Abstract

Strong-motion records from 23 November 1980 earthquake in Irpinia-Basilicata, Southern Italy, recorded less than 59 km to the source show strong S-wave phases coherent up to 10 Hz. At more distant stations, the signal is only coherent up to 5 Hz, even when recordings contain signal up to 10 Hz. Hodogram plots of the coherent signal in the 5 to 10 Hz range show Lissajous figures of SH and SV waves radiated by the nearest point with strong seismic radiation, by the terminations of the rupture, or by other relatively compact features. The largest acceleration, recorded at the Sturno station, is due to high-frequency pulses radiated by a source south of the station and 25 km from the epicenter.

Hodogram plots of low frequencies show polarizations due to the propagation of the large-scale rupture along the fault. Lissajous figures help to discriminate P and S arrivals and reveal their direction of propagation. On the basis of this kind of analysis, it is confirmed that the 1980 Irpinia-Basilicata earthquake (ML = 6.5) was composed of three main sub-events, at 0 sec, 19 sec, and 39 sec in relative time. The first and second events broke the same structure striking NW. There is also evidence of bilateral propagation. The third event broke a second, smaller structure also striking NW and placed very close to the alignment of the previous ones.

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