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%

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We acknowledge

• Suchan Lamichhane

• Technical contributions from Woody Savage, Jeff Wagoner, Alex Goya

• Funding from DOE, UNLV

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