Download Seismic Hazard Forecasting

Seismic Hazard
Forecasting
Andrew B. Lockman
Skylar L. Primm
6 April 2004
General Outline
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Seismic Hazards in Southern California:
Probably Earthquakes, 1994 to 2024
Prospects for Larger or More Frequent
Earthquakes in the Los Angeles
Metropolitan Region
Bigger Jolts Are on the Way for Southern
California
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(State of California, 2003)
Probability Modeling
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Steps:
1. Identify Faults
2. Determine Characteristic Magnitudes and
Recurrence Intervals
3. Develop Probabilistic Shake Map
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Sometimes, this is very straightforward
More often, complicated analysis is
necessary
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Identifying Faults
(WGCEP, 1995)
Identifying Faults
(WGCEP, 1995)
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Identifying Faults
(WGCEP, 1995)
Type A Fault Zones
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Characteristic Magnitude
M = µ⋅ H ⋅ L⋅ D
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M = seismic moment (N*m)
µ = crustal rigidity (3*1010 N*m-2)
H = brittle crustal thickness (11,000 m)
L = fault segment length (m)
D = characteristic displacement (m)
Then: m = 2 3⋅ log10 M − 6
m = moment magnitude
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Recurrence Interval
T = D /V
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T = time between successive earthquakes (yr)
D = displacement in earlier earthquake (m)
V = long-term slip rate (m/yr)
Note: possibility of cascading ruptures means that
rupture rates are not necessarily the same as
earthquake rates
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Type B & C Fault Zones
Probabilistic Methods
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Segmentation, displacement, and
paleoseismicity are not known, so the
straightforward mathematical methods are
not applicable
Relevant Data:
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Historical Earthquakes
Regional Strain Accumulation (Geodetics)
Geological Slip Estimates
Surface/Blind Thrusts
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Historical Earthquakes
Record goes back as far as 1852
 Most magnitudes prior to 1932 are uncertain,
but may be approximated using the Mercalli
Intensity Scale and historical records
 Future earthquakes are more likely to occur
at historical earthquake sites
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Historical Earthquakes
(http://www.tnema.org/EP/EP_EQ.htm)
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Regional Strain Accumulation
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Geodetics (GPS) can show areas of strain
accumulation along locked faults (known and
unknown)
Strain accumulation is directly related to seismic
moment accumulation, hence areas with greater
strain rates have greater earthquake potential
Bulk strain is found to be distributed throughout
the broader region, rather than only being
concentrated on certain discrete faults
Regional Strain Accumulation
(WGCEP, 1995)
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Geological Slip Estimates
Used to identify surficial faults with slip rates
> 3 mm/yr
 These faults probably exhibit recurrence
intervals of hundreds of years
 Magnitude 7 earthquakes are possible on
some of these segments, though this has
not occurred in the historical record
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Thrust Faults
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Surface Thrusts
 Slip rates can be estimated using stratigraphy,
structures, and geomorphology
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Blind Thrusts
 Slip rates are generally unknown, though may
be related to fold growth rates
 Recurrence intervals and characteristic
behavior are unknown
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Thrust Faults
(WGCEP, 1995)
Regional Earthquake Potential
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Given that we know where the faults are and
have some kind of measure or estimate of
their potential magnitudes and recurrence
intervals, we can finally turn to hazard
potentials
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Regional Earthquake Potential
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Characteristic earthquake potential is based
directly on recurrence interval
For randomly distributed earthquakes, it is based
on the Gutenberg-Richter distribution
N ( m) = 10 a−b⋅ m
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In general, the annual number of distributed
earthquakes of magnitude ≥ m is related to the
characteristic seismicity rate (f) and the maximum
magnitude (m
(mx, assumed to be the characteristic
magnitude), with the number approaching 0 as m
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approaches mx
Regional Earthquake Potential
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Type A Zones
 Characteristic earthquakes are dealt with in the
cascading earthquake model
 Remaining (distributed) seismicity rate is
determined from the earthquake catalog, with
characteristic earthquakes removed
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Regional Earthquake Potential
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Type B Zones
 Distributed seismicity rate is determined from
the complete earthquake catalog
 Remaining (characteristic) seismicity rate is
determined from geological and geodetic
information
Regional Earthquake Potential
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Type C Zones
 There are no characteristic earthquakes, and
distributed seismicity rate is the average of the
earthquake catalog rate and geodetic rate
 Maximum seismicity mx is assumed from global
seismic catalogs, so it’
it’s not at all wellconstrained
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Regional Earthquake Potential
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The upshot is that for each of the 65 seismic
source zones, they produce:
 Predicted rates of distributed (m ≥ 6) and
characteristic (m = mx) earthquake ruptures
 Predicted moment rates for both
Regional Earthquake Potential
0.61/yr
0.32/yr
0.067/yr
0.035/yr
(WGCEP, 1995)
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Regional Earthquake Potential
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According to the data, there is an 81-90%
probability of an m ≥ 7 earthquake in Southern
California between 1994 and 2024
An alternative model (assuming different cascade
behavior and increased maximum magnitudes)
matches the observed distribution somewhat
better, and predicts an 80-89% probability of an m
≥ 7 earthquake in Southern California between
1994 and 2024
Uncertainty is probably greater than implied here,
because of various assumptions in the data
Finally, Seismic Hazard Analysis
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Seismic hazard analysis takes into account all
ground-motion effects, including smaller
earthquakes
It also accounts for attenuation of ground motion
with distance from the seismic source
The authors use a peak ground acceleration (PGA)
of 0.2 g (≈
(≈ 1.96 m/s2) as a baseline
The probability of PGA at a given site exceeding
0.2 g is related to the annual seismicity rates,
earthquake magnitudes, and earthquake
probabilities at each of the 65 seismicity zones,
summed using multiple integrals
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Seismic Hazard Analysis at
Los Angeles City Hall
(WGCEP, 1995)
Seismic Hazard Analysis for
Southern California
(WGCEP, 1995)
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Future Work
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Models could be improved through:
 A better understanding of site-specific
responses to ground motion
 Increased understanding of earthquake size
versus fault geometry
 Increased study of blind thrusts (segmentation,
strain accumulation, relationships with other
faults)
 Wider and higher resolution geodetic
measurements
Conclusion/Segue
Earthquakes with m ≥ 7 have been less
frequent than predicted for the last 150 yr,
though an m ≥ 7 earthquake by 2024 might
make up for the deficit
 How would this affect the Los Angeles
Metropolitan Area?
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Los Angeles' Seismic Deficit
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There have been too few earthquakes in the
Los Angeles area over the past 200 years to
account for the amount of strain
accumulation over the same period
Major Fault Systems in the
Los Angeles Region
(Dolan et al., 1995)
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Earthquake Data Regressions
(Dolan et al., 1995)
Potential Moderate Earthquake
(Mw ~ 6.7) Sources
(Dolan et al., 1995)
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Conclusions
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Two scenarios:
 Moderate-Earthquake: predicts one Mw 6.7 earthquake
every 11 years
 Large-Earthquake: predicts one Mw 7.2 to 7.5
earthquake every 140 years
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In either case, there is a seismic deficit (either a
quiet period between earthquake clusters or
between major events)
Dolan et al. prefer the Large-Earthquake Scenario
Further Implications
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Buildings constructed according to the
California Building Code of 1991 may not be
able to withstand the predicted earthquakes,
particularly along the fault rupture
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