Developing an integrated 3D geological-seismological model at urban scale in Basel, Switzerland

<p>Assessment of seismic risk at a local scale is fundamental to the adoption of efficient risk mitigation strategies for urban areas with spatially distributed building portfolios and infrastructure systems. An important component of such a study is to estimate the spatial distribution of the expected seismic ground motion induced by site response. The current work presents a detailed seismic site response study at urban scale, performed in the context of developing an earthquake risk model for the Swiss canton of Basel-Stadt. Different studies undertaken over last two decades in the area concluded that unconsolidated sediments were responsible for inducing resonances and significant amplification of seismic waves over a range of frequencies pertinent to engineering interest. Therefore, we make a step forward in this study by attempting to develop a three-dimensional (3D) integrated geological-seismological model, which will explicitly account for the complex geological conditions at the surface and at depth. Thanks to the past projects, there is an abundance of geological, geophysical and seismological data for Basel. Earthquake recordings are available from an operating network of more than 20 permanent stations as well as from several former and six current temporary stations. Ambient noise measurements are available from several hundred single stations and more than 25 passive seismic arrays. In addition, a number of active seismic measurements and borehole logs are also available. An updated 3D model of subsurface geological structure of the area has been provided by the team of Applied and Environmental Geology (AUG) of University of Basel.</p><p>We use dispersion characteristics of surface waves from ambient vibration array data for imaging subsurface shear wave velocity (Vs) profiles. We apply a novel approach based on a Multizonal Transdimensional Inversion (MTI), formulated in the Bayesian probabilistic framework, in order to retrieve 1D Vs profiles from ambient vibration arrays. A joint inversion of multimodal Rayleigh and Love wave dispersion curves along with Rayleigh wave ellipticity curve is performed. This is a major improvement as such joint inversions were performed only for few sites in this area. The key advantages of MTI are that the model complexity in terms of number of layers and distribution of associated parameters are determined self-adaptively from the measured data, and model uncertainties can be assessed quantitatively. Additional constraints on the depths of intermediate layers are drawn from the 3D geological model and boreholes for the multizonal inversion. Moreover, the solution of the transdimensional Bayesian inversion enables reconstruction of the posterior probability density function of prior model parameters and their properties from the ensemble of inverted models. Hence, the model uncertainty can be duly propagated from dispersion curves to Vs profiles. The initial results seem very promising in resolving the interfaces corresponding to major velocity contrasts, especially in the complex sedimentary structure of the Rhine Graben formation. The ongoing analysis will also better identify composition, geometry, thickness and topography of the surficial unconsolidated sediments as well as the underlying more consolidated layers, which will form the basis for future numerical simulations of earthquake ground motion.</p>

A Computational Tool for Ground-Motion Simulations Incorporating Regional Crustal Conditions

This article introduces a computational tool, namely ground-motion simulation system (GMSS), for generating synthetic accelerograms based on stochastic simulations. The distinctive feature of GMSS is that it has two independently developed upper-crustal models (expressed in the form of shear-wave velocity profiles), which have been built into the program for deriving the frequency-dependent crustal factors, and one of these models was originally developed by the authors. GMSS also has provisions to allow the user to specify their own preferred crustal profile. Sufficient details of both crustal models (forming part of the seismological model) and the accelerogram simulation methodology are presented herein in one article, to allow any person who has programming skills (on a user-friendly platform such as MATLAB, see Data and Resources), to develop their computational tools to implement any further innovations in crustal modeling for direct engineering applications. Crustal properties deep into bedrock can only be accounted for implicitly by conventional ground-motion prediction equation (GMPE) as much depends on the region where the ground motion was recorded. This limitation of existing GMPEs poses a challenge to engineering in regions that are not well represented by any strong-motion database. Toward the end of this article, readers are enlightened with the potential transdisciplinary utility of using GMSS, to facilitate the retrieval and scaling of accelerograms sourced from a database of real earthquake records through the construction of a conditional mean spectrum.

Field-wide reservoir compressibility estimation through inversion of subsidence data above the Groningen gas field

Not long after discovery of the Groningen field, gas-production-induced compaction and consequent land subsidence was recognised to be a potential threat to groundwater management in the province of Groningen, in addition to the fact that parts of the province lie below sea level. More recently, NAM's seismological model also pointed to a correlation between reservoir compaction and the observed induced seismicity above the field. In addition to the already existing requirement for accurate subsidence predictions, this demanded a more accurate description of the expected spatial and temporal development of compaction.Since the start of production in 1963, multiple levelling campaigns have gathered a unique set of deformation measurements used to calibrate geomechanical models. In this paper we present a methodology to model compaction and subsidence, combining results from rock mechanics experiments and surface deformation measurements. Besides the optical spirit-levelling data, InSAR data are also used for inversion to compaction and calibration of compaction models. Residual analysis, i.e. analysis of the difference between measurement and model output, provides confidence in the model results used for subsidence forecasting and as input to seismological models.

Development of statistical geomechanical models for forecasting seismicity induced by gas production from the Groningen field

This paper reviews the evolution of a sequence of seismological models developed and implemented as part of a workflow for Probabilistic Seismic Hazard and Risk Assessment of the seismicity induced by gas production from the Groningen gas field. These are semi-empirical statistical geomechanical models derived from observations of production-induced seismicity, reservoir compaction and structure of the field itself. Initial versions of the seismological model were based on a characterisation of the seismicity in terms of its moment budget. Subsequent versions of the model were formulated in terms of seismic event rates, this change being driven in part by the reduction in variability of the model forecasts in this domain. Our approach makes use of the Epidemic Type After Shock model (ETAS) to characterise spatial and temporal clustering of earthquakes and has been extended to also incorporate the concentration of moment release on pre-existing faults and other reservoir topographic structures.

THE EFFECTIVENESS OF DECONVOLUTION PROCESS IN PRESENCE OF RANDOM NOISE ON THE BASE OF ROCK SALT DEPOSITS FROM “BYTOM ODRZAŃSKI” AREA

A number of synthetic pseudoacoustic impedance sections are presented in order to test the effectiveness of deconvolution process when random noise distorted the seismic traces. A simplified seismological model is created on the basis of geological data from Bytom Odrzański area in NW part of the Legnica-Głogów Copper District (LGOM). The synthetic sections are constructed for different noise levels. Acoustic impedance is one of the basic factors characterising physical features of rocks. The main idea is the inversion of seismic sections into pseudoacoustic impedance sections. All inhomogeneities of salt deposits must be predicted before the underground storage location is fixed. The accuracy and reliability of interpretation decreases when the noise in seismic data increases. It should be realised that the inversion procedure is not a unique process. So, modelling pseudoacoustic impedance sections is recommended for verification of the effectiveness of deconvolution process for the accuracy and reliability interpretation.