Forecasting Magnitude from Fault Geometry Bill Ellsworth, USGS Menlo Park, CA.
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Transcript of Forecasting Magnitude from Fault Geometry Bill Ellsworth, USGS Menlo Park, CA.
Forecasting Magnitude from Fault Geometry
Bill Ellsworth, USGS Menlo Park, CA
Magnitude – Area RelationsM = log(A) + k
• Wells and Coppersmith (W&C, 1994) widely used in hazard analysis.
• Good agreement between W&C and kinematic rupture models derived from seismic waves.
• Application of W&C to WG02 fault model overpredicts historical seismicity rate.
• WG02 adopted 3 relations for large earthquakes:
M = 3.98 + 1.02 log(A) (W&C) M = 4.2 + log(A) (Ellsworth) M = 3.03 + 4/3 log(A) (Hanks & Bakun)
where A = Length x Width x R (seismic coupling factor)
• Length (L): easy• Width (W): difficult;
disagreement between seismic and geodetic rupture models
• Aseismic slip factor (R): shallow creep – do-able; brittle-ductile transition – hard
• Trade-off between W and R: M = log(L) + log(WR) + k
Magnitude – Area RelationsM = log(LWR) + k
M(A) and the Earthquake Cycle
Seismologists observethe coseismic rupture
M = log(LW) + k
Forecast modelsmust account for the
total slip budget
M = log(LWR) + k
In this exampleR = 0.7
orlog(R) = -0.15
If the coseismic ruptureis described by
M = log(LW) + 4.0
the forecast rupture is
M = log(LWR) + 4.15
WG02 Approach to Determining W and R
Define W as the depth of the brittle-ductiletransition determinedfrom seismicity andthermal data
Use geodetic data to determine R given W
Depth of Seismicity and Depth to Brittle-Ductile transition in the San Francisco Bay Area
Colin Williams USGS, Menlo Park
WG02 Fault L, W and R Values
R factor accounts for creep but not for aseismicslip at the brittle-ductile transition
If great earthquakes rupture into the brittle-ductiletransition W and R will be incorrect
2002 Mw 7.9 Denali Fault, Alaska Earthquake
Denali Aftershocks LocationsRathkoviski et al.
East (km)
No
rth (
km
)
-20 -10 0 10 20
-20
-10
010
20
1 m
Near-Fault Displacements from GPS Surveyand Rupture Depth of a Uniform Dislocation
Distance to Denali Fault (km)
Fau
lt P
ara
lle
l S
lip
(m
)
-20 -15 -10 -5 0 5 10 15
-3-2
-10
12
3
Displacement (m)
De
pth
of
Fau
ltin
g (
km
)
3 4 5 6 7 8 9
51
015
20
0.90.99
Best model: 5.95 m slip from 0 to 11.6 km
Fault displacement vectors along the
Trans-Alaskan Pipeline
Comparing Depth of Rupture EstimatesObtained from Geodesy and Seismology
Depth range ofseismic rupture models
Depth range ofgeodetic models
Concluding Remarks
• Different approaches to the definition of W lead to different M(A) relations.
• The trade-off between W and R will be difficult to resolve with available data.
• The discrepancy between M(A) relations derived from earthquake slip models and those derived from earthquake cycle considerations can be explained by the R-factor.
• If rupture in large magnitude earthquakes routinely extends below the depth of complete locking the R-value in the current Working Group model will need to be modified if the W&C relation is used.