Theoretical Investigations on the Nakamura\u27s Technique

We use synthetic calculations to investigate the spectral ratio between horizontal and vertical components (H/V ratio), derived from noise simulation, in order to appreciate the reliability of the so-called Nakamura\u27s method for site effects applications. This ratio shows a peak whose position generally coincides with the fundamental resonance frequency. We show that this position is independent of the source function, whereas it is characteristic of the geological structure. We also compare these results with those obtained for vertical S waves and Rayleigh waves, in order to better understand the significance of this HIV peak. Finally, we show that the amplitude of the H/V ratio cannot be used directly to derive the amplification for body waves, as suggested by Nakamura (1989), since it is very sensitive to parameters such as the Poisson\u27s ratio and the source-receiver distance

Comparison between seismic vulnerability models and experimental dynamic properties of existing buildings in France

Elastic fundamental frequency is a key-parameter of simplified seismic design and vulnerability assessment methods. Empirical relationships exist in codes to estimate this frequency but they miss experimental data to validate them accounting for national feature of building design and, above all, corresponding uncertainties. Even if resonance frequency extracted from ambient vibrations may be larger than the elastic frequency (at yield) generally used in earthquake engineering, ambient vibration recordings may provide a large set of data for statistical analysis of periods versus building characteristics relationships. We recorded ambient vibrations and estimated the fundamental frequency of about 60 buildings of various types (RC and masonry) in Grenoble City (France). These data complete the set existing yet, made of 26 RC-buildings of Grenoble (Farsi and Bard 2004) and 28 buildings in Nice (France) (Dunand 2005). Statistical analysis of these experimental data was performed for fundamental frequencies of RC shear wall structures and the results are compared with existing relationships. Only building height or number of stories has a statistical relevancy to estimate the resonance frequency but the variability associated to the proposed relationships is large. Moreover, we compared the elastic part of capacity curves of RC and masonry buildings used in the European Risk-UE method for vulnerability assessment with the experimental frequencies. The variability is also large and the curves may not be consistent with French existing building

Maximum Likelihood Estimation of Closed Queueing Network Demands from Queue Length Data

Resource demand estimation is essential for the application of analyical models, such as queueing networks, to real-world systems. In this paper, we investigate maximum likelihood (ML) estimators for service demands in closed queueing networks with load-independent and load-dependent service times. Stemming from a characterization of necessary conditions for ML estimation, we propose new estimators that infer demands from queue-length measurements, which are inexpensive metrics to collect in real systems. One advantage of focusing on queue-length data compared to response times or utilizations is that confidence intervals can be rigorously derived from the equilibrium distribution of the queueing network model. Our estimators and their confidence intervals are validated against simulation and real system measurements for a multi-tier application

Reef response to sea-level and environmental changes during the last deglaciation: Integrated Ocean Drilling Program Expedition 310, Tahiti Sea Level

The last deglaciation is characterized by a rapid sea-level rise and coeval abrupt environmental changes. The Barbados coral reef record suggests that this period has been punctuated by two brief intervals of accelerated melting (meltwater pulses, MWP), occurring at 14.08-13.61 ka and 11.4-11.1 ka (calendar years before present), that are superimposed on a smooth and continuous rise of sea level. Although their timing, magnitude, and even existence have been debated, those catastrophic sea-level rises are thought to have induced distinct reef drowning events. The reef response to sea-level and environmental changes during the last deglacial sea-level rise at Tahiti is reconstructed based on a chronological, sedimentological, and paleobiological study of cores drilled through the relict reef features on the modern forereef slopes during the Integrated Ocean Drilling Program Expedition 310, complemented by results on previous cores drilled through the Papeete reef. Reefs accreted continuously between 16 and 10 ka, mostly through aggradational processes, at growth rates averaging 10 mm yr-1. No cessation of reef growth, even temporary, has been evidenced during this period at Tahiti. Changes in the composition of coralgal assemblages coincide with abrupt variations in reef growth rates and characterize the response of the upward-growing reef pile to nonmonotonous sea-level rise and coeval environmental changes. The sea-level jump during MWP 1A, 16 ± 2 m of magnitude in ~350 yr, induced the retrogradation of shallow-water coral assemblages, gradual deepening, and incipient reef drowning. The Tahiti reef record does not support the occurrence of an abrupt reef drowning event coinciding with a sea-level pulse of ~15 m, and implies an apparent rise of 40 mm yr-1 during the time interval corresponding to MWP 1B at Barbados. © 2012 Geological Society of America

Basin Effects in Strong Ground Motion: A Case Study from the 2015 Gorkha, Nepal Earthquake

The term "basin effects" refers to entrapment and reverberation of earthquake waves in soft sedimentary deposits underlain by concave basement rock structures. Basin effects can significantly affect the amplitude, frequency and duration of strong ground motion, while the cone-like geometry of the basin edges gives rise to large amplitude surface waves through seismic wave diffraction and energy focusing, a well-known characteristic of basin effects. In this research, we study the role of basin effects in the mainshock ground motion data recorded at the Kathmandu basin, Nepal during the 2015 Mw7.8 Gorkha earthquake sequence. We specifically try to understand the source of the unusual low frequency reverberating pulse that appeared systematically across the basin, and the unexpected depletion of the ground surface motions from high frequency components, especially away from the basin edges. In order to do that we study the response of a 2D cross section of Kathmandu basin subjected to vertically propagating plane SV waves. Despite the scarcity of geotechnical information and of strong ground motion recordings, we show that an idealized plane-strain elastic model with a simplified layered velocity structure can capture surprisingly well the low frequency components of the basin ground response. We finally couple the 2D elastic simulation with a 1D nonlinear analysis of the shallow basin sediments. The 1D nonlinear approximation shows improved performance over a larger frequency range relative to the first order approximation of a 2D elastic layered basin response.Comment: Geotechnical Earthquake Engineering and Soil Dynamics V, Austin, Texas (2018

Modeling seismic wave propagation and amplification in 1D/2D/3D linear and nonlinear unbounded media

To analyze seismic wave propagation in geological structures, it is possible to consider various numerical approaches: the finite difference method, the spectral element method, the boundary element method, the finite element method, the finite volume method, etc. All these methods have various advantages and drawbacks. The amplification of seismic waves in surface soil layers is mainly due to the velocity contrast between these layers and, possibly, to topographic effects around crests and hills. The influence of the geometry of alluvial basins on the amplification process is also know to be large. Nevertheless, strong heterogeneities and complex geometries are not easy to take into account with all numerical methods. 2D/3D models are needed in many situations and the efficiency/accuracy of the numerical methods in such cases is in question. Furthermore, the radiation conditions at infinity are not easy to handle with finite differences or finite/spectral elements whereas it is explicitely accounted in the Boundary Element Method. Various absorbing layer methods (e.g. F-PML, M-PML) were recently proposed to attenuate the spurious wave reflections especially in some difficult cases such as shallow numerical models or grazing incidences. Finally, strong earthquakes involve nonlinear effects in surficial soil layers. To model strong ground motion, it is thus necessary to consider the nonlinear dynamic behaviour of soils and simultaneously investigate seismic wave propagation in complex 2D/3D geological structures! Recent advances in numerical formulations and constitutive models in such complex situations are presented and discussed in this paper. A crucial issue is the availability of the field/laboratory data to feed and validate such models.Comment: of International Journal Geomechanics (2010) 1-1

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

Phase Response Curves of Coupled Oscillators

Many real oscillators are coupled to other oscillators and the coupling can affect the response of the oscillators to stimuli. We investigate phase response curves (PRCs) of coupled oscillators. The PRCs for two weakly coupled phase-locked oscillators are analytically obtained in terms of the PRC for uncoupled oscillators and the coupling function of the system. Through simulation and analytic methods, the PRCs for globally coupled oscillators are also discussed.Comment: 5 pages 4 figur

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)