Ck. Saikia et al., MODELING OF ENERGY AMPLIFICATION RECORDED WITHIN GREATER LOS-ANGELES USING IRREGULAR STRUCTURE, Bulletin of the Seismological Society of America, 84(1), 1994, pp. 47-61
We have investigated energy amplification observed within Greater Los
Angeles basin by analyzing regional waveforms recorded from several Ne
vada Test Site (NTS) nuclear explosions. Although the stations are loc
ated nearly at the same azimuth (distances ranging from 350 to 400 km)
, the seismograms recorded in Compton (the central part of the basin),
Long Beach (the southern edge of the basin), and downtown Los Angeles
are remarkably different, even for a common explosion. Following the
onset of L(g) waves, the Long Beach sites have recorded surface waves
for more than 100 sec. From one explosion, the sites within downtown L
os Angeles have recorded seismograms with strong 3-sec surface waves.
These waves are not observed on the seismograms recorded in the neighb
oring hard-rock site California Institute of Technology (CIT) station.
Thus, they must have been generated by local wave guides. Numerically
, we modeled these 3-sec waves by convolving the CIT seismogram with t
he response of a sedimentary strata dipping gently (about 6-degrees) f
rom CIT toward downtown. We also examined the irregular basin effect b
y analyzing the variation of cumulative temporal energy across the bas
in relative to the energy recorded at CIT from the same explosion. Var
iation up to a factor of 30 was observed. To model the energy variatio
n that is caused by extended surface waves in the Long Beach area, we
used numerically simulated site transfer functions (STF) from a NNE-SS
W oriented two-dimensional basin structure extending from Montebello t
o Palos Verdes that included low-velocity sedimentary material in the
uppermost layers. These STFs were convolved with the CIT seismogram re
corded from the MAST explosion. To simulate elongated duration of surf
ace waves, we introduced in the upper sedimentary structure some disco
ntinuous microbasin structures of varying size, each microbasin delayi
ng the seismic waves propagating through them. Consequently, the surfa
ce-reflected phases through these structures are delayed and reflected
into the upper medium by the underlying interfaces. This mechanism he
lps delayed energy to appear at a later time and result in a longer ti
me duration at sites located at southern edge of the basin.