AN ALTERNATIVE ANALYSIS AN ALTERNATIVE ANALYSIS
OF THE PROBABILISTIC OF THE PROBABILISTIC
SEISMIC HAZARD FOR LAS SEISMIC HAZARD FOR LAS
VEGAS VALLEY, VEGAS VALLEY, NEVADANEVADA
Barbara Luke
Wanda J. Taylor
ACTIVE FAULTS IN AND NEAR
LAS VEGAS VALLEY• sources of M>6 earthquakes since 1.6 Ma• distant sources• Sources within Las Vegas Valley
• Las Vegas Valley fault system and Frenchman Mtn fault
100 km radius
LAS VEGAS VALLEY FAULT
SYSTEM
• EF splay -MRE 2245 BCE
• VVF splay –MRE 14,500 14C BP (dePolo et al., 2006)
• 1400+ well logs
• Sediment ranges from
LAS VEGAS
VALLEY
WELL DATA
ranges from fine to coarse
• Alluvial fans around basin
• Interfingeredgrain sizes near LVVFS
LAS VEGAS VALLEY BASIN FILL AT
DEPTH
� Well data show that ~2/3 of the basin-fill is > ~5 Ma� <5 Ma basin has less paleorelief along its base, is
broader and more symmetric� Shape difference and depocenter shift caused by E-
dipping LVFS
bedrock
>~5 Ma< ~5 Ma
SUPERIMPOSED BASINS
• Non-planar faults• Upper basin controlled
by LVVFS and FMF
Probabilistic seismic hazard analysis (PSHA): Combined hazard due to multiple earthquake sources
- 3 ground shaking parameters- Peak Ground Acceleration
(PGA)- 0.2-s spectral acceleration
OUR SEISMIC HAZARD
ACCORDING TO USGS (2008)Hypothetical bedrock outcrop *0.2-s spectral acceleration2% PE in 50 years ~ 2500 year return period
Acceleration, g
- 0.2-s spectral acceleration- 1.0-s spectral acceleration
- 10%, 5%, 2% probability of exceedance (PE) in 50 years
- Uses logic tree formalism- Fault parameters- Fault recurrence- Ground motion relations http://earthquake.usgs.gov/hazards/products/conterminous/2008/maps/
0.35 – 0.65 g
Acceleration, g
Model does not explicitly consider most of Las Vegas Valley Fault System (LVVFS)
* Does not address effects of basin-fill sediments
USGS FAULT CLASS “B”Not considered explicitly as fault source in PSHA
NEW PSHA
- SUCHAN LAMICHHANE, PH.D.
All known sources within 200 km• 29 Latest Quaternary• 1 Late Quaternary
(Frenchman Mountain)Added
Lamichhane, S., Luke, B. Taylor, W. 2014. An alternative analysis of the probabilistic seismic hazard for Las Vegas Valley, Nevada. Bulletin of the Seismological Society of America 104:741-768.
What is different from USGS (2008)?
• Faults added : Cashman, Decatur, Valley View, Whitney Mesa, Rock Valley, West Specter Range, Pahrump Valley, Yucca Mountain – up to M7.2
• Faults characteristics modified :• Eglington: slip rate• Black Hills: magnitude and slip rate
Added
USGS 2008
Heavy outlines: strike-slip; otherwise normal
FAULTS
Added
200 km
USGS quaternary fault and fold databasehttp://earthquake.usgs.gov/hazards/qfaults/google.php October 2011
WMF: Whitney Mesa Fault VVF: Valley View Fault CF: Cashman Fault DF: Decatur Fault
Added
PSHA USES LOGIC TREEAddresses uncertainties due to lack of knowledge and understanding (“epistemic”)
Ground motion prediction equation
WeightDifferent with this analysis
10% PE in 50 years
2% PE in 50 years
Reference location EZ-FRISK v. 7.62
Total hazard curves
Uniform hazard spectra
PSHA OUTCOME
2% PE in 50 years:~70% increase
72 %
69 % 71 %
Period (s)
5% damping
PSHA OUTCOME441 locations, ~ 3 km grid
Background & gridded sources0.15 – 0.55 g
LVVFS
DeaggregationsPGA
Peak ground acceleration
4 seconds period
Distant faults
Reference location
2% PE in 50 years
Long period (4 s)
5% PE in 50 years2% PE in 50 years 10% PE in 50 years
~ 0.2 - 0.6 g ~ 0.1 - 0.3 g ~ 0.07 - 0.1 g
PGA
Compare to USGS 2008
PGA is:
� Consistently higher
� Maximum increase ~150%
� Greatest impact in north-central, least in south
News:
USGS next iteration maps (2014 preliminary):
Hazard increases by ~ 30%
PR
ELI
MIN
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sgs.
gov/
haza
rds/
2014
prel
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We acknowledge
• Suchan Lamichhane
• Technical contributions from Woody Savage, Jeff Wagoner, Alex Goya
• Funding from DOE, UNLV
PR
ELI
MIN
AR
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tp://
eart
hqua
ke.u
sgs.
gov/
haza
rds/
2014
prel
im/