531 research outputs found
Ăber die Anwendung von Satellitengradiometrie und terrestrischer Gravimetrie zur Identifikation regionaler Stressanomalien im nordchilenischen Subduktionssystem
The effect of the lithospheric density distribution on physical coupling of the subduction system in the area of the Central Andes (18°â35°S) has been investigated. Amongst the composition of the subducting oceanic plate and properties of the subduction interface itself, there has also been evidence that increased coupling of the system may be linked to excess masses above the descending plate. High plate coupling is associated with an increased risk for megathrust earthquakes which occur at respective locations when the accumulated stresses are released and dispersed as seismic energy.
Methodological considerations, forwardâmodelling as well as GPE- and stress calculations â that base on the utilisation and interpretation of the gravity field and its gradient tensor â have been applied in order to examine and analyse the density- and stress distribution in the target area. Particular attention has also been paid to the evaluation of gravity data from the GOCE satellite mission, which, for the first time, provides nearâglobally measured fullâtensor gradiometric data. Methodological analysis concerning the resolving capacity of gravity and gradiometry reveal that measured gravity gradients of the GOCE mission are just sensitive enough to receive information at the order of the expected coastal batholiths' gravity anomaly in North Chile. Those are believed to significantly affect coupling of the
subduction system through their relative excess mass. Synthetic 3D density forward modelling of a standard subduction setting has been applied to further test the analysis. It confirms that density contrasts beyond resolvability for gravity data at the orbit height (~250 km) are just resolved in the gradient tensor and its invariants though. The spatial resolution of most recent potential field models that represent measurements from satellite missions is not better than sphericalâharmonic degree and order (d/o) 300
(~67 km halfâwavelength). This is still not sufficient for very detailed lithospheric or even crustal studies. It is mainly attributed to the signalâtoânoise ratio conditions and to overlapping signals in the measurement systems at large distance to the sourceâmasses. However, satellite-based data are virtually globally available and they are acquired and processed in homogeneous manner. They may therefore be consistently handled, analysed and interpreted. Terrestrial data, on the other hand, are strongly heterogeneously distributed, sometimes inadequately
processed and meta-data is often not available. Their advantage is a good spatial resolution because of the short distance to the sourceâmasses which allows the distinction of signals. When satelliteâderived data are coâlocated with groundâbased data, the respective advantages of both setsâhomogeneous coverage and processing of satellite data and high resolution of terrestrial dataâmay be jointly utilized in one combined potential field model. Here, the combined regional gravity field model IMOSAGA01C has been employed which incorporates terrestrial gravity data of the Central Andes from more than 20 years and satelliteâbased data from the GOCO03S gravity field model. It has been used on a 6x6âminute
(~11 km) grid at 8 km altitude to optimize a preâexisting 3Dâdensity model of North Chile between 18â31.5°S and 66â73°W. By applying geometry adjustments and density inversion, the standard deviation of the residual anomaly could be reduced by 62.3% to 6.3X10e-5 m/sÂČ for the model area. When also parts of the model area are considered that are not covered by terrestrial gravity data, this correspond to an overall error decrease by 68.7%. The adjusted geometry and density information of the density model served as an input to the computation of static stress anomalies on top of the subducting Nazca Plate. The interfaceânormal rotated component of the lithostaticallyâinduced stress anomaly exhibits a
clear segmentation of the forearc. It is characterized by a sequence of positive stress anomalies of up to 80 MPa along the coastal Jurassic batholith belt. It correlates well with the major seismicity of the active margin in North Chile and is attributed to mass excess in the continental crust and lithosphere above the subduction interface. A joint analysis with coupling coefficients from GPSâmodelling revealed that positive stress anomalies in the order of 0â100 MPa act as an approximate threshold for the minimum plate coupling (in per cent) within the scope of the seismogenic zone between 18.75°S and 21.75°S. Thus, there must exist patches along the margin where the plate coupling is generally higher than in areas with less relative load (except co-seismic state). Furthermore, a systematic analysis in the area of the April 2014 megathrust earthquake offshore Pisagua/Iquique revealed that coupling and stress anomalies in the area of the fracture plane
form virtual loops in their common parameterâspace. This could be understood as a strong indicator for the validity of the stressâinduced coupling hypothesis. In the future, this finding could help to better understand the seismoâperiodic state of a subduction system. This work comes to the conclusion that potential field data from the GOCE mission, despite its high sensitivity, must be understood to reside at the very edge of an appropriate resolution
for detailed lithospheric studies. The IMOSAGA01C combined gravity field model however, which collocates the satelliteâ and surfaceâbased gravity data, clearly outperforms the existing combined models in this regard. Hereby the quality of density models can be
improved, which in turn leads to better constrained derivatives such as dynamic models or stress anomalies. From the latter it could finally be concluded that the hypothesis for the North Chile case is acceptable, that relative mass excess above the subducting Nazca Plate coâgenerates asperities with increased potential for megathrust earthquakes.Mit der vorliegenden Arbeit wurde der Einfluss der Dichteverteilung kontinentaler LithosphĂ€re auf die physikalische Kopplung des Subduktionssystems im Bereich der Zentralanden (18°â35°S) untersucht. Neben der Beschaffenheit der subduzierenden ozeanischen Platte und den Eigenschaften der GrenzflĂ€che der Subduktion selbst, gibt es Hinweise darauf, dass ein relativer MassenĂŒberschuss oberhalb der abtauchenden Platte in Zusammenhang mit erhöhter
Kopplung des Systems steht. Gleichzeitig erhöht sich die Gefahr durch Starkbeben, die an entsprechenden Lokationen auftreten, wenn die aufgebauten Spannungen an den Blockaden gelöst werden und in Form seismischer Energie freigesetzt werden. Methodische Ăberlegungen, ForwĂ€rtsmodellierungen, sowie GPE- und Stressberechnungen, die auf der Interpretation des Schwerefeldes und seines Gradiententensors beruhen, wurden angewendet, um die Dichte- und Stressverteilung im Untersuchungsgebiet nĂ€her zu
bestimmen und zu analysieren. Ein besonderes Augenmerk lag auch auf der Evaluierung der Schweredaten der Satellitenmission GOCE, welche erstmals gemessene gradiometrische Daten des gesamten SchwereâTensors nahezu global zur VerfĂŒgung stellte. Methodische Analysen zum Auflösungsvermögen der Schwereâ und GradiometrieâDaten ergaben, dass die gemessenen Gradienten der GOCE Mission hinreichend sensitiv sind, um Informationen in der GröĂenordnung der erwarteten Schwereanomalie der KĂŒstenbatholithe in Nordchile zu registrieren. Es wird angenommen, dass diese durch ihren MassenĂŒberschuss die Kopplung des Subduktionssystems signifikant beeinflussen. Die synthetische 3D-Dichtemodellierung eines standardisierten Subduktions-Settings wurde verwendet, um diese Ăberlegung zu ĂŒberprĂŒfen. Sie bestĂ€tigt das Ergebnis, dass Dichtekontraste, die sich fĂŒr Schweredaten in Orbithöhe (~250 km) jenseits der Auflösbarkeit befinden, durch den Schweregradienten-Tensor und seine Invarianten hingegen gerade noch aufgelöst werden können. Die rĂ€umliche Auflösung aktueller PotenzialfeldâModelle, welche Messungen von Satellitenmissionen wiedergeben, ist nicht besser als Grad/Ordnung 300 (~67 km HalbwellenlĂ€nge). Das ist fĂŒr sehr detaillierte Studien der LithosphĂ€re oder Kruste noch unzureichend. Ursachen hierfĂŒr sind in erster Linie die Kondition des SignalâRauschâVerhĂ€ltnisses und sich ĂŒberlagernde Signale im Messsystem bei groĂer Distanz zu den verursachenden Massen. DafĂŒr sind Satellitenâbasierte Daten quasiâglobal verfĂŒgbar und homogen aufgenommen. Sie können konsistent prozessiert, analysiert und interpretiert werden. Terrestrische Daten hingegen sind zumeist extrem heterogen verteilt, sehr unterschiedlich prozessiert und Metadaten stehen oft nicht zur VerfĂŒgung. Der Vorteil bei ihrer Verwendung liegt darin, dass sie aufgrund des geringen Abstandes zu den Quellen ein hohes rĂ€umliches Auflösungsvermögen haben und eine entsprechend klare Signaltrennung möglich ist. Die jeweiligen Vorteile beider Datenarten â homogene Aufnahme, Abdeckung und Prozessierung der Satellitendaten sowie hohe Auflösung der terrestrischen Daten â können in einer kombinerten Bearbeitung vereinigt werden, wenn beide DatensĂ€tze ĂŒber Kollokation zusammengefĂŒhrt werden. In der vorliegenden Arbeit wurde das kombinerte regionale Schwerefeldmodell IMOSAGA01C verwendet, welches terretrische Schweredaten aus mehr als 20 Jahren Bodenmessungen in den Zentralanden, sowie Satelliten-basierte Daten aus dem GOCO03S Schwerefeldmodell zusammenfĂŒhrt. Hier wurde ein 6x6âMinuten (~11 km) Grid in 8 km Höhe verwendet, um ein vorhandenes 3D Dichtemodell von Nordchile im Bereich 18â31.5°S und 66â73°W zu optimieren. Durch Anpassungen der Modellgeometrie und durch DichteâInversion konnte die Standardabweichung der Residualanomalie im Bereich der Modellierung um 62,3% auf 6.3x10e-5 m/sÂČ gesenkt werden. Unter zusĂ€tzlicher BerĂŒcksichtigung der Modellgebiete, die nicht mit terrestrischen Schweremessungen abgedeckt sind, entspricht dies einer Verbesserung um 68,7%. Die angepassten Geometrieâ und Dichteinformationen des Dichtemodells dienten als EingangsgröĂen zur Berechnung statischer Stressanomalien auf der subduzierten NazcaâPlatte. Die normal zur Subduktion rotierte Komponente der lithostatisch induzierten Stressanomalie weist eine deutliche Segmentierung des Forearcs auf. Diese zeichnet sich durch ein Band mit positiven Stressanomalien von bis zu 80 MPa im Bereich der Jurassischen KĂŒstenbatholithe aus. Es korreliert mit der vorherrschenden SeismizitĂ€t des aktiven Kontinentalrandes in Nordchile und ist auf MassenĂŒberschĂŒsse innerhalb der kontinentalen Kruste und LithosphĂ€re oberhalb der Subduktions-GrenzflĂ€che zurĂŒckzufĂŒhren. Im Gebiet der seismogenen Zone zwischen 18.75°S und 21.75°S zeigte eine gemeinsame Analyse mit Kopplungs-Koeffizienten aus GPSâModellen, dass positive Stressanomalien im Bereich 0â100 MPa jeweils als nĂ€herungsweise Schwellenwerte fĂŒr die minimale Kopplung (in Prozent) eingesetzt werden können. Demnach muss es Regionen geben, in denen die Kopplung der Platten, bis auf den co-seismischen Zustand, stĂ€ndig höher ist als in Regionen mit geringerer relativer Auflast. DarĂŒber hinaus zeigte eine systematische Analyse im Bereich des Starkbebens von Pisagua/Iquique vom April 2014, dass Plattenkopplung und Stressanomalien innerhalb ihres gemeinsamen Parameterraumes fĂŒr den Bereich der BruchflĂ€che ĂŒber virtuelle Schleifen verknĂŒpft sind. Dies kann als Kennzeichen fĂŒr die GĂŒltigkeit der These zur Stressâinduzierten Kopplung gewertet werden. Diese Erkenntnis könnte zukĂŒnftig dabei helfen, den seismozyklischen Zustand eines Subduktionssystems besser zu verstehen. Die vorliegende Arbeit kommt zu dem Schluss, dass sich die PotenzialfeldâDaten der GOCE Mission trotz der hohen SensitivitĂ€t im Grenzbereich der notwendigen Auflösung fĂŒr detaillierte LithosphĂ€renstudien liegen. Das kombinierte Schwerefeldmodell IMOSAGA01C hingegen, welches die Satellitenâ und BodenâgestĂŒtzten Schweredaten zusammenfĂŒhrt, ĂŒbertrifft existierende kombinierte Modelle in dieser Hinsicht deutlich. Hierdurch lĂ€sst sich die QualitĂ€t der Dichtemodelle erhöhen, was zu einer besser validierten Zusatzinterpretation fĂŒhrt, wie zum Beispiel dynamische Modelle oder Stressanomalien zeigen. Aus letzteren konnte abgeleitet werden, dass die Hypothese, derzufolge relative MassenĂŒberschĂŒsse oberhalb der subduzierenden NazcaâPlatte Asperities mit erhöhtem GefĂ€hrdungspotenzial fĂŒr Starkbeben mitverursachen, fĂŒr das Untersuchungsgebiet in Nordchile gĂŒltig ist
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