Teleseismic earthquake swarms at ultraslow spreading ridges: indicator for dyke intrusions?

Abstract

SUMMARY Earthquakes at mid‐ocean ridges reflect the active magmatic and tectonic processes that form new oceanic crust. Studies of large earthquakes observed on land and smaller earthquakes observed locally or regionally by ocean bottom seismometers or autonomous underwater hydrophones have greatly contributed to our understanding of the structure and active spreading processes at the mid‐ocean ridges of the Atlantic and Pacific Ocean opening with velocities in excess of 25 mm yr–1. At spreading rates below 20 mm yr–1 full rate, the appearance and the accretion processes of mid‐ocean ridges undergo fundamental changes as the melt supply is drastically reduced. The active spreading processes at these so‐called ultraslow spreading ridges are still poorly known, as the main representatives, the Arctic Ridge System and the Southwest Indian Ridge, are poorly accessible and neither autonomous underwater hydrophone nor ocean bottom seismometer records of local seismicity are available. In an attempt to compare on a large scale the accretion style of ultraslow spreading ridge sections, I analyse the teleseismically recorded seismicity in 11 sections of the Arctic Ridge System and the Southwest Indian Ridge spanning altogether 7200 km. Epicentres located within 30–35 km of the rift axis were extracted from the catalogue of the International Seismological Centre for a time period of 35 yr. On the basis of a single‐link cluster analysis, I identified 27 swarms with eight or more events. These swarms occur almost exclusively at centres of focussed magmatism suggesting that the swarms are probably initiated by magmatism. Normal faults along several tens of kilometres surrounding the volcanic centres react in large earthquakes (M > 5) to dyke emplacement. The routine generation of large earthquakes in the cold, brittle lithosphere of ultraslow spreading ridges makes the teleseismic record a valuable means to study ultraslow accretion processes and to provide a global framework for the interpretation of the limited local and regional seismicity studies.

Dependence of seismic coupling on normal fault style along the Northern Mid‐Atlantic Ridge

“…the number of declustered events (both above and below the completion size) divided by the initial number of earthquakes in each section. A low ratio indicates a high degree of spatio-temporal clustering, perhaps indicative of a magmatic swarm as opposed to a mainshock-aftershock sequence that follows Omori's law [Schlindwein, 2012]. However, we found no statistically robust difference in declustering ratio between sections, and can therefore not conclude on the physical interpretation of the clustering.…”

Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere

“…1, site 3) where an episode of teleseismic earthquake swarms occurred between 1996 and 2003 (ref. 5).…”

“…1, site 3). The repeated teleseismic earthquake swarms between 1996 and 2003 at the Segment-8 volcano potentially mark a phase of magmatic activity 5 . The complete absence of seismicity underneath the volcano Letter reSeArCH during our survey in 2012-2013 may therefore be a result of increased temperatures caused by recent magmatism.…”

“…To assess the relevance of our local seismicity surveys we examined the teleseismic earthquake record of ultraslow-spreading ridges 5 . The average numbers of earthquakes per kilometre of rift axis (for calculation see Methods) confirm the different deformation styles of predominantly magmatic and amagmatic rift segments (Figs 1 and 2 and Extended Data Fig.…”

Characteristics of tectonomagmatic earthquake swarms at the Southwest Indian Ridge between 16°E and 25°E

“…Hence, the regionally detected swarms can also be identified in teleseismic data, even if event rates are an order of magnitude lower ( cf. Schlindwein 2012).…”