Download An Alternative Analysis of the Probabilistic Seismic Hazard for Las Vegas Valley, Nevada 2014

AN ALTERNATIVE ANALYSIS
OF THE PROBABILISTIC
SEISMIC HAZARD FOR LAS
VEGAS VALLEY, NEVADA
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)
LAS VEGAS
VALLEY
WELL DATA
• 1400+ well logs
• Sediment
ranges from
fine to coarse
• Alluvial fans
around basin
• Interfingered
grain 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 Edipping LVFS
< ~5 Ma
>~5 Ma
bedrock
SUPERIMPOSED BASINS
• Non-planar faults
• Upper basin controlled
by LVVFS and FMF
OUR SEISMIC HAZARD
ACCORDING TO USGS (2008)
Probabilistic seismic hazard
analysis (PSHA): Combined
hazard due to multiple earthquake
sources
Hypothetical bedrock outcrop *
0.2-s spectral acceleration
2% PE in 50 years ~ 2500 year return period
0.35 – 0.65 g
- 10%, 5%, 2% probability of
exceedance (PE) in 50 years
- Uses logic tree formalism
- Fault parameters
- Fault recurrence
- Ground motion relations
Acceleration, g
- 3 ground shaking parameters
- Peak Ground Acceleration
(PGA)
- 0.2-s spectral acceleration
- 1.0-s spectral acceleration
http://earthquake.usgs.gov/hazards/products/conterminous/2008/maps/
* Does not address effects of basin-fill sediments
Model does not explicitly consider most of Las Vegas Valley Fault System (LVVFS)
USGS FAULT CLASS “B”
Not considered explicitly as fault source in PSHA
NEW PSHA
- SUCHAN LAMICHHANE, PH.D.
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.
All known sources within 200 km
• 29 Latest Quaternary
• 1 Late Quaternary
(Frenchman Mountain)
Added
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
USGS
2008
Heavy outlines: strike-slip; otherwise normal
FAULTS
200 km
Added
Added
USGS quaternary fault and fold database
http://earthquake.usgs.gov/hazards/qfaults/google.php October 2011
WMF: Whitney Mesa Fault
CF: Cashman Fault
VVF: Valley View Fault
DF: Decatur Fault
PSHA USES LOGIC TREE
Addresses uncertainties due to lack of knowledge and understanding (“epistemic”)
Ground motion
prediction equation
Weight
Different with this analysis
PSHA OUTCOME
Reference location
EZ-FRISK v. 7.62
Total hazard curves
10% PE in 50 years
2% PE in 50 years
Uniform hazard spectra
72 %
2% PE in 50 years:
~70% increase
5% damping
69 %
71 %
Period (s)
PSHA OUTCOME
441 locations, ~ 3 km grid
Deaggregations
PGA
0.15 – 0.55 g
LVVFS
Background &
gridded sources
Peak ground acceleration
Distant faults
Reference
location
4
seconds
Long
periodperiod
(4 s)
2% PE in 50 years
PGA
2% PE in 50 years
~ 0.2 - 0.6 g
5% PE in 50 years
~ 0.1 - 0.3 g
Compare to USGS 2008
PGA is:
Consistently higher
Maximum increase ~150%
Greatest impact in north-central, least in south
10% PE in 50 years
~ 0.07 - 0.1 g
News:
USGS next iteration maps
(2014 preliminary):
Hazard increases by ~ 30%
http://earthquake.usgs.gov/hazards/2014prelim/
://earthquake.usgs.gov/hazards/2014prelim/
PRELIMINARY
We acknowledge
•
Suchan Lamichhane
•
Technical contributions
from Woody Savage,
Jeff Wagoner, Alex Goya
•
Funding from DOE,
UNLV