Site Amplification and Basin Response on Strong Motion in Las Vegas

Feng Su, John G. Anderson, Shean Der Ni and Yuehua Zeng

The June 29, 1992 Little Skull Mountain earthquake (Mw=5.6) was
recorded at 9 strong motion stations in Las Vegas.  Even though the
epicenter is over a hundred kilometers away, the strong motion
seismograms recorded in the sedimentary sites of the Las Vegas valley
show a remarkable long duration of shaking.  Using these strong motion
records, we studied the site effect and basin response in Las Vegas.
We found site response in Las Vegas varies dramatically.  Ground motion
at sedimentary sites is much greater than that at rock sites,
especially at low frequencies.  Several stations on sediment have about
a factor of 10 times amplifications below 2 Hz compared to the average
of the rock stations.  These site amplifications are greater than those
previously reported by Murphy and Hewlett (1975) because there is a
strong site effect at the station they used for a reference.  The
strong site amplifications in Las Vegas thus appear to have a great
impact to the earthquake hazard prospect in this area, especially for
tall buildings.  Large earthquakes at moderate distances, which would
excite strong long period waves are a particular concern.  For example,
the Death Valley fault system, which is among the largest and most
active seismic sources in the Basin and Range, is just 150 km away from
Las Vegas.  To display the possible consequences of a large earthquake
on the  Death Valley fault system, we used a composite source model
approach to generate scenario ground motions in Las Vegas based on two
assumed earthquakes of magnitude 7.4 from this fault system, including
the site amplification.  We obtained a peak ground accelerations of
nearly 0.13g at the rock and 0.31g at the sediment sites.  The peak
ground velocity at one of the sediment sites reaches about 61 cm/s.