Matteo Picozzi

Ambient noise measurements for preliminary site-effects characterization in the urban area of Florence, Italy

Abstract Environmental noise measurements and borehole data were used for a preliminary seismic characterization of the Plio-Quaternary sedimentary basin underlying the city of Florence (Italy). An extensive survey of ambient vibrations was conducted by using the seismic noise horizontal-to-vertical (H/V) spectral ratio technique (HVSR) to map the fundamental resonance frequency of the sedimentary cover. In order to use this information to infer main features of the basin subsurface geometry, an estimate of the S -wave velocity profile in the sediment layer was obtained from array noise recordings along with information on resonance frequency and direct measurements of the sediment thickness available at 23 borehole sites. A first-order reconstruction of the seismic bedrock topography in the area was provided, which shows a good consistency with available geological/log data. From these pieces of information, a preliminary soil classification of the whole area was attempted in order to identify situations where possible seismic amplification effects are expected during future earthquakes and where more accurate investigations are needed to parameterize the local seismic response.

Statistical Analysis of Noise Horizontal-to-Vertical Spectral Ratios (hvsr)

Statistical properties of the horizontal to vertical spectral ratios (hvsr) applied to noise recording are analyzed in order to define optimal strategies for numerical processing and identification of possible artifacts. To this purpose, two time series have been analyzed: one constituted by environmental seismic noise in the presence of a genuine physical signal and one relative to pure instrumental noise, both obtained with the same experimental apparatus. By means of suitable statistics, some guidelines for the hvsr analysis are provided. A statistical test proposed by Albarello (2001) for the identification of artifacts in the hvsr function has been analyzed and invalidated.

Interferometric Analysis of Strong Ground Motion for Structural Health Monitoring: The Example of the L’Aquila, Italy, Seismic Sequence of 2009

Abstract Structural health monitoring (SHM) aims to improve knowledge of the safety and maintainability of civil structures. The usage of recording systems exploiting wireless communication technology is particularly suitable for SHM, especially for rapid response following earthquakes. In this study, both of these issues are combined, and we report on the application of seismic interferometry to SHM using a dataset of seven earthquakes collected using a novel wireless system of accelerometers during the L’Aquila, Italy, seismic sequence in 2009. We show that interferometric analysis allows the estimation of the shear-wave velocity of seismic phases propagating throughout a structure, and, most important for SHM purposes, allows the monitoring of the velocity variations during the aftershock sequence. Moreover, innovatively we apply the S transform to the building response functions retrieved by interferometry to estimate the fundamental resonance frequency and the quality factor Q .

Evidence for a difference in rupture initiation between small and large earthquakes

The process of earthquake rupture nucleation and propagation has been investigated through laboratory experiments and theoretical modelling, but a limited number of observations exist at the scale of earthquake fault zones. Distinct models have been proposed, and whether the magnitude can be predicted while the rupture is ongoing represents an unsolved question. Here we show that the evolution of P-wave peak displacement with time is informative regarding the early stage of the rupture process and can be used as a proxy for the final size of the rupture. For the analysed earthquake set, we found a rapid initial increase of the peak displacement for small events and a slower growth for large earthquakes. Our results indicate that earthquakes occurring in a region with a large critical slip distance have a greater likelihood of growing into a large rupture than those originating in a region with a smaller slip-weakening distance.

Rayleigh wave dispersion curves from seismological and engineering-geotechnical methods: a comparison at the Bornheim test site (Germany)

Active and passive procedures for estimating the local seismic response from surface-wave measurements are compared for a test site in the Bornheim area (Germany), where independent geophysical and geological information is available. Recording was done using geophones, as well as seismometers, in various configurations. Five popular and standardized techniques were used for analysing the data: multichannel analysis of surface waves (MASW), the refraction microtremor technique (ReMi), the extended spatial autocorrelation technique (ESAC) and frequency–wavenumber analysis (beam-forming and maximum likelihood methods). The resulting surface wave dispersion curves are largely consistent, but differ in their respective low-frequency ranges due to the resolving capabilities of the respective acquisition geometries. Two joint inversions of dispersion and H/V curves, one for the lower frequency range (2.3–9.2 Hz) and the other for the complete range (2.3–45 Hz) of the dispersion curves resulted in fairly similar S-wave profiles, but increasing the frequency range allowed better estimates for the lower velocities at shallow depths. The results also compare well with borehole information. The site responses obtained from the two S-wave profiles are very similar, even at higher frequencies. The use of combined procedures (geotechnical-engineering and seismological) allows a high quality estimation of the S-wave velocity structure to be obtained, both at shallow and large depth. However, if a combined approach is not possible, for site response estimation at sites with sedimentary cover thicker than 30 to 50 m and where knowledge of the average S-wave velocity is more important than higher resolution estimates at shallower depths, the use of passive seismological 2D arrays is strongly recommended.

GFZ Wireless Seismic Array (GFZ-WISE), a Wireless Mesh Network of Seismic Sensors: New Perspectives for Seismic Noise Array Investigations and Site Monitoring

Over the last few years, the analysis of seismic noise recorded by two dimensional arrays has been confirmed to be capable of deriving the subsoil shear-wave velocity structure down to several hundred meters depth. In fact, using just a few minutes of seismic noise recordings and combining this with the well known horizontal-to-vertical method, it has also been shown that it is possible to investigate the average one dimensional velocity structure below an array of stations in urban areas with a sufficient resolution to depths that would be prohibitive with active source array surveys, while in addition reducing the number of boreholes required to be drilled for site-effect analysis. However, the high cost of standard seismological instrumentation limits the number of sensors generally available for two-dimensional array measurements (i.e., of the order of 10), limiting the resolution in the estimated shear-wave velocity profiles. Therefore, new themes in site-effect estimation research by two-dimensional arrays involve the development and application of low-cost instrumentation, which potentially allows the performance of dense-array measurements, and the development of dedicated signal-analysis procedures for rapid and robust estimation of shear-wave velocity profiles. In this work, we present novel low-cost wireless instrumentation for dense two-dimensional ambient seismic noise array measurements that allows the real–time analysis of the surface-wavefield and the rapid estimation of the local shear-wave velocity structure for site response studies. We first introduce the general philosophy of the new system, as well as the hardware and software that forms the novel instrument, which we have tested in laboratory and field studies.

QUICK ESTIMATES OF SOFT SEDIMENT THICKNESSES FROM AMBIENT NOISE HORIZONTAL TO VERTICAL SPECTRAL RATIOS: A CASE STUDY IN SOUTHERN ITALY

An approach devoted to quickly assess the thickness of soft sedimentary cover in areas of unknown subsurface morphology is applied in this study. In particular, soil thickness (h) is derived by combining estimates of the resonance frequency (f r ) relative to soft sediments with the local shear-wave velocity (Vs) profile. For this purpose, (f r ) values are assessed from horizontal to vertical (H/V) spectral ratios of seismic noise recordings and the (V s ) profile is obtained by considering information from shallow seismic surveys. Results obtained for a Quaternary sedimentary basin in Southern Italy are discussed. Since in the investigated area only weak independent constraints are available, special emphasis is given to the assessment of uncertainties involved in this estimate of soil thickness.

Exploring the feasibility of a nationwide earthquake early warning system in Italy

When accompanied by appropriate training and preparedness of a population, Earthquake Early Warning Systems (EEWS) are effective and viable tools for the real-time reduction of societal exposure to seismic events in metropolitan areas. The Italian Accelerometric Network, RAN, which consists of about 500 stations installed over all the active seismic zones, as well as many cities and strategic infrastructures in Italy, has the potential to serve as a nationwide early warning system. In this work, we present a feasibility study for a nationwide EEWS in Italy obtained by the integration of the RAN and the software platform PRobabilistic and Evolutionary early warning SysTem (PRESTo). The performance of the RAN-PRESTo EEWS is first assessed by testing it on real strong motion recordings of 40 of the largest earthquakes that have occurred during the last 10 years in Italy. Furthermore, we extend the analysis to regions that did not experience earthquakes by considering a nationwide grid of synthetic sources capable of generating Gutenberg-Richter sequences corresponding to the one adopted by the seismic hazard map of the Italian territory. Our results indicate that the RAN-PRESTo EEWS could theoretically provide for higher seismic hazard areas reliable alert messages within about 5 to 10 s and maximum lead times of about 25 s. In case of large events (M > 6.5), this amount of lead time would be sufficient for taking basic protective measures (e.g., duck and cover, move away from windows or equipment) in tens to hundreds of municipalities affected by large ground shaking.

Monitoring the structural dynamic response of a masonry tower: comparing classical and time-frequency analyses

The monitoring of the evolution of structural dynamic response under transient loads must be carried out to understand the physical behaviour of building subjected to earthquake ground motion, as well as to calibrate numerical models simulating their dynamic behaviour. Fourier analysis is one of the most used tools for estimating the dynamic characteristics of a system. However, the intrinsic assumption of stationarity of the signal imposes severe limitations upon its application to transient earthquake signals or when the dynamic characteristics of systems change over time (e.g., when the frequency of vibration of a structure decreases due to damage). Some of these limitations could be overcome by using the Short Time Fourier Transform (STFT). However, the width of the moving window adopted for the analysis has to be fixed as a function of the minimum frequency of interest, using the best compromise between resolution in both the time and frequency domains. Several other techniques for time-frequency analysis of seismic signals recorded in buildings have been recently proposed. These techniques are more suitable than the STFT for the application described above, although they also present drawbacks that should be taken into account while interpreting the results. In this study, we characterize the dynamic behaviour of the Falkenhof Tower (Potsdam, Germany) while forced by ambient noise and vibrations produced by an explosion. We compare the results obtained by standard frequency domain analysis with those derived by different time-frequency methods. In particular, the results obtained by the standard Transfer Function method, Horizontal to Vertical Spectral Ratio (HVSR), Short Time Fourier Transform (STFT), Empirical Mode Decomposition (EMD) and S-Transform are discussed while most of the techniques provide similar results, the EMD analyses suffer some problems derived from the mode mixing in most of the Intrinsic Mode Functions (IMFs).

A wireless mesh sensing network for early warning

Earthquake early warning systems should provide reliable warnings as quickly as possible with a minimum number of false and missed alarms. Wireless meshed networks, coupled with low-cost seismometers for monitoring, evaluation, and information about seismic vibrations in space and time are introducing a new generation of warning infrastructures for mega-cities. The use of a cooperative method for signal analysis makes it possible to distinguish earthquakes (with a certain minimal magnitude) from other ground shaking in a city. The paper gives a short overview of our approach for developing decentralized early warning systems and an evaluation based on experiences gained from model investigations, testbeds in Berlin, and prototype installations in Istanbul.