Seismic noise in the shallow subsurface: Methods for using it in earthquake hazard assessment.

摘要

The primary focus of this work has been characterization of the shallow subsurface for seismic hazard using naturally occurring seismic noise. Three studies chronicle the further development of the refraction microtremor method for determining shear-wave velocity-depth structure, which is a predictor of earthquake shaking amplification. These studies present results from the first uses of the refraction microtremor method to determine earthquake hazard across entire urban basins. Improved field methods led to speed and efficiency in these deployments. These spatially dense geophysical measurements of shallow shear-wave velocity were conducted to broadly define shaking hazard and to determine the accuracy of earlier methods of prediction. The refraction microtremor method agrees well with borehole and other shear-velocity methods. In Chapter 2, I present results from the first long urban transect, 16 km across the Reno, Nevada basin. In 45 of the 55 (82%) measurements of shear velocity averaged to 30 m depth (Vs30) the result was above 360 m/s. The National Earthquake Hazards Reduction Program (NEHRP) defines Vs30 of 360 m/s as the boundary between site hazard class C and class D, with class C above 360 m/s. Mapped geologic and soil units are not accurate predictors of Vs30 on this transect, and would have predicted most of the transect as NEHRP-D. In Chapter 3, I present Vs30 results along a 13 km-long transect parallel to Las Vegas Blvd. (The Strip), along with borehole and surface-wave measurements of 30 additional sites. Again, our transect measurements correlate poorly against geologic map units, which do not predict Vs30 at any individual site with sufficient accuracy for engineering application. Two models to predict Vs30 were reported in this study. In Chapter 4, I present aggregate results from the Reno and Las Vegas transects and include results from our 60 km-long transect across the Los Angeles basin. Our statistical analyses suggest that the lateral heterogeneity of our shear-velocity transects can be characterized by fractal dimensions (D) within a narrow range. Appendix A gives results of tests of a small electromagnetic vibrator that I devised, which was driven by unusual waveforms.