Spectra of the earthquake sequence February-March, 1981, in south-central Sweden

On February 13, 1981 a relatively strong earthquake occurred in the Lake Vänern region in south-central Sweden. The shock had a magnitude ofM_L = 3.3 and was followed within three weeks by three aftershocks, with magnitudes 0.5 ≤ M_L ≤ 1.0. The focal mechanism solution of the main shock indicates reverse faulting with a strike in the N-S or NE-SW direction and a nearly horizontal compressional stress. The aftershocks were too small to yield data for a full mechanism solution, but first motions of P-waves, recorded at two stations, are consistent for the aftershocks. Dynamic source parameters, derived from Pg- and Sg-wave spectra, show similar stress drops for the main shock (2 bar) and the aftershocks (1 bar), while the differences in seismic moment (1.5·10²⁰ resp. 4·10¹⁸dyne cm), fault length (0.7 resp. 0.2 km) and relative displacement (0.15 resp. 0.03 cm) are significant.     

Radiative decay of the 4d5(6S)5p z5,7Po states in Tc ii: comparison along the homologous and isoelectronic sequences. Application to astrophysics

Using three independent theoretical approaches (CA, HFR + CP, AUTOSTRUCTURE), oscillator strengths have been calculated for a set of Tc  ii transitions of astrophysical interest and the reliability of their absolute scale has been assessed. The examination of the spectra emitted by some Ap stars has allowed the identification of Tc  ii transitions in HD 125248. This Tc  ii detection should however await confirmation from spectral synthesis relying on dedicated model atmospheres. New partition functions are also provided for Tc  i , Tc  ii and Tc  iii for temperatures ranging between 4000 and 13 000 K.     

A technique for modeling the Earth’s gravity field on the basis of satellite accelerations

A technique is proposed for Earths gravity field modeling on the basis of satellite accelerations that are derived from precise orbit data. The functional model rests on Newtons second law. The computational procedure is based on the pre-conditioned conjugate-gradient (PCCG) method. The data are treated as weighted average accelerations rather than as point-wise ones. As a result, a simple three-point numerical differentiation scheme can be used to derive them. Noise in the orbit-derived accelerations is strongly dependent on frequency. Therefore, the key element of the proposed technique is frequency-dependent data weighting. Fast convergence of the PCCG procedure is ensured by a block-diagonal pre-conditioner (approximation of the normal matrix), which is derived under the so-called Colombo assumptions. Both uninterrupted data sets and data with gaps can be handled. The developed technique is compared with other approaches: (1) the energy balance approach (based on the energy conservation law) and (2) the traditional approach (based on the integration of variational equations). Theoretical considerations, supported by a numerical study, show that the proposed technique is more accurate than the energy balance approach and leads to approximately the same results as the traditional one. The former finding is explained by the fact that the energy balance approach is only sensitive to the along-track force component. Information about the cross-track and the radial component of the gravitational potential gradient is lost because the corresponding force components do no work and do not contribute to the energy balance. Furthermore, it is shown that the proposed technique is much (possibly, orders of magnitude) faster than the traditional one because it does not require the computation of the normal matrix. Hints are given on how the proposed technique can be adapted to the explicit assembling of the normal matrix if the latter is needed for the computation of the model covariance matrix.Acknowledgments. Professor R. Klees is thanked for support of the project and for numerous fruitful discussions. The authors are also thankful to Dr. J. Kusche for useful remarks and to Dr. E. Schrama, his solid background in satellite geodesy proved to be very helpful. A large number of valuable comments were made by Dr. S.-C. Han, Dr. P. Schwintzer, and an anonymous reviewer; their contribution is greatly acknowledged. The satellite orbits used in the numerical study were kindly provided by Dr. P. Visser (Aerospace Department, Delft University of Technology). Access to the SGI Origin 3800 computer was provided by Stichting Nationale Computerfaciliteiten (NCF), grant SG-027.     

Transition probabilities in singly ionized promethium and the identification of Pm ii lines in Przybylski's star and HR 465★

The first theoretical transition probabilities are obtained for a set of 46 Pm  ii transitions of astrophysical interest. These data fill in a gap in astrophysics and will allow to establish, on a firmer basis, the presence of some lines of this radioactive element in the spectra of chemically peculiar stars and, consequently, a quantitative investigation of the stellar Pm abundance. A search for Pm  ii lines in Przybylski's star (HD 101065) and in HR 465 is reported and discussed, supporting the detection of this ion. A more detailed quantitative analysis is awaiting the availability of dedicated model atmospheres for these stars.     

An autonomous instrument for time series analysis of TCO2 from oceanographic moorings

The design and testing of a robotic analyzer for autonomous TCO₂ measurement from oceanographic moorings is described. The analyzer employs a conductimetric method of TCO₂ measurement wherein CO₂ from an acidified sample diffuses across a semi-permeable membrane into a NaOH solution decreasing the conductivity of the base. The instrument is capable of ～850 analyses over a period of at least six months. It is designed to operate to depths of at least 1000 m. TCO₂ calibration is based on in situ standardization throughout a deployment.We report both laboratory and in situ tests of the analyzer. In the laboratory automated analyses over a period of 38 days at temperatures ranging from 8° to 25 °C yielded a TCO₂ accuracy and precision of ±2.7 μmol/kg. In situ tests were conducted at the WHOI dock with a deployment of 8 weeks at in situ temperatures of 5°–13 °C. The accuracy and precision of TCO₂ analyses over the deployment period, based on in situ calibration, was ±3.6 μmol/kg.Laboratory tests of reagent and standard solution stability are also reported. Standards, based on Certified Reference Material were followed for periods of up to 2 years. In all cases TCO₂ increased. Drift of the standards was the equivalent of ～1 to 3 μmol/kg per 6 months. The conductivity indicator solution was found to be stable for at least 2 months.     

Intrinsic absorption and scattering attenuation in the southern part of the Netherlands

We present the first systematic study of attenuation derived from the S-wave coda in the frequency range 1-32 Hz for the southern part of the Netherlands and its surroundings. For this we used two methods, the codaQ (Q _c) method and the Multiple Lapse Time Window (MLTW) method. In the interpretation of the results both single and multiple scattering in a half space are considered. Our aim is to validate these interpretations in our region and to try to identify theeffects of attenuation due to intrinsic absoprtion (Q _i)and scattering attenuation (Q _s). For this we analyzedmore than 100 3-component high-quality digital seismograms from 43 crustalevents and 23 different stations in the Netherlands, Germany and Belgium.Coda Q results show smaller Q _c (=Q ₀fⁿ) values for epicentral distances shorter than 25 km (Q ₀=90) compared to larger epicentral distances (Q ₀=190), but similar frequency dependence (f^-0.9). Interpretation of MLTW results provided a seismic albedo smaller then 0.5, suggesting that the intrinsic absorption dominates over scattering in this region. Both Q _i and Q _s show similar frequency dependences as Q _c. These results are comparable to those obtained in other areas, but we also show that more sophisticated models are required to remove ambiguities in the interpretation. For short lapse times and shortevent-station distances we find for the simple half space model a correspondinginterpretation of both methodologies, where Q _c correspondsto Q _t, suggesting that a model with single scattering in ahalf space is appropriate. For long lapse times and long event station distances, however, we find that the S-wave coda is, most probably, too much influenced by crust-mantel heterogenities and more sophisticated Qinversion models using larger data sets are required for more reliable attenuation estimates.     

The Roer Valley Graben earthquake of 13 April 1992 and its seismotectonic setting

A moderately sized pure, normal dip-slip earthquake occurred in the Roer Valley Graben (RVG) near Roermond, The Netherlands on 13 April 1992 at 1h 20m UTC. This contribution presents an overview of the locations, fault-plane solutions and magnitudes obtained for the mainshock and the aftershocks by the different scientific groups involved in their analysis. The observed maximum intensity of VII is compared with that of other earthquakes in the region to illustrate the relatively low level of damage caused by the mainshock.
Using SH and Lg waves recorded at seven local and regional broadband stations, we determine a seismic moment of 1.4 × 10¹⁷ Nm, a static stress drop of 9.7 MPa and an average displacement of 33 cm over a rupture surface of approximately 11 km².
The seismotectonics of the region extending from the RVG to the city of Liège including the western part of the Rhenish Massif (WRM) and the eastern part of the Brabant Massif (EBM) is analysed based on the Roermond earthquake studies and data collected since 1985 by the Belgian seismic network. The geographical distribution of focal mechanism reveals four different seismotectonic regimes in this area. From stress tensor inversion we find that ₃ coincides with the minimum horizontal stress component in the RVG, the WRM and possibly in the EBM, while in the Liège region ₃ is approximately vertical. The minimum horizontal stress component shows a 30° rotation to the north in the WRM and the Liège region and possibly 50° in the EBM when compared with the minimum horizontal stress component in the RVG.