Near-field Response of a 1D-structure Alluvial Site

Near-field Response of a 1D-structure Alluvial Site

Here, you find a case history of the complex strong motion responses at a site with an almost 1-D structure (Buia) in the near-field of four 5.2-6.0 Mw earthquakes in the Friuli Plain of Italy. In one case (September 15th 1976 09:21 GMT, Mw=6.0; see Figure 17), the 47 m alluvial cover of Buia experienced spectral amplitudes lower than its reference site on rock, due to the different back-azimuths from the source. The largest peaks at Buia occurred during an Mw 5.6 event. For practical purposes, we tested (i) subjectivity when preparing the 1-D models (using a blind test with seven experts); (ii) 1-D linear, linear-equivalent or nonlinear modeling in common practice; and (iii) the current seismic regulations. We also used noise recordings and the weak motion velocities recorded for local events (with digital triplets on the top and the bottom of the alluvial cover). Buia’s response to weak events was more stable than its response to strong ones. A simple 1-D model (five-layer) under linear conditions with good knowledge of the propagation velocities was able to reproduce the frequencies of the two relative strong motion maxima with an average amplitude underestimation of 1.5. The nonlinear approach simulated the two recorded maxima with slight frequency shifts. Since the disaggregation of the seismic hazard showed that the four destructive shocks studied represent the magnitude and distance ranges that dominate the hazard of Buia, we can conclude that the response spectra of the new Italian (and European) rules were sufficiently preventive, notwithstanding the aforementioned complications in the near-field.

Sandron D., Sirovich L., and F. Pettenati

The paper is in press, 2011, in the Bulletin of the Seismological Society of America; below, we present three sample figures.

FIGURE CAPTIONS

Figure 1 Map view of the case-study area. The two strong motion stations (white triangles), the epicenters of the four shocks of September 1976 (stars) and their focal mechanisms (beach-ball size proportional to magnitude) are shown. The contour solid lines show the radiation pattern of the S-waves calculated for a bilateral rupture from the nucleation of the 11 September, 1976 16:35 event with the KF formula (Sirovich, 1996b).

Figure 13 The black solid line is the mean empirical transfer function Buia/S. Rocco out of five recordings (see text). The solid light grey line is the NS component of the 11 September, 16:35 event. See the colors for the nonlinear response analyses. a) calculated using NERA; b) calculated with Deepsoil.

Figure 17 The same area as Figure 1. Fault-normal Peak Ground Velocities (PGV_FN, calculated); the surface projection of the causative fault and nucleation (star) adopted to model the Mw 6.0 event of 15 September, 1976, 09:21; the existing accelerometric stations (here, Buia is BUI and San Rocco is SRC). From Smerzini et al., 2010. This figure explains why in this earthquake San Rocco (on sound rock) experienced higher amplitudes than on alluvium in Buia; it was a near-field source effect.