Earthquake potential and ground shaking hazard at the Los Alamos National Laboratory
Ivan G. Wong, Keith Kelson, Susan Olig, Jacqueline Bott, Robert Green, Thomas Kolbe, Mark Hemphill-Haley, Jamie N. Gardner, Steven Reneau, and Walter. Silva

Abstract:

A four-year program of geologic, seismologic, geophysical and geotechnical investigations recently evaluated the potential seismic hazards at the Los Alamos National Laboratory (LANL). In this study, 25 faults and four seismic source zones were identified as seismic sources potentially significant to LANL in terms of strong ground shaking. The source zones, such as the Rio Grande rift, in which the Laboratory is located, account for the hazard from "background" earthquakes that do, not repeatedly rupture the surface and cannot be associated with known faults or tectonic features. All seismic sources were characterized in terms of their location, geometry, maximum magnitude and earthquake recurrence. The three most significant and closest faults to LANL (Pajarito, Guaje Mountain and Rendija Canyon faults, which comprise the Pajarito fault system) were the focus of detailed paleoseismic studies. The main 41-km-long Pajarito fault is located along the western margin of LANL and is a down-to-the-east normal fault, dipping beneath the Laboratory. The size of the maximum earthquake for the Pajarito fault is estimated to be moment magnitude (M w) 6.9 ± 0.3 based on empirical relationships for rupture length, rupture area and displacement. The average long-term slip rate for the fault is about 0.1 mm/yr. The subsurface geology beneath LANL was characterized through a program of borehole exploration and velocity measurements, dynamic laboratory testing of borehole samples, and compilation and evaluation of all relevant borehole and geotechnical data. Site-specific geologic and velocity profiles for several LANL facility sites were developed based on these data. These geologic data and the seismic source characterization were input into a probabilistic seismic hazard analysis performed using a logic tree approach. Peak horizontal accelerations estimated for LANL are approximately 0.15, 0.30 and 0.56 g for the return periods of 500, 2000, and 10,000 years, respectively. The dominant seismic source contributing to the hazard at LANL is the Pajarito fault system and, to a lesser extent, the background earthquakes within the Rio Grande rift source zone.


Citation:

  1. Wong, Ivan G.; Kelson, Keith; Olig, Susan; Bott, Jacqueline; Green, Robert; Kolbe, Thomas; Hemphill-Haley, Mark; Gardner, Jamie N.; Reneau, Steven; Silva, Walter., 1996, Earthquake potential and ground shaking hazard at the Los Alamos National Laboratory, in: The Jemez Mountains Region, Goff, Fraser; Kues, Barry S.; Rogers, Margaret Ann; McFadden, Les D.; Gardner, Jamie N., New Mexico Geological Society, Guidebook, 47th Field Conference, pp. 135-142.

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