Hilmar Bungum

On the Use of Logic Trees for Ground-Motion Prediction Equations in Seismic-Hazard Analysis

Logic trees are widely used in probabilistic seismic hazard analysis as a tool to capture the epistemic uncertainty associated with the seismogenic sources and the ground-motion prediction models used in estimating the hazard. Combining two or more ground-motion relations within a logic tree will generally require several conversions to be made, because there are several definitions available for both the predicted ground-motion parameters and the explanatory parameters within the predictive ground-motion relations. Procedures for making conversions for each of these factors are presented, using a suite of predictive equations in current use for illustration. The sensitivity of the resulting ground-motion models to these conversions is shown to be pronounced for some of the parameters, especially the measure of source-to-site distance, highlighting the need to take into account any incompatibilities among the selected equations. Procedures are also presented for assigning weights to the branches in the ground-motion section of the logic tree in a transparent fashion, considering both intrinsic merits of the individual equations and their degree of applicability to the particular application.

The SHARE European Earthquake Catalogue (SHEEC) 1000–1899

In the frame of the European Commission project “Seismic Hazard Harmonization in Europe” (SHARE), aiming at harmonizing seismic hazard at a European scale, the compilation of a homogeneous, European parametric earthquake catalogue was planned. The goal was to be achieved by considering the most updated historical dataset and assessing homogenous magnitudes, with support from several institutions. This paper describes the SHARE European Earthquake Catalogue (SHEEC), which covers the time window 1000–1899. It strongly relies on the experience of the European Commission project “Network of Research Infrastructures for European Seismology” (NERIES), a module of which was dedicated to create the European “Archive of Historical Earthquake Data” (AHEAD) and to establish methodologies to homogenously derive earthquake parameters from macroseismic data. AHEAD has supplied the final earthquake list, obtained after sorting duplications out and eliminating many fake events; in addition, it supplied the most updated historical dataset. Macroseismic data points (MDPs) provided by AHEAD have been processed with updated, repeatable procedures, regionally calibrated against a set of recent, instrumental earthquakes, to obtain earthquake parameters. From the same data, a set of epicentral intensity-to-magnitude relations has been derived, with the aim of providing another set of homogeneous Mw estimates. Then, a strategy focussed on maximizing the homogeneity of the final epicentral location and Mw, has been adopted. Special care has been devoted also to supply location and Mw uncertainty. The paper focuses on the procedure adopted for the compilation of SHEEC and briefly comments on the achieved results.

Seismicity and seismotectonics of Norway and nearby continental shelf areas

The earthquake activity of Norway and nearby offshore areas is low to intermediate, with few events above magnitude 5. Recent significant improvements in instrumental coverage in parallel with a better utilization of older (including historical) data have shown that the seismicity in the south is predominantly confined to the coastal areas and to the Viking Graben, while from the northern North Sea to Svalbard the earthquakes in a broad sense follow the continental margin. Fifty-one focal mechanism solutions from these areas, about half of them new, reveal stress directions that clearly indicate a connection to the plate tectonic “ridge push” force, at least for the areas at a minimum distance from the continental margin. Along the margin, stress directions also indicate a possible connection to postglacial uplift as well as to lithospheric loading effects. A dominance of normal faulting on the landward side and reverse faulting on the oceanic side agrees with this interpretation. On a regional level, the seismicity in these areas correlate quite well with geologic features such as grabens, fault zones, fault complexes, fracture zones, and the margin itself, indicating that these structures act in a general sense as weakness zones in the presence of a regionally more stable stress field. In the northern North Sea, however, an area with quite anomalous stress orientations, with strike-slip faulting, is found in a region transitional between normal and reverse faulting. Most of the earthquake foci are confined to the presumably brittle parts of the crust, but many events are also located quite close to, and on both sides of, the Moho discontinuity.

Composite Ground-Motion Models and Logic Trees: Methodology, Sensitivities, and Uncertainties

Logic trees have become a popular tool in seismic hazard studies. Commonly, the models corresponding to the end branches of the complete logic tree in a probabalistic seismic hazard analysis (psha) are treated separately until the final calculation of the set of hazard curves. This comes at the price that information regarding sensitivities and uncertainties in the ground-motion sections of the logic tree are only obtainable after disaggregation. Furthermore, from this end-branch model perspective even the designers of the logic tree cannot directly tell what ground-motion scenarios most likely would result from their logic trees for a given earthquake at a particular distance, nor how uncertain these scenarios might be or how they would be affected by the choices of the hazard analyst. On the other hand, all this information is already implicitly present in the logic tree. Therefore, with the ground-motion perspective that we propose in the present article, we treat the ground-motion sections of a complete logic tree for seismic hazard as a single composite model representing the complete state-of-knowledge-and-belief of a particular analyst on ground motion in a particular target region. We implement this view by resampling the ground-motion models represented in the ground-motion sections of the logic tree by Monte Carlo simulation (separately for the median values and the sigma values) and then recombining the sets of simulated values in proportion to their logic-tree branch weights. The quantiles of this resampled composite model provide the hazard analyst and the decision maker with a simple, clear, and quantitative representation of the overall physical meaning of the ground-motion section of a logic tree and the accompanying epistemic uncertainty. Quantiles of the composite model also provide an easy way to analyze the sensitivities and uncertainties related to a given logic-tree model. We illustrate this for a composite ground-motion model for central Europe. Further potential fields of applications are seen wherever individual best estimates of ground motion have to be derived from a set of candidate models, for example, for hazard maps, sensitivity studies, or for modeling scenario earthquakes.

Ground-motion prediction equations based on data from the himalayan and zagros regions

This study derives ground-motion prediction equations for the horizontal elastic response spectral acceleration for 5% damping for application to the Indian Himalayas. The present equations include a consideration of site category (rock/soil) and style-of-faulting (strike-slip/reverse). Due to a lack of near-field data from India, additional strong-motion data have been included from the Zagros region of Iran, which has comparable seismotectonics to the Himalayas (continental compression). A set of 201 records from 16 earthquakes were used within the regression. The derived model predicts similar ground motions to previously published equations for the Himalayan region but with lower standard deviations.

Submarine landslide tsunamis: how extreme and how likely?

A number of examples are presented to substantiate that submarine landslides have occurred along most continental margins and along several volcano flanks. Their properties of importance for tsunami generation (i.e. physical dimensions, acceleration, maximum velocity, mass discharge, and travel distance) can all gain extreme values compared to their subaerial counterparts. Hence, landslide tsunamis may also be extreme and have regional impact. Landslide tsunami characteristics are discussed explaining how they may exceed tsunamis induced by megathrust earthquakes, hence representing a significant risk even though they occur more infrequently. In fact, submarine landslides may cause potentially extreme tsunami run-up heights, which may have consequences for the design of critical infrastructure often based on unjustifiably long return periods. Giant submarine landslides are rare and related to climate changes or glacial cycles, indicating that giant submarine landslide tsunami hazard is in most regions negligible compared to earthquake tsunami hazard. Large-scale debris flows surrounding active volcanoes or submarine landslides in river deltas may be more frequent. Giant volcano flank collapses at the Canary and Hawaii Islands developed in the early stages of the history of the volcanoes, and the tsunamigenic potential of these collapses is disputed. Estimations of recurrence intervals, hazard, and uncertainties with today’s methods are discussed. It is concluded that insufficient sampling and changing conditions for landslide release are major obstacles in transporting a Probabilistic Tsunami Hazard Assessment (PTHA) approach from earthquake to landslide tsunamis and that the more robust Scenario-Based Tsunami Hazard Assessment (SBTHA) approach will still be most efficient to use. Finally, the needs for data acquisition and analyses, laboratory experiments, and more sophisticated numerical modelling for improved understanding and hazard assessment of landslide tsunamis are elaborated.

Ground-motion prediction equations for southern Spain and southern Norway obtained using the composite model perspective

In this paper, two sets of earthquake ground-motion relations to estimate peak ground and response spectral acceleration are developed for sites in southern Spain and in southern Norway using a recently published composite approach. For this purpose seven empirical ground-motion relations developed from recorded strong-motion data from different parts of the world were employed. The different relations were first adjusted based on a number of transformations to convert the differing choices of independent parameters to a single one. After these transformations, which include the scatter introduced, were performed, the equations were modified to account for differences between the host and the target regions using the stochastic method to compute the host-to-target conversion factors. Finally functions were fitted to the derived ground-motion estimates to obtain sets of seven individual equations for use in probabilistic seismic hazard assessment for southern Spain and southern Norway. The relations are compared with local ones published for the two regions. The composite methodology calls for the setting up of independent logic trees for the median values and for the sigma values, in order to properly separate epistemic and aleatory uncertainties after the corrections and the conversions.

Earthquake Occurrence and Seismotectonics in Norway and Surrounding Areas

Recent major improvements in our knowledge about the seismicity (historical and contemporary) in the continental margin areas of Norway has now made it possible to obtain a reasonably balanced and complete picture of the earthquake occurrence from the Central Graben in the south to the Svalbard Platform in the north. The same areas have recently been subject also to extensive geophysical and geological investigations, which have provided an excellent basis for correlating the seismicity with the geology in a meaningful way. Such correlations have been found to be present for a variety of geological structures that are interpreted to act as weakness zones in the presence of a stress field that seems to be dominated by ridge push forces. The potential seismotectonic significance of likely local variations in the stress field remains to be elucidated.

Long-period ground-motions for large European earthquakes, 1905–1992, and comparisons with stochastic predictions

Data from European earthquakes in themagnitude range 5 to 8 between 1905 and1992 have been collected and collated, fordistances between 200 and 3400 km. The datainclude both analog and digital records,with priority on wave paths traversingnorthern Europe. Historical analog recordsfrom Uppsala and De Bilt have beendigitised, and appropriate responsefunctions established. New estimates ofseismic moments and moment magnitudes havebeen obtained, which together with momentmagnitudes from other sources have beencompared to surface wave magnitudes. Thelocation of the largest north Europeanearthquakes substantiate earliersuggestions that rifted regions (passivemargins, rifts and grabens) may have thelargest seismic potentials. Arandom-vibration (stochastic) model forprediction of observed peak amplitude,period and Fourier acceleration spectra hasbeen developed and calibrated againstintermediate and long-period observations.Reasonably good correspondence betweenpredictions and observations are obtainedwhen using a simple Brune source spectrum,new values for seismic moments andmoment-magnitude relations, together withreasonable assumptions for stress drop,geometrical spreading and anelasticattenuation. The model is useful first ofall for predicting broad regional averages,but as such it is robust, and it also hasthe potential to be used in an engineeringcontext for predicting spectral responseand peak ground accelerations. Some of theempirical data have also been studied interms of pseudo-relative spectral velocityand compared to strong-motion responsespectral prediction models established fornorthwestern Europe, again for lowfrequencies. Irrespective of these prediction models we emphasize, however, thatthe establishment of the data base itselfhas been an independent and importantpurpose of this study.

Probabilistic seismic hazard : a review of the seismological frame of reference with examples from Norway

Abstract A probabilistic seismic hazard analysis (PSHA) utilizes, in the conventional Cornell–McGuire approach, a quantitative model of the earthquake activity implying major simplifications which are important to assess in terms of their contributions to uncertainty. The goal is one of the basic principles in science, namely to establish a minimum parameter model that depicts nature with the optimum representativity (Occams razor). All too often, underlying seismological issues remain obscure in PSHA analyses. On the basis of a specific analysis conducted in Norway we highlight how a combined seismicity analysis using both modern network data and historical data can be utilized in order to provide realistic insights into location precision and to establish magnitude homogeneity. All of this is aimed at improving the reliability of the seismic source models (i.e. the activity parameters), and to improve, without over-interpretation the earthquake catalog data, the spatial differentiation of the seismogenic zones.