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Source parameter analysis from strong motion records of the Friuli, Italy, earthquake sequence (1976-1977)

OSSERVATORIO VESUVIANO, 80056 ERCOLANO, Italy
LABORATOIRE DE SISMOLOGIE INSTITUT DE PHYSIQUE DU GLOBE, PARIS, France
DIPARTIMENTO SCIENZE DELLA TERRA UNIVERSITÁ "LA SAPIENZA", 00100 ROME, Italy

Abstract

Time and frequency domain analyses are applied to strong motion data recorded in Friuli, Italy, during 1976 to 1977. An inversion procedure to estimate spectral parameters (low frequency level, corner frequency, and high frequency decay) has been applied to displacement spectra using a simple earthquake source model with a single corner frequency.

The data were digitized accelerograms from ENEA-ENEL portable and permanent networks. Instrument-corrected SH waves were selected from a set of 138 three-component, hand-digitized records and 28 automatically digitized records. Thirty-eight events with stations having 8 to 32 km epicentral distance were studied.

Different stress drop estimates were performed showing high values (200 to 300 bars, on the average) with seismic moments ranging from 2.8 x 10²² to 8.0 x 10²⁴ dyne-cm.

The observation of systematic higher values of Brune stress drop (obtained from corner frequencies) with respect to other time and frequency domain estimates of stress release, and the evidence on time series of multiple rupture episodes suggest that the observed corner frequencies are most probably related to subevent ruptures rather than the overall fault size.

Seven events recorded at more than one station show a good correlation between rms, Brune, and dynamic stress drops, and a constant scaling of this parameter as a function of the seismic moment. When single station events are also considered, a slight moment dependence of these three stress drop estimates is observed differently. This may be explained by an inadequacy of the ^–2 high-frequency decay of the source model or by high-frequency attenuation due to propagation effects.

The high-frequency cutoff of acceleration spectra indicates the presence of an F_max in the range of 5 to 14 Hz, except for the stations where local site effects produce spectral peaks.

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