SWEAP: Southwest Indian Ridge Earthquakes and Plumes: First Results from a Comparative Seismicity Study of Magmatic and Amagmatic Spreading

Cruise ANT-29/8 (SWEAP) of RV Polarstern in November 2013 headed for the Oblique Supersegment of the Southwest Indian Ridge (SWIR) in the “Furious Fifties” to recover 10 ocean bottom seismometers (OBS) after recording earthquake activity for a period of one year. The OBS recovery was flanked by a multidisciplinary science program that searched in difficult sea conditions for signs of hydrothermal venting, examined deep-sea fauna and determined the thermal regime of this rift section.

The seismic activity that accompanies crustal generation at ultraslow spreading mid-ocean ridges is hardly known. We present here preliminary results from the first-ever long-term deployment of OBS networks at two locations of the SWIR. We instrumented the segment 8 volcano near 65°E with 8 OBS and the amagmatically spreading Oblique Supersegment with 10 OBS, two of which returned no data. The networks had dimensions of 60 km x 40 km and a comparable station spacing of about 15 km. A first data analysis suggests that the seismic activity of the magmatic segment is about 4 times as high as that of the magmatically starved Oblique Supersegment. Interestingly, the segment 8 volcano itself displays a prominent seismic gap with a complete absence even of small earthquakes while the adjacent rift valley hosts earthquakes down to 15 km depth indicating a cold lithosphere. This spatial earthquake distribution may reflect an up-doming lithosphere-asthenosphere boundary that has been postulated to guide melts towards the widely spaced volcanoes of ultraslow spreading ridges.