The 1997–1998 Umbria‐Marche sequence (central Italy): Source, path, and site effects estimated from strong motion data recorded in the epicentral area

Abstract

We apply a nonparametric spectral inversion scheme to calculate source spectra, S wave attenuation, and site transfer functions from strong motion records in the epicentral region of the 1997–1998 Umbria‐Marche seismic sequence (central Italy). We use records from moderate size earthquakes (4.6 ≤ M l ≤ 5.9) to parameterize the spectral amplitude decay in the distance range from 5 to 40 km. We find that the average quality factor Q can be approximated by Q ( f ) = 49 f 0.9 , between 0.5 and 8 Hz, and the geometrical spreading by r −0.9 . At high frequencies ( f > 8 Hz) the dependence of Q on frequency weakens, and it takes an approximate constant value of 318. We fit the source spectra to the ω‐square model and calculate an average stress drop of (2 ± 1) MPa. The average value is consistent with the previous estimates from the weak events (1.4 < M l < 4.5) of the Umbria‐Marche seismic sequence. The most remarkable site effects are found in correspondence of large sedimentary basins, filled by alternation of sandy‐clayey deposits. The estimated spectral parameters are used to simulate acceleration spectra recorded during several earthquakes of the Umbria‐Marche sequence. Both point source and finite fault effects are considered. Furthermore, attenuation relationships for peak ground velocity and ground acceleration are estimated using synthetic data, and compared to existing relationships.