The microzonation of the Memphis, Tennessee area

Abstract

Although the probability of a major earthquake in the central U.S. is only one-tenth of the Californian events, any such event is expected to result in damage ten times greater than that anticipated in California. The city of Memphis, which is situated very close to the inferred epicenter of one of the three major 1811-1812 earthquakes, is in a potentially hazardous zone which will be susceptible to the usual seismic hazards. By recognizing the high level of seismicity in the New Madrid area, this study attempts to microzone the potential hazards in the city of Memphis. This study examines the pertinent criteria necessary for any microzonation study. The subjects considered include: (i) The seismicity of the central United States, (ii) Design earthquakes, (iii) Response analysis which allows us to construct the necessary microzonation maps. The seismicity of the region is evaluated according to the state-of-the-art literature available as there is no recorded strong motion data available for the central U.S. The maximum credible earthquakes which are likely to affect Memphis are evaluated as the one in a thousand year occurrence. However, earthquakes of a lower intensity (and lower recurrence rates) are selected as design earthquakes to permit a more realistic microzonation to be performed. For the response analysis, these earthquakes are simulated using synthetically generated accelerograms which exhibit the relevant parameters. These accelerograms displaying the significant characteristics of amplitude, predominant frequency and duration were selected to display, as accurately as possible, the anticipated nature of the horizontal bedrock-motions at Memphis. The horizontal motions (SH-waves) were applied at a depth of 45m below ground surface at numerous sites in the city of Memphis, where the soil stratigraphy had been conceptualized from borehole data. The soils data made available by local sources was the only information used for this study as any laboratory testing was outside the scope of this study. The dynamic soil properties were thus established from the available Standard Penetration Resistances and soil classifications. The results of the response analysis were transformed into microzonation maps depicting (i) zones showing qualitative estimates of ground response, (ii) zones showing the natural frequency of the soils, (iii) zones showing the peak spectral acceleration for 2% damping ratio, (iv) zones of liquefaction potential. These maps are essentially useful for preliminary investigation and we do not expect them to be used on a quantitative basis. However, further investigation is necessary in determining the stratigraphy and soil properties to a more accurate level if an earthquake hazard is anticipated.