Rayleigh-wave dispersion curve: a proxy for site effect estimation?

One of the open issues on the effects of surface geology regards the estimation of site response when limited resources are available. In that restrictive context, one solution is to use soil characteristics as proxy. Despite its extensive use, the most common proxy, Vs30, is presently criticized because it cannot carry alone the main physics of site response. We propose here a statistical investigation of the capabilities of another proxy, the Rayleigh-wave dispersion curve, DC. When considered over a broad enough frequency band, it can provide deeper information missing in the single Vs30 parameter. A set of shear-wave velocity profiles measured for more than 600 Japanese KiK-net stations is used to compute theoretical dispersion curves (DC) and theoretical SH transfer functions (SH), while instrumental surface/downhole spectral ratios were calculated in a previous work (Cadet et al., 2011a). Canonical correlation techniques are applied to this large data set to analyze the relationship between DC and theoretical or empirical site responses. The results indicate very encouraging qualitative statistical relationships between DC and site amplification for numerically derived SH transfer functions, showing significant canonical couples with correlations up to 0.95. Results for instrumental surface/downhole transfer functions correspond to lower correlations (up to 0.73) but still allow the development of quantitative relationships

Statistical investigation of site ef f ects with emphasis on sedimentary basins, using earthquake and ambient noise recordings

During the last two decades, three empirical methods for assessing site effects have been widely used: the Standard Spectral Ratio (SSR), the Horizontal-to-Vertical Spectral Ratio from earthquake recordings (HVSR) and the Horizontal-to-Vertical Spectral Ratio from ambient noise recordings (HVN). The SSR is considered the reference empirical method to detect amplification as a function of frequency, while the HVSR and the HVN realistically indicate fundamental frequency but, for the majority of the worldwide examined sites, they cannot give reliable amplification curves as a function of frequency. Given the fact that HVSR and especially HVN can be easily obtained, it is challenging to search for any correlation with SSR amplification functions. We used recordings from 168 sites worldwide, for which all three types of spectral ratios were homogeneously processed (Haghsenas et al., Bull. Earthquake Eng. 2008). On this data set we applied standard multivariate statistical analyses, namely, factor analysis and canonical correlation, to investigate and quantify -where it is possible- any correlation between spectral ratios for a certain number of the examined frequency bins. Results show that the correlation between HVN and HVSR is very good. Moreover, their correlation with broad band SSR can be statistically quantified and receive a satisfactory physical explanation. In addition, we looked for the correlation of SSR, HVSR and HVN collected in sedimentary basins (a subset of the previous database) with geometrical and geophysical parameters. T hese attempts were constrained by the limited amount of reliable in-situ data. Among many, we select 5 parameters: Vs30, Hb, Vs_average/Hb, Hb/W_valley, Hb/W_edge (where Hb is the bedrock’s depth below the station; Vs_average is the average Vs from surface to bedrock; W_valley is 2D-width of the valley; W_edge is the distance from the closest valley’s edge). T he analysis assesses that larger are the first 4 parameters, larger is the low-frequency amplification in HVSR and HVN, and lower the high-frequency contribution. Although additional data would improve our statistical investigation and better establish quantitative correlation between spectral ratios and geophysical or/and geometrical characteristics of sedimentary basins, our results clearly show that statistical correlation between SSR and HVN-HVSR is present and modulated in specific frequency domains. T his study has been performed in the framework of the T oK IT SAK-GR EC project (2006-2010)

Analisi statistica degli effetti di sito da dati di terremoti e di rumore ambientale.

L’ultimo ventennio ha visto lo sviluppo e l’uso di tre metodi empirici per l’analisi degli effetti di sito: Standard Spectral Ratio (SSR), Horizontal-to-Vertical Spectral Ratio da registrazioni sismiche (HVSR) e Horizontal-to-Vertical Spectral Ratio da registrazioni di rumore ambientale (HVN). SSR è considerato il metodo empirico di riferimento per rilevare le amplificazioni in funzione della frequenza. HVSR e HVN, invece, danno una stima realistica della frequenza fondamentale ma, generalmente, non riescono a fornire valori affidabili di amplificazione. Nel presente lavoro sono state utilizzate le registrazioni sismiche effettuate in 168 siti provenienti da diverse aree geografiche e per cui sono stati calcolati tutti e tre i tipi di rapporti spettrali (Haghsenas et al., 2008). Su questi dati abbiamo applicato delle analisi statistiche multivariate quali la correlazione canonica (Davis, 2002), con lo scopo di mettere in evidenza e quantificare le correlazioni tra i differenti rapporti spettrali nell’intero intervallo di frequenza compreso tra 0.2Hz e 10Hz. Questo tipo di analisi permette inoltre di associare alle correlazioni una stima della loro significatività ed è stata già utilizzata da Theodulidis et al. (2008) per studiare la relazione tra HVN e danneggiamento in aree urbane. I risultati mostrano che la correlazione tra HVN e HVSR è molto buona ad esclusione delle basse frequenze e che, per entrambe le tecniche, la presenza di un picco di amplificazione nell’intervallo 0.6-2 Hz è correlato ad un minimo per frequenze 3-10Hz. I picchi di amplificazione evidenziati da queste due tecniche sono inoltre correlabili con un più largo intervallo di frequenze nei rapporti SSR. Abbiamo quindi esteso l’analisi per correlare SSR, HVSR e HVN in bacini sedimentari (un subset dei dati utilizzati) con parametri geofisici e geometrici. La riduzione del numero dei dati deriva dall’esigenza di avere siti con una buona qualità di informazioni geofisiche e geometriche. Sono stati scelti cinque parametri indicatori delle velocità medie delle onde S e delle caratteristiche geometriche 2D della valle. Sebbene un più esteso data-set migliorerebbe l’analisi statistica, stabilendo migliori stime quantitative della correlazione tra rapporti spettrali e le caratteristiche geofisiche e geometriche dei bacini sedimentari, i nostri risultati mostrano chiaramente che le correlazioni tra SSR e HVN-HVSR esistono e si modulano in specifici intervalli di frequenza. Questo studio è stato condotto nell’ambito del progetto ToK ITSAK-GR EC (2006-2010)

Statistical estimation of earthquake site response from noise recordings

Standard spectral ratio from earthquake recordings (SSR) is considered the reference empirical method for assessing site effects as a function of frequency. However, other estimates can be easily obtained from noise measurements (i.e., Horizontal-to-Vertical Spectral Ratio, HVN), even though their reliability in terms of amplitude is controversial. In the framework of the ToK ITSAK-GR (2006-2010) EC project, Cultrera et al. (2010) analyzed recordings from 64 sites worldwide, founding that it is possible to have linear combinations of the HVN amplitudes significantly correlated to linear combinations of the SSR. In the present paper we show how to estimate the SSR spectral ratios when only noise measurements are available, using the results of the canonical correlation analysis between SSR and HVN recorded at several sites. The SSR evaluation has been tested by a cross validation procedure: the expected SSR at each validation site are in turn estimated by a weighted average of the SSR values measured at the other sites; the weights are properly set to account more for the sites with similar behavior in terms of the canonical correlation between HVN and SSR. To evaluate the goodness of the estimation, we compared all the inferred and original SSR, and we performed a critical analysis on the spectral characteristics of earthquake site response that can be easily recovered from noise measurements

Ground motion scenarios for the 1997 Colfiorito, central Italy, earthquake

In this paper we report the results of several investigations aimed at evaluating ground motion scenarios for the September 26th, 1997 Colfiorito earthquake (Mw 6.0, 09:40 UTC). We model the observed variability of ground motions through synthetic scenarios which simulate an earthquake rupture propagating at constant rupture velocity (2.7 km/s) and the inferred directivity. We discuss the variability of kinematic source parameters, such as the nucleation position and the rupture velocity, and how it influences the predicted ground motions and it does not account for the total standard deviation of the empirical predictive model valid for the region. Finally, we used the results from the scenario studies for the Colfiorito earthquake to integrate the probabilistic and deterministic approaches for seismic hazard assessment

Ground motion shaking scenarios for the 1997 Colfiorito earthquake

In the recent years, two Italian research projects have been devoted to the simulation of ground shaking scenarios in different areas. A large part of the activities has been performed in the Umbria region and was in particular related to the 1997 Colfiorito earthquake. In general the statistical-deterministic approach was adopted for evaluating the scenarios for strong motion parameters (peak values, spectral ordinates, signal integral quantities, and so on) associated with the occurrence of a characteristic earthquake on a given fault. This approach is based on the realistic occurrence of a single earthquake related to the fracture of an a priori well identified active fault. According to the characteristic earthquake model, an earthquake rupture can repeatedly occurs along the same fault (or fault system) with an almost constant geometry, mechanism and seismic moment, these parameters being mainly related to the direction and intensity of the large scale tectonic stress regime. These ideas are supported by numerous paleoseismic studies of active faults in different tectonic environments [e.g., Pantosti and Valensise, 1990]. On the other hand, each faulting process may not repeat the same style of nucleation, propagation and arrest during successive rupture episodes occurring along a given fault zone, depending these characteristics on the pre-fracturing conditions of rock strength and/or yielding stress along the fault zone. It is therefore assumed that the large scale source characteristics (i.e., fault size and position, focal mechanism and seismic moment) are a priori known as the result of previous geological, geophysical and historical seismicity investigations. The variability of the rupture process is expected to produce variable strong ground motions at the earth surface, depending on the distribution of the kinematic parameters (final slip distribution, rupture velocity, slip duration …) along the faulting surface. In order to account for the possible variation of the source process from one rupture event to another, a large number of synthetic seismograms should be computed for different (and possible) rupture histories occurring along the characteristic fault selected, so to provide a representative set of strong motion records to be used for hazard estimation. By this strategy, the massive computation of synthetics for different possible rupture models does not provide a single earthquake scenario (as for the standard deterministic approach) but a set of possible scenarios whose variability substantially reflects the heterogeneity of the source process. The advantage of this approach is that the variability of the selected strong ground motion parameter at a given site can be described by the statistical quantities inferred from the large number of simulations available. The earthquake scenario can then be represented, for example, by a couple of maps, one describing the spatial distribution of the mean value of the considered ground motion parameter and the other representing the associated variability for example in terms of standard deviation