ALBERT

Description: Global earthquake locations are often associated with very large systematic travel-time residuals even for clear arrivals, especially for regional and near-regional stations in subduction zones because of their strongly heterogeneous velocity structure. Travel-time corrections can drastically reduce travel-time residuals at regional stations and, in consequence, improve the relative location accuracy. We have extended the shrinking-box source-specific station terms technique to regional and teleseismic distances and adopted the algorithm for probabilistic, nonlinear, global-search location. We evaluated the potential of the method to compute precise relative hypocentre locations on a global scale. The method has been applied to two specific test regions using existing P - and pP -phase picks. The first data set consists of 3103 events along the Chilean margin and the second one comprises 1680 earthquakes in the Tonga-Fiji subduction zone. Pick data were obtained from the GEOFON earthquake bulletin, produced using data from all available, global station networks. A set of timing corrections varying as a function of source position was calculated for each seismic station. In this way, we could correct the systematic errors introduced into the locations by the inaccuracies in the assumed velocity structure without explicitly solving for a velocity model. Residual statistics show that the median absolute deviation of the travel-time residuals is reduced by 40–60 per cent at regional distances, where the velocity anomalies are strong. Moreover, the spread of the travel-time residuals decreased by ~20 per cent at teleseismic distances (〉28°). Furthermore, strong variations in initial residuals as a function of recording distance are smoothed out in the final residuals. The relocated catalogues exhibit less scattered locations in depth and sharper images of the seismicity associated with the subducting slabs. Comparison with a high-resolution local catalogue reveals that our relocation process significantly improves the hypocentre locations compared to standard locations.

Description: Seismological agencies play an important role in seismological research and seismic hazard mitigation by providing source parameters of seismic events (location, magnitude, mechanism), and keeping these data accessible in the long term. The availability of catalogues of seismic source parameters is an important requirement for the evaluation and mitigation of seismic hazards, and the catalogues are particularly valuable to the research community as they provide fundamental long-term data in the geophysical sciences. This work is well motivated by the rising demand for developing more robust and efficient methods for routine source parameter estimation, and ultimately generation of reliable catalogues of seismic source parameters. Specifically, the aim of this work is to develop some methods to determine hypocentre location and temporal evolution of seismic sources based on regional and teleseismic observations more accurately, and investigate the potential of these methods to be integrated in near real-time processing. To achieve this, a location method that considers several events simultaneously and improves the relative location accuracy among nearby events has been provided. This method tries to reduce the biasing effects of the lateral velocity heterogeneities (or equivalently to compensate for limitations and inaccuracies in the assumed velocity model) by calculating a set of timing corrections for each seismic station. The systematic errors introduced into the locations by the inaccuracies in the assumed velocity structure can be corrected without explicitly solving for a velocity model. Application to sets of multiple earthquakes in complex tectonic environments with strongly heterogeneous structure such as subduction zones and plate boundary region demonstrate that this relocation process significantly improves the hypocentre locations compared to standard locations. To meet the computational demands of this location process, a new open-source software package has been developed that allows for efficient relocation of large-scale multiple seismic events using arrival time data. Upon that, a flexible location framework is provided which can be tailored to various application cases on local, regional, and global scales. The latest version of the software distribution including source codes, a user guide, an example data set, and a change history, is freely available to the community. The developed relocation algorithm has been modified slightly and then its performance in a simulated near real-time processing has been evaluated. It has been demonstrated that applying the proposed technique significantly reduces the bias in routine locations and enhance the ability to locate the lower magnitude events using only regional arrival data. Finally, to return to emphasis on global seismic monitoring, an inversion framework has been developed to determine the seismic source time function through direct waveform fitting of teleseismic recordings. The inversion technique can be systematically applied to moderate- size seismic events and has the potential to be performed in near real-time applications. It is exemplified by application to an abnormal seismic event; the 2017 North Korean nuclear explosion. The presented work and application case studies in this dissertation represent the first step in an effort to establish a framework for automatic, routine generation of reliable catalogues of seismic event locations and source time functions.

Description: Seismologische Dienste spielen eine wichtige Rolle in der seismologischen Forschung und Gefährdungsanalyse, indem sie Quellparameter für seismische Ereignisse (Lokalisierung, Stärke, Mechanismus) bereitstellen und diese Daten langfristig verfügbar machen. Die Verfügbarkeit von Katalogen seismischer Quellparameter ist eine wichtige Voraussetzung für die Bewertung der seismischen Gefährdung und Minderung der Auswirkungen von Erdbeben. Die seismischen Kataloge sind für die Forschungsgemeinschaft besonders wertvoll, da sie grundlegende Hintergrundsdaten über lange Zeiträume liefern. Die vorliegende Arbeit ist motiviert durch die steigende Nachfrage nach der Entwicklung robusterer und effizienterer Methoden für die routinemäßige Schätzung von Quellparametern und schließlich die Verbesserung der Qualität und Quantität der seismischen Kataloge. Ziel dieser Arbeit ist es insbesondere Methoden zu entwickeln, um die Lokalisierung von Hypozentren und die zeitliche Entwicklung der Momentfreisetzung bei einem Erdbeben auf der Grundlage regionaler und teleseismischer Beobachtungen genauer zu bestimmen, und das Potenzial dieser Methoden für eine zeitnahe Verarbeitung zu untersuchen. Um dies zu erreichen, wurde eine Lokalisierungsmethode bereitgestellt, die mehrere Ereignisse gleichzeitig berücksichtigt und die relative Genauigkeit der Hypozentren zwischen benachbarten Ereignissen verbessert. Dieses Verfahren versucht, die Verzerrungen durch Geschwindigkeits-heterogenitäten zu reduzieren, oder äquivalent Einschränkungen und Ungenauigkeiten in dem angenommenen Geschwindigkeitsmodell auszugleichen, indem richtungsabhängige Zeitkorrekturen für jede seismische Station berechnet werden. Die systematischen Fehler, die durch die Ungenauigkeiten in der angenommenen Geschwindigkeitsstruktur in die Lokalisierung projiziert werden, können korrigiert werden, ohne explizit nach einem Geschwindigkeitsmodell zu lösen. Die Anwendung auf Ensemble von Erdbeben in tektonischen Situationen mit stark heterogener, komplexer Struktur wie Subduktionszonen und Plattengrenzregionen zeigt, dass diese Relokalisierung die Hypozentren im Vergleich zu Standardlokalisierungen signifikant verbessert. Um den rechnerischen Anforderungen der Lokalisierung gerecht zu werden, wurde ein neues Open-Source-Softwarepaket entwickelt, das eine effiziente Relokalisierung einer großen Anzahl von Erdbeben unter Verwendung von Ankunftszeitdaten ermöglicht. Daraufhin wird ein flexibler Lokalisierungsrahmen bereitgestellt, der an verschiedene Fälle auf lokaler, regionaler und globaler Ebene angepasst werden kann. Die neueste Version der Software einschließlich Quellcodes, Benutzerhandbuch, Beispieldatensatz und Änderungsverlauf ist für die Öffentlichkeit frei verfügbar. Der entwickelte Relokalisierungs-Algorithmus wurde geringfügig verändert und anschließend seine Leistung in einem simulierten Echtzeit Prozess bewertet. Es zeigte sich, dass die Anwendung der vorgeschlagenen Technik die Verzerrung der Hypozentren der Routineauswertung erheblich verringert und die Fähigkeit zum Lokalisieren der Ereignisse mit niedriger Magnitude unter Verwendung nur regionaler Ankunftsdaten verbessert. Um sich wieder auf die globale seismische Überwachung zu konzentrieren, wurde schließlich ein Inversionssystem entwickelt, um die Zeitfunktion der seismischen Quelle durch direkte Wellenformanpassung von teleseismischen Aufzeichnungen zu bestimmen. Die Inversionstechnik kann systematisch auf seismische Ereignisse mittlerer Größe angewendet werden und hat das Potenzial, in nahezu Echtzeitanwendungen ausgeführt zu werden. Dies wird beispielhaft durch die Anwendung auf ein abnormales seismisches Ereignis veranschaulicht: die nordkoreanische Atomexplosion 2017. Die in dieser Dissertation vorgestellten Arbeits- und Anwendungsfallstudien stellen den ersten Schritt dar, um einen Rahmen für die automatische, routinemäßige Erzeugung zuverlässiger Kataloge von seismischen Ereignisorten und Quellzeitfunktion zu schaffen.

Description: There has been growing recognition of the importance of the accurate seismic locations in quantitative seismological studies, such as seismic hazard analyses, fault zone characterization, and Earth's deformation. Accurate estimation of seismic locations is critical since a wrong estimate of the seismic source location will result in wrong interpretations in the subsequent analyses. We present SCOTER, an open-source Python program package that is designed to relocate multiple seismic events by using P- and S-wave station correction terms. The package implements static and shrinking-box source-specific station terms techniques extended to regional and teleseimic distances and adopted for probabilistic, non-linear, global-search location for large-scale multiple-event location. This program provides robust relocation results for seismic event sequences over a wide range of spatial and temporal scales by applying empirical corrections for the biasing effects of 3-D velocity structure. Written in the Python programming language, SCOTER is run as a stand-alone command-line tool (requiring no knowledge of Python) and also provides a set of sub-commands to develop inputs (dataset, configuration etc) and export results (hypocenter parameters, travel-time residuals etc) { routine but non-trivial tasks that can consume much user time. This package can be used for relocation in local, regional, and teleseimic scales. We describe SCOTER's functionality, design and technical implementation, accompanied by an overview of its use cases. As an illustration, we demonstrate the applicability of this tool through two examples based on (1) a catalogue of several hundred events in the Arctic plate boundary region using regional and teleseismic arrival times and (2) a small dataset of low-magnitude seismic events recorded by dense, local stations at the western Iberia, central Portugal. The relocated datasets highlight the future potential for applying the SCOTER relocation tool to greatly improve the relative location accuracy among nearby events.

Description: We present SCOTER, an open-source Python programming package that is designed to relocate multiple seismic events by using direct P- and S-wave station correction terms. The package implements static and shrinking-box source-specific station terms techniques extended to regional and teleseimic distances and adopted for probabilistic, non-linear, global-search location for large-scale multiple-event location. This program provides robust relocation results for seismic event sequences over a wide range of spatial and temporal scales by applying empirical corrections for the biasing effects of 3-D velocity structure. Written in the Python programming language, SCOTER is run as a stand-alone command-line tool (requiring no knowledge of Python) and also provides a set of sub-commands to develop required input files (e.g. phase files, travel-time grid files, configuration) and export relocation results (such as hypocenter parameters, travel-time residuals) in different formats -- routine but non-trivial tasks that can consume much user time. This package can be used for relocating data sets in local, regional, and teleseimic scales.

Description: Pyrocko is an open source seismology toolbox and library, written in the Python programming language. It can be utilized flexibly for a variety of geophysical tasks, like seismological data processing and analysis, calculation of Green's functions and earthquake models' synthetic waveforms and static displacements (InSAR or GPS). Those can be used to characterize extended earthquake ruptures, point sources (moment tensors) and other seismic sources. This publication includes the Pyrocko core, a library providing building blocks for researchers and students wishing to develop their own applications.

Description: The sedimentary basin infill of continental passive margins is considered as geo-reactors with temperature-related physical and chemical processes. The aim of this study is to derive regional variations in geothermal gradient at depth for two differently aged passive margins and to explore the controlling factors for these variations. Hence, we analyzed two previously published 3D conductive and lithospheric-scale thermal models of the Southwest (SW) African and the Norwegian passive margins. These 3D models differentiate various sedimentary, crustal and mantle units and integrate different geophysical data such as seismic observations and the gravity field. We extracted the temperature-depth distributions in 1 km intervals down to 6 km below the upper thermal boundary condition (UBC). The geothermal gradient was then calculated for these intervals between the UBC and the respective depth levels (1, 2, 3, 4, 5, and 6 km below the UBC). According to our results, the geothermal gradient decreases with increasing depth and shows different trends and values for these two different margins. It has the least lateral variations over the area at 6 km below the UBC by a range of 16°C/km for the SW African margin and 26°C/km for the Norwegian margin. In the onshore parts of the SW African margins (variably covered by sediments) the geothermal gradient differs by 28-34°C/km and varies with depth. In contrast, the Norwegian onshore domains (with outcropping basement) show a variation as low as 15-17°C/km throughout the different depth intervals. Offshore, at the Norwegian margin, the geothermal gradient increases oceanward. However, at the SW African margin, the geothermal gradient declines from the sedimentary basins towards the distal parts of the shelf and reaches the minimum value in the oceanic crustal domain. These results indicate the ongoing process of oceanic mantle cooling at the young Norwegian margin compared with the old SW African passive margin that seems to be thermally equilibrated.