Near-source attenuation of peak horizontal acceleration

Strong-motion data recorded within 50 km of the rupture zone were used to study near-source attenuation characteristics of horizontal peak ground acceleration for worldwide earthquakes of magnitudes 5.0 to 7.7. The data base consisted of 229 horizontal components of peak acceleration recorded from 27 earthquakes, including the 15 October 1979, Imperial Valley earthquake. These data were found to be adequately represented by the functional relationship PGA = a exp ( b M ) [ R + C ( M ) ] − d where PGA represents the mean of the peak values scaled from the two horizontal components of each recording, M is Richter magnitude, and R is distance from the fault rupture zone. Peak acceleration was found to be lognormally distributed with a standard error representing a 45 per cent increase in the median estimate. The regression analysis statistically confirmed the results of earthquake simulation studies that have predicted peak acceleration to become independent of magnitude and distance in the near field. An extensive sensitivity study showed that predictions based on our attenuation relationships are stable with respect to reasonable model and parameter variations. An analysis of residuals was used to investigate the behavior of peak acceleration with respect to various earthquake, site, and recording parameters, the more significant findings being: (1) a similarity in the level of acceleration recorded on soil or rock; (2) larger than average accelerations recorded at sites located on shallow soils or in areas of steep topography; (3) larger than average accelerations associated with earthquakes having reverse fault mechanisms; and (4) lower than average accelerations recorded in large embedded structures.