Seismic Source Characterization Research Articles

A seismic hazard assessment and the results of detailed seismic zoning for urban territories of Sakhalin Island

This work briefly discusses the main features of probabilistic seismic hazard analysis (PSHA). Special attention is paid to the identification and quantification of uncertainties related to seismic source characteristics and seismic engineering models for prediction of strong ground motions. The principal seismic models and the results of PSHA application for detailed seismic zoning of urban territories in Sakhalin Island are presented.

Probabilistic seismic hazard assessment of Eastern Marmara Region

The objective of this study is to evaluate the seismic hazard in Eastern Marmara Region using an improved probabilistic seismic hazard assessment methodology. Two significant improvements over the previous seismic hazard assessment practices are accomplished in this study: advanced seismic source characterization models in terms of source geometry and recurrence relationships are developed, and improved global ground motion models (NGA-W1 models) are employed to represent the ground motion variability. Planar fault segments are defined and a composite magnitude distribution model is used for all seismic sources in the region to properly represent the characteristic behavior of the North Anatolian Fault without the need for an additional background zone. Multi-segment ruptures are considered using the rupture model proposed by the Working Group on California Earthquake Probabilities (2003). Events in the earthquake catalogue are attributed to the fault zones and scenario weights are determined by releasing the accumulated seismic energy. The uniform hazard spectra at 10 % probability of exceedance in 50 years hazard level for different soil conditions (soil and rock) are revealed for specific locations in the region (Adapazari, Duzce, Gemlik, Izmit, Iznik and Sapanca). Hazard maps of the region for rock site conditions at the selected hazard levels are provided to allow the readers perform site-specific hazard assessment and develop site-specific design spectrum for local site conditions.

Seismic monitoring of soft-rock landslides: the Super-Sauze and Valoria case studies

This work focuses on the characterization of seismic sources observed in clay–shale landslides. Two landslides are considered: Super-Sauze (France) and Valoria (Italy). The two landslides are developed in reworked clay–shales but differ in terms of dimensions and displacement rates. Thousands of seismic signals have been identified by a small seismic array in spite of the high-seismic attenuation of the material. Several detection methods are tested. A semi-automatic detection method is validated by the comparison with a manual detection. Seismic signals are classified in three groups based on the frequency content, the apparent velocity and the differentiation of P and S waves. It is supposed that the first group of seismic signals is associated to shearing or fracture events within the landslide bodies, while the second group may correspond to rockfalls or debris flows. A last group corresponds to external earthquakes. Seismic sources are located with an automatic beam-forming location method. Sources are clustered in several parts of the landslide in agreement with geomorphological observations. We found that the rate of rockfall and fracture events increases after periods of heavy rainfall or snowmelt. The rate of microseismicity and rockfall activity is also positively correlated with landslide displacement rates. External earthquakes did not influence the microseismic activity or the landslide movement, probably because the earthquake ground motion was too weak to trigger landslide events during the observation periods.

Seismic Source Characteristics of Nuclear and Chemical Explosions in Granite from Hydrodynamic Simulations

Seismic source characteristics of low-yield (0.5–5 kt) underground explosions are inferred from hydrodynamic simulations using a granite material model on high-performance (parallel) computers. We use a non-linear rheological model for granite calibrated to historical near-field nuclear test data. Equivalent elastic P-wave source spectra are derived from the simulated hydrodynamic response using reduced velocity potentials. Source spectra and parameters are compared with the models of Mueller and Murphy (Bull Seism Soc Am 61:1675–1692, 1971, hereafter MM71) and Denny and Johnson (Explosion source phenomenology, pp 1–24, 1991, hereafter DJ91). The source spectra inferred from the simulations of different yields at normal scaled depth-of-burial (SDOB) match the MM71 spectra reasonably well. For normally buried nuclear explosions, seismic moments are larger for the hydrodynamic simulations than MM71 (by 25 %) and for DJ91 (by over a factor of 2), however, the scaling of moment with yield across this low-yield range is consistent for our calculations and the two models. Spectra from our simulations show higher corner frequencies at the lower end of the 0.5–5.0 kt yield range and stronger variation with yield than the MM71 and DJ91 models predict. The spectra from our simulations have additional energy above the corner frequency, probably related to non-linear near-source effects, but at high frequencies the spectral slopes agree with the f −2 predictions of MM71. Simulations of nuclear explosions for a range of SDOB from 0.5 to 3.9 show stronger variations in the seismic moment than predicted by the MM71 and DJ91 models. Chemical explosions are found to generate higher moments by a factor of about two compared to nuclear explosions of the same yield in granite and at normal depth-of-burial, broadly consistent with comparisons of nuclear and chemical shots at the US Nevada Test Site (Denny, Proceeding of symposium on the non-proliferation experiment, Rockville, Maryland, 1994). For all buried explosions, the region of permanent deformation and material damage is not spherical but extends along the free surface above and away from the source. The effect of damage induced by a normally buried nuclear explosion on seismic radiation is explored by comparing the motions from hydrodynamic simulations with those for point-source elastic Green’s functions. Results show that radiation emerging at downward takeoff angles appears to be dominated by the expected isotropic source contribution, while at shallower angles the motions are complicated by near-surface damage and cannot be represented with the addition of a simple secondary compensated linear vector dipole point source above the shot point. The agreement and differences of simulated source spectra with the MM71 and DJ91 models motivates the use of numerical simulations to understand observed motions and investigate seismic source features for underground explosions in various emplacement media and conditions, including non-linear rheological effects such as material strength and porosity.

Long period seismic source characterization at Popocatépetl volcano, Mexico

The seismicity of Popocatépetl is dominated by long‐period and very‐long period signals associated with hydrothermal processes and magmatic degassing. We model the source mechanism of repetitive long‐period signals in the 0.4–2 s band from a 15‐station broadband network by stacking long‐period events with similar waveforms to improve the signal‐to‐noise ratio. The data are well fitted by a point source located within the summit crater ∼250 m below the crater floor and ∼200 m from the inferred magma conduit. The inferred source includes a volumetric component that can be modeled as resonance of a horizontal steam‐filled crack and a vertical single force component. The long‐period events are thought to be related to the interaction between the magmatic system and a perched hydrothermal system. Repetitive injection of fluid into the horizontal fracture and subsequent sudden discharge when a critical pressure threshold is met provides a non‐destructive source process.

Estimation of earthquake parameters in the Alborz seismic zone, Iran using generalized inversion method

Generalized inversion of the S-wave amplitude spectra from the strong-motion network data in the Alborz, Iran has been used to estimate simultaneously source parameters, site response and S-wave attenuation (Qs). To obtain an optimum inverse solution, and also for decreasing the uncertainty level, a frequency-dependent site amplification as a constraint, was imposed to five reference site responses. This constraint removes the undetermined degree of freedom in the inversion. Furthermore, for removing the trade-off between geometrical spreading and anelastic attenuation, a geometrical spreading factor was adopted from the Motazedian [20] study. A point source model has been calibrated against the resulting source terms and consequently source parameters, like corner frequency, moment magnitude and high frequency fall off coefficient, for each event has been determined separately. Also, based on the available data and their connectivity two sub-regions including western and eastern parts of Alborz located east and west of 52.5°E have been considered to see that if there is any possible systematic difference in their seismic source characteristics. The average stress drops obtained are about 182 and 116bars, respectively for eastern and western Alborz. Another result of the study is the site responses, which have been determined for all of 81 stations individually. Though soil nonlinearity was detected at the Ab-bar station (experienced strong ground shaking, i.e., PGA>0.5g) near to the epicenter of Manjil M7.4 earthquake of June 20, 1990, but an analysis of residuals showed generally a weak influence of soil nonlinearity (i.e., dependence of amplification on shaking level); probably because of the relatively weak levels of acceleration in our database. Finally, the shear wave quality factor (i.e., Qs) has been determined as a function of frequency represented by a linear equation in logarithmic scale. To evaluate the outcomes of the current study, the results have been compared with similar studies wherever it was available. The results of the current study are of utmost importance for seismic hazard assessment of the metropolitan area of Tehran, where 15 million people live, one-fifth of the population of Iran.

Ground-motion predictions in Shillong region, northeast India

We deliver ground-motion prediction equations for Shillong region, northeast India, based on a database generated by finite-fault stochastic simulations. An examination of the regional seismic source characteristics is carried out beforehand. Micro/minor earthquakes (M W < 5.0) nucleating at hypocentral depth <21 km in the region recorded at broadband seismic stations are observed to have Brune stress-drop ranging between 2.8 and 99.9 bars. Likewise, macroseismic intensity data for the 1897 Shillong Earthquake that nucleated at a hypocentral depth of ~35 km places the associated stress-drop at 100−200 bars. The apparent variation of the stress-drop parameter with depth is considered with two source zones namely lower-crust and upper-crust. Equations for the lower-crust predict higher ground-motion levels and exhibit affinity to those developed for stable continental region of Eastern North America. The ground-motion levels predicted by the equations for the upper-crust are relatively lower but are still higher compared with those predicted for tectonically active regions, viz., the Himalayas and Western North America.

Characterization of Regional Seismic Source Zones in and around India

Online material : Earthquake catalog; coordinates of source zones; list of events in each zone; frequency magnitude distribution plots. Identification and characterization of seismic sources are essential input for seismic hazard analysis. A complete and consistent catalog of earthquakes in a region can offer good data for studying the distribution of earthquakes with respect to space, time, and magnitude. Earthquakes are known to have occurred in the region of the Indian subcontinent from historic times. Most of the activities, including many great earthquakes, have occurred in the northern subcontinent and in the Andaman and Nicobar regions. In the southern peninsula, damaging earthquakes have occurred, but their frequency was less and magnitudes were lower than the plate boundary earthquakes. The tectonic framework of the Indian subcontinent—an area of about 3.2 million square kilometers—is complex and varied spatially. This necessitates identifying different regions of similar seismicity. In this study we attempt to identify and characterize regional seismic source zones based on a homogeneous catalog of earthquakes in and around India from 250 BC until 2010. We identified different regional seismic source zones and created separate catalogs for each respective zone. Most earthquake catalogs do not report earthquake magnitudes consistently over time, which may hinder defining seismicity patterns or assessing seismic hazards. The magnitude scales used for earthquake catalogs in India are not homogeneous. Compiling a complete catalog of earthquakes with uniform magnitudes is essential for defining seismicity patterns and assessing seismic hazards for a region (Chen and Tsai 2008). Hence the original magnitudes of earthquakes have to be converted to a common and reliable magnitude scale using appropriate magnitude correlations. In the raw catalog, many events can be dependent events that occurred in association with a mainshock in a cluster. These aftershocks and foreshocks have to be removed from the catalog using …

Quantifying a Potential Bias in Probabilistic Seismic Hazard Assessment: Seismotectonic Zonation with Fractal Properties

The accurate definition and characterization of relevant seismic sources are critical steps in probabilistic seismic hazard assessment (PSHA). This is particu- larly challenging in low-seismicity regions because observation periods are relatively short, seismicity is often diffuse, and active faults are difficult to identify. In such regions, seismogenic sources are typically represented as areal sources, zones with equal seismic potential. However, observed seismicity is never truly uniformly dis- tributed but clusters at all scales, a reflection of the seismotectonic forces and tectonic structures. In this study, we use a fractal scaling approach to explore a more realistic characterization of the seismicity distribution within each source zone. For a hypothe- tical square source zone, we compute hazard curves and hazard maps resulting from simulations of uniformly distributed seismicity (fractal dimension D 2:0), and we compare these with those resulting from simulations of clustered seismicity (1:0 <D< 2:0). We find that the assumption of uniform distribution of events leads systematically to a higher estimate of hazard within the source zone. This overestima- tion is increasing for lower probability levels. Equally important, the assumed uniform distribution underestimates the uncertainty of the hazard by up to a factor of three. We perform an extensive sensitivity study of PSHA input parameters, illustrating the complexity of the interaction between the specific setup and the resulting impact on the PSHA. We apply the fractal scaling approach to the seismicity of Switzerland and measure the fractal dimension of instrumental seismicity for the past 30 yr (D 1:5); using this value for synthetic catalogs, we build a fractal seismic zonation and hazard model. We find that, in general, the assumption of uniform distribution of events over- estimates the mean hazard in Switzerland by 3% to 20%, and the uncertainty distribu- tion estimation is 50% to 100% narrower than in fractal distribution, depending on the location and the probability level of interest.

Seismogenic sources in the Bay of Bengal vis-à-vis potential for tsunami generation and its impact in the northern Bay of Bengal coast

Geodynamic status, seismo-tectonic environment, and geophysical signatures of the Bay of Bengal do not support the occurrence of seismogenic tsunami. Since thrust fault and its intensity and magnitude of rupture are the key tectonic elements of tsunamigenic seismic sources, the study reveals that such characteristics of fault-rupture and seismic sources do not occur in most of the Bay of Bengal except a small segment in the Andaman–Nicobar subduction zone. The inferred segment of the Andaman–Nicobar subduction zone is considered for generating a model of the deformation field arising from fluid-driven source. The model suggests local tsunami with insignificant inundation potential along the coast of northern Bay of Bengal. The bathymetric profile and the sea floor configuration of the northern Bay of Bengal play an important role in flattening the waveform through defocusing process. The direction of motion of the Indian plate makes an angle of about 30° with the direction of the opening of Andaman Sea. The opening of Andaman Sea and the direction of plate motion of the Indian plate results in the formation of Andaman trench where the subducting plate dives more obliquely than that in the Sunda trench in the south. The oblique subduction reduces significantly the possibilities of dominant thrust faulting in the Andaman subduction zone. Further, north of Andaman subduction in the Bengal–Arakan coast, there is no active subduction. On the otherhand, much greater volume of sediments (in excess of 20 km) in the Bengal–Arakan segment reduces the possibilities of mega rupture of the ocean floor. The water depth (≈1,000 m) along most of the northern Bay of Bengal plate margin is not optimum for any significant tsunami generation. Hence, very weak possibility of any significant tsunami is suggested that based on the interpretation of geodynamic status, seismo-tectonic environment, and geophysical signatures of the Andaman subduction zone and the Bengal–Arakan coast.

New approach to detect seismic surface waves in 1Hz-sampled GPS time series

Recently, co-seismic seismic source characterization based on GPS measurements has been completed in near- and far-field with remarkable results. However, the accuracy of the ground displacement measurement inferred from GPS phase residuals is still depending of the distribution of satellites in the sky. We test here a method, based on the double difference (DD) computations of Line of Sight (LOS), that allows detecting 3D co-seismic ground shaking. The DD method is a quasi-analytically free of most of intrinsic errors affecting GPS measurements. The seismic waves presented in this study produced DD amplitudes 4 and 7 times stronger than the background noise. The method is benchmarked using the GEONET GPS stations recording the Hokkaido Earthquake (2003 September 25th, Mw = 8.3).

Exploring the potentials and limitations of the time-reversal imaging of finite seismic sources

Abstract. The characterisation of seismic sources with time-reversed wave fields is developing into a standard technique that has already been successful in numerous applications. While the time-reversal imaging of effective point sources is now well-understood, little work has been done to extend this technique to the study of finite rupture processes. This is despite the pronounced non-uniqueness in classic finite source inversions. The need to better constrain the details of finite rupture processes motivates the series of synthetic and real-data time reversal experiments described in this paper. We address questions concerning the quality of focussing in the source area, the localisation of the fault plane, the estimation of the slip distribution and the source complexity up to which time-reversal imaging can be applied successfully. The frequency band for the synthetic experiments is chosen such that it is comparable to the band usually employed for finite source inversion. Contrary to our expectations, we find that time-reversal imaging is useful only for effective point sources, where it yields good estimates of both the source location and the origin time. In the case of finite sources, however, the time-reversed field does not provide meaningful characterisations of the fault location and the rupture process. This result cannot be improved sufficiently with the help of different imaging fields, realistic modifications of the receiver geometry or weights applied to the time-reversed sources. The reasons for this failure are manifold. They include the choice of the frequency band, the incomplete recording of wave field information at the surface, the excitation of large-amplitude surface waves that deteriorate the depth resolution, the absence of a sink that should absorb energy radiated during the later stages of the rupture process, the invisibility of small slip and the neglect of prior information concerning the fault geometry and the inherent smoothness of seismologically inferred Earth models that prevents the beneficial occurrence of strong multiple-scattering. The condensed conclusion of our study is that the limitations of time-reversal imaging – at least in the frequency band considered here – start where the seismic source stops being effectively point-localised.

Scenarios for earthquake-generated tsunamis on a complex tectonic area of diffuse deformation and low velocity: The Alboran Sea, Western Mediterranean

The tsunami impact on the Spanish and North African coasts of the Alboran Sea generated by several reliable seismic tsunamigenic sources in this area was modeled. The tectonic setting is complex and a study of the potential sources from geological data is basic to obtain probable source characteristics. The tectonic structures considered in this study as potentially tsunamigenic are: the Alboran Ridge associated structures, the Carboneras Fault Zone and the Yusuf Fault Zone. We characterized 12 probable tsunamigenic seismic sources in the Alboran Basin based on the results of recent oceanographical studies. The strain rate in the area is low and therefore its seismicity is moderate and cannot be used to infer characteristics of the major seismic sources. These sources have been used as input for the numerical simulation of the wave propagation, based on the solution of the nonlinear shallow water equations through a finite-difference technique. We calculated the Maximum Wave Elevations, and Tsunami Travel Times using the numerical simulations. The results are shown as maps and profiles along the Spanish and African coasts. The sources associated with the Alboran Ridge show the maximum potential to generate damaging tsunamis, with maximum wave elevations in front of the coast exceeding 1.5 m. The Carboneras and Yusuf faults are not capable of generating disastrous tsunamis on their own, although their proximity to the coast could trigger landslides and associated sea disturbances. The areas which are more exposed to the impact of tsunamis generated in the Alboran Sea are the Spanish coast between Malaga and Adra, and the African coast between Alhoceima and Melilla.

Spatial distribution of site effects and wave propagation properties in Thessaloniki (N. Greece) using a 3-D finite difference method

SUMMARY The detailed wave propagation characteristics and the spatial distribution of site effects in the metropolitan area of Thessaloniki are studied using a 3-D finite-difference method. Fourier amplitude spectra (FAS) and standard spectra ratios (SSR) are computed for various scenarios and their spatial distribution is examined throughout the Thessaloniki metropolitan area, in order to study the spatial variability of site-response. The variability of the contribution of different model layers in site amplification at different frequency windows and the identification of high amplification areas due to the presence of trapped waves in the surficial layers of the model identified along selected cross-sections, verify the impact of the complex 3-D wave propagation on the computed synthetics. Moreover, examination of the spatial distribution of the fundamental period (TSSR0) from synthetic SSR and comparisons with results from ambient noise measurements show that for (thicker soil formations) the average H/V results for the fundamental period from ambient noise measurements, TNoiseH/V, tend to overestimate (by roughly 30–35 per cent) the average fundamental periods from 3-D synthetics, . The characteristics of the computed time-series and the type and properties of the dominating seismic waves are also examined along the same typical cross-sections spanning the study area, revealing the selective propagation of Love surface waves for various seismic scenarios. The previous results, as well as the strong spatial and interscenario variability of various measures of seismic motion such as Fourier spectra, peak ground velocity, cumulative energy and Housner intensity show a complicated 3-D wave propagation pattern, affecting the final site-effect distribution, both in the time and frequency domain. In most cases, even for areas with relatively simple shallow structure, the final site-response strongly depends on the seismic source characteristics, indicating the necessity of specific earthquake scenario 3-D synthetics for the study of complex 3-D geometry sedimentary basins, such as the broader Thessaloniki region.

Scaling of the Source Dimensions of Interface and Intraslab Subduction-zone Earthquakes with Moment Magnitude

This paper derives source scaling relations between rupture dimensions and moment magnitude for subduction-zone earthquakes, separating between interface events occurring at the contact of the subducting and overriding tectonic plates, and intraslab events, which occur within the subducting slab. These relations are then compared with existing scaling relations, which are predominantly based on data from crustal events. Relations between the dimensions of the rupture zone of earthquakes and the amount of energy released as measured by the seismic moment, M , or equivalently moment magnitude, Mw , (Hanks and Kanamori 1979), are of great practical use in engineering seismology. Early relations ( e.g. , Kanamori and Anderson 1975; Wyss 1979) were derived with the purpose of using rupture dimensions to constrain estimates of magnitude. Additionally, the relation between independently determined rupture dimensions and seismic moment also was used to draw inferences in terms of source scaling from comparisons between observed data and predictions of theoretical seismological models ( e.g. , Kanamori and Anderson 1975; Astiz et al. 1987). Nowadays, moment magnitude is routinely estimated from instrumental recordings, and the scaling relations described above are predominantly used to infer the probable dimensions of an earthquake of given magnitude. Applications include distance calculations using finite-fault distance metrics ( e.g. , Chiou and Youngs 2006), characterization of seismic sources in seismic hazard analysis, and theoretical studies involving forward-modeling of fault slip and resulting ground motions ( e.g. , Atkinson and Macias 2009; Somerville et al. 2008). However, the reciprocal relations giving moment magnitude as a function of rupture dimensions may still be useful for estimating the moment magnitude of either historical or hypothetical scenario events for which an estimate of the rupture dimensions is available, for instance on the basis of the dimensions of an observed seismic gap or from fault …

Scenario-Based Seismic Risk Analysis: An Engineering Approach to the Development of Source and Site-Specific Ground Motion Time Histories in Areas of Low Seismicity

Modern engineering design methods require ground motion time histories as input for non-linear dynamic structural analysis. Non-linear dynamic methods of analysis are increasingly applied in the context of probabilistic risk assessments and for cost-effective design of critical infrastructures. In current engineering practice artificial time histories matching deterministic design spectra or probabilistic uniform hazard spectra are most frequently used for engineering analysis. The intermediate step of generation of response spectra can lead to a biased estimate of the potential damage from earthquakes because of insufficient consideration of the true energy content and strong motion duration of earthquakes. Thus, assessment of seismic risk may seem unrealistic. An engineering approach to the development of three-component ground motion time histories has been established which enables consideration of the typical characteristics of seismic sources, regional ground motion attenuation, and the main geotechnical characteristics of the target site. Therefore, the approach is suitable for use in scenario-based risk analysis a larger number of time histories are required for representation of the seismic hazard. Near-field effects are implemented in the stochastic source model using engineering approximations. The approach is suggested for use in areas of low seismicity where ground motion records of larger earthquakes are not available. Uncertainty analysis indicates that ground motions generated by individual earthquakes are well constrained and that the usual lognormal model is not the best choice for predicting the upper tail of the distribution of the ground motions.

Spatial Distribution Characteristics of Seismic Ground Motion Intensities in the Kanto Region

本研究は、各種機関の地震観測網によって観測された記録を用いて、関東地方における揺れやすさの空間的分布特性について検討したものである。揺れやすさの空間的分布特性は、観測記録と経験的に得られる平均的な地震動強さとの差異として抽出した。ここで、地震動強さとしてはPGA、PGV、計測震度の3種類を対象としている。72地震の観測記録を用いて統計的処理により検討した結果、PGVと計測震度の空間的分布特性については、表層地盤の増幅特性の影響を強く反映していることが分かった。また、いずれの地震動強さ指標の空間的分布特性にも、表層地盤の増幅特性だけでなく、震源特性や伝播経路特性などが影響していることが明らかになった。

Publication of a series of papers dealing with Seismic Source Characterization in Switzerland and adjacent areas

Publication of a series of papers dealing with Seismic Source Characterization in Switzerland and adjacent areas

Methodology and main results of seismic source characterization for the PEGASOS Project, Switzerland

Under the direction of National Cooperative for the Disposal of Radioactive Waste (NAGRA), a probabilistic seismic hazard analysis was conducted for the Swiss nuclear power plant sites. The study has become known under the name “PEGASOS Project.” This is the first of a group of papers in this volume that describes the seismic source characterization methodology and the main results of the project. A formal expert elicitation process was used, including dissemination of a comprehensive database, multiple workshops for identification and discussion of alternative models and interpretations, elicitation interviews, feedback to provide the experts with the implications of their preliminary assessments, and full documentation of the assessments. A number of innovative approaches to the seismic source characterization methodology were developed by four expert groups and implemented in the study. The identification of epistemic uncertainties and treatment using logic trees were important elements of the assessments. Relative to the assessment of the seismotectonic framework, the four expert teams identified similar main seismotectonic elements: the Rhine Graben, the Jura / Molasse regions, Helvetic and crystalline subdivisions of the Alps, and the southern Germany region. In defining seismic sources, the expert teams used a variety of approaches. These range from large regional source zones having spatially-smoothed seismicity to smaller local zones, to account for spatial variations in observed seismicity. All of the teams discussed the issue of identification of feature-specific seismic sources (i.e. individual mapped faults) as well as the potential reactivation of the boundary faults of the Permo-Carboniferous grabens. Other important seismic source definition elements are the specification of earthquake rupture dimensions and the earthquake depth distribution. Maximum earthquake magnitudes were assessed for each seismic source using approaches that consider the magnitudes of observed earthquakes within analogous tectonic regions. All four expert teams used the PEGASOS earthquake catalogue for estimating earthquake recurrence parameters. This catalogue was developed by the Swiss Seismological Service and provided all historical and instrumental events in a uniform moment magnitude. The teams evaluated alternative declustering approaches and used available historical data to assess catalog completeness as a function of location, magnitude, and time period.

Seismic source characterization of the Alpine foreland in the context of a probabilistic seismic hazard analysis by PEGASOS Expert Group 1 (EG1a)

Seismic source characterization is performed as part of the PEGASOS project for the assessment of the seismic hazard at the 4 sites of the Swiss Nuclear Power Plants. The analysis is performed according to the Level 4 procedures for expert elicitation defined in the guidelines of the US Nuclear Regulatory Committee whereby the quantification of uncertainties plays a crucial role. According to our analysis, which is one amongst four that were performed in the frame of PEGASOS, the most important epistemic uncertainty is related to the question as to weather basement-rooted faults at the margins of pre-existing Permo-Carboniferous troughs are prone for compressive or transpressive reactivation under the present-day stress field or not. The question after the present-day style of deformation in the Alpine foreland (thick-skinned versus thin-skinned) is closely related to this key question. Together with the consideration of uncertainties regarding the mapping of seismogenic zones and/or line sources alternative zonations are presented in form of a logic tree with 21 branches. Area sources play a predominant role in the working area located at the margin of a diffuse plate boundary. Earthquake recurrence relationships are discussed by taking into account a series of uncertainties. These concern the evaluation of b-values and the evaluation of a-values once the b-values were fixed. Both parameters in the Gutenberg-Richter law are based on non-perfect and incomplete catalogue data that were carefully analysed beforehand. Since PEGASOS demanded an analysis of annual probabilities down to one event in 107 years, the question after the value of the maximum possible earthquake magnitude M max and related error in M max estimates plays a crucial role. We estimate M max by using geological as well as statistical methods. M max = 6.9 cannot be excluded in most areas, in the Basel area M max = 7.3 is possible. Uncertainties in a, b and M max are again discussed in form of a logic tree, this time with 18 branches. Hence the final logic tree has 378 branches and represents the seismic source characterization input into PSHA that takes account of all uncertainties we are aware of.