A CASE OF THE INFLUENCE OF RADIATION-PATTERN ON PEAK ACCELERATIONS

The strong ground shaking of the 23 November 1980 earthquake in southe rn Italy seems to have been conditioned by the dimension of the source , its focal mechanism, and by the distance from the shallow portion of the source. There was only a low, and doubtful, directivity effect. T hese results come from a comparison of the azimuthal distribution of t he recorded peak ground horizontal accelerations with that of the tota l, dimensionless, radiation pattern of S waves in the horizontal plane at each site (radiation from the closest point of the fault, and appr opriate azimuth and takeoff angles were considered). The recorded maxi ma were obtained from hodogram plots of each couple of automatically d igitized horizontal components in 13 stations with negligible local si te effects at a distance of up to 78 km from the epicenter. The analys is indicates the strong influence of the strike-slip component on the azimuthal distribution of motion. The fault mechanism best fitting the recorded maxima is as follows: strike 318 degrees, dip 64 degrees, ra ke 317 degrees. This picture does not change if acceleration maxima in the frequency bands 0.1 to 5 Hz, 1 to 5 Hz, or 1 to 2 Hz are used. In a segment of the southern Apennines, where the strong-motion energy r adiation in the near/intermediate field of a repetitive series of shoc ks from the seventeenth century up to 1980 seems to be controlled by t he gross features of the source, it could be useful to include radiati on patterns into algorithms for regional seismic hazard calculations. Conversely, because of the fact that drawing isoseismal lines results in a smoothing of at least the very local site effects, it might be po ssible to infer information about the gross features of the sources of historical earthquakes from macroseismic data.