Seismizität am Segment-8 Vulkan, Südwestindischer Rücken

Vera.Schlindwein [ at ]


New oceanic crust is formed the mid-oceanic ridges (MOR), where the lithospheric plates drift apart. The structure and characteristics of MOR differ with spreading rates being either fast (80-100mm/a), intermediate (55-70mm/a), slow (20-55mm/a), or ultraslow spreading ridges (<20mm/a). In comparison with faster spreading ridges the ultraslow spreading ridges are poorly investigated, their spreading mechanisms and parameters for generating volcanic centres are relatively unknown. Local earthquakes analyses can be used for a better understanding of the thermal and mechanical condition of the lithosphere at MOR. The Southwest Indian ridge (SWIR) separates the Antarctic and African plates and with a full spreading rate of 14mm/a belongs to the ultraslow spreading ridges. The segment-8 volcano (65.60E, 27.65S) is located at the north eastern part near the Rodriguez triple junction. I processed nine months of continuous seismic data recorded by the first local network of eight Ocean Bottom Seismometers (OBS) in this area. After automatic event identification, arrivals of P- and S-phases were hand-picked. Locations of 2996 hypocentres could be calculated using the linear location algorithm Hyposat. Furthermore, I used the relative location algorithm hypoDD to improve the standard earthquake location and additional waveform cross-correlation of Pphases to increase the accuracy of phase onset times. With detailed analyses of the parameters for the inversion with hypoDD I could relocated 2083 events. I received a sharp image of seismicity confined to the rift valley with local magnitudes in the range -0.9 up to 3.8. Relative locations are on average accurate to each other within 0.025 km. I identified spatial clusters around the segment-8 volcano with considerable background seismicity throughout the recording period. Receding from the centre of the volcano, focal depths increase continuously to >20 km, defining the boundary between brittle-ductile behaviour of the lithosphere. The results show a noticeable zone with a radius of 15km without any occurrences of earthquakes located beneath the central volcano. This aseismic zone has a remarkable low-velocity anomaly, which indicates increased temperatures. Along-axis, I find another aseismic zones wich I charactirized as serpentized peridotis because of the losses of the shear strength. With the impressiv depths of the hypocenters I find evidence that supportes the idea of a cold, thick and brittle Lithosphere of ultraslow spreading ridges. Depending on the model from Standish et al. [2008], I could established the hypothetical conception of a pronounced Lithosphere- Asthenosphere-Boundary (LAB) which provides the focussing melt supply over large distances towards the isolated volcanic centers along-axis.

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Plötz, A. (2015): Seismizität am Segment-8 Vulkan, Südwestindischer Rücken , Master thesis, Universitaet Leipzig.

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