Abstract

We have investigated energy amplification observed within Greater Los Angeles basin by analyzing regional waveforms recorded from several Nevada Test Site (NTS) nuclear explosions. Although the stations are located 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 Lg waves, the Long Beach sites have recorded surface waves for more than 100 sec. From one explosion, the sites within downtown Los Angeles have recorded seismograms with strong 3-sec surface waves. These waves are not observed on the seismograms recorded in the neighboring 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 the response of a sedimentary strata dipping gently (about 6°) from CIT toward downtown. We also examined the irregular basin effect by analyzing the variation of cumulative temporal energy across the basin relative to the energy recorded at CIT from the same explosion. Variation up to a factor of 30 was observed. To model the energy variation that is caused by extended surface waves in the Long Beach area, we used numerically simulated site transfer functions (STF) from a NNE-SSW oriented two-dimensional basin structure extending from Montebello to Palos Verdes that included low-velocity sedimentary material in the uppermost layers. These STFs were convolved with the CIT seismogram recorded from the MAST explosion. To simulate elongated duration of surface waves, we introduced in the upper sedimentary structure some discontinuous microbasin structures of varying size, each microbasin delaying the seismic waves propagating through them. Consequently, the surface-reflected phases through these structures are delayed and reflected into the upper medium by the underlying interfaces. This mechanism helps delayed energy to appear at a later time and result in a longer time duration at sites located at southern edge of the basin.

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