an alternative analysis of the probabilistic seismic hazard for las vegas valley, nevada barbara...
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AN ALTERNATIVE ANALYSIS OF THE PROBABILISTIC SEISMIC
HAZARD FOR LAS VEGAS VALLEY, NEVADA
Barbara LukeWanda 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 fine to coarse
• Alluvial fans around basin
• Interfingered grain sizes near LVVFS
LAS VEGAS VALLEY WELL DATA
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- 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
OUR SEISMIC HAZARD ACCORDING TO USGS (2008)
Hypothetical bedrock outcrop *0.2-s spectral acceleration
2% PE in 50 years ~ 2500 year return period
Acceleratio
n, 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)
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
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.
FAULTS
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
Added
200 km
PSHA USES LOGIC TREE
Weight
Addresses uncertainties due to lack of knowledge and understanding (“epistemic”)
Different with this analysis
Ground motion prediction equation
10% PE in 50 years
2% PE in 50 years
Reference location EZ-FRISK v. 7.62
2% PE in 50 years:~70% increase
72 %
69 % 71 %
Total hazard curves
Uniform hazard spectra
PSHA OUTCOME
Period (s)
5% damping
PSHA OUTCOME441 locations, ~ 3 km grid
Peak ground acceleration
Background & gridded sources
4 seconds period
Distant faults
0.15 – 0.55 g
Reference location
LVVFS
Deaggregations
2% PE in 50 years
Long period (4 s)
PGA
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
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%
PGA
PR
ELI
MIN
AR
Yht
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2014
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We acknowledge
• Suchan Lamichhane
• Technical contributions from Woody Savage, Jeff Wagoner, Alex Goya
• Funding from DOE, UNLV