Conrad Lindholm

Postglacial uplift, neotectonics and seismicity in Fennoscandia

Abstract Fennoscandia has experienced major uplift in postglacial time, which is assumed to reflect a glacial isostatic process connected to the melting of the last ice sheets. Extensive modelling of the isostatic movements show that the applied deglaciation and uplift model fit the observations well. There are, however, areas with significant deviations between uplift measurements and regional model predictions. The misfit between observations and the isostatic uplift modelling is interpreted here to reflect a tectonic component of the uplift. The objective of the present investigation is to isolate this tectonic uplift component. Interestingly enough, the areas found partly correspond to areas with pronounced seismic activity, and the assumption that the postglacial rebound is responsible for much of the observed onshore seismicity is substantiated. We conclude that there seems to be present-day deformation along the shoreline of mid-Norway, southern Norway (shoreline and mountain areas), and along the Swedish east coast with the centre northeast of the Gulf of Bothnia that cannot be explained by glacial isostasy. Not all of the deformations in these areas are necessarily co-seismic. The study suggests that such vertical deformations are small in magnitude and overprint the glacial rebound. The deformations may be a consequence of the Plio-Pleistocene erosional pattern, which is of glacial origin.

Crustal stress and tectonics in Norwegian regions determined from earthquake focal mechanisms

Abstract A database of 109 earthquake focal mechanisms for Scandinavia and Svalbard has been compiled, comprising 29 new solutions. Stress directions extracted from these earthquake focal mechanisms have been calculated for southeastern and southwestern Norway, the Norwegian Sea and the Barents-Finnmark region. The principal horizontal stress is c. N45°W, and indicates a clockwise rotation as one moves from south to north, in accordance with stress trajectories that can be expected from ridge push. The data indicate that in southern Norway normal faulting regimes are more dominant in onshore regions, whereas thrust faulting dominates the offshore seismic activity. Offshore mid-Norway a preference for normal faulting is also found, whereas the shallow earthquakes in Finnmark, northern Norway, are all reverse. Focal depth distributions were studied in the northern North Sea region, revealing substantial differences with depths in the range 5–30 km.

Crustal stress in and around Norway: an evaluation of stress-generating mechanisms

Abstract Recent stress observations from in situ measurements and earthquake focal mechanisms in the Norwegian onshore and offshore areas are evaluated with the aim of characterizing the most important mechanisms for the present stress field in and around Norway. The evaluation is based on a set of stress indicators that include both shallow measurements (overcoring and borehole breakouts) and deep data (earthquake focal mechanisms). Computer simulations of simple stress-generation models were used to constrain the relative importance of different stress-generating mechanisms. The ridge push force associated with sea-floor spreading in the North Atlantic is considered to be the primary source of the compressional stress field observed in Norway. Regional influences from the continental margin density contrast, topography and flexure induced by sediment loading are of limited lateral extent, but are important in reorienting the stress field in certain areas. The observed tectonics and stresses are generally also in accord with tectonics expected from Fennoscandian uplift.

Seismo- and neotectonics in Finnmark, Kola and the southern Barents Sea, part 2: Seismological analysis and seismotectonics

Abstract A detailed review and analysis of seismological data acquired since 1979 from Finnmark, Kola and the southern Barents Sea has provided a much improved delineation of the earthquake activity in that region. In the Barents Sea the seismicity is confined only to the western areas, while the Finnmark–Kola coastal areas show some seismicity only north and east of Murmansk, in the Rybachi–Cape Tereberski region. Further inland there are several clear seismicity lineations that to some extent can be correlated with postglacial faults, including the NE–SW striking Stuoragurra Fault in Finnmark. The spatial correlation between seismicity and mapped faults is interpreted in general as a reflection of broad zones of weakness that respond to the present-day stress field. A detailed analysis of the earthquakes in this area has also shown a dynamic consistency in that they reveal a reverse faulting pattern that is consistent with a detailed geological study that includes a core drilling. The dominant horizontal stress direction in this region is NNW–SSE, consistent with the “ridge push” effect.

A LOGIC TREE EXTENSION OF THE CAPACITY SPECTRUM METHOD DEVELOPED TO ESTIMATE SEISMIC RISK IN OSLO, NORWAY

The city of Oslo, Norway, was affected by a magnitude 5.4 earthquake in 1904 causing widespread minor damage. The earthquake occurred around 100 km south of Oslo within the Permean rift structure that runs North-South along the Oslofjord, and deep clay deposits under the city contributed to the damages. A seismic risk scenario including soil amplification and buildings classifications has been conducted with two earthquake sources, one very close to the city and one near the 1904 epicenter. Both scenarios exhibit strong dependencies on the soft clays underlying large parts of Oslo. The results confirm the 1904 effects, but also show a strong dependency on the applied attenuation functions. All computations are based on the capacity-spectrum method, and the predefined pushover curves and vulnerability functions were adopted from the HAZUS code. With this basis, the computational scheme was developed independent from the GIS framework, and a weighted logic tree formulation was implemented for appropriate treatment of epistemic uncertainties.

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.

A New Evaluation of Seismic Hazard for the Central America Region

A new evaluation of seismic hazard in the Central America region has been carried out, in the frame of the cooperation project RESIS II, financed by the Norway Cooperation Agency (NORAD). Different experts in seismic hazard from Costa Rica, Guatemala, Nicaragua , El Salvador, Norway and Spain participated in the study, which was aimed at obtaining results suitable for seismic design purposes. The analysis started with an exhaustive revision of the seismic catalogues of each country from which a global catalogue for CA has been configured and homogenised at moment magnitude, Mw. Seismotectonic models proposed for the region were revised and a regional zonation was proposed, taking into account seismotectonic data, seismicity, focal mechanisms, GPS observations and other evidences useful for defining seismic sources. In parallel, attenuation models for subduction and volcanic crustal zones were revised and the more suitable models were calibrated with strong motion data. Taking the previous inputs, the seismic hazard analysis was developed in terms of peak ground acceleration, PGA and spectral accelerations SA (T) for periods of 0.1, 0.5, 1 and 2 s, through the PSHA methodology (Probabilistic Seismic Hazard Assessment). As a result, different hazard maps were obtained for the quoted parameters, together with Uniform Hazard Spectra (UHS) in the main populations of Central America. This is the first study developed at regional scale after the last earthquakes that have occurred in the region and as a result the new generation of maps will be useful in the revision of seismic codes of the area.

Socioeconomic Clustering in Seismic Risk Assessment of Urban Housing Stock

A seismic risk assessment methodology based on socioeconomic clustering of urban habitat is presented in this paper. In this methodology, the city is divided into different housing clusters based on socioeconomic level of occupants, representing reasonably uniform seismic risk. It makes an efficient utilization of high resolution satellite data and stratified random sample survey to develop the building stock database. Ten different classes of socioeconomic clusters found in Indian cities are defined and 34 model building types (MBTs) prevalent on the Indian subcontinent have been identified and compared with the Medvedev-Sponheuer-Karnik (MSK) scale, European macroseismic scale (EMS), parameterless scale of seismic intensity (PSI), and HAZUS classifications. Lower and upper bound damage probability matrices (DPMs) are estimated, based on the MSK and EMS intensity scales and experience from past earthquakes in India. A case study of Dehradun, a city in the foothills of Himalayas, is presented. The risk estimates using the estimated DPMs have been compared with those obtained using the PSI scale. It has been observed that poorer people are subjected to higher seismic risk, both in terms of casualties and in terms of percent economic losses.

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.

Crustal stress in and around Norway: a compliation of in situ stress observations

Abstract In-situ rock stresses yield information about geodynamic processes in the crust, and are important input data for almost all kinds of geomechanics work. Compilations of rock stress measurements, such as the World Stress Map, have been used to characterize the regional stress field. On the basis of new data from the Norwegian region, a map of the maximum horizontal stress (σH) direction, for both on- and offshore Norway, has been established. Four main stress provinces with different regional stress trends are identified. The Barents Sea and northern Norway province exhibits a very consistent N-S σH direction, with high horizontal stresses. In the Norwegian Sea and mid-Norway province the σH direction has rotated towards NW-SE. Also here, high horizontal stresses are measured and a compressional regime appears. In the northern North Sea and western Norway province the σH direction is more scattered. A WNW-ESE direction dominates, but a NNE-SSW trend, parallel to the major structures in the area, also appears. The stress regime is primarily compressional, but in the Stord area normal faulting earthquakes are observed. The southern North Sea province yields a very scattered σH direction. However, a NW-SE trend, which aligns with the stress system in western Europe, is identified. The high variation, both laterally and with depth, indicates near-isotropic horizontal stresses and influence by local features. The different stress determination techniques applied (overcoring, borehole breakouts and earthquake focal mechanisms) yield approximately the same stress orientation, and regional and internal variation, although they cover different depth intervals. They further manifest high horizontal stresses in the Norwegian area, generally exceeding the vertical stress. This indicates the presence of compressional tectonic stresses penetrating deep into the crust. The first-order stress pattern in Norway and adjacent offshore regions show a rotation in σH direction from N-S in the Barents Sea to WNW-ESE in the North Sea. This rotation can be explained by the ridge push effect alone, but other effects may also be contributing factors. The increased scatter in σH direction in the south is believed to be due to changes in the tectonic stress magnitudes, or related to relative changes in angle between the continental margin and the ridge push force.