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Abstract Submission No. | ABS-2022-13-0466 |
Title of Abstract | Hydroacoustic observations of seismic swarms along the Southwest Indian Ridge of the Indian Ocean |
Authors | Vaibhav Vijay INGALE*, Sara BAZIN, Jean-Yves ROYER |
Organisation | University of Brest and CNRS, Lab Geo-Ocean |
Address | CNRS, Lab Geo-Ocean, Institut Universitaire Europe´en de la Mer, Place Copernic, 29280 Plouzane´, Fr Copernic, Plouzané, France Pincode: 29280 Mobile: 7038073806 E-mail: vaibhavvijay.ingale@univ-brest.fr |
Country | France |
Presentation | Oral |
Abstract | Seismic swarms of volcanic and tectonic events along mid-oceanic ridges are inherent to seafloor spreading. Due to the rapid attenuation of seismic waves in the solid Earth, landbased seismic networks lack the low-level seismicity associated with such swarms. However, regional studies using autonomous underwater hydrophones overcome this difficulty due to their sensitivity to low-frequency hydroacoustic T-waves, that travel in SOund Fixing And Ranging (SOFAR) channel over very long distances with little attenuation. A network of 7-9 hydrophones, OHASISBIO, moored in the SOFAR channel, has been monitoring the seismicity in the Indian Ocean for more than 10 years. During this period, the land-based seismic networks have detected 15 major seismic swarms along the ultraslow spreading Southwest Indian Ridge (SWIR) with spreading rate of 14-15 mm/yr. We have, so far, examined three of these swarms using hydroacoustic records from the OHASISBIO network combined with that of the permanent stations of the Comprehensive Test Ban Treaty Organization (CTBTO). Two of these seismic swarms along the SWIR occurred in 2018. The first swarm, located at the ridge-transform intersection with Novara Fracture Zone (58°E), comprised 231 International Seismological Center (ISC) catalogue events between July 06 and July 18 (13 days). Out of them, 7 events display normal faulting mechanisms parallel to ridge axis (Global Centroid Moment Tensor (GCMT) solutions). In the same period, we detected 1109 hydroacoustic events, an increase of about 5 times compared to terrestrial detections. The second swarm, located near a ridge-axis discontinuity (67.5°E), 220 km west of the Rodrigues Triple Junction (RTJ), comprised 92 ISC catalogue events between September 25 and October 27 (33 days), 6 of which display normal faulting mechanisms (GCMT), vs 4880 hydroacoustic events, a ~53-fold increase relative to land-based detections. The third swarm occurred in 2016 near the Melville FZ (61°E) and comprised 259 ISC events, 17 of which display normal faulting mechanisms, between June 09, 2016 and March 25, 2017. In a preliminary analysis from June 09 to July 11, 2016, we detected 4273 hydroacoustic events vs 28 ISC events, a ~150-fold increase in the event detections. All hydroacoustic events were detected over two iterations of T-wave picking, which improved the uncertainties to ~0.4 km in latitude and longitude, and ~0.2 s in origin time. Peculiar to these three SWIR swarms is the detection of ~110 highly energetic and short duration (~10s) impulsive events compared to the longer (~100s) duration of local earthquakes. These events are all located on slopes of seamounts near the ridge axis. Another hydroacoustic experiment has demonstrated that similar impulsive events were associated with active and fresh lava flows. So, we interpret them as thermal explosions resulting from direct magma supplies on the seafloor. Series of other hydroacoustic events indicate migration patterns within the swarms, interpreted as dyke migrations. However, no observations of strong mainshock-aftershock sequence, typical of tectonic events, are found in these swarms. So, overall, combined observations of impulsive events, clustered near seamounts, migrating events and absence of aftershock sequences point to a magmatic origin for these three distinct swarms with active sources located near local seamounts. |