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AN ALTERNATIVE ANALYSIS OF THE PROBABILISTIC SEISMIC HAZARD FOR LAS VEGAS VALLEY, NEVADA Barbara Luke Wanda J. Taylor.

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Presentation on theme: "AN ALTERNATIVE ANALYSIS OF THE PROBABILISTIC SEISMIC HAZARD FOR LAS VEGAS VALLEY, NEVADA Barbara Luke Wanda J. Taylor."— Presentation transcript:

1 AN ALTERNATIVE ANALYSIS OF THE PROBABILISTIC SEISMIC HAZARD FOR LAS VEGAS VALLEY, NEVADA Barbara Luke Wanda J. Taylor

2 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

3 LAS VEGAS VALLEY FAULT SYSTEM EF splay - MRE 2245 BCE VVF splay – MRE 14, C BP (dePolo et al., 2006)

4 1400+ well logs Sediment ranges from fine to coarse Alluvial fans around basin Interfingered grain sizes near LVVFS LAS VEGAS VALLEY WELL DATA

5 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

6 SUPERIMPOSED BASINS Non-planar faults Upper basin controlled by LVVFS and FMF

7 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 – 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 Acceleration, g Model does not explicitly consider most of Las Vegas Valley Fault System (LVVFS) * Does not address effects of basin-fill sediments

8 USGS FAULT CLASS “B” Not considered explicitly as fault source in PSHA

9 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 An alternative analysis of the probabilistic seismic hazard for Las Vegas Valley, Nevada. Bulletin of the Seismological Society of America 104:

10 FAULTS USGS quaternary fault and fold database October 2011 WMF: Whitney Mesa Fault VVF: Valley View Fault CF: Cashman Fault DF: Decatur Fault Added 200 km

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12 PSHA USES LOGIC TREE Weight Addresses uncertainties due to lack of knowledge and understanding (“epistemic”) Different with this analysis Ground motion prediction equation

13 10% PE in 50 years 2% PE in 50 years Reference location EZ-FRISK v % PE in 50 years: ~70% increase 72 % 69 % 71 % Total hazard curves Uniform hazard spectra PSHA OUTCOME Period (s) 5% damping

14 PSHA OUTCOME 441 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

15 5% PE in 50 years2% PE in 50 years10% PE in 50 years ~ g~ g~ 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

16 PRELIMINARY We acknowledge Suchan Lamichhane Technical contributions from Woody Savage, Jeff Wagoner, Alex Goya Funding from DOE, UNLV


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