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%

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We acknowledge

• Suchan Lamichhane

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

• Funding from DOE, UNLV