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.

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

This paper presents the results from the Deflo-hydroacoustic experiment in the Southern Indian Ocean using three autonomous underwater hydrophones, complemented by two permanent hydroacoustic stations. The array monitored for 14 months, from November 2006 to December 2007, a 3000 x 3000 km wide area, encompassing large segments of the three Indian spreading ridges that meet at the Indian Triple Junction. A catalogue of 11 105 acoustic events is derived from the recorded data, of which 55 per cent are located from three hydrophones, 38 per cent from 4, 6 per cent from five and less than 1 per cent by six hydrophones. From a comparison with land-based seismic catalogues, the smallest detected earthquakes are m(b) 2.6 in size, the range of recorded magnitudes is about twice that of land-based networks and the number of detected events is 5-16 times larger. Seismicity patterns vary between the three spreading ridges, with activity mainly focused on transform faults along the fast spreading Southeast Indian Ridge and more evenly distributed along spreading segments and transforms on the slow spreading Central and ultra-slow spreading Southwest Indian ridges; the Central Indian Ridge is the most active of the three with an average of 1.9 events/100 km/month. Along the Sunda Trench, acoustic events mostly radiate from the inner wall of the trench and show a 200-km-long seismic gap between 2 degrees S and the Equator. The array also detected more than 3600 cryogenic events, with different seasonal trends observed for events from the Antarctic margin, compared to those from drifting icebergs at lower (up to 50 degrees S) latitudes. Vocalizations of five species and subspecies of large baleen whales were also observed and exhibit clear seasonal variability. On the three autonomous hydrophones, whale vocalizations dominate sound levels in the 20-30 and 100 Hz frequency bands, whereas earthquakes and ice tremor are a dominant source of ambient sound at frequencies < 20 Hz.

/pdf/seafloor-seismicity-antarctic-ice-sounds-cetacean-2xrt16jwlq.pdf

Seafloor seismicity, Antarctic ice-sounds, cetacean vocalizations and long-term ambient sound in the Indian Ocean basin

The 11 April 2012 M  8.6 Indian Ocean earthquake was an unusually large intraoceanic strike‐slip event. For several days, the global M ≥4.5 and M ≥6.5 seismicity rate at remote distances (i.e., thousands of kilometers from the mainshock) was elevated. The strike‐slip mainshock appears through its Love waves to have triggered a global burst of strike‐slip aftershocks over several days. But the M ≥6.5 rate subsequently dropped to zero for the succeeding 95 days, although the M ≤6.0 global rate was close to background during this period. Such an extended period without an M ≥6.5 event has happened rarely over the past century, and never after a large mainshock. Quiescent periods following previous large ( M ≥8) mainshocks over the past century are either much shorter or begin so long after a given mainshock that no physical interpretation is warranted. The 2012 mainshock is unique in terms of both the short‐lived global increase and subsequent long quiescent period. We believe that the two components are linked and interpret this pattern as the product of dynamic stressing of a global system of faults. Transient dynamic stresses can encourage short‐term triggering, but, paradoxically, it can also inhibit rupture temporarily until background tectonic loading restores the system to its premainshock stress levels.

The profound reach of the 11 April 2012 M 8.6 Indian Ocean earthquake: Short‐term global triggering followed by a longer‐term global shadow

We have developed a set of long-term autonomous hydrophones dedicated to long-term monitoring of low-frequency sounds in the ocean (<120Hz). Deploying arrays of such hydrophones (at least 4 instruments) proves a very efficient approach to monitor acoustic events of geological origin (earthquakes, icequakes), sea state or large baleen whales, over large and remote areas of the world ocean (up to 1000 km between instruments or more). Such approach takes advantage of the high sensitivity of hydrophones and of the long-range acoustic properties of the water column. Our instrument have been designed to be deployed for more than one year, continuously recording the ocean sounds at 240Hz with high dynamic 24-bit resolution.

/pdf/long-term-autonomous-hydrophones-for-large-scale-38477nkil0.pdf

Long-term autonomous hydrophones for large-scale hydroacoustic monitoring of the oceans

/pdf/seismicity-and-active-accretion-processes-at-the-ultraslow-3rtic6z9ee.pdf

Seismicity and active accretion processes at the ultraslow-spreading Southwest and intermediate-spreading Southeast Indian ridges from hydroacoustic data

[1] The seismicity of the North Atlantic was monitored from May 2002 to September 2003 by the ‘SIRENA array’ of autonomous hydrophones. The hydroacoustic signals provide a unique data set documenting numerous low-magnitude earthquakes along the section of the Mid-Atlantic Ridge (MAR) located in a ridge-hot spot interaction context. During the experiment, 1696 events were detected along the MAR axis between 40°N and 51°N, with a magnitude of completeness level ofmb≈ 2.4. Inside the array, location errors are in the order of 2 km, and errors in the origin time are less than 1 s. From this catalog, 15 clusters were detected. The distribution of source level (SL) versus time within each cluster is used to discriminate clusters occurring in a tectonic context from those attributed to non-tectonic (i.e. volcanic or hydrothermal) processes. The location of tectonic and non-tectonic sequences correlates well with regions with positive and negative Mantle Bouguer Anomalies (MBAs), indicating the presence of thinner/colder and thicker/warmer crust respectively. At the scale of the entire array, both the complete and declustered catalogs derived from the hydroacoustic signals show an increase of the seismicity rate from the Azores up to 43°30′N suggesting a diminishing influence of the Azores hot spot on the ridge-axis temperature, and well correlated with a similar increase in the along-axis MBAs. The comparison of the MAR seismicity with the Residual MBA (RMBA) at different scales leads us to think that the low-magnitude seismicity rates are directly related to along-axis variations in lithosphere rheology and temperatures.

/pdf/spatiotemporal-distribution-of-the-seismicity-along-the-mid-1ncdmxn085.pdf

Spatiotemporal distribution of the seismicity along the Mid-Atlantic Ridge north of the Azores from hydroacoustic data: Insights into seismogenic processes in a ridge-hot spot context

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Spatiotemporal distribution of the seismicity along the Mid-Atlantic Ridge north of the Azores from hydroacoustic data: Insights into seismogenic processes in a ridge-hot spot context