Fault scarp statistics at the Galapagos spreading centre from Deep Tow data

What is it about?

The vertical reliefs of faults are interpreted from Deep Tow sediment profiler data, which allow the basement topography beneath pelagic sediments to be interpreted. The results showed no increase in scarp relief with seafloor age, but rather a decrease, to 40 km off-axis (1.5 Ma seafloor). This most likely is due to temporal effects in the structure of the ridge, not reversal of fault offsets, as no compressional earthquake mechanisms have been observed in this area. Areas of near-uniform pelagic sediment thickness interpreted as due to large lava flows suggest that some relief may have become obscured by later lava flowage into fault hanging wall depressions. Pelagic sediments form steep deposits on the crests of fault scarps. We carried out a simple pseudo-static calculation using their gradients and geotechnical properties from nearby DSDP cores to estimate the possible peak horizontal ground acceleration that represents their limits of stability. We found the scarps would be sensitive to accelerations of 0.1-0.2 g, typical of areas within 5-10 km of magnitude 4-5 earthquakes with normal focal mechanisms. This also supports the lack of fault movement on the ridge flanks.

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Why is it important?

This study illustrates how fault statistics combined with information on slope stability can be used to investigate the possibility of fault movements in the flanks of mid-ocean ridges.