HR: 08:35h
AN: S41C-03 INVITED     [Abstracts]
TI: Site Reading Blues in the Mississippi Embayment
AU: * Langston, C A
EM: clangstn@memphis.edu
AF: CERI, University of Memphis, 3876 Central Ave., Suite 1, Memphis, TN 38152 United States
AB: The Mississippi embayment of the central U.S. is the location of a major intraplate seismic zone with high earthquake strong motion hazards combined with unique geological structures that only serve to confound the usual notions of "source" excitation, wave propagation along the "path", and "site" response. Thick, unconsolidated sediments of the embayment are the usual suspects in assuming that strong ground motions from nearby earthquakes will be significantly amplified through their dramatic decrease in impedance from deeper rocks and through near-surface, site resonance effects. Near-surface resonance can be inferred from local microearthquake recordings and dominates the waveform of P and S waves. Indeed, the universal character of near-surface velocity structure has been inferred from detailed modeling of ground motions from an extraterrestrial source - the atmospheric acoustic shock wave from a large bolide that impacted the atmosphere last November in the region. Acoustic wave coupling with the ground shows that S wave and, often, P wave velocities are significantly less than the P wave velocity of air and that the substrate at depths of about 12 m produces a sharp boundary causing significant high frequency resonance effects. Nevertheless, engineering and seismological wisdom suggests that the thick unconsolidated sediments should be significantly anelastic at high frequency so that strong ground motions should be essentially damped away. Results of the Embayment Seismic Excitation Experiment demonstrate that previously inferred low seismological Qs's of 30 or less in the sediments are severely underestimated and that the bulk Qs is greater than 100 for Rayleigh waves that propagate entirely within the unconsolidated sedimentary column. Empirical analysis of regional S wave coda (Jemberie and Langston, this meeting) shows significant amplification of coda waves within the embayment of factors up to 18 relative to stations outside of the embayment. Although interpreted as a "site" effect, simple plane wave models for this amplification cannot predict the magnitude of the amplification. The long duration and amplitude of reverberations seen in the regional S waves also require significant deeper structure within the Paleozoic basins of the Reelfoot rift. Plane wave models demonstrate that "site" response is a complicated function of the underside reflectivity of structure in the upper 10 km of the crust and the decrease in impedance of the unconsolidated sediments. However, the distinction of source-path-site becomes completely blurred if earthquakes can rupture into the sediments exciting large, high frequency surface waves.
DE: 7223 Seismic hazard assessment and prediction
DE: 7205 Continental crust (1242)
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2004 AGU Fall Meeting