ALBERT

Description: The ultraslow spreading Southwest Indian Ridge (SWIR) is a prominent end member of the global mid-ocean ridge system. It spreads with a full-rate of 14 mm/y to 16 mm/y and shows several segments of various obliquities. The western SWIR consists of the Oblique and Orthogonal Supersegments lying at an epicentral distance of ~21° to the VNA2 seismic array operated by the German Neumayer station in East Antarctica. The array monitors backazimuth, apparent velocity and signal-to-noise ratio of arriving waves and provides a dataset of seismicity from the western SWIR over several years. Compared to the global seismological network, its detection threshold for earthquakes occurring at the western SWIR is more than 0.5 mb lower enabling a more comprehensive study of mid-ocean ridge processes than the teleseismic earthquake catalogues. We identified a total number of 743 earthquakes occurring at the western part of the SWIR and calculated the body-wave magnitudes (mb) from P-wave amplitude picks on the VNA2 broadband sensor obtaining a magnitude range from mb 3.18 to mb 5.34. In the years of 2001, 2004, 2005 and 2008, significantly increased event rates indicated four earthquake swarms with up to 164 events lasting for several days. All swarms had strong events registered in the International Seismological Centre (ISC) catalogue. The relocalization of these events confirmed that all swarms occurred in the same region on the Orthogonal Supersegment. We analysed event and moment release rate histories, b-values and aftershock decay rates (Modified Omori Law) finding that the swarms of 2001, 2004 and 2005 have similarities in the temporal distribution of seismic moment and event numbers. The swarm of 2008 is smaller with high magnitude events at the swarm’s onset which represent shear failure on normal faults. The application of the Modified Omori Law and the b-value show that the earthquakes of the swarms do not follow the classical mainshock-aftershock pattern of purely tectonic earthquake sequences. At the Orthogonal Supersegment a continuous positive magnetic anomaly along the rift axis, a negative Mantle Bouguer Anomaly, basalts at the seafloor and potentially volcanic edifices indicate robust magmatic crustal accretion. The high-resolution bathymetry shows ubiquitous rift-parallel ridges with steep flanks toward the rift axis indicating high angle normal faults. The high magnitude earthquakes detected teleseismically during the swarms are generated there. We interpret that the swarms are caused by magmatic accretion episodes at a suggested volcanic centre of the Orthogonal Supersegment and that possible magma injection activates the steeply dipping fault planes.