Giovanna Cultrera

Finite Fault Modeling of Strong Ground Motions Using a Hybrid Deterministic–Stochastic Approach

A hybrid deterministic-stochastic method (dsm) is developed to calculate synthetic time series of ground accelerations radiated from an extended source. The main goal of the proposed methodology is to include in the classical point-source stochastic method (pssm) the effects of the rupture propagation on a finite fault. This purpose is achieved through two important modifications of the pssm technique. First, the envelope does not have a predetermined functional form; rather, it is calculated deterministically following the isochron formulation with a kinematic rupture model. Second, we have generalized the various parameters of the point-source ground motion spectrum to account for the extended fault: corner frequency, distance from the fault, and radiation pattern are evaluated through the kinematic modeling. The guiding principal in all these modifications has been to develop a robust methodology capable of capturing the complexity of near-source ground motion even when input information about earthquake source, propagation medium, and site characteristics are of a very schematic nature. We show that the synthetic envelope contains the required information on the rupture process on extended fault, such as directivity effects and azimuthal variations depending on the source-to-receiver geometry. The method’s capability is demonstrated by modeling strong ground motions of the 1992 M w 7.3 Landers, California, earthquake and comparing them with the recorded accelerograms, which are clearly affected by directivity effects. The proposed technique reproduces the main characteristics of strong-motion recordings, and can be implemented using only a limited number of parameters to describe the source (dimension and geometry), the propagation medium (wave velocities and layers), and the site effects (transfer function). These characteristics are important for a methodology aimed to simulate ground-shaking scenarios for which a more complete description of the faulting process is not available.

The Role of Site Effects on the Intensity Anomaly of San Giuliano di Puglia Inferred from Aftershocks of the Molise, Central Southern Italy, Sequence, November 2002

The ML 5.4 Molise earthquake of 31 October 2002 caused damage of Mercalli-Cancani-Sieberg (MCS) intensity VIII-IX to the small town of San Giuliano di Puglia. In contrast, the other towns in the epicentral area did not exceed MCS intensity VII. Building vulnerability and near-surface geology were suspected to be potentially responsible for the high level of damage. However, early results of en- gineering studies in San Giuliano di Puglia (Dolce et al., 2004) indicate that vulner- ability of the strongest damage (European Macroseismic Scale, (EMS) intensity VII- VIII) zone was not higher than vulnerability of the remaining part of the town (EMS intensity VI). We use the aftershock recordings in the town to investigate the local amplification effect due to the lateral variations of near-surface geology. The wave- form analysis shows that in the high-damage zone, where clay deposits outcrop, direct S waves are characterized by a large initial pulse that is a factor of 6 larger than S waves recorded on a nearby rock outcrop, a few hundred meters away. Moreover, the strong S pulse is followed by a 10-sec long amplification of ground motion between 4 and 7 Hz. This frequency band corresponds to the fundamental resonance frequencies of two- and three-storied buildings, which are the most common type of construction in San Giuliano di Puglia. Since the duration of the strongest shaking is estimated to have been longer than 10 sec during the main shock, we conclude that the highly damaging effect in the clayey zone could have been due to the com- bination of the large initial pulse with time-persistent amplification at the resonant frequencies of buildings.

Statistical correlation of earthquake and ambient noise spectral ratios

The Horizontal-to-Vertical Spectral Ratio from earthquake (HVSR) and from ambient noise (HVN) recordings realistically indicate the fundamental frequency of soil response but, for the majority of the worldwide examined sites, they do not provide reliable amplification curves as calculated by the earthquake standard Spectral Ratio (SSR). Given the fact that HVSR and especially HVN can be easily obtained, it is challenging to search for a meaningful correlation with SSR amplification functions for the entire frequency band and to use the results for the SSR estimate at a further site where only noise measurements are available. To this aim we used recordings from 75 sites worldwide and we applied a multivariate statistical approach (canonical correlation analysis) to investigate and quantify any correlation among spectral ratios. The canonical correlation between SSR and HVN is then used to estimate the expected SSR at each site by a weighted average of the SSR values measured at the other sites; the weights are properly set to account more for sites with similar behaviour in terms of the canonical correlation results between HVN and SSR. This procedure, repeated for all sites in turn, constitutes the basis of a cross validation. The comparison between the inferred and the original SSR highlights the improvements of site response estimation with respect to the use of ambient noise techniques. The goodness and limitations of the reconstruction procedure are explained by specific geological settings.