Stalagmites' reactions to ground motion studied using modified Raspberry Shake and nodal sensors

editorial reviewedKarstic zones are numerous on Earth and offer a particular field of study to evaluate the ground motion levels that occurred in the past in support of regional seismic hazard assessment. Indeed, some fine and slender candlestick stalagmites are intact and therefore indicate that a certain level of ground motion has not been exceeded since they exist. Many parameters must be considered in the behaviour of stalagmites to earthquakes such as their shape, their mechanical properties and their natural frequency. A good way to better understand and characterize the reaction of these stalagmites to earthquakes is to study their reaction to the current permanent ground motion. To do this, a study based on the measurement of ambient seismic noise is underway in the cave of Han-sur-Lesse (Ardenne, Belgium). The ambient seismic noise is measured both at the surface (above the limestone massif and in the nearest village), on the floor of the cave and on the stalagmites themselves. Different three-component seismic sensors are used in parallel: three SmartSolo IGU-16HR 3C and two Raspberry Shake 3D Personal Seismographs, one of which has been adapted to be easily attached to the stalagmites. This parallel configuration during two-week recording periods made it possible to determine the eigenfrequencies and the polarization of the associated movements of 16 stalagmites. In addition, daily and weekly variations in ambient noise and transient events are measured such as earthquakes, quarry explosions or flooding in the cave. The presence of sensors in different places over the same period also makes it possible to study the possible impact of the caves local characteristics on these measurements

Study of space weather impact on Antarctica ionosphere from GNNS data

The impact of solar activity on the ionosphere at polar latitudes is not well known compare to low and mid-latitudes due to lack of experimental observations, especially over Antarctica. Consequently, one of the present challenges of the Space Weather community is to better characterize (1) the climatological behavior of the polar ionosphere in response to variations of the solar activity and (2) the different response of the ionosphere at high latitudes during extreme solar events and geomagnetic storms. For that, the combination of GNSS measurements (e.g. GPS, GLONASS and Galileo) on two separate frequencies allows determining the ionospheric delay between a ground receiver and a satellite. This delay is function of the integrated number of electrons encountered in the ionosphere along the signal ray path, called the Total Electron Content (TEC). It is thus possible to study the behavior of ionospheric TEC at different time and spatial scales from the observations of a network of permanent GNSS stations. In the frame of GIANT-LISSA and IceCon projects we installed since 2009 five GNSS stations around the Princess Elisabeth station. We used these stations additionally to other stations from the IGS global network to estimate the ionospheric TEC at different locations over Antarctica. This study presents this regional data set during different solar activity levels and discusses the different climatological behaviors identified in the ionosphere at these high latitudes. Finally, we will show few examples of typical TEC disturbances observed during extreme solar events

Mécanisme au foyer des tremblements de terre et contraintes tectoniques : le cas de la zone intraplaque belge

Doctorat en sciences physiques -- UCL, 199

La séquence sismique dans la région de Dour de février à mai 1987

Une séquence séismique de 60 événements localisés dans la région de Dour (Hainaut-Belgique) a été enregistrée durant la période février-mai 1987 par les stations permanentes du réseau séismologique belge ainsi que par quatre stations temporaires implantées dans la zone épicentrale au début du mois de mars. Leur magnitude est comprise entre 0,2 et 2,6. Trois de ces secousses ont été ressenties avec une intensité maximale de IV (échelle M.S.K.) sur une étendue d'environ 150 km2. Quatre stations étant situées à moins de 10 km de la zone épicentrale, 18 événements de la séquence ont pu être localisés avec une grande précision. La qualité de ces déterminations a été testée en variant les différents paramètres du modèle crustal. La précision absolue sur la position de l'épicentre est de l'ordre de 0,5 km, tandis que celle de la profondeur du foyer est meilleure que 1 km. La similarité de la distribution des hypocentres pour les modèles de croûte les plus vraisemblables met en évidence une faille dont l'azimut est quasi Est-Ouest et dont le pendage est proche de 30° Sud. Ces résultats sont compatibles avec la faille normale mise en évidence par l'étude du mécanisme au foyer du séisme le plus important et de la solution composite obtenue à l'aide de 8 autres événements. La grande majorité de ces tremblements de terre ont leur profondeur comprise entre 6 et 7 km. Quelques événements en fin de séquence sont néanmoins moins profonds (4 à 5 km). La profondeur des foyers et la mise en évidence d'une zone faillée en accord avec le mécanisme au foyer montrent que la séquence résulte de la relaxation de contraintes de cisaillement le long d'une surface de faille et non d'un effondrement cavitaire provoqué par la dissolution d'évaporites dans le substratum paléozoïque. Les déformations induites par ces tremblements de terre entraînent localement une extension Nord-Sud du Bassin de Möns.Camelbeeck Thierry. La séquence sismique dans la région de Dour de février à mai 1987. In: Bulletin de la Classe des sciences, tome 74, 1988. pp. 96-116

Electrical resistivity tomography data across the Hockai Fault Zone (Ardenne, Belgium)

In this work, we present the result of a large-scale geophysical survey that had the objective of identifying the subsurface characteristics and the NE–SW extension of the Hockai Fault Zone: a major NNW–SSE oriented crustal-rooted fault zone crossing the Stavelot-Venn Massif (Eastern Belgium). 31 two-dimensional electrical resistivity tomography (ERT) profiles are presented, resulting in 10,679 m of 2D sections. All profiles were acquired between 2008 and 2010 using a single channel ABEM Terrameter SAS1000 instrument connected to a 64 electrodes setup of maximum 315 m extent which was often extended using the roll-along technique. Major findings based on the data presented here are reported in the manuscript "A geophysical cross-section of the Hockai Fault Zone (Eastern Belgium)" (Lecocq and Camelbeeck, Submitted for publication) [1]

Wind-induced seismic noise at the Princess Elisabeth Antarctica Station

Icequakes are the result of processes occurring within the ice mass or between the ice and its environment. Studying icequakes provides a unique view on ice dynamics, specifically on the basal conditions. Changes in conditions due to environmental or climate changes are reflected in icequakes. Counting and characterizing icequakes is thus essential to monitor them. Most of the icequakes recorded by the seismic station at the Belgian Princess Elisabeth Antarctica Station (PE) have small amplitudes corresponding to maximal displacements of a few nanometres. Their detection threshold is highly variable because of the rapid and strong changes in the local seismic noise level. Therefore, we evaluated the influence of katabatic winds on the noise measured by the well-protected PE surface seismometer. Our purpose is to identify whether the lack of icequake detection during some periods could be associated with variations in the processes generating them or simply with a stronger seismic noise linked to stronger wind conditions. We observed that the wind mainly influences seismic noise at frequencies greater than 1ĝ€¯Hz. The seismic noise power exhibits a bilinear correlation with the wind velocity, with two different slopes at a wind velocity lower and greater than 6ĝ€¯mĝ€¯s-1 and with, for example at a period of 0.26ĝ€¯s, a respective variation of 0.4ĝ€¯dBĝ€¯(mĝ€¯-1ĝ€¯s) and 1.4ĝ€¯dBĝ€¯(mĝ€¯-1ĝ€¯s). These results allowed a synthetic frequency and wind-speed-dependent noise model to be presented that explains the behaviour of the wind-induced seismic noise at PE, which shows that seismic noise amplitude increases exponentially with increasing wind speed. This model enables us to study the influence of the wind on the original seismic dataset, which improves the observation of cryoseismic activity near the PE station.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Macroseismic intensity data points for shallow 20th century earthquakes in the Hainaut coal area and the 1983 Liège earthquake (Belgium)

<p>This dataset contains macroseismic data points and source parameters for 28 shallow 20th-century earthquakes in the Hainaut area, as well as for the 1983-11-08 Liège earthquake, all in Belgium. For each earthquake, there is 1 CSV-file containing minimum and maximum evaluated macroseismic intensity, latitude, longitude, commune name, epicentral distance and azimuth. The source parameters (origin time, epicentral coordinates, hypocentral depth, magnitude, maximum intensity, macroseismic radius and number of observations) are listed in a XLSX file. The ID_EARTH column in this file corresponds to the first part of the CSV filenames.<br>The most significant difference with respect to the original dataset is an update of the coordinates of several Belgian localities that are used to locate the IDPs, resulting mainly in insignificant changes (<1 km difference for 96% of the IDPs used here), but a few outliers up to a difference of 22 km occur as well. This can result in significantly higher or lower epicentral distances for a few IDPs. Other adjustments to the data include the addition of intensity 1 values (not felt) and the removal, addition or modification of some IDPs (>20 IDPs in total). For the 1983 Liège earthquake (ID_EARTH=651), more significant changes were made to the IDP dataset as part of a major update of the ROB traditional macroseismic database, such as the addition of 297 new IDPs (~90% not felt), the modification of intensity values of 19 IDPs and the removal of 3 IDPs that were previously assigned to the wrong municipality.</p&gt