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A case of the influence of radiation pattern on peak accelerations

Osservatorio Geofisico Sperimentale, 34016 Trieste Italia

Abstract

The strong ground shaking of the 23 November 1980 earthquake in southern 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.

These results come from a comparison of the azimuthal distribution of the recorded peak ground horizontal accelerations with that of the total, dimensionless, radiation pattern of S waves in the horizontal plane at each site (radiation from the closest point of the fault, and appropriate azimuth and take-off angles were considered).

The recorded maxima were obtained from hodogram plots of each couple of automatically digitized horizontal components in 13 stations with negligible local site effects at a distance of up to 78 km from the epicenter.

The analysis 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°, dip 64°, rake 317°. 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 radiation in the near/intermediate field of a repetitive series of shocks from the seventeenth century up to 1980 seems to be controlled by the gross features of the source, it could be useful to include radiation 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 possible to infer information about the gross features of the sources of historical earthquakes from macroseismic data.

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