Seismicity of ultraslow spreading mid-ocean ridges at local, regional and teleseismic scales: A case study of contrasting segments

Christine.Laederach [ at ]


Of the global mid-ocean ridge system, ultraslow spreading ridges represent a different class of spreading mechanism. Below a full spreading rate of 20 mm y-1, the melt supply per increment of plate boundary drastically decreases resulting in a ridge morphology which is different from the morphology of faster spreading ridges. The main representatives of ultraslow spreading ridges, the Arctic Ridge System (ARS) and the Southwest Indian Ridge (SWIR), are located in remote areas which is the reason for the few number of case studies conducted at these ridges compared to manifold studies performed at faster spreading ridges. Nevertheless, several studies of the ARS and the SWIR have revealed that these ridges are composed of magmatic and amagmatic segments which represent different styles of crustal accretion. At the magmatic segments, the presence of basalt at the seafloor and a continuous positive magnetic anomaly at the ridge axis (CMA) indicate robust magmatism. At amagmatic segments, volcanic activity seems to be absent or to be concentrated on isolated volcanic centres where magma accumulates. Away from these volcanic centres, the seafloor is mainly composed of peridotite and lacks a CMA. Spreading at such segments is interpreted to take place by extension and thinning of the crust along normal faults or detachment faults exposing mantle material to the seafloor. At the ARS, the perennial ice cover limits ship operation including seismic profiling and the deployment of ocean bottom instrumentation. Similarly, the SWIR is located in latitudes with stromy weather limiting local studies accordingly. Thus, it is not well understood how spreading takes place at these ridges and what factors lead to the formation of volcanic centres. In order to learn more about spreading processes at ultraslow spreading mid-ocean ridges, seismicity can be monitored as earthquakes indicate active processes and give information about the mechanical and thermal state of the lithosphere. This project includes two case studies at an amagmatic and a magmatic segment of the ARS and the SWIR. The amagmatic Lena Trough, part of the ARS and located between Greenland and Svalbard, spreads obliquely and shows an asymmetric distribution of the teleseismically detected earthquakes west of the ridge axis. In order to verify this atypical earthquake distribution, I relocalized the teleseismic dataset confirming that the seismicity at the southern Lena Trough is focussed west of the ridge axis. Furthermore, our working group deployed several seismic arrays on ice floes in 2008 and 2009 to record the microseismicity of Lena Trough bridging the gap between the teleseismic dataset and this local dataset. Unfortunately, this was only partly successful as the seismic activity during the survey periods was too low to be detected by the land stations located in Greenland and Svalbard. Nevertheless, the case study of Lena Trough presents new results of a complex interaction of a long-term shear movement and the onset of spreading at a nascent oceanic rift which we published together with a compilation of high-resolution multibeam bathymetry and a newly compiled magnetic dataset. The acquisition technique of local seismicity in ice covered regions by seismic arrays on drifting ice floes is a rather young method and the further development of this application was an important part of this PhD project. Describing the methodology and showing data examples, the second publication means to introduce our method and its broad application field to the scientific community. The second case study concerns the ultraslow spreading Orthogonal Supersegment of the western SWIR where the occurrence of earthquake swarms is supposed. This segment shows robust magmatism and the presence of a volcanic centre has been proposed where, in the teleseismic dataset, a cluster of seismicity is observed. As no stations of the global station network are located nearby, a potentially vast amount of earthquakes is missed. To overcome this, I accessed the regional dataset of the seismic array operated by the German Neumayer station in East Antarctica. The small-aperture array with its central broadband sensor VNA2 lies in a distance of ~2100 km to the Orthogonal Supersegment and continuously monitors backazimuth and apparent velocity of incoming waves. This allows the extraction of earthquakes occurring at a specific region. Thus, 743 earthquakes within an 8-year period located at this section of the SWIR could be identified and body-wave magnitudes (mb) were calculated. The Neumayer seismic record clearly showed four periods of drastically increased event rates. These swarm periods contained large earthquakes which were teleseismically recorded allowing to bridge the gap between teleseismic and regional seismicity records. The relocalization of the teleseismic earthquakes confirmed that these swarms repetitively occurred at the same location. They are most probably caused by repetitive magmatic accretion episodes at an assumed volcanic centre. Completed with a detailed swarm characterization and the comparison of the teleseismic and regional dataset, this analysis is summarized in a third paper that has been submitted recently. The two case studies, the newly developed data acquisition technique and the processing of the corresponding dataset have yielded new possibilities of monitoring seismicity of ultraslow spreading mid-ocean ridges. Local and regional datasets allow a comprehensive evaluation of the earthquakes of a specific region lowering the detection threshold compared to the global station network. Thus, active accretion episodes can be monitored in greater detail showing the vast differences of spreading mechanisms between these contrasting amagmatic and magmatic ridge segments.

Item Type
Thesis (PhD)
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Eprint ID
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Läderach, C. (2012): Seismicity of ultraslow spreading mid-ocean ridges at local, regional and teleseismic scales: A case study of contrasting segments , PhD thesis, University of Bremen/Alfred Wegener Institute for Polar and Marine Research.

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