The San Salvador area lies along the axis of the Central American volcanic chain, which is the product of the northeast-directed subduction of the Cocos plate beneath the Caribbean plate. The destructive earthquakes that occurred in El Salvador during the last few decades are either shallow, moderate-sized earthquakes with epicenters in the volcanic zone, or large earthquakes from the subduction zone. The earthquakes of 13 January and 13 February 2001 are the most recent examples of both types of events. A considerable part of the damage related to these earthquakes comes from the local site effects. The present study describes the local site effects in the metropolitan area of San Salvador based on the spectral ratio technique through a comparison between three data sets from microtremors, weak motion, and strong motion.

Data from four sites within the metropolitan area are used to investigate the local site response in San Salvador. Spectral ratios of the horizontal components from the four sedimentary sites with respect to a reference site are calculated and compared between microtremors, earthquakes from the subduction zone (weak motion), and the earthquake of October 10, 1986 (strong motion). Results indicate site amplification factors ranging from 4 to 10 at all sites on all three data sets within the frequency band 1.0–10.0 Hz. However, within this band, no correlation is found at any dominant frequency. This is probably due to the vertical and lateral variations in the thickness and the properties of the sedimentary layers. Average amplification factors obtained from the three data sets systematically show a decreasing trend from microtremors to weak motion to strong motion. Estimates on all four sites indicate higher values for microtremors compared to the weak-motion data, whereas strong-motion data show the lowest. In order to obtain reliable estimates of site amplification, nonlinear sediment response has to be taken into account. Unless calibrated with the strong-motion data, caution is recommended in using microtremors.