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 - Rumpf, Michael A1 - Tronicke, Jens T1 - Assessing uncertainty in refraction seismic traveltime inversion using a global inversion strategy JF - Geophysical prospecting N2 - To analyse and invert refraction seismic travel time data, different approaches and techniques have been proposed. One common approach is to invert first-break travel times employing local optimization approaches. However, these approaches result in a single velocity model, and it is difficult to assess the quality and to quantify uncertainties and non-uniqueness of the found solution. To address these problems, we propose an inversion strategy relying on a global optimization approach known as particle swarm optimization. With this approach we generate an ensemble of acceptable velocity models, i.e., models explaining our data equally well. We test and evaluate our approach using synthetic seismic travel times and field data collected across a creeping hillslope in the Austrian Alps. Our synthetic study mimics a layered near-surface environment, including a sharp velocity increase with depth and complex refractor topography. Analysing the generated ensemble of acceptable solutions using different statistical measures demonstrates that our inversion strategy is able to reconstruct the input velocity model, including reasonable, quantitative estimates of uncertainty. Our field data set is inverted, employing the same strategy, and we further compare our results with the velocity model obtained by a standard local optimization approach and the information from a nearby borehole. This comparison shows that both inversion strategies result in geologically reasonable models (in agreement with the borehole information). However, analysing the model variability of the ensemble generated using our global approach indicates that the result of the local optimization approach is part of this model ensemble. Our results show the benefit of employing a global inversion strategy to generate near-surface velocity models from refraction seismic data sets, especially in cases where no detailed a priori information regarding subsurface structures and velocity variations is available. KW - Inversion KW - Seismic refraction KW - Uncertainty Y1 - 2015 U6 - https://doi.org/10.1111/1365-2478.12240 SN - 0016-8025 SN - 1365-2478 VL - 63 IS - 5 SP - 1188 EP - 1197 PB - Wiley-Blackwell CY - Hoboken ER - TY - JOUR A1 - Asari, Seiki A1 - Wardinski, Ingo T1 - On magnetic estimation of Earth's core angular momentum variation JF - Journal of geophysical research : Solid earth N2 - We study systematically the estimation of Earth's core angular momentum (CAM) variation between 1962.0 and 2008.0 by using core surface flow models derived from the recent geomagnetic field model C(3)FM2. Various flow models are derived by changing four parameters that control the least squares flow inversion. The parameters include the spherical harmonic (SH) truncation degree of the flow models and two Lagrange multipliers that control the weights of two additional constraints. The first constraint forces the energy spectrum of the flow solution to follow a power law l-p, where l is the SH degree and p is the fourth parameter. The second allows to modulate the solution continuously between the dynamical states of tangential geostrophy (TG) and tangential magnetostrophy (TM). The calculated CAM variations are examined in reference to two features of the observed length-of-day (LOD) variation, namely, its secular trend and 6year oscillation. We find flow models in either TG or TM state for which the estimated CAM trends agree with the LOD trend. It is necessary for TM models to have their flows dominate at planetary scales, whereas TG models should not be of this scale; otherwise, their CAM trends are too steep. These two distinct types of flow model appear to correspond to the separate regimes of previous numerical dynamos that are thought to be applicable to the Earth's core. The phase of the subdecadal CAM variation is coherently determined from flow models obtained with extensively varying inversion settings. Multiple sources of model ambiguity need to be allowed for in discussing whether these phase estimates properly represent that of Earth's CAM as an origin of the observed 6year LOD oscillation. KW - Core KW - Earth rotation KW - Inversion KW - Geomagnetic field KW - rapid variations KW - Geodynamo Y1 - 2015 U6 - https://doi.org/10.1002/2014JB011458 SN - 2169-9313 SN - 2169-9356 VL - 120 IS - 10 SP - 6740 EP - 6757 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Garofalo, F. A1 - Foti, S. A1 - Hollender, F. A1 - Bard, Pierre-Yves A1 - Cornou, C. A1 - Cox, B. R. A1 - Ohrnberger, Matthias A1 - Sicilia, D. A1 - Asten, M. A1 - Di Giulio, G. A1 - Forbriger, T. A1 - Guillier, B. A1 - Hayashi, K. A1 - Martin, A. A1 - Matsushima, Satoru A1 - Mercerat, D. A1 - Poggi, V. A1 - Yamanaka, H. T1 - InterPACIFIC project: Comparison of invasive and non-invasive methods for seismic site characterization. Part I: Intra-comparison of surface wave methods JF - Soil Dynamics and Earthquake Engineering N2 - The main scope of the InterPACIFIC (Intercomparison of methods for site parameter and velocity profile characterization) project is to assess the reliability of in-hole and surface-wave methods, used for estimating shear wave velocity. Three test-sites with different subsurface conditions were chosen: a soft soil, a stiff soil and a rock outcrop. This paper reports the surface-wave methods results. Specifically 14 teams of expert users analysed the same experimental surface-wave datasets, consisting of both passive and active data. Each team adopted their own strategy to retrieve the dispersion curve and the shear-wave velocity profile at each site. Despite different approaches, the dispersion curves are quite in agreement with each other. Conversely, the shear-wave velocity profiles show a certain variability that increases in correspondence of major stratigraphic interfaces. This larger variability is mainly due to non-uniqueness of the solution and lateral variability. As expected, the observed variability in V-s,V-30 estimatesis small, as solution non-uniqueness plays a limited role. (C) 2015 Elsevier Ltd. All rights reserved. KW - Surface-wave methods KW - Dispersion curve KW - Inversion KW - V-s,V-30 KW - Site characterization KW - MASW KW - Microtremors KW - Rayleigh waves KW - Geophysical methods Y1 - 2016 U6 - https://doi.org/10.1016/j.soildyn.2015.12.010 SN - 0267-7261 SN - 1879-341X VL - 82 SP - 222 EP - 240 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Lontsi, Agostiny Marrios A1 - Ohrnberger, Matthias A1 - Krüger, Frank T1 - Shear wave velocity profile estimation by integrated analysis of active and passive seismic data from small aperture arrays JF - Journal of applied geophysics N2 - We present an integrated approach for deriving the 1D shear wave velocity (Vs) information at few tens to hundreds of meters down to the first strong impedance contrast in typical sedimentary environments. We use multiple small aperture seismic arrays in 1D and 2D configuration to record active and passive seismic surface wave data at two selected geotechnical sites in Germany (Horstwalde & Lobnitz). Standard methods for data processing include the Multichannel Analysis of Surface Waves (MASW) method that exploits the high frequency content in the active data and the sliding window frequency-wavenumber (f-k) as well as the spatial autocorrelation (SPAC) methods that exploit the low frequency content in passive seismic data. Applied individually, each of the passive methods might be influenced by any source directivity in the noise wavefield. The advantages of active shot data (known source location) and passive microtremor (low frequency content) recording may be combined using a correlation based approach applied to the passive data in the so called Interferometric Multichannel Analysis of Surface Waves (IMASW). In this study, we apply those methods to jointly determine and interpret the dispersion characteristics of surface waves recorded at Horstwalde and Lobnitz. The reliability of the dispersion curves is controlled by applying strict limits on the interpretable range of wavelengths in the analysis and further avoiding potentially biased phase velocity estimates from the passive f-k method by comparing to those derived from the SPatial AutoCorrelation method (SPAC). From our investigation at these two sites, the joint analysis as proposed allows mode extraction in a wide frequency range (similar to 0.6-35 Hz at Horstwalde and similar to 1.5-25 Hz at Lobnitz) and consequently improves the Vs profile inversion. To obtain the shear wave velocity profiles, we make use of a global inversion approach based on the neighborhood algorithm to invert the interpreted branches of the dispersion curves. Within the uncertainty given by the apparent spread of forward models we find that besides a well defined sediment velocity range also a reasonable minimum estimate of bedrock depth and bedrock velocity can be achieved. The Vs estimate for the best model in Horstwalde ranges from similar to 190 m/s at the surface up to similar to 390 m/s in the bottom of the soft sediment column. The bedrock starts earliest around 200 m depth and bedrock velocities are higher than 1000 m/s. In Lobnitz, we observe slightly lower velocities for the sediments (similar to 165-375 m/s for the best model) and a minimum thickness of 75 m. (C) 2016 Elsevier B.V. All rights reserved. KW - Active seismic KW - Passive seismic KW - Virtual active seismic KW - Dispersion curves KW - Inversion KW - V-s profiles Y1 - 2016 U6 - https://doi.org/10.1016/j.jappgeo.2016.03.034 SN - 0926-9851 SN - 1879-1859 VL - 130 SP - 37 EP - 52 PB - Elsevier CY - Amsterdam ER -