geosciences Article Hydroacoustic Observations of Two Contrasted Seismic Swarms along the Southwest Indian Ridge in 2018 Vaibhav Vijay Ingale * , Sara Bazin and Jean-Yves Royer Laboratoire Geosciences Ocean, University of Brest & CNRS, 29280 Plouzané, France; [email protected] (S.B.); [email protected] (J.-Y.R.) * Correspondence: [email protected] Abstract: In 2018, two earthquake swarms occurred along spreading ridge segments of the ultra-slow Southwest Indian Ridge (SWIR). The first swarm was located at the spreading-ridge intersection with the Novara Fracture Zone, comprising 231 events (ISC catalogue) and spanning over 6 days (10 July to 15 July). The second swarm was more of a cluster of events focusing near a discontinuity, 220 km west of the Rodrigues Triple Junction, composed of 92 events and spanning over 31 days (27 September to 27 October). We examined these two swarms using hydroacoustic records from the OHASISBIO network with seven to nine autonomous hydrophones moored on either side of the SWIR. We detected 1109 hydroacoustic events spanning over 13 days (6 July to 18 July) in the first swarm and 4880 events spanning over 33 days in the second swarm (25 September to 27 October). The number of events per day was larger, and the hydroacoustic magnitude (source level) was, on average, smaller during the second swarm than the first. The spatio-temporal distribution of events from both swarms indicates a magmatic origin initiated by dike intrusions and followed by a Citation: Ingale, V.V.; Bazin, S.; readjustment of stresses in the surrounding crust. Royer, J.-Y. Hydroacoustic Observations of Two Contrasted Keywords: seismic swarm; hydroacoustics; mid-ocean ridge; Indian Ocean; impulsive events Seismic Swarms along the Southwest Indian Ridge in 2018. Geosciences 2021, 11, 225. https://doi.org/10.3390/ geosciences11060225 1. Introduction Seismic swarms are sequences of earthquakes with no major mainshock. They do not Academic Editors: Jesus have a well-defined temporal order in the magnitude of events and seismicity rate [1,2]. Martinez-Frias, Maria Grazia Ciaccio, Seismic swarms at mid-ocean ridges have been investigated since the 1970s (e.g., [3–8]). Paola Baccheschi and Diana Latorre Swarms of volcanic and tectonic events along mid-ocean ridges are inherent to seafloor spreading [9]. Because of the remoteness of most of the mid-ocean ridges and due to the Received: 5 April 2021 rapid attenuation of seismic waves in the solid Earth, land-based seismic networks lack Accepted: 18 May 2021 Published: 24 May 2021 the low-level seismicity associated with seafloor spreading processes [10,11]. Since the late 1980s, local studies using ocean-bottom seismometers (OBS) (e.g., [12–14]) and regional Publisher’s Note: MDPI stays neutral studies using autonomous underwater hydrophones (AuH) (e.g., [15–17]) have greatly with regard to jurisdictional claims in contributed to a detailed understanding of mid-ocean ridge seismicity [18–20]. This is published maps and institutional affil- possible because seismic events produce low-frequency hydroacoustic waves, known as iations. T-waves, that are excited in the water column by earthquakes through the conversion of seismic waves into acoustic waves at the solid–liquid interface (i.e., sea-bottom) [10,15,20] and which propagate through the sound fixing and ranging (SOFAR) channel over long distances with little attenuation [21–23]. The Southwest Indian Ridge (SWIR) is a major seafloor spreading ridge separating Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Africa and Antarctica. It extends from the Bouvet Triple Junction in the southern Atlantic This article is an open access article Ocean to the Rodrigues Triple Junction (RTJ) in the Indian Ocean [24–26]. The western end distributed under the terms and of SWIR is older than its eastern end due to the lengthening and eastward propagation of conditions of the Creative Commons the ridge axis toward the RTJ [27,28]. The SWIR is among the world’s slowest spreading Attribution (CC BY) license (https:// ridges, with a full spreading rate of ~14 mm/a [24,29,30]. It is also characterized by creativecommons.org/licenses/by/ several large-offset transform faults that divide the ridge into spreading segments of 4.0/). various lengths [31,32] with several magmatic and amagmatic ridge segments [33] and a Geosciences 2021, 11, 225. https://doi.org/10.3390/geosciences11060225 https://www.mdpi.com/journal/geosciences Geosciences 2021, 11, x FOR PEER REVIEW 2 of 19 Geosciences 2021, 11, 225 propagation of the ridge axis toward the RTJ [27,28]. The SWIR is among the world’s2 of 18 slowest spreading ridges, with a full spreading rate of ~14 mm/a [24,29,30]. It is also characterized by several large-offset transform faults that divide the ridge into spreading segments of various lengths [31,32] with several magmatic and amagmatic ridge segments deep[33] and axial a deep valley axial bounded valley by bounded ~3 km highby ~3 ridges km high [26 ,ridges28,32]. [26,28,32]. The cyclic The nature cyclic of nature volcanic of constructionvolcanic construction and tectonic and dismembermenttectonic dismemberment across the across SWIR the is SWIR responsible is responsible for the for ridge the crestsridge andcrests rugged and rugged morphology morphology of this spreading of this spreading ridge [32 ].ridge As shown[32]. As in shown Figure1 in, the Figure SWIR 1, isthe mostly SWIR characterizedis mostly characterized by the obliquity by the ofobliquity the ridge of the axis ridge with axis respect with to respect the main to the fracture main zone’s (FZ) orientation [34,35]. The section of the ridge between the Gallieni (52◦ E) and fracture zone’s ◦(FZ) orientation [34,35]. The section◦ of the ridge between the Gallieni (52° MelvilleE) and Melville FZ (60 E)FZ shows (60° E) an shows obliquity an ofobliqui 40 andty of contains 40° and several contains long-lived several transformlong-lived and non-transform discontinuities. The section between Melville FZ and RTJ (70◦ E) has an transform and◦ non-transform discontinuities. The section between Melville FZ and RTJ obliquity(70° E) has of an 25 obliquityand is continuous of 25° and withis continuous minor discontinuities. with minor discontinuities. FigureFigure 1.1. Bathymetric map of the Southwest Indian Indian Ri Ridge,dge, between between 50° 50 ◦EE and and 70° 70 ◦E,E, showing showing the the geographicgeographic and temporal distribution of of swarms swarms reported reported in in the the ISC ISC catalogue catalogue from from 2010 2010 to to 2020. 2020. The numbers next to the symbols refer to the number of teleseismic events in the ISC catalogue for The numbers next to the symbols refer to the number of teleseismic events in the ISC catalogue for each individual swarm. The blue dots show the location of the two 2018 swarms investigated in each individual swarm. The blue dots show the location of the two 2018 swarms investigated in this paper. this paper. Only a few seismic swarms have been reported along the SWIR based on teleseismic Only a few seismic swarms have been reported along the SWIR based on teleseismic observations (e.g., [36,37]), hydroacoustic observations (e.g., [38,39]) or local OBS surveys observations (e.g., [36,37]), hydroacoustic observations (e.g., [38,39]) or local OBS surveys (e.g.,(e.g., [[40,41]).40,41]). InIn aa comprehensive studystudy ofof teleseismicteleseismic earthquakesearthquakes betweenbetween 19761976 andand 20102010 atat ultra-slowultra-slow spreadingspreading ridges,ridges, SchlindweinSchlindwein [[42]42] showedshowed thatthat earthquakeearthquake swarmsswarms occuroccur atat volcanicvolcanic centerscenters or or segments segments with with magmatic magmatic accretion accretion that that are separatedare separated by stretches by stretches of non- of volcanicnon-volcanic seafloor. seafloor. These These swarms swarms are mostly are most associatedly associated with diking with events.diking Inevents. the western In the partwestern of the part SWIR, of the based SWIR, on based regional on regional and teleseismic and teleseismic earthquakes, earthquakes, Läderach Läderach et al. et [43 al.] showed[43] showed that earthquake that earthquake swarms swarms are associated are associated with along-axis with meltalong-axis flow mechanisms. melt flow Seismicmechanisms. swarms Seismic observed swarms in local observed OBS studies in local in OBS the easternstudies partin the of eastern the SWIR part are of also the interpreted as the result of magma movement related to diking episodes [7,40,44]. From 2010 to 2020, in the International Seismological Centre catalogue [45], we found only nine seismic swarms between 50◦ E and 70◦ E. We considered as swarms, the occur- rence of at least five consecutive events within 15 days clustered in a grid of 3 × 3 degrees. Geosciences 2021, 11, x FOR PEER REVIEW 3 of 19 SWIR are also interpreted as the result of magma movement related to diking episodes [7,40,44]. From 2010 to 2020, in the International Seismological Centre catalogue [45], we found only nine seismic swarms between 50° E and 70° E. We considered as swarms, the occurrence of at least five consecutive events within 15 days clustered in a grid of 3 × 3 degrees. Figure 1 shows their distribution: eight of them occurred west of Melville FZ and only one east of it. Most of them count about 10 or less (teleseismic) events, except three swarms: one in 2016 with 215 events and two in 2018 with 231 and 92 events. The latter Geosciencestwo are2021 , remarkable:11, 225 the one west of Melville FZ included the most Mw > 5 events3 (7) of 18 compared to the 2016 swarm (5); the one east of Melville FZ was the only one occurring W in the nearly 1000 kmFigure long1 shows continuous their distribution: section eight of the of them SWIR occurred and west also of Melvilleincluded FZ and6 M only > one 5 events.