Investigation of earthquake and microtremor ground motions

The nature of strong earthquake ground shaking has been investigated based on a study of 15 accelerograms recorded at El Centro in southern California. It is concluded that the characteristics of the source mechanism and the transmission path play a dominant role in determining the details of strong ground shaking at the site. No local site periodicities could be clearly identified, which suggests that source and transmission path effects overshadow the influence of local site conditions. The method of using microtremor measurements to determine local site characteristics has been tested by direct comparison with strong motion measurements. Microtremor ground motions were recorded at five sites in the El Centro area and measurements were repeated at three of these sites after a period of 24 hours. These low amplitude ground motions have been found to be widely different from the motions caused by strong earthquake ground shaking. Their nonstationary nature over a period of a day or so makes the interpretation of such data from a single microtremor measurement very unreliable. It has been concluded that these microtremor ground motions are forced oscillations of the ground caused by nearby sources of excitation. The microtremor acceleration spectra do not indicate prominent peaks that could be correlated with local site conditions. At this site the use of microtremor measurements to define local subsoil conditions would evidently not be feasible.