Benjamin Edwards

Predicting ground motion from induced earthquakes in geothermal areas

Induced seismicity from anthropogenic sources can be a significant nuisance to a local population and in extreme cases lead to damage to vulnerable structures. One type of induced seismicity of particular recent concern, which, in some cases, can limit development of a potentially important clean energy source, is that associated with geothermal power production. A key requirement for the accurate assessment of seismic hazard (and risk) is a ground-motion prediction equation (GMPE) that predicts the level of earthquake shaking (in terms of, for example, peak ground acceleration) of an earthquake of a certain magnitude at a particular distance. Few such models currently exist in regard to geothermal-related seismicity, and consequently the evaluation of seismic hazard in the vicinity of geothermal power plants is associated with high uncertainty. Various ground-motion datasets of induced and natural seismicity (from Basel, Geysers, Hengill, Roswinkel, Soultz, and Voerendaal) were compiled and processed, and moment magnitudes for all events were recomputed homogeneously. These data are used to show that ground motions from induced and natural earthquakes cannot be statistically distinguished. Empirical GMPEs are derived from these data; and, although they have similar characteristics to recent GMPEs for natural and miningrelated seismicity, the standard deviations are higher. To account for epistemic uncertainties, stochastic models subsequently are developed based on a single corner frequency and with parameters constrained by the available data. Predicted ground motions from these models are fitted with functional forms to obtain easy-to-use GMPEs. These are associated with standard deviations derived from the empirical data to characterize aleatory variability. As an example, we demonstrate the potential use of these models using data from Campi Flegrei. Online Material: Sets of coefficients and standard deviations for various groundmotion models.

Derivation of a Reference Shear-Wave Velocity Model from Empirical Site Amplification

Abstract The definition of a reference bedrock condition representative of a region of interest is of great significance in seismic-hazard assessment. It is highly beneficial when ground-motion prediction equations are referenced to a specific site condition, particularly in the case of site-specific seismic-hazard analyses. When known, the effect of any given site with respect to the reference can then be applied to the predicted ground motion. However, the choice of a reference velocity profile is not straightforward, mainly due to the high variability of the velocity structure in the shallower layers. A new method to define the regional reference rock profile is proposed. The method relates quarter-wavelength average velocity at a site to frequency-dependent amplification. A reference bedrock velocity profile can then be directly defined in relation to expected amplification characteristics over a number of sites. We compare 27 quarter-wavelength velocity profiles from seismic station locations in Switzerland with empirical amplification functions derived from spectral modeling. From this comparison, a set of frequency-dependent calibration relationships is established. Assuming that the reference profile is defined by a lack of any relative amplification, the quarter-wavelength velocity profile that corresponds to unitary spectral amplification can be extracted. The reference velocity profile can then be obtained through an inversion procedure and defines the reference for the ground-motion prediction equation (GMPE). The proposed reference velocity profile is compared with previous reference velocity profiles. A good agreement is found between the different methods. Additionally, an estimation of the transfer function for the Swiss reference rock condition is provided. This can be used to correct recorded or estimated spectral amplitudes for the local response of the reference site. Finally, it is shown that the coefficients from the aforementioned correlations can be used to estimate a generic amplification function at any site with a known quarter-wavelength velocity profile.

A Stochastic Earthquake Ground‐Motion Prediction Model for the United Kingdom

Low‐seismicity regions such as the United Kingdom (UK) pose a challenge for seismic hazard analysis in view of the limited amount of locally recorded data available. In particular, ground‐motion prediction is faced with the problem that most of the instrumental observations available have been recorded at large distances from small earthquakes. Direct extrapolation of the results of regression on these data to the range of magnitudes and distances relevant for the seismic hazard analysis of engineered structures generally leads to unsatisfactory predictions. The present study presents a new ground‐motion prediction equation (GMPE) for the UK in terms of peak ground acceleration (PGA), peak ground velocity (PGV), and 5% damped pseudospectral acceleration (PSA), based on the results of numerical simulations using a stochastic point‐source model calibrated with parameters derived from local weak‐motion data. The predictions from this model are compared with those of previous GMPEs based on UK data, other GMPEs derived for stable continental regions (SCRs), as well as recent GMPEs developed for the wider European area.

Assessment of Site Effects in Alpine Regions through Systematic Site Characterization of Seismic Stations

Abstract In the framework of the renewal project of the Swiss Strong Motion Network (SSMNet), a procedure for site characterization has been established. The aim of the procedure was to systematically derive realistic 1D velocity profiles at each station. It is mainly based on the analysis of surface waves, particularly from passive experiments, and includes cross checks of the derived amplification functions with those obtained through spectral modeling of recorded earthquakes. The systematic use of three component surface‐wave analysis, allowing the derivation of both Rayleigh and Love dispersion curves, also contributes to the improvement of the quality of the retrieved profiles. The procedure is applied to the 30 SSMNet stations installed on various site types within the project, covering different aspects of seismic risk. The characterization of these 30 sites gives an overview of the variety of possible effects of surface geology on ground motion in the Alpine area. Such effects ranged from deamplification at hard‐rock sites to amplification up to a factor of 15 in lacustrine sediments with respect to the Swiss reference rock velocity model. The derived velocity profiles are shown to reproduce observed amplification functions from empirical spectral modeling. Although many sites are found to exhibit 1D behavior, the procedure allows the detection and qualification of 2D and 3D effects. The sites are therefore classified with respect to the occurrence of 2D/3D resonance and edge‐generated surface waves. In addition to the large and deeply incised alpine valleys of the Rhone, the Rhine, and the Aar, smaller structures such as local alpine valleys and alluvial fans are shown to exhibit 2D/3D behavior.

Developing an Application‐Specific Ground‐Motion Model for Induced Seismicity

Abstract A key element of quantifying both the hazard and risk due to induced earthquakes is a suite of appropriate ground‐motion prediction equations (GMPEs) that encompass the possible shaking levels due to such events. Induced earthquakes are likely to be of smaller magnitude and shallower focal depth than the tectonic earthquakes for which most GMPEs are derived. Furthermore, whereas GMPEs for moderate‐to‐large magnitude earthquakes are usually derived to be transportable to different locations and applications, taking advantage of the limited regional dependence observed for such events, the characteristics of induced earthquakes warrant the development of application‐specific models. A preliminary ground‐motion model for induced seismicity in the Groningen gas field in The Netherlands is presented as an illustration of a possible approach to the development of these equations. The GMPE is calibrated to local recordings of small‐magnitude events and captures the epistemic uncertainty in the extrapolation to larger magnitude considered in the assessment of the resulting hazard and risk.

A Comparative Study on Attenuation and Source-Scaling Relations in the Kantō, Tokai, and Chubu Regions of Japan, Using Data from Hi-Net and KiK-Net

Abstract Attenuation relations are derived for central Japan (broadly spanning the Kantō, Tokai, and Chubu regions) using recordings of small earthquakes (2.0≤ M JMA ≤4.0 [Japan Meterological Agency magnitude]) and moderate- to large-magnitude earthquakes (3.0≤ M JMA ≤7.2). We independently analyze data from both small-magnitude and moderate- to large-magnitude earthquakes to provide an insight into the use of attenuation relations derived in regions of low seismic activity. A strong correlation is found between the attenuation parameters derived from each dataset. We find that Q is strongly depth dependent and that apparent geometrical decay increases with increasing hypocentral distance. This is modeled by using a three segment decay function, with the initial decay forced to 1/ R . Moment magnitudes are close to the published M JMA magnitude, but are, on average, slightly higher. An increase in stress drop with magnitude is required in order to model both the small-magnitude and moderate- to large-magnitude datasets. Alternatively we show that a constant stress-drop model is suitable to model the response spectra of both small-magnitude and moderate- to large-magnitude earthquakes when considering the saturation of the source-corner frequency due to a static site filter such as f max or κ . We test our ability to predict strong ground motion by using our attenuation and source-scaling relations derived from the small-magnitude recordings to stochastically simulate peak ground acceleration, peak ground velocity, and 5% damped response spectral ordinates over a range of magnitudes and distances. The residuals of this simulation are found to be largely independent of distance and magnitude. We compare our attenuation relations against other relations derived for Japan. The residuals of these relations are analyzed and compared against those obtained from the model found in this study. We find that, in this study, the prediction of strong ground motions is possible using small-magnitude data and that the validity of the prediction extends across all magnitudes available for comparison (2.0≤ M JMA ≤7.8). On the other hand, by using an alternative published predictive relation for Japan, derived using large-magnitude events, peak ground acceleration is significantly overestimated for small-magnitude earthquakes.

Framework for a Ground-Motion Model for Induced Seismic Hazard and Risk Analysis in the Groningen Gas Field, The Netherlands

The potential for building damage and personal injury due to induced earthquakes in the Groningen gas field is being modeled in order to inform risk management decisions. To facilitate the quantitative estimation of the induced seismic hazard and risk, a ground motion prediction model has been developed for response spectral accelerations and duration due to these earthquakes that originate within the reservoir at 3 km depth. The model is consistent with the motions recorded from small-magnitude events and captures the epistemic uncertainty associated with extrapolation to larger magnitudes. In order to reflect the conditions in the field, the model first predicts accelerations at a rock horizon some 800 m below the surface and then convolves these motions with frequency-dependent nonlinear amplification factors assigned to zones across the study area. The variability of the ground motions is modeled in all of its constituent parts at the rock and surface levels.

Reference S‐Wave Velocity Profile and Attenuation Models for Ground‐Motion Prediction Equations: Application to Japan

Defining the reference rock or soil condition related to ground‐motion prediction is an important aspect of seismic‐hazard analysis. In a previous study by the authors, a method was proposed to establish a reference rock profile for Switzerland through the comparison of empirical amplification functions with shear‐wave velocity profiles at 27 selected sites of the Swiss National Seismic Network. The retrieved velocity profile served as reference for a regional ground‐motion prediction equation. However, a lacking piece of information remained: the anelastic attenuation for such a reference profile. Reference attenuation is essential to correctly model and interpret amplification at high frequencies. In the present study we extended our approach to simultaneously model both the reference shear‐wave velocity profile and the corresponding attenuation for Japan. We compared site‐specific attenuation measurements with quarter‐wavelength average velocities at 36 soil and rock sites from the Japanese KiK‐net strong‐motion network. The selected sites are characterized by a lack of observed resonance phenomena in order to avoid trade‐off between amplification and attenuation effects. We establish a parametric model through regression analysis. The resulting model gives us the possibility to estimate anelastic attenuation of a rock site with a given velocity profile and provides the base for host‐to‐target adjustments of real or modeled ground motion.

Characterizing the Vertical‐to‐Horizontal Ratio of Ground Motion at Soft‐Sediment Sites

A predictive equation to obtain the vertical‐to‐horizontal ratio (V/H) of ground motion for rock sites has been established in a previous article. The method was based on the comparison between V/H of Fourier and response spectra of earthquakes with the quarter‐wavelength average velocity at discrete frequencies. We extend this approach to account for resonance phenomena at soft‐sediment sites. In order to do so, a new parameter is defined and included in the comparison with the V/H spectra. The new parameter is directly derived from the quarter‐wavelength velocity and represents the frequency‐dependent seismic impedance contrast at the site. We show that extending the correlation in this three‐dimensional space is beneficial to reconstruct V/H of the 5%‐damped response spectra at soft‐sediment sites ( V S 30<800  m/s) for which a shear‐wave velocity profile is available. In this study we analyze 220 sites of the Japanese KiK‐net strong‐motion network. These sites were selected from the entire network through comparison of the fundamental frequencies estimated from the recordings and by indirect modeling methods. From the analysis, two types of predictive equations are then established, the first based on frequency‐dependent and the second on frequency‐independent correlations. These can subsequently be used to reconstruct the V/H spectrum at any site with a known shear‐wave velocity profile. For both equations, uncertainties of the V/H models are provided, and a sensitivity study to magnitude–distance dependence is presented. Finally, we show an example of the application of the model at four selected soft‐sediment sites of the Swiss Seismic Network.

Scenario dependence of linear site effect factors for short-period response spectral ordinates

Ground-motion models for response spectral ordinates commonly partition site-response effects into linear and nonlinear components. The nonlinear components depend upon the earthquake scenario being considered implicitly through the use of the expected level of excitation at some reference horizon. The linear components are always assumed to be independent of the earthquake scenario. This article presents empirical and numerical evidence as well as a theoretical explanation for why the linear component of site response depends upon the magnitude and distance of the earthquake scenario. Although the impact is most pronounced for small-magnitude scenarios, the finding has significant implications for a number of applications of more general interest including the development of site-response terms within ground-motion models, the estimation of ground-motion variability components φS2S and φSS, the construction of partially nonergodic models for site-specific hazard assessments, and the validity of the convolution approach for computing surface hazard curves from those at a reference horizon, among others. All of these implications are discussed in the present article.