TY - JOUR A1 - Di Giacomo, Domenico A1 - Bindi, Dino A1 - Parolai, Stefano A1 - Oth, Adrien T1 - Residual analysis of teleseismic P-wave energy magnitude estimates: inter- and intrastation variability JF - Geophysical journal international N2 - P>Computing the magnitude of an earthquake requires correcting for the propagation effects from the source to the receivers. This is often accomplished by performing numerical simulations using a suitable Earth model. In this work, the energy magnitude M(e) is considered and its determination is performed using theoretical spectral amplitude decay functions over teleseismic distances based on the global Earth model AK135Q. Since the high frequency part (above the corner frequency) of the source spectrum has to be considered in computing M(e), the influence of propagation and site effects may not be negligible and they could bias the single station M(e) estimations. Therefore, in this study we assess the inter- and intrastation distributions of errors by considering the M(e) residuals computed for a large data set of earthquakes recorded at teleseismic distances by seismic stations deployed worldwide. To separate the inter- and intrastation contribution of errors, we apply a maximum likelihood approach to the M(e) residuals. We show that the interstation errors (describing a sort of site effect for a station) are within +/- 0.2 magnitude units for most stations and their spatial distribution reflects the expected lateral variation affecting the velocity and attenuation of the Earth's structure in the uppermost layers, not accounted for by the 1-D AK135Q model. The variance of the intrastation error distribution (describing the record-to-record component of variability) is larger than the interstation one (0.240 against 0.159), and the spatial distribution of the errors is not random but shows specific patterns depending on the source-to-station paths. The set of coefficients empirically determined may be used in the future to account for the heterogeneities of the real Earth not considered in the theoretical calculations of the spectral amplitude decay functions used to correct the recorded data for propagation effects. KW - Time series analysis KW - Earthquake source observations KW - Body waves KW - Site effects KW - Wave propagation Y1 - 2011 U6 - https://doi.org/10.1111/j.1365-246X.2011.05019.x SN - 0956-540X VL - 185 IS - 3 SP - 1444 EP - 1454 PB - Wiley-Blackwell CY - Malden ER - TY - JOUR A1 - Tran Thanh Tuan, A1 - Scherbaum, Frank A1 - Malischewsky, Peter G. T1 - On the relationship of peaks and troughs of the ellipticity (H/V) of Rayleigh waves and the transmission response of single layer over half-space models JF - Geophysical journal international N2 - One of the key challenges in the context of local site effect studies is the determination of frequencies where the shakeability of the ground is enhanced. In this context, the H/V technique has become increasingly popular and peak frequencies of H/V spectral ratio are sometimes interpreted as resonance frequencies of the transmission response. In the present study, assuming that Rayleigh surface wave is dominant in H/V spectral ratio, we analyse theoretically under which conditions this may be justified and when not. We focus on 'layer over half-space' models which, although seemingly simple, capture many aspects of local site effects in real sedimentary structures. Our starting point is the ellipticity of Rayleigh waves. We use the exact formula of the H/V-ratio presented by Malischewsky & Scherbaum (2004) to investigate the main characteristics of peak and trough frequencies. We present a simple formula illustrating if and where H/V-ratio curves have sharp peaks in dependence of model parameters. In addition, we have constructed a map, which demonstrates the relation between the H/V-peak frequency and the peak frequency of the transmission response in the domain of the layer's Poisson ratio and the impedance contrast. Finally, we have derived maps showing the relationship between the H/V-peak and trough frequency and key parameters of the model such as impedance contrast. These maps are seen as diagnostic tools, which can help to guide the interpretation of H/V spectral ratio diagrams in the context of site effect studies. KW - Site effects KW - Theoretical seismology KW - Wave propagation Y1 - 2011 U6 - https://doi.org/10.1111/j.1365-246X.2010.04863.x SN - 0956-540X VL - 184 IS - 2 SP - 793 EP - 800 PB - Wiley-Blackwell CY - Malden ER - TY - JOUR A1 - Pilz, Marco A1 - Parolai, Stefano A1 - Stupazzini, Marco A1 - Paolucci, Roberto A1 - Zschau, Jochen T1 - Modelling basin effects on earthquake ground motion in the Santiago de Chile basin by a spectral element code JF - Geophysical journal international N2 - Simulations of strong ground motion within the Santiago de Chile Metropolitan area were carried out by means of 3-D deterministic wave propagation tool based on the spectral element method. The simulated events take into account the pronounced interface between the low-velocity sedimentary basin and the bedrock as well as topography of the area. To verify our model we simulated a regional earthquake recorded by a dense network installed in the city of Santiago for recording aftershock activity after the 2010 February 27 Maule main shock. The results proof the alluvial basin amplification effects and show a strong dependence of spectral amplification in the basin on the local site conditions. Moreover, we studied the seismic response due to a hypothetical M(w) = 6.0 event occurring along the active San Ramon Fault, which is crossing the eastern edge of the city. The scenario earthquakes exhibit that an unfavourable interaction between fault rupture, radiation mechanism and complex geological and topographic conditions in the near-field region may give rise to large values of peak ground velocity in the basin. Finally, 3-D numerical predictions of ground motion are compared with the one computed according to ground motion prediction equations selected among the next generation attenuation relationships, in terms of ground motion peak values and spectral acceleration. The comparison underlines that the 3-D scenario simulations predict a significantly higher level of ground motion in the Santiago basin, especially over deep alluvial deposits. Moreover, also the location of the rupture nucleation largely influences the observed shaking pattern. KW - Earthquake ground motions KW - Site effects KW - Wave propagation KW - South America Y1 - 2011 U6 - https://doi.org/10.1111/j.1365-246X.2011.05183.x SN - 0956-540X VL - 187 IS - 2 SP - 929 EP - 945 PB - Wiley-Blackwell CY - Malden ER - TY - JOUR A1 - Pilz, Marco A1 - Parolai, Stefano A1 - Picozzi, Matteo A1 - Zschau, Jochen T1 - Evaluation of proxies for seismic site conditions in large urban areas the example of Santiago de Chile JF - Physics and chemistry of the earth N2 - Characterizing the local site response in large cities is an important step towards seismic hazard assessment. To this regard, single station seismic noise measurements were carried out at 146 sites in the northern part of Santiago de Chile. This extensive survey allowed the fundamental resonance frequency of the sedimentary cover, derived from horizontal-to-vertical (H/V) spectral ratios, to be mapped. By inverting the spectral ratios under the constraint of the thickness of the sedimentary cover, known from previous gravimetric measurements, local S-wave velocity profiles have been retrieved. After interpolation between the individual profiles, the resulting high resolution 3D S-wave velocity model allows the entire area, as well as deeper parts of the basin, to be represented in great detail. Since one lithology shows a great scatter in the velocity values only a very general correlation between S-wave velocity in the uppermost 30 m (v(s)(30)) and local geology is found. Local S-wave velocity profiles can serve as a key factor in seismic hazard assessment, since they allow an estimate of the amplification potential of the sedimentary cover. Mapping the intensity distribution of the 27 February 2010 Maule, Chile, event (Mw = 8.8) the results indicate that local amplification of the ground motion might partially explain the damage distribution and encourage the use of the low cost seismic noise techniques for the study of seismic site effects. KW - Ambient seismic noise KW - H/V ratio KW - Inversion KW - S-wave velocity KW - Site effects Y1 - 2011 U6 - https://doi.org/10.1016/j.pce.2011.01.007 SN - 1474-7065 VL - 36 IS - 16 SP - 1259 EP - 1266 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Strollo, Angelo A1 - Parolai, Stefano A1 - Bindi, Dino A1 - Chiauzzi, Leonardo A1 - Pagliuca, Rossella A1 - Mucciarelli, Marco A1 - Zschau, Jochen T1 - Microzonation of Potenza (Southern Italy) in terms of spectral intensity ratio using joint analysis of earthquakes and ambient noise JF - Bulletin of earthquake engineering : official publication of the European Association for Earthquake Engineering N2 - A temporary seismic network composed of 11 stations was installed in the city of Potenza (Southern Italy) to record local and regional seismicity within the context of a national project funded by the Italian Department of Civil Protection (DPC). Some stations were moved after a certain time in order to increase the number of measurement points, leading to a total of 14 sites within the city by the end of the experiment. Recordings from 26 local earthquakes (M-l 2.2-3.8 ) were analyzed to compute the site responses at the 14 sites by applying both reference and non-reference site techniques. Furthermore, the Spectral Intensity (SI) for each local earthquake, as well as their ratios with respect to the values obtained at a reference site, were also calculated. In addition, a field survey of 233 single station noise measurements within the city was carried out to increase the information available at localities different from the 14 monitoring sites. By using the results of the correlation analysis between the horizontal-to-vertical spectral ratios computed from noise recordings (NHV) at the 14 selected sites and those derived by the single station noise measurements within the town as a proxy, the spectral intensity correction factors for site amplification obtained from earthquake analysis were extended to the entire city area. This procedure allowed us to provide a microzonation map of the urban area that can be directly used when calculating risk scenarios for civil defence purposes. The amplification factors estimated following this approach show values increasing along the main valley toward east where the detrital and alluvial complexes reach their maximum thickness. KW - Site effects KW - Seismic noise KW - Earthquakes KW - Spectral intensity KW - Correlation Y1 - 2012 U6 - https://doi.org/10.1007/s10518-011-9256-4 SN - 1570-761X VL - 10 IS - 2 SP - 493 EP - 516 PB - Springer CY - Dordrecht ER - TY - JOUR A1 - Hobiger, M. A1 - Cornou, C. A1 - Wathelet, M. A1 - Di Giulio, G. A1 - Knapmeyer-Endrun, B. A1 - Renalier, F. A1 - Bard, Pierre-Yves A1 - Savvaidis, Alexandros A1 - Hailemikael, S. A1 - Le Bihan, N. A1 - Ohrnberger, Matthias A1 - Theodoulidis, N. T1 - Ground structure imaging by inversions of Rayleigh wave ellipticity sensitivity analysis and application to European strong-motion sites JF - Geophysical journal international N2 - The knowledge of the local soil structure is important for the assessment of seismic hazards. A widespread, but time-consuming technique to retrieve the parameters of the local underground is the drilling of boreholes. Another way to obtain the shear wave velocity profile at a given location is the inversion of surface wave dispersion curves. To ensure a good resolution for both superficial and deeper layers, the used dispersion curves need to cover a wide frequency range. This wide frequency range can be obtained using several arrays of seismic sensors or a single array comprising a large number of sensors. Consequently, these measurements are time-consuming. A simpler alternative is provided by the use of the ellipticity of Rayleigh waves. The frequency dependence of the ellipticity is tightly linked to the shear wave velocity profile. Furthermore, it can be measured using a single seismic sensor. As soil structures obtained by scaling of a given model exhibit the same ellipticity curve, any inversion of the ellipticity curve alone will be ambiguous. Therefore, additional measurements which fix the absolute value of the shear wave velocity profile at some points have to be included in the inversion process. Small-scale spatial autocorrelation measurements or MASW measurements can provide the needed data. Using a theoretical soil structure, we show which parts of the ellipticity curve have to be included in the inversion process to get a reliable result and which parts can be omitted. Furthermore, the use of autocorrelation or high-frequency dispersion curves will be highlighted. The resulting guidelines for inversions including ellipticity data are then applied to real data measurements collected at 14 different sites during the European NERIES project. It is found that the results are in good agreement with dispersion curve measurements. Furthermore, the method can help in identifying the mode of Rayleigh waves in dispersion curve measurements. KW - Inverse theory KW - Surface waves and free oscillations KW - Site effects KW - Computational seismology KW - Wave propagation Y1 - 2013 U6 - https://doi.org/10.1093/gji/ggs005 SN - 0956-540X VL - 192 IS - 1 SP - 207 EP - 229 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Lontsi, Agostiny Marrios A1 - Jose Sanchez-Sesma, Francisco A1 - Camillo Molina-Villegas, Juan A1 - Ohrnberger, Matthias A1 - Krüger, Frank T1 - Full microtremor H/V(z,f) inversion for shallow subsurface characterization JF - Geophysical journal international N2 - The H/V spectral ratio has emerged as a single station method within the seismic ambient noise analysis field by its capability to quickly estimate the frequency of resonance at a site and through inversion the average profile information. Although it is easy to compute from experimental data, its counter theoretical part is not obvious when building a forward model which can help in reconstructing the derived H/V spectrum. This has led to the simplified assumption that the noise wavefield is mainly composed of Rayleigh waves and the derived H/V often used without further correction. Furthermore, only the right (and left) flank around the H/V peak frequency is considered in the inversion for the subsurface 1-D shear wave velocity profile. A new theoretical approach for the interpretation of the H/V spectral ratio has been presented by Sanchez-Sesmaet al. In this paper, the fundamental idea behind their theory is presented as it applies to receivers at depth. A smooth H/V(z, f) spectral curve on a broad frequency range is obtained by considering a fine integration step which is in turn time consuming. We show that for practical purposes and in the context of inversion, this can be considerably optimized by using a coarse integration step combined with the smoothing of the corresponding directional energy density (DED) spectrum. Further analysis shows that the obtained H/V(z, f) spectrum computed by the mean of the imaginary part of Green's function method could also be recovered using the reflectivity method for a medium well illuminated by seismic sources. Inversion of synthetic H/V(z, f) spectral curve is performed for a single layer over a half space. The striking results allow to potentially use the new theory as a forward computation of the H/V(z, f) to fully invert the experimental H/V spectral ratio at the corresponding depth for the shear velocity profile (Vs) and additionally the compressional velocity profile (Vp) using receivers both at the surface and in depth. We use seismic ambient noise data in the frequency range of 0.2-50 Hz recorded at two selected sites in Germany where borehole information is also available. The obtained 1-D Vs and Vp profiles are correlated with geological log information. Results from shallow geophysical experiment are also used for comparison. KW - Inverse theory KW - Interferometry KW - Site effects Y1 - 2015 U6 - https://doi.org/10.1093/gji/ggv132 SN - 0956-540X SN - 1365-246X VL - 202 IS - 1 SP - 298 EP - 312 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Wathelet, Marc A1 - Guillier, B. A1 - Roux, P. A1 - Cornou, C. A1 - Ohrnberger, Matthias T1 - Rayleigh wave three-component beamforming BT - signed ellipticity assessment from high-resolution frequency-wavenumber processing of ambient vibration arrays JF - Geophysical journal international N2 - The variation of Rayleigh ellipticity versus frequency is gaining popularity in site characterization. It becomes a necessary observable to complement dispersion curves when inverting shear wave velocity profiles. Various methods have been proposed so far to extract polarization from ambient vibrations recorded on a single three-component station or with an array of three-component sensors. If only absolute values were recovered 10 yr ago, new array-based techniques were recently proposed with enhanced efficiencies providing also the ellipticity sign. With array processing, higher-order modes are often detected even in the ellipticity domain. We suggest to explore the properties of a high-resolution beamforming where radial and vertical components are explicitly included. If N is the number of three-component sensors, 2N x 2N cross-spectral density matrices are calculated for all presumed directions of propagation. They are built with N radial and N vertical channels. As a first approach, steering vectors are designed to fit with Rayleigh wave properties: the phase shift between radial and vertical components is either -Pi/2 or Pi/2. We show that neglecting the ellipticity tilt due to attenuation has only minor effects on the results. Additionally, we prove analytically that it is possible to retrieve the ellipticity value from the usual maximization of the high-resolution beam power. The method is tested on synthetic data sets and on experimental data. Both are reference sites already analysed by several authors. A detailed comparison with previous results on these cases is provided. KW - Fourier analysis KW - Time-series analysis KW - Site effects KW - Surface waves and free oscillations KW - Wave propagation Y1 - 2018 U6 - https://doi.org/10.1093/gji/ggy286 SN - 0956-540X SN - 1365-246X VL - 215 IS - 1 SP - 507 EP - 523 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Zhu, Chuanbin A1 - Pilz, Marco A1 - Cotton, Fabrice Pierre T1 - Which is a better proxy, site period or depth to bedrock, in modelling linear site response in addition to the average shear-wave velocity? JF - Bulletin of earthquake engineering : official publication of the European Association for Earthquake Engineering N2 - This study aims to identify the best-performing site characterization proxy alternative and complementary to the conventional 30 m average shear-wave velocity V-S30, as well as the optimal combination of proxies in characterizing linear site response. Investigated proxies include T-0 (site fundamental period obtained from earthquake horizontal-to-vertical spectral ratios), V-Sz (measured average shear-wave velocities to depth z, z = 5, 10, 20 and 30 m), Z(0.8) and Z(1.0) (measured site depths to layers having shear-wave velocity 0.8 and 1.0 km/s, respectively), as well as Z(x-infer) (inferred site depths from a regional velocity model, x = 0.8 and 1.0, 1.5 and 2.5 km/s). To evaluate the performance of a site proxy or a combination, a total of 1840 surface-borehole recordings is selected from KiK-net database. Site amplifications are derived using surface-to-borehole response-, Fourier- and cross-spectral ratio techniques and then are compared across approaches. Next, the efficacies of 7 single-proxies and 11 proxy-pairs are quantified based on the site-to-site standard deviation of amplification residuals of observation about prediction using the proxy or the pair. Our results show that T-0 is the best-performing single-proxy among T-0, Z(0.8), Z(1.0) and V-Sz. Meanwhile, T-0 is also the best-performing proxy among T-0, Z(0.8), Z(1.0) and Z(x-infer) complementary to V-S30 in accounting for the residual amplification after V-S30-correction. Besides, T-0 alone can capture most of the site effects and should be utilized as the primary site indicator. Though (T-0, V-S30) is the best-performing proxy pair among (V-S30, T-0), (V-S30, Z(0.8)), (V-S30, Z(1.0)), (V-S30, Z(x-infer)) and (T-0, V-Sz), it is only slightly better than (T-0, V-S20). Considering both efficacy and engineering utility, the combination of T-0 (primary) and V-S20 (secondary) is recommended. Further study is needed to test the performances of various proxies on sites in deep sedimentary basins. KW - Site effects KW - Amplification KW - Site proxy KW - Surface-to-borehole spectral ratios KW - KiK-net KW - Earthquake Y1 - 2019 U6 - https://doi.org/10.1007/s10518-019-00738-6 SN - 1570-761X SN - 1573-1456 VL - 18 IS - 3 SP - 797 EP - 820 PB - Springer CY - Dordrecht ER - TY - JOUR A1 - Lontsi, Agostiny Marrios A1 - Garcia-Jerez, Antonio A1 - Camilo Molina-Villegas, Juan A1 - Jose Sanchez-Sesma, Francisco A1 - Molkenthin, Christian A1 - Ohrnberger, Matthias A1 - Krüger, Frank A1 - Wang, Rongjiang A1 - Fah, Donat T1 - A generalized theory for full microtremor horizontal-to-vertical [H/V(z,f)] spectral ratio interpretation in offshore and onshore environments JF - Geophysical journal international N2 - Advances in the field of seismic interferometry have provided a basic theoretical interpretation to the full spectrum of the microtremor horizontal-to-vertical spectral ratio [H/V(f)]. The interpretation has been applied to ambient seismic noise data recorded both at the surface and at depth. The new algorithm, based on the diffuse wavefield assumption, has been used in inversion schemes to estimate seismic wave velocity profiles that are useful input information for engineering and exploration seismology both for earthquake hazard estimation and to characterize surficial sediments. However, until now, the developed algorithms are only suitable for on land environments with no offshore consideration. Here, the microtremor H/V(z, f) modelling is extended for applications to marine sedimentary environments for a 1-D layered medium. The layer propagator matrix formulation is used for the computation of the required Green’s functions. Therefore, in the presence of a water layer on top, the propagator matrix for the uppermost layer is defined to account for the properties of the water column. As an application example we analyse eight simple canonical layered earth models. Frequencies ranging from 0.2 to 50 Hz are considered as they cover a broad wavelength interval and aid in practice to investigate subsurface structures in the depth range from a few meters to a few hundreds of meters. Results show a marginal variation of 8 per cent at most for the fundamental frequency when a water layer is present. The water layer leads to variations in H/V peak amplitude of up to 50 per cent atop the solid layers. KW - Numerical modelling KW - Earthquake hazards KW - Seismic interferometry KW - Site effects KW - Theoretical seismology KW - Wave propagation Y1 - 2019 U6 - https://doi.org/10.1093/gji/ggz223 SN - 0956-540X SN - 1365-246X VL - 218 IS - 2 SP - 1276 EP - 1297 PB - Oxford Univ. Press CY - Oxford ER -