Are NEHRP and Earthquake-based Site Effects in Greater Los Angeles Compatible?

Abstract

Realistic determination of earthquake ground-motion site amplification is an important aspect of seismic hazard analysis. The state of the practice is to characterize sites based on the average shear-wave velocity of the top 30 meters of underlying materials ( V30 ) and use empirical relationships developed by Borcherdt (1994) to quantify the site amplification. The choice of 30-meter depth comes from the fact that most engineering bore logs do not exceed this depth. Borcherdt (1992, 1994), Seed et al. (1992), Dobry et al. (1992), Martin and Dobry (1994), and Boore et al. (1997) have shown supportive evidence for the application of this empirical procedure for site amplification analysis. The 1994 Northridge earthquake and its aftershocks, as well as other earthquakes in southern California, have provided seismologists with a large amount of data with which to study regional and local site-amplification phenomena in the greater Los Angeles area (Hartzel et al. , 1996, 1997; Mermonte et al. , 1996; Bonilla et al. , 1997; Gao et al. , 1996; Field and Hough, 1997; Harmsen 1997). Using this information, Hartzel et al. (1998) published three site-amplification maps for the Los Angeles region for the frequency bands of 1–3, 3–5, and 5–7 Hz. The maps clearly show areas of high amplification values within the Los Angeles and San Fernando basins. There are similarities and differences between the site amplification values from these maps and those estimated based on NEHRP V30 procedure. It is the objective of this study to investigate how these different estimates of the site-amplification values affect the results of the probabilistic seismic hazard analysis (PSHA) in different areas of southern California. Considering that the seismic design criteria in the International Building Codes (IBC) is based on the NEHRP V30 procedure, the results of …