1,721,124 research outputs found
Acceleration of buoyancy-driven fractures and magmatic dikes beneath the free surface
No full textBy performing buoyancy-driven fracture experiments in brittle gelatin we observe that the ascent velocity of a fracture containing a finite volume of fluid increases when approaching the free surface. We theoretically describe this free-surface effect and quantify it by introducing an effective depth-dependent fracture toughness and developing an ascent model on the basis of linear fracture mechanics. We develop a successful inversion approach and resolve the actual and critical fracture length and the ascent velocity far away from the free surface from the observation of the fracture tip migration alone. Other parameters, as the fluid volume included in the fracture and the in situ fracture toughness, can be derived. Applying the model and inversion to the 1998 eruption at Piton de la Fournaise, Reunion Island, reveals estimates of the length and critical length of the feeding magma batch, the magma batch volume and the in situ fracture toughness. It further indicates that the ascent velocity of the magma batch was probably much smaller at greater depths and that the batch might have been initiated several months or years before the eruption
A probabilistic approach for the classification of earthquakes as ‘triggered’ or ‘not triggered’
No full textThe occurrence time of earthquakes can be anticipated or delayed by external phenomena that induce strain energy changes on the faults. ‘Anticipated’ earthquakes are generally called ‘triggered’; however, it can be controversial to label a specific earthquake as such, mostly because of the stochastic nature of earthquake occurrence and of the large uncertainties usually associated to stress modelling. Here we introduce a combined statistical and physical approach to quantify the probability that a given earthquake was triggered by a given stress-inducing phenomenon. As an example, we consider an earthquake that was likely triggered by a natural event: the M = 6.2 13 Jan 1976 Kópasker earthquake on the Grímsey lineament (Tjörnes Fracture Zone, Iceland), which occurred about 3 weeks after a large dike injection in the nearby Krafla fissure swarm. By using Coulomb stress calculations and the rate-and-state earthquake nucleation theory, we calculate the likelihood of the earthquake in a scenario that contains only the tectonic background and excludes the dike and in a scenario that includes the dike but excludes the background. Applying the Bayes’ theorem, we obtain that the probability that the earthquake was indeed triggered by the dike, rather than purely due to the accumulation of tectonic strain, is about 60 to 90 %. This methodology allows us to assign quantitative probabilities to different scenarios and can help in classifying earthquakes as triggered or not triggered by natural or human-induced changes of stress in the crust
A multibranch, multitarget neural network for rapid point-source inversion in a microseismic environment: examples from the Hengill Geothermal Field, Iceland
Full text linkDespite advanced seismological techniques, automatic source characterization for microseismic earthquakes remains difficult and challenging since current inversion and modelling of high-frequency signals are complex and time consuming. For real-time applications such as induced seismicity monitoring, the application of standard methods is often not fast enough for true complete real-time information on seismic sources. In this paper, we present an alternative approach based on recent advances in deep learning for rapid source-parameter estimation of microseismic earthquakes. The seismic inversion is represented in compact form by two convolutional neural networks, with individual feature extraction, and a fully connected neural network, for feature aggregation, to simultaneously obtain full moment tensor and spatial location of microseismic sources. Specifically, a multibranch neural network algorithm is trained to encapsulate the information about the relationship between seismic waveforms and underlying point-source mechanisms and locations. The learning-based model allows rapid inversion (within a fraction of second) once input data are available. A key advantage of the algorithm is that it can be trained using synthetic seismic data only, so it is directly applicable to scenarios where there are insufficient real data for training. Moreover, we find that the method is robust with respect to perturbations such as observational noise and data incompleteness (missing stations). We apply the new approach on synthesized and example recorded small magnitude (M ≤ 1.6) earthquakes at the Hellisheiði geothermal field in the Hengill area, Iceland. For the examined events, the model achieves excellent performance and shows very good agreement with the inverted solutions determined through standard methodology. In this study, we seek to demonstrate that this approach is viable for microseismicity real-time estimation of source parameters and can be integrated into advanced decision-support tools for controlling induced seismicity
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Wellenfeldmethoden zur Auswertung von passiven seismischen Ozeanbodendaten
No full textIn this study different full waveform techniques was developed for the investigation of seismic ocean bottom single station data. These techniques were applied to data obtained in a pilot ocean bottom experiment in the Tyrrhenian Sea/Italy (TySea experiment) from December 2000 to May 2001. A network of broadband seven ocean bottom seismometers and seven ocean bottom hydrophones was installed above the subducting Ionian plate which descends from Southeast to Northwest. Local and teleseismic earthquakes were recorded by the stations. The newly developed techniques produce very promising results in reconstructing the sea floor structure beneath the stations and in attenuating waveform effects generated by water layer multiples. Additionally the techniques offer a possibility to determine the orientation of free fall ocean bottom seismometers. The main results are:
1. The waveform recorded at the seafloor differs from waveforms recorded at land stations. This is primarily due to multiple reflections in the water layer. These multiple reflections show different patterns on seismometer and hydrophone recordings depending on the seafloor structure. This opens the possibility to constrain the P-wave velocity structure beneath the station by means of a full waveform inversion. Seismometer and hydrophone traces of 13 local deep earthquakes were used to resolve the seafloor structure below the single stations of the experiment in terms of P-wave velocity. It was found that the average P-wave velocity of the uppermost gradient layer varies from 1630 m/s to 1690 m/s. The layer itself is 95 m to 190 m thick. The results are well determined and comparable with findings of other studies undertaken in this region of the Tyrrhenian Sea.
2. The multiple reflections of waves in the water layer also influence the analysis of teleseismic events, in receiver function studies or for tomographic studies where relative arrival times are usually estimated by cross correlation techniques. The wavefield decomposition using seismometer and hydrophone traces separates the up- and downgoing wavefield. The effects of multiple reflections can be attenuated and the signal-to-noise-ratio can be improved with a decomposition analysis. As a by-product an in situ calibration of the sensors is possible as well as the estimation of the impedance contrast at the seafloor. The impedance contrast was found to have a value of 1.1 to 1.3. With these improvements the data of land and ocean bottom stations may be combined in future.
3. In the recorded seismograms an energy signal which is mainly horizontal linearly polarized occurs between 0.4 s and 1.2 s after the P-onset from local deep earthquakes. This is interpreted as a converted P to S-wave which is trapped in the upper sediments. This energy signal is used to estimate the orientation of the ocean bottom seismometer and to resolve the seafloor structure in terms of S-wave velocity. For the TySea experiment the obtained S-wave velocity in the uppermost 10 m to 50 m deep layer is in the range of 10 m/s to 100 m/s.In dieser Arbeit wurden verschiedene vollständige Wellenformmethoden zur Untersuchung von seismischen Daten von einzelnen Ozeanbodenstationen entwickelt. Diese Methoden wurden auf Daten angewendete, die in einem Pilotprojekt im Tyrrhenischen Meer/Italien (TySea Experiment) zwischen Dezember 2000 und Mai 2001 auf dem Ozeanboden gewonnen wurden. Ein Netzwerk aus sieben Ozeanbodenseismometern und sieben Ozeanbodenhydrofonen wurde oberhalb der nach Nordwesten abtauchenden Ionischen Platte installiert. Lokale und teleseismische Erdbeben wurden auf den Stationen aufgezeichnet. Die neu entwickelten Methoden liefern vielversprechende Ergebnisse beim Rekonstruieren der Struktur des Ozeanbodens unterhalb der Stationen und beim Abschwächen von Wellenformeffekten, die durch multiple Reflexionen in der Wasserschicht entstehen. Darüber hinaus bietet die Technik eine Möglichkeit zur Bestimmung der Orientierung der ausgesetzten Ozeanbodenseismometern. Die Hauptergebnisse sind:
1. Wellenformen, welche am Ozeanboden aufgezeichnet werden, unterscheiden sich von Wellenformen, welche auf Landstationen aufgezeichnet wurden. Dies ist vor allem auf die multiplen Reflexionen in der Wasserschicht zurückzuführen. Diese multiplen Reflexionen zeigen unterschiedliches Verhalten auf den Aufzeichnungen des Seismometers und des Hydrofons, welches von der Untergrundsstruktur abhängt. Dies eröffnet die Möglichkeit, mittels einer vollständigen Wellenforminversion die Geschwindigkeitsstruktur der P-welle unterhalb der Station zu bestimmen. Seismometer- und Hydrofonspuren von 13 lokale Tiefbeben wurden dafür verwendet. Es wurde herausgefunden, dass die mittlere P-wellengeschwindigkeit für die oberste Gradientenschicht zwischen 1630 m/s und 1690 m/s variiert. Die Schicht ist zwischen 95 m und 195 m dick. Die Ergebnisse sind vertrauenswürdig und sind vergleichbar mit den Ergebnissen andere Studien, die in der Region des Tyrrhenischen Meeres durchgeführt wurden.
2. Die multiplen Reflexionen in der Wasserschicht beeinflussen auch die Auswertung von teleseismischen Erdbeben in Studien der Receiver Funktionen oder der seismischen Tomografie, bei der relative Ankunftzeiten üblicherweise durch Kreuzkorrelationtechniken bestimmt werden. Die Wellenfeldzerlegung, die Seismometer- und Hydrofonspuren verwendet, trennt hoch- und runterlaufende Wellenfelder. Die Effekte der multiplen Reflexionen können durch die Wellenfeldzerlegung abgeschwächt und das Signal-Rausch-Verhältnis verbessert werden. Als Nebenprodukt ist sowohl eine In-situ Kalibration der Sensoren also auch die Bestimmung des Impedanzkontrastes am Ozeanboden möglich. Es wurde ein Impedanzkontrast zwischen 1.1 und 1.3 gefunden. Mit diesen Verbesserungen ist es in Zukunft möglich, Daten von Land- und Ozeanbodenstationen zu kombinieren.
3. In den aufgezeichneten Daten von Lokalbeben taucht zwischen 0.4 s und 1.2 s nach dem P-Einsatz ein Signal auf, welches hauptsächlich horizontal linear polarisiert ist. Dieses Signal wird als eine konvertierte P zu S-welle interpretiert, die in den oberen Sedimenten eingeschlossen ist. Mit diesem Signal wurde die Orientierung der Ozeanbodenseismometer bestimmt und die Geschwindigkeitsstruktur der S-welle aufgelöst. Im TySea Experiment wurden eine S-wellengeschwindigkeit für die obersten 10 m bis 50 m von 10 m/s bis 100 m/s gefunden
Eine robuste Methode zur Abschätzung von kinematischen Erdbeben-Herdparametern
No full textAutomatic methods to determine earthquake source parameters have become essential tools in modern seismology. Currently, most such methods are based on point source (i.e. moment tensor) approximations of earthquake rupture. This simple model presents a restriction for some applications. Especially in the scope of rapid hazard assessment and tsunami early warning, automatic methods revealing more details about extension and temporal evolution of the rupture process (kinematic source parameters) are of great importance. A main problem inherent to many earlier attempts in this direction is their tendency to produce unstable and ambiguous results due to overparameterization.
The aim of the work presented in the following was to investigate the possibilities to robustly determine, based on teleseismic and regional recordings, not only point source but also kinematic earthquake source parameters. The main challenges targeted, were how to identify and prevent ambiguities and how to properly quantify uncertainties of the results.
The methodical requirements were met by a combination of several advances: A new source model has been introduced, the eikonal source, which has been especially designed to avoid overparameterization. An adaptive data weighting scheme has been proposed to gain a robust and balanced procedure with respect to heterogeneous input data. The misfit function used is based on an L1-norm between real and synthetic data to reduce the influence of outliers. Large portions of parameter-space are searched in order to detect ambiguities inherent to the specific setup of each investigated event.
To meet the computational demands of this non-linear inverse problem, I have developed a set of tools to efficiently calculate synthetic seismograms for extended earthquake source models based on pre-calculated Green's functions. Upon that, a flexible inversion framework is provided which can be tailored to various application cases on local, regional, and global scales.
In this work I explain the methodical tools which have been developed and used, and present an automatic procedure to estimate point source and kinematic source parameters for global earthquakes. It is exemplified by application to the Mw 6.3 L'Aquila earthquake of 2009. Details of the method are investigated through test applications to synthetic datasets. Finally, the usability of the method is shown by comparing several test cases with published results.Ein Ziel bei der Untersuchung von Erdbeben ist die Ermittlung von Herdparametern. Im einfachsten Fall wird der Bruchvorgang als Punktquelle beschrieben. In vielen Fällen ist man nur an den Effekten dieser ersten Näherung interessiert, hierfür existieren bereits verschiedene automatisierte Auswertungsverfahren. Die näherungsweise Betrachtung eines Erdbebens als punktförmiges Ereignis ist für bestimmte Anwendungen, wie z.B. im Rahmen von Tsunami-Frühwarnsystemen und bei der schnellen Abschätzung von Schadens-Szenarien oft unzureichend. Man möchte dort zusätzlich Informationen über Ausdehnung und zeitlichen Ablauf des Bruchprozesses aus den Beobachtungen gewinnen. Diese werden durch die kinematischen Herdparameter beschrieben. Ein Problem bei ihrer Bestimmung ist das häufige Auftreten von mehrdeutigen Lösungen, unter anderem wegen der Überparametrisierung der vorhandenen Modelle.
Ziel dieser Arbeit war es, ein robustes und automatisierbares Verfahren zum Abschätzen von kinematischen Herdparametern aus teleseismischen und regionalen Datensätzen zu entwickeln. Schwerpunkte lagen hierbei im Untersuchen von Mehrdeutigkeiten und in der Quantifizierung der Unsicherheiten der Ergebnisse.
Das Verfahren beruht auf mehreren methodischen Neuerungen: Ein neues vereinfachtes Modell für den Bruchprozess, welches die Gefahr der oben genannten Mehrdeutigkeiten minimiert, wurde vorgestellt, das sogenannte Eikonalmodell. Eine Methode zur adaptiven Gewichtung der seismischen Daten wurde entwickelt, um Fehlgewichtungen zu vermeiden. Im Gegensatz zu anderen Ansätzen wurden hier die Abweichungen zwischen echten und modellierten Daten mit einer L1-Norm gemessen. Eine variable Kombination von verschiedenen Suchalgorithmen ermöglicht eine ausreichend vollständige Untersuchung des gesamten Parameterraums.
Um den numerischen Anforderungen dieses nichtlinearen Inversionsproblems gerecht zu werden, habe ich ein Softwarepaket entwickelt, mit dessen Hilfe man synthetische Seismogramme aus im voraus berechneten Greenschen Funktionen für ausgedehnte Herdmodelle effizient berechnen kann. Darauf aufbauend wurde ein flexibles System zur Umsetzung von Inversionsschemata erstellt, welches sich leicht an lokale, regionale und globale Anwendungen anpassen lässt.
Neben der detaillierten Beschreibung der Theorie des Verfahrens wird seine Funktionsfähigkeit mit Hilfe mehrerer Tests gezeigt. Die Anwendung wird anhand des Erdbebens von L'Aquila (Mw 6.3, 2009) exemplarisch dargestellt. Weitere Erdbeben mit verschiedenen Quellgeometrien werden analysiert und die Resultate mit Referenzergebnissen verglichen
MARINE GRAVIMETRIC AND MAGNETIC INVESTIGATIONS WITHIN THE AREA OF THE SOUTHEASTERN MEDITERRANEAN SEA AND THE NORTHERN SECTION OF EGYPT
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