ausblenden:
Schlagwörter:
Berlin, FU, Diss. 2017
seismolgy; subduction; Andes; tectonics
Zusammenfassung:
In this thesis I use local earthquake data to investigate physical rock properties near the nucleation point of small earthquakes in the Central Andean subduction system at 21° S in order to better understand the dynamics of subduction and the deep water cycle.
I observe abundant seismicity in the overriding and within the subducting plate. I localize the seismic events with high precision. The oceanic lithosphere clearly exhibits a Wadati-Benioff-zone that is structured in three slab-parallel bands. One is located at the plate interface at shallow depth, one near the oceanic Moho, and a third one 20 km below the oceanic Moho within the lithospheric mantle. The comparison with results from the ANCORP’96 active source seismic experiment yields a tight correlation between seismicity and reflectivity. Seismicity of the continental crust is pervasive to lower crustal levels onshore towards the trench. It is constrained to the upper crust towards the magmatic arc. The sharp lower limit of seismicity delineates the brittle-ductile boundary.
In order to investigate the acting forces and to identify pre-defined structures within the subducting slab, I compute focal mechanisms using first motion polarities and P/S-amplitude ratios and further perform a full waveform inversion for the whole moment tensor at relatively short periods. The results show that shallow seismicity is dominated by compressive ridge-push and collisional forces, and intermediate depth seismicity by dilatational slab-pull. An overall steepening of the tension-direction of the focal mechanisms pleads for a successive intensification of the downward-pulling force component.
To test the mineral dehydration hypothesis for the nucleation of intermediate depth earthquakes, I construct a petrophysical model that allows me to predict the mineral assemblage of the subducting oceanic crust and mantle. I compute elastic rock properties and investigate for selected mineralogies the influences of fluid-filled pore-space and elastic anisotropy. The results show that high seismic P- to S-wave velocity ratios (Vp/Vs-ratios) are indicative for the presence of fluids in pressurized compliant pores. Low Vp/Vs-values can be attributed to an anisotropic rock fabric. I critically review a method to estimate Vp/Vs-ratios in the direct vicinity of a set of seismic events from arrival time measurements of elastic body waves. I apply a revised version of this method to the entire seismic event cloud. At shallow depth I find values that are in accordance with the expexted lithologies. At intermediate depth I find anomalous Vp/Vs values that probably indicate the presence of a compliant network of interconnected, fluid-filled, and pressurized veins.
Altogether, the results deliver a coherent picture from the perspective of observational seismology of the driving forces of subduction, the transfer of stress to the overriding plate, the transport of seawater below the continent, its release there and the attributed successive densification of the descending lithosphere.
