using geodetic rates in seismic hazard mapping march 30, 20011 geodetic and geologic slip rate...
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![Page 1: Using Geodetic Rates in Seismic Hazard Mapping March 30, 20011 Geodetic and Geologic slip rate estimates for earthquake hazard assessment in Southern California](https://reader030.vdocument.in/reader030/viewer/2022032801/56649d545503460f94a30928/html5/thumbnails/1.jpg)
Using Geodetic Rates in Seismic Hazard MappingMarch 30, 2001 1
Geodetic and Geologic slip rate estimates for earthquake hazard
assessment in Southern California
Ken HudnutUSGS
March 30, 2001
University of Utah
![Page 2: Using Geodetic Rates in Seismic Hazard Mapping March 30, 20011 Geodetic and Geologic slip rate estimates for earthquake hazard assessment in Southern California](https://reader030.vdocument.in/reader030/viewer/2022032801/56649d545503460f94a30928/html5/thumbnails/2.jpg)
Using Geodetic Rates in Seismic Hazard MappingMarch 30, 2001 2
North American and Pacific plate relative motions
- is it 52 or 56 mm/yr?
San Andreas fault zone
- is it 22 mm/yr or 34 mm/yr along the Mojave segment?
Eastern CaliforniaShear Zone
– Estimated rates range between 6 and 12 mm/yr (geological & geodetic)
– Is this difference in rates real?– Does it indicate elevated strain rate during
a clustering of seismic (and aseismic) events?
– Is the ECSZ energetically ‘easier’ than the Big Bend?
Different path integrals yield differences
Measuring rates by geology or geodesy are both difficult and prone to pitfalls
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Using Geodetic Rates in Seismic Hazard MappingMarch 30, 2001 3
SCEC crustal motion map
• Combined
EDM, VLBI,
survey-mode
and continuous
GPS rigorously
• Released as a
SCEC product
• Set the bar very
high for the
SCIGN project
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Using Geodetic Rates in Seismic Hazard MappingMarch 30, 2001 4
SCEC Crustal Motion Map - Version 3
• 315 GPS vectors added for a total of 678 vectors (version 2 had 363 vectors)
• Early SCIGN site velocities now included
• Version 3 will include time series for all stations
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Using Geodetic Rates in Seismic Hazard MappingMarch 30, 2001 5
Generalized fault map of southern California with known slip rates circa 1990
from Tom Rockwell’s SCEC2 talk
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Using Geodetic Rates in Seismic Hazard MappingMarch 30, 2001 6
Effects of lower crust viscosity on geodetic velocity field
Weak lower crust => large variations in time
Strong lower crust
early
late
Borrowed from Brad Hager’s SCEC2 talk
‘Spectacular’AgreementIn CarrizoPlain
Rates may disagree the most wherethe lower crust is the weakest…
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Using Geodetic Rates in Seismic Hazard MappingMarch 30, 2001 7
Examples of differences in rate• Garlock fault
– Geologic rate 7 +/- 2 mm/yr– Geodetic rate 2 +/- 2 mm/yr– Geodesy < Geology => weak lower crust
• Eastern California Shear Zone– Geologic rate summed over all faults is ~6 mm/yr– Geodetic rate across ECSZ is ~10–12 mm/yr– Geodesy > Geology => clustering or new higher tectonic rate?
• Imperial Valley– Geologic rate of 20 mm/yr– Geodetic rate across valley of ~50 mm/yr => missing a major fault?
• Sierra Madre – Cucamonga fault zone– Geologic rate of 0.5 mm/yr– Geodetic rate of a
• Raymond fault– Geologic rate of 1.5-4 mm/yr – Geodetic rate of b a + b ~ 6-8 mm/yr
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Using Geodetic Rates in Seismic Hazard MappingMarch 30, 2001 8
Conclusions• In the past decade, our understanding of rates has changed
quite radically in So. Calif. – both in terms of geodetic and geologic estimates
• Where we now think there are differences, it is important to confirm the observations and then attempt to explain the differences
• Possible to explain these differences in a variety of ways– Bad data (e.g., missing a fault, off-fault deformation, etc.)– Bad modelling (e.g., dipping fault & layered structure, or variable
strength modelled as elastic half-space)– Weak lower crust or strong lower crust (earthquake cycle)
• Hypothesis testing approach needed where rates differ• Don’t assume geologic rate to be correct as a default position