S-wave attenuation and site effects in the region of Friuli, Italy

R.R. Castro, F. Pacor, A. Sala, C. Petrungaro
Division Ciencias de la Tierra, CICESE, Apartado Postal 2732, Ensenada, Baja California, 22860, Mexico

Abstract:

We used strong-motion records from the 1976 Friuli earth- quake (M 6.4) and ten of the biggest aftershocks recorded by the National Accelerograph Network of the ENEL spA ( the Electrical Power Company of Italy ) to estimate the quality factor Q of S waves in this region. The wide distance range of the recordings (10 < r <190 km) permits us to analyze the spectral amplitude decay of the records using a non-parame- tric approach (e.g.,Anderson and Quaas, 1988; Castro et al., 1990; Anderson, 1991). We obtained attenuation functions for a set of 18 frequencies ranging between 0.4 and 25.0 Hz. The values of Q retrived from the attenuation functions obtained follow the frequency-dependent relation Q= 20.4 f. A test of the method was made using a second data set consisting of digital seismograms from the Friuli-Venezia Giulia Seismograph Network. In spite of the different size of the volume sampled by this data (10 < r < 131 km), the frequency dependence of Q obtained (Q = 16.1 f) is similar to that obtained with the strong-motion data set. The near-surface attenuation was also estimated using the model proposed by Anderson and Hough (1984) and Anderson (1991). We found that _O is smaller for the strong-motion stations loca- ted on rock compared to stations located on either shallow or soft sediments. To estimate the site response of the strong-motion stations, we corrected the spectral records for the attenuation effect and then inverted the corrected records to separate source and site effects using the inver- sion scheme proposed by Andrews (1986). To veryfy the site amplification estimates obtained, we also calculated the transfer function of each site using Nakamura's method for S wave (e.g., Lermo and Chavez-Garcia, 1993). In general, the shapes of the site functions obtained with the inversion are consistent with the transfer functions obtained calculating the horizontal to vertical component ratio.

AGU Index Terms: 7200 SEISMOLOGY; 7212 Earthquake ground motions and engineering; 5144 Wave attenuation; 7205 Continental crust
Keywords/Free Terms: S-wave attenuation, site effects, Friuli, Italy.

JGR-Solid Earth 96JB02295