Evaluation of Linear and Nonlinear Site Effects for the MW 6.3, 2009 L’Aquila Earthquake
by C. Nunziata, M.R. Costanzo, F. Vaccari and G.F. Panza
An effective strategy for the seismic risk mitigation needs the use of advanced seismological methodologies for a realistic estimate of the seismic hazard and, consequently, to reduce earthquake damage through a preventive evaluation of vulnerability and actions for structure safety. Prediction of earthquakes and their related effects (expressed in terms of ground shaking) can be performed either by a probabilistic approach or by using modelling tools based, on one hand, on the theoretical knowledge of the physics of the seismic source and of wave propagation and, on the other hand, on the rich database of geological, tectonic, historical information already available. Strong earthquakes are very rare phenomena and it is therefore statistically very difficult to assemble a representative database of recorded strong motion signals that could be analyzed to define ground motion parameters suitable
for seismic hazard estimations. That is, the probabilistic estimation of the seismic hazard is a very gross approximation, and often a severe underestimation, of reality. A realistic and reliable estimate of the expected ground motion can be performed by using
the Neo-Deterministic Seismic Hazard Analysis (NDSHA), an innovative modelling technique that takes into account source, propagation and local site effects (for a recent review see Panza et al., 2011). This is done using basic principles of physics about wave generation and propagation in complex media, and does not require to resort to convolutive approaches, that have been proven to be quite unreliable, mainly when dealing with complex geological structures, the most interesting from the practical point
of view. The NDSHA approach has been used, among others, in the framework of the UNESCO- IUGS-IGCP project 414 "Seismic Ground Motion in Large Urban Areas", to evaluate ground motion of a group of Large Urban Areas and Megacities in the world representative of a broad spectrum of seismic hazard severity (Panza et al., 2004). A MW 6.3 earthquake struck on 6 April 2009, at 01.32 GMT, the Abruzzo region (central Italy). The L’Aquila town, located few km northeast to the main shock epicentre, and several
villages located nearby, suffered heavy damages and the casualties were about 300. The damage level generally corresponded to intensity ≤ VIII MCS, with few maximum values ≥ IX MCS generally associated to construction vulnerability and, in some cases, to site amplification effects (Fig. 1). In the past, destructive earthquakes originated in the L’Aquila basin such as the 1349, I=IX–X; the 1461, L'Aquila, I=X and the 1703, I=X (CPTI working group, 2004). Seismic hazard maps based on geological fault slip-rate data show that strong events (intensities ~IX) can hit L’Aquila with short recurrence time of approximately 250±50 years (Roberts et al., 2004). Boncio et al. (2004) estimated a maximum expected earthquake magnitude of 6.1–6.4 for the L’Aquila fault segment in Paganica, and stronger events are expected for other segments of the same fault system or other neighboring faults for their impressive post-glacial fault scarps (Papanikolaou et al., 2010 and references therein). Based on trenching investigations, Galli et al. (2002) support that the Campo Imperatore fault, only 20 km away from L’Aquila, can give a Magnitude 7.0 earthquake. Moreover, the 1703 (MW ~6.7) earthquake produced surface ruptures >10 km and a maximum vertical displacement of 1m in the neighbouring Arischia fault (Blumetti, 1995). These ruptures are almost one order of magnitude larger than the ruptures produced by the 6 April L’Aquila earthquake (0.1-0.3 m), implying that the surrounding faults have the capacity to generate significantly stronger events. For seismic hazard assessment, in order to prevent damage from even more energetic and dangerous earthquakes at L’Aquila, it is necessary to compute realistic seismograms. Aim of this paper is to compute the seismic ground motion at L’Aquila for the 6 April 2009 earthquake by the NDSHA approach and evaluate nonlinear effects with equivalent-linear approach, by assuming literature variations of shear modulus and damping with strain.
Active and Passive Experiments for S-Wave Velocity Measurements in Urban Areas
by C. Nunziata, G. De Nisco and M.R. Costanzo
One of the key parameters for the study of the effects of local site conditions is the S-wave velocity structure of unconsolidated sediments and the S-wave velocity contrast between bedrock and overlying sediments. Detailed VS profiles with depth can be measured with standard borehole logging and hole measurements, like down-hole and cross-hole. Such measurements are expensive, very local (point measurements) and may be not representative of large areas. Powerful methods for VS measurements, that do not need drillings, are all based on the dispersion properties of
Rayleigh wave phase and group velocities. Methods for phase velocity measurement of surface waves need recordings along dense arrays, with small geophone spacing, to avoid spatial aliasing, or, in case of 2 receivers, there is the problem of getting the right number of cycles and, hence, the analysis may lead to wrong values (Nunziata, 2005). Instead, the group velocity dispersion curve of the fundamental mode of Rayleigh waves can be extracted from the recorded signal at a single station by using the FTAN (Frequency Time Analysis) method (Nunziata, 2010 and references therein). FTAN is appropriate to process surface wave data both for the identification and the separation of signals and for the measurement of signal characteristics other than phase and group velocities, like attenuation, polarization, amplitude and phase spectra. When not only the fundamental mode but also the higher modes are excited, FTAN method lets to estimate the gross Q values too. In fact, the comparison between synthetic seismograms computed with extreme Q values and experimental data is based on the relative amplitude of fundamental and higher modes (Nunziata et al., 1999). FTAN method is successfully employed both in seismological and engineering field (e.g. Nunziata et al., 2009; Nunziata, 2010). At urban sites, the impossible use of explosive sources or heavy masses blows, limits the penetration depth to the uppermost 20-30 m, depending upon the rock velocities. Recently, cross correlations of long time series of ambient seismic noise have been demonstrated to recover surface wave dispersion (Green function) over a broad range of distances, from a few hundred metres to several hundred kilometres (e.g. Weaver & Lobkis, 2001; Bensen et al., 2007; Nunziata et al., 2009). Detailed VS profiles with depth are then obtained from the non linear inversion (Hedgehog method) of the average dispersion curve of the fundamental mode of Rayleigh group velocities. Aim of this paper is to present examples of the FTAN and Hedgehog methods applied to both active and passive experiments, to obtain reliable VS profiles to depths of 2 km in a complex urban area like Napoli, with high seismic and volcanic risk.