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

International audienceThis 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

Hydroacoustic monitoring of oceanic spreading centers : past, present, and future

Author Posting. © The Oceanography Society, 2012. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 25, no. 1 (2012): 116–127, doi:10.5670/oceanog.2012.10.Mid-ocean ridge volcanism and extensional faulting are the fundamental processes that lead to the creation and rifting of oceanic crust, yet these events go largely undetected in the deep ocean. Currently, the only means available to observe seafloor-spreading events in real time is via the remote detection of the seismicity generated during faulting or intrusion of magma into brittle oceanic crust. Hydrophones moored in the ocean provide an effective means for detecting these small-magnitude earthquakes, and the use of this technology during the last two decades has facilitated the real-time detection of mid-ocean ridge seafloor eruptions and confirmation of subseafloor microbial ecosystems. As technology evolves and mid-ocean ridge studies move into a new era, we anticipate an expanding network of seismo-acoustic sensors integrated into seafloor fiber-optic cabled observatories, satellite-telemetered surface buoys, and autonomous vehicle platforms.SOSUS studies discussed in this paper were supported by the NOAA Vents Program and during 2006–2009 by the National Science Foundation, Grant OCE-0623649

Explosive Processes during the 2015 Eruption of Axial Seamount, as Recorded by Seafloor Hydrophones

Following the installation of the Ocean Observatories Initiative cabled array, the 2015 eruption of Axial Seamount, Juan de Fuca ridge, became the first submarine eruption to be captured in real time by seafloor seismic and acoustic instruments. This eruption also marked the first instance where the entire eruption cycle of a submarine volcano, from the previous eruption in 2011 to the end of the month-long 2015 event, was monitored continuously using autonomous ocean bottom hydrophones. Impulsive sounds associated with explosive lava-water interactions are identified within hydrophone records during both eruptions. Explosions within the caldera are acoustically distinguishable from those occurring in association with north rift lava flows erupting in 2015. Acoustic data also record a series of broadband diffuse events, occurring in the waning phase of the eruption, and are interpreted as submarine Hawaiian explosions. This transition from gas-poor to gas-rich eruptive activity coincides with an increase in water temperature within the caldera and with a decrease in the rate of deflation. The last recorded diffuse events coincide with the end of the eruption, represented by the onset of inflation. All the observed explosion signals couple strongly into the water column, and only weakly into the solid Earth, demonstrating the importance of hydroacoustic observations as a complement to seismic and geodetic studies of submarine eruptions. Plain Language Summary: Axial Seamount, a submarine volcano on the Juan de Fuca ridge, erupted in 2015. This eruption was recorded in real-time by an array of seafloor seismometers and hydrophones located on the volcano, and connected to shore by a power and data cable. Hydrophones recording the sounds generated by the eruption reveal several different types of explosions, including short bursts interpreted as lava-water interactions, and prolonged signals thought to be due to explosive ejection of gas and ash. These signals provide a window into the dynamics of the undersea eruption and are an excellent complement to other types of data including earthquakes and ground deformation

Long-term seismicity of the Reykjanes Ridge (North Atlantic) recorded by a regional hydrophone array

The seismicity of the northern Mid-Atlantic Ridge was recorded by two hydrophone networks moored in the sound fixing and ranging (SOFAR) channel, on the flanks of the Mid-Atlantic Ridge, north and south of the Azores. During its period of operation (05/2002-09/2003), the northern 'SIRENA' network, deployed between latitudes 40 degrees 20'N and 50 degrees 30'N, recorded acoustic signals generated by 809 earthquakes on the hotspot-influenced Reykjanes Ridge. This activity was distributed between five spatio-temporal event clusters, each initiated by a moderate-to-large magnitude (4.0-5.6 M) earthquake. The rate of earthquake occurrence within the initial portion of the largest sequence (which began on 2002 October 6) is described adequately by a modified Omori law aftershock model. Although this is consistent with triggering by tectonic processes, none of the Reykjanes Ridge sequences are dominated by a single large-magnitude earthquake, and they appear to be of relatively short duration (0.35-4.5 d) when compared to previously described mid-ocean ridge aftershock sequences. The occurrence of several near-equal magnitude events distributed throughout each sequence is inconsistent with the simple relaxation of main shock-induced stresses and may reflect the involvement of magmatic or fluid processes along this deep (>2000 m) section of the Reykjanes Ridge.info:eu-repo/semantics/publishedVersio

January 2006 seafloor-spreading event at 9°50′N, East Pacific Rise: Ridge dike intrusion and transform fault interactions from regional hydroacoustic data

An array of autonomous underwater hydrophones is used to investigate regional seismicity associated with the 22 January 2006 seafloor-spreading event on the northern East Pacific Rise near 9°50′N. Significant earthquake activity was observed beginning 3 weeks prior to the eruption, where a total of 255 earthquakes were detected within the vicinity of the 9°50′N area. This was followed by a series of 252 events on 22 January and a rapid decline to background seismicity levels during the subsequent 3 days. Because of their small magnitudes, accurate locations could be derived for only 20 of these events, 18 of which occurred during a 1-h period on 22 January. These earthquakes cluster near 9°45′N and 9°55′N, at the distal ends of the young lava flows identified posteruption, where the activity displays a distinct spatial-temporal pattern, alternating from the north to the south and then back to the north. This implies either rapid bilateral propagation along the rift or the near-simultaneous injection of melt vertically from the axial magma lens. Short-duration T wave risetimes are consistent with the eruption of lavas in the vicinity of 9°50′N on 22 January 2006. Eruptions on 12 and 15–16 January also may be inferred from the risetime data; however, the locations of these smaller-magnitude events cannot be determined accurately. Roughly 15 h after the last earthquakes were located adjacent to the eruption site, a sequence of 16 earthquakes began to the north-northeast at a distance of 25–40 km from the 9°50′N site. These events are located in vicinity of the Clipperton Transform and its western inside corner, an area from which the regional hydrophone network routinely detects seismicity. Coulomb stress modeling indicates that a dike intrusion spanning the known eruptive zone to the south (9°46′–9°56′N) would act to promote normal faulting or a combination of normal faulting and transform slip within this region, with stress changes on the order of 1–10 kPa

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

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[subscript]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 °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°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.Keywords: Mid-ocean ridge processes, Indian Ocean, Hydrogeophysics, Acoustic propertie

A Pulsed-air Model of Blue Whale B Call Vocalizations

Blue whale sound production has been thought to occur by Helmholtz resonance via air flowing from the lungs into the upper respiratory spaces. This implies that the frequency of blue whale vocalizations might be directly proportional to the size of their sound-producing organs. Here we present a sound production mechanism where the fundamental and overtone frequencies of blue whale B calls can be well modeled using a series of short-duration (\u3c1 \u3es) wavelets. We propose that the likely source of these wavelets are pneumatic pulses caused by opening and closing of respiratory valves during air recirculation between the lungs and laryngeal sac. This vocal production model is similar to those proposed for humpback whales, where valve open/closure and vocal fold oscillation is passively driven by airflow between the lungs and upper respiratory spaces, and implies call frequencies could be actively changed by the animal to center fundamental tones at different frequency bands during the call series

Acoustic response of submarine volcanoes in the Tofua Arc and northern Lau Basin to two great earthquakes

Using a short-baseline hydrophone array, persistent volcanoacoustic sources are identified within the ambient noise field of the Lau Basin during the period between 2009 January and 2010 April. The submarine volcano West Mata and adjacent volcanic terrains, including the northern Matas and Volcano O, are the most active acoustic sources during the 15-month period of observation. Other areas of long-term activity include the Niua hydrothermal field, the volcanic islands of Hunga Ha’apai, Founalei, Niuatoputapu and Niuafo’ou, two seamounts located along the southern Tofua Arc and at least three unknown sites within the northern Lau Basin. Following the great Samoan earthquake on 2009 September 29, seven of the volcanoacoustic sources identified exhibit increases in the rate of acoustic detection. These changes persist over timescales of days-to-months and are observed up to 900 km from the earthquake hypocentre. At least one of the volcanoacoustic sources that did not respond to the 2009 Samoan earthquake exhibits an increase in detection rate following the great Mw 8.8 Chile earthquake that occurred at a distance of ∼9500 km on 2010 February 27. These observations suggest that great earthquakes may have undocumented impacts on Earth’s vast submarine volcanic systems, potentially increasing the short-term flux of magma and volcanic gas into the overlying ocean.Keywords: Volcano seismology, Backarc basin processes, Subaqueous volcanism, Volcanic arc processe

Hydroacoustic Investigations of Submarine Landslides at West Mata Volcano, Lau Basin

Submarine landslides are an important process in volcano growth yet are rarely observed and poorly understood. We show that landslides occur frequently in association with the eruption of West Mata volcano in the NE Lau Basin. These events are identifiable in hydroacoustic data recorded between ~5 and 20 km from the volcano and may be recognized in spectrograms by the weak and strong powers at specific frequencies generated by multipathing of sound waves. The summation of direct and surface-reflected arrivals causes interference patterns in the spectrum that change with time as the landslide propagates. Observed frequencies are consistent with propagation down the volcano’s north flank in an area known to have experienced mass wasting in the past. These data allow us to estimate the distance traveled by West Mata landslides and show that they travel at average speeds of ~10–25m/s

Seismic tremor at the 9°50′N East Pacific Rise eruption site

Ocean bottom seismic observations within the 9°50′N region of the East Pacific Rise indicate persistent, low-amplitude tremor activity throughout the October 2003 through February 2007 period of monitoring. These signals exhibit either monochromatic or polychromatic spectral characteristics, with a ∼6 Hz fundamental frequency and up to two harmonics. Individual events cannot be correlated between nearby (<1 km) stations, implying the presence of multiple, small-amplitude sources positioned within the shallow crust. Tremor exhibits a semidiurnal periodicity, with some stations recording activity during times of increasing tidal extension and others detecting tremor signals during times of increasing compression. The amplitude, duration, and rate of activity also correlate positively with fortnightly changes in the amplitude of the tides. These spatiotemporal patterns are consistent with tremor generation in response to tidally modulated fluid flow within a network of shallow cracks. Tremor energy flux is spatially and temporally heterogeneous; however, there are extended periods of greater and lesser activity that can be tracked across portions of the array. Despite their shallow crustal origin, changes in tremor amplitude and spectral character occur in the months prior to a major microearthquake swarm and inferred seafloor spreading event on 22 January 2006, with an increase in the degree of correlation between tremor activity and tidal strain in the weeks leading up to this event. After the spreading event, two eruption-surviving stations near the axis continue to show high rates of tremor activity, whereas these signals are suppressed at the single station recovered from the near-axis flanks. This off-axis quiescence may result from the dike-induced closing of cracks or perhaps from the emplacement of impermeable flows near the station