Basin Effects on Strong Motion in Las Vegas

Feng Su and John G. Anderson

The June 29, 1992 Little Skull Mountain earthquake (ML=5.6) was recorded at
9 strong motion stations in Las Vegas.  The epicenter is located more than a
hundred kilometers away.  Several of the strong motion seismograms recorded
in the Las Vegas Valley show a surprisingly long duration.  To study the site
effect and basin response on ground motion in Las Vegas Valley, we used a new
method we developed recently to calculate S-wave site amplification relative
to a regional layered crust model.  In this method, the site response
function is defined as a ratio of ground response of empirical Green's
function to synthetic Green's function.  To apply this method, we first
computed the synthetic Green's function in a layered elastic model using an
improved reflectivity method of Luco and Apsel (1983) with a Brune's rise
time function (1970, 1971) at all the recording sites.  Then we took the
ratio of the observed acceleration data to the synthetics we calculated at
the same site, with proper adjustment of the earthquake size and corner
frequency.  The logarithm of the ratio is defined as the site amplification
function at that site.  Our result show more than a factor of 10 difference
in site amplification for some sediment stations compared with a rock station
at frequencies below 2 Hz.  These site amplifications are much greater than
those previously observed by Murphy and Hewlett (1975) because there
is a strong site effect at the station they used for a reference.  Basin
response and site effects thus appear to have a great impact to the
earthquake hazard prospect in the Las Vegas area.  Large earthquakes
at moderate distances, which would excite strong long period waves are
a particular concern.  Based on the above result, we are looking for the
implication of the major faults, such as the Death Valley fault, to the Las
Vegas region.