EARTHQUAKE RECURRENCE Crucial for hazards,
earthquake physics tectonics (seismic versus
aseismic deformation)
Recordings of the east-west component of motion
made by Galitzin instruments at DeBilt, the
Netherlands. Recordings from the 1922 earthquake
(shown in black) and the 1934 and 1966 events at
Parkfield (shown in red) are strikingly similar,
suggesting virtually identical ruptures.
2
EARTHQUAKE FREQUENCY - MAGNITUDE LOG-LINEAR
Gutenberg-Richter RELATION
3
LEVEL OF ACTIVITY (a value) VARIES REGIONALLY
BUT b 1
4
CHALLENGE INFER UNKNOWN RATE OF LARGEST
EARTHQUAKES FROM RECORDED RATE OF SMALLER
ONESUse standard log-linear Gutenberg-Richter
relationship
Global Earthquakes
Continental Intraplate
Number per year
Triep Sykes, 1997
Stein Wysession, 2003
Magnitude (Ms)
Magnitude (Ms)
With seismological data only, log-linear
relation breaks down Largest earthquakes (M gt
7-7.5) less frequent than expected, presumably
due to fault finiteness (large event lengths gtgt
width)
5
Romanowicz, 1992
Most earthquakes between solid lines with slope
1/3, showing M0 proportional to L3. However,
strike-slip earthquakes (solid diamonds) have
moments higher than expected for their fault
lengths, because above a certain moment fault
width reaches maximum, so fault grows only in
length.
6
MOMENTS HAVE SIMILAR CURVE TO MAGNITUDES
7
Total global seismic moment release dominated by
few largest events Total moment for 1976-1998
1/3 that of giant 1960 Chilean earthquake
8
AFTERSHOCKS FOLLOWING MAINSHOCK HAVE A
CHARACTERISTIC DECAY IN SIZE AND TIME
Largest aftershock is usually more than a
magnitude unit smaller than the main shock, and
the aftershocks have a size distribution with b
1, so the total energy released by aftershocks
is usually lt10 of that of the main shock.
9
GUTENBERG-RICHTER RELATIONSHIP INDIVIDUAL FAULTS
Wasatch
Basel, Switzerland
instrumental data
historical data
Uncharacteristic
Characteristic
paleoseismic data
paleoseismic data
Meghraoui et al., 2001
Youngs Coppersmith, 1985
Largest events deviate in either direction, often
when different data mismatch When more frequent
than expected termed characteristic earthquakes.
Alternative are uncharacteristic
earthquakes Could these differences - at least in
some cases - be artifacts?
10
EARTHQUAKE RECURRENCE IS HIGHLY VARIABLE
Sieh et al., 1989
Extend earthquake history with paleoseismology
Mgt7 mean 132 yr s 105 yr Estimated
probability in 30 yrs 7-51
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ESTIMATING EARTHQUAKE PROBABILITIES A game of
chance, with unknown rules, and very little data
from which to infer them
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CHALLENGE DONT KNOW WHAT PROBABILITY
DISTRIBUTION DESCRIBES EARTHQUAKE RECURRENCE TIMES
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POISSON DISTRIBUTION TIME INDEPENDENT MODEL OF
EARTHQUAKE PROBABILITY Used to describe rare
events include volcanic eruptions, radioactive
decay, and number of Prussian soldiers killed by
their horses
14
TIME INDEPENDENT VERSUS TIME DEPENDENT MODEL
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GAUSSIAN DISTRIBUTION TIME DEPENDENT MODEL OF
EARTHQUAKE PROBABILITY Probability of large
earthquake a time t after the past one is p(t,
?, ? ) Depends on average and variability of
recurrence times, described by the mean ? and
standard deviation ? p is probability that
recurrence time for this earthquake will be t,
given an assumed distribution of recurrence times.
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CONDITIONAL PROBABILITY Use the fact that we
know the next earthquake hasnt already happened
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SAN ANDREAS FAULT PALLETT CREEK SEGMENT Gaussian
(time dependent) model
Gaussian
In 1983, estimate 9 probability by 2003,
increases with time
18
SAN ANDREAS FAULT PALLETT CREEK SEGMENT Poisson
(time independent) model
Gaussian
In 1983, estimate 10 probability by 2003,
constant with time
19
SYNTHETIC EARTHQUAKE HISTORIES Gaussian model
yields more periodic series Poisson model yields
clustering
Which looks more like earthquake history?
20
SEISMIC GAP MODEL Long plate boundary like the
San Andreas or an oceanic trench ruptures in
segments Expect steady plate motion to cause
earthquakes that fill in gaps that have not
ruptured for a long time Gap exists when it has
been long enough since the last major earthquake
that time-dependent models predict earthquake
probability much higher than expected from
time-independent models Sounds sensible but
seems not to work well, for unknown reasons
GAP?
NOTHING YET
21
EARTHQUAKE FORECASTS EASY TO MAKE, HARD TO TEST
Hard to prove right or wrong Because the
estimates must be tested using data that were
not used to derive them, hundreds or thousands
of years (multiple recurrences) will be needed to
assess how well various models predict large
earthquakes on specific faults or fault segments.
The first challenge is to show that a model
predicts future earthquakes significantly better
than the simple time-independent Poissonian
model Given human impatience, attempts have been
made to conduct alternative tests using smaller
earthquakes or many faults over a short time
interval. To date, results are not encouraging.


22
RECENT SEISMICITY MAY NOT REFLECT
LONG-TERM PATTERN WELL
Random seismicity simulation for fault along
which probability of earthquake is
uniform Apparent seismic gaps develop May take
long time to fill compared to length of
earthquake record
Stein Wysession, 2003
23
PARKFIELD, CALIFORNIA SEGMENT OF SAN ANDREAS
Characterized by smaller earthquakes that occur
more frequently and appear much more periodic
than other segments. Earthquakes of M 5-6
occurred in 1857, 1881, 1901, 1922, 1934, and
1966. Average recurrence is 22 yr linear fit
made 1988 likely date of the next event. In
1985, predicted at 95 confidence level that the
next earthquake would occur by 1993 Actually
didnt occur till 2004 (16 years
late) Problems Limitations of statistical
approach in prediction (including omission of
1934 earthquake on the grounds that was premature
and should have occurred in 1944) Unclear
whether Parkfield shows such unusual
quasi-periodicity because it differs from other
parts of San Andreas (in which case predicting
earthquakes there might not be that helpful for
others), or results simply from the fact that
given enough time fault segments, random
seismicity can yield apparent periodicity
somewhere


24
GLOBAL TEST OF SEISMIC GAP HYPOTHESIS Gap map
forecasting locations of major earthquakes did
no better than random guessing. Many more large
earthquakes occurred in areas identified as low
risk than in presumed higher-risk gaps (reverse
colors?) Result appears inconsistent with ideas
of earthquake cycles and seismic gaps
Within 10 years of prediction, 10 large events
occurred in these areas. None were in high- or
intermediate-risk areas 5 were in low-risk areas.
Kagan Jackson, 1991
25
EARTHQUAKE PROBABILITY MAPS
Hard to assess utility of such maps for many
years Major uncertainties involved Perhaps only
meaningful to quote probabilities in broad
ranges, such as low (lt10), intermediate
(10-90), or high (gt90).
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MITIGATING EARTHQUAKE RISK
Useful to distinguish between hazards and risks
for earthquakes or other natural
disasters. Hazard is natural occurrence of
earthquakes and the resulting ground motion and
other effects. Risk is the danger the hazard
poses to life and property. Hazard is
unavoidable geological fact, risk is affected by
human actions. Areas of high hazard can have low
risk because few people live there, and areas of
modest hazard can have high risk due to large
populations and poor construction. Earthquake
risks can be reduced by human actions, whereas
hazards cannot.
27
In US, earthquake risk is primarily to property,
though there are deaths Property loss can be
high 20 billion damage from the Northridge
earthquake Some other countries have much
greater risk The most destructive earthquakes
occur where large populations live near plate
boundaries. Highest property losses occur in
developed nations where more property is at
risk Fatalities are highest in developing
nations.
28
PROBABILISTIC SEISMIC HAZARD ASSESSMENT
(PSHA) Seek to quantify risk in terms of
maximum expected acceleration in some time period
(2 or 10 in 50 yr, or once in 2500 or 500
yr) Maps made by assuming Where and how often
earthquakes will occur How large they will
be How much ground motion they will
produce Because these factors are not well
understood, especially on slow moving boundaries
or intraplate regions where large earthquakes
are rare, hazard estimates have considerable
uncertainties and it will be a long time before
we know how well theyve done A game of chance
of which we still don't know all the rules"


29
STRONG GROUND MOTION DECAYS RAPIDLY WITH DISTANCE
30
EARTHQUAKE-RESISTANT CONSTRUCTION REDUCES
EARTHQUAKE RISKS
0.2 g Damage onset for modern buildings
Earthquakes don't kill people buildings kill
people."
31
10 EXCEEDENCE PROBABILITY (90 NON
EXCEEDENCE) WITHIN 50 YEARS


Jimenez, Giardini, Grünthal (2003)
32
SHORT RECORD OF SEISMICITY HAZARD ESTIMATE
Africa-Eurasia convergence rate varies smoothly
NUVEL-1 Argus et al., 1989
Predicted hazard from historic seismicity is
highly variable Likely overestimated near recent
earthquakes, underestimated elsewhere More
uniform hazard seems more plausible - or opposite
if time dependence considered Map changes after
major earthquakes
GSHAP
33
SHORT RECORD OF SEISMICITY HAZARD ESTIMATE
Africa-Eurasia convergence rate varies smoothly
NUVEL-1 Argus et al., 1989
Predicted hazard from historic seismicity is
highly variable Likely overestimated near recent
earthquakes, underestimated elsewhere More
uniform hazard seems more plausible - or opposite
if time dependence considered Map changes after
major earthquakes
2003
2004
GSHAP 1998
34
EARTHQUAKE PREDICTION? Because little is known
about the fundamental physics of faulting, many
attempts to predict earthquakes searched for
precursors, observable behavior that precedes
earthquakes. To date, search has proved
generally unsuccessful In one hypothesis, all
earthquakes start off as tiny earthquakes, which
happen frequently, but only a few cascade via
random failure process into large
earthquakes This hypothesis draws on ideas from
nonlinear dynamics or chaos theory, in which
small perturbations can grow to have
unpredictable large consequences. These ideas
were posed in terms of the possibility that the
flap of a butterfly's wings in Brazil might set
off a tornado in Texas, or in general that
minuscule disturbances do not affect the overall
frequency of storms but can modify when they
occur If so, there is nothing special about
those tiny earthquakes that happen to grow into
large ones, the interval between large
earthquakes is highly variable and no observable
precursors should occur before them. Thus
earthquake prediction is either impossible or
nearly so. Its hard to predict earthquakes,
especially before they happen