AN ALTERNATIVE ANALYSIS OF THE PROBABILISTIC SEISMIC HAZARD

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 14 C BP (de. Polo 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 n n n 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 M 7. 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 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 PGA 0. 15 – 0. 55 g Deaggregations LVVFS Background & gridded sources Peak ground acceleration Distant faults Reference location 4 seconds Long period (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/2014 prelim/ PRELIMINARY We acknowledge • Suchan Lamichhane • Technical contributions from Woody Savage, Jeff Wagoner, Alex Goya • Funding from DOE, UNLV