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CITATION Dziak, R.P., D.R. Bohnenstiehl, and D.K. Smith. 2012. Hydroacoustic monitoring of oceanic spreading centers: Past, present, and future. 25(1):116–127, http://dx.doi.org/10.5670/oceanog.2012.10.

DOI http://dx.doi.org/10.5670/oceanog.2012.10

COPYRIGHT This article has been published inOceanography , Volume 25, Number 1, a quarterly journal of The Oceanography Society. Copyright 2012 by The Oceanography Society. All rights reserved.

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downloaded from http://www.tos.org/oceanography Oceanic Spreading Center Processes | Ridge 2000 Program Research

Hydroacoustic Monitoring of Oceanic Spreading Centers past, Present, and Future

By Robert P. Dziak, DelWayne R. Bohnenstiehl, and Deborah K. Smith

Ocean bottom hydrophone within the caldera of Axial , Juan de Fuca Ridge. Photo courtesy of William Chadwick, Oregon State University/NOAA

116 Oceanography | Vol. 25, No. 1 Abstract. Mid- ridge and extensional faulting are the magmatic explosions from degassing. fundamental processes that lead to the creation and rifting of oceanic , yet these All three -acoustic sources events go largely undetected in the deep ocean. Currently, the only means available can be useful for remote detection of to observe seafloor-spreading events in real time is via the remote detection of the volcanism on MORs because of their seismicity generated during faulting or intrusion of magma into brittle . unique signal characteristics and because Hydrophones moored in the ocean provide an effective means for detecting these they are the loudest natural sounds small-magnitude earthquakes, and the use of this technology during the last two recorded in the ocean. decades has facilitated the real-time detection of mid-ocean ridge seafloor eruptions Because of the combined dependence and confirmation of subseafloor microbial ecosystems. As technology evolves and of ocean sound speed on pressure and mid-ocean ridge studies move into a new era, we anticipate an expanding network temperature, much of the global ocean of seismo-acoustic sensors integrated into seafloor fiber-optic cabled observatories, exhibits a low-velocity region known satellite-telemetered surface buoys, and autonomous vehicle platforms. as the SOund Fixing And Ranging (SOFAR) channel (Figure 2). The chan- Introduction first discovery of seismically gener- nel’s axis lies ~ 1,000 m below the New seafloor is magmatically emplaced, ated acoustic phases in the 1920s up surface in equatorial regions, becoming cooled, and then faulted within the mid- to the present, and speculates upon progressively shallower and then disap- ocean ridge (MOR) system, forming future applications of these techniques. pearing near the poles in response to one of the most active and longest belts The spreading rate of an MOR largely the changing temperature structure of of seismicity on the planet (Figure 1). determines the character of seismicity it the . Seismically generated Detection of MOR magma intrusion and produces. Fast-spreading ridges exhibit acoustic energy may become trapped rifting events is critical to our under- significant magmatic activity; however, in the SOFAR channel, where it propa- standing of the global pace of volcanism the earthquakes tend to be small and gates laterally in range (r) via a series of and the ephemeral physical and chemical more difficult to detect regionally upward- and downward-turning refrac- impacts these events have on the ocean (Figure 1). At slow-spreading ridges, tions and has little interaction with the and seafloor ecosystems. MOR magmatic fault processes dominate, exemplified seafloor or sea surface. The attenuation and tectonic activity, however, typically by more frequent, large-magnitude due to geometric spreading is cylin- has no expression at the sea surface, and earthquakes but sporadic magmatic drical (~ 1/r) for SOFAR guided waves, therefore our only means of identifying activity. MOR volcanism can be detected making transmission significantly more these events is via remote detection by (1) earthquakes caused from magma efficient relative to solid Earth phases of small-magnitude (typically M < 4) intrusion through oceanic crust and/ that undergo spherical spreading loss earthquakes caused by intrusion of or faulting resulting from the intrusion, (~ 1/r2) and allowing acoustic phases to magma and faulting of brittle oceanic (2) volcanic tremor produced by flow propagate thousands of kilometers with crust. One of the most effective means of of either magma or hydrothermal fluid little energy loss. detecting small-magnitude, deep-ocean through oceanic crust, or (3) violent seismicity is to record an earthquake’s hydroacoustic phase, or “T-phase,” Robert P. Dziak ([email protected]) is Professor, Cooperative Institute for Marine that enters and propagates within the Resources Studies/Oregon State University, Hatfield Marine Science Center, Newport, ocean’s sound channel. OR, USA. DelWayne R. Bohnenstiehl is Associate Professor, Department of Marine, This article discusses the hydro- Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC, USA. acoustic monitoring methods used Deborah K. Smith is Senior Scientist, and Department, Woods Hole to study mid-ocean ridges from the Oceanographic Institution, Woods Hole, MA, USA.

Oceanography | March 2012 117 100°E 120°E 140°E 160°E 180° 160°W 140°W 120°W 100°W 80°W 60°W 40°W 20°W0° 20°E 40°E 60°E 80°E

North American Knipovich SovancExploreor Plate Greenland Middle 75°N Valley Mohns Juan de Axial Fuca Plate Cobb JuanBlanco de Fuca

Pacic 60°N Plate Gorda Gorda Reykjanes

40° N Asia Plate Asia North Europe America NMAR 45°N Lucky Strike 30°N Wake Pacic Ocean Atlantic Africa 15°N Ocean Central Indian Galapagos Ridge 0° Ascension Diego EPR South Garcia 15°S America SMAR Australia 30°S Juan Cape Fernandez Leeuwin Indian 45°S SW Indian Ocean Ridge Crozet SE Indian Ridge 60°S Pacic- Antarctic Ridge

Antarctica

Figure 1. Global map of seismicity from the National Earthquake Information Center catalog, M ≥ 4.5, 1976– 2009. Mid-ocean ridges and oceanic transforms are defined by narrow bands of shallow hypocenter earthquakes. Spreading centers and approximate full spreading rates: • (EPR), ~ 110–140 mm yr–1 • Pacific-Antarctic Ridge, 65 mm yr–1 • Galápagos Spreading Center, ~ 45–60 mm yr–1 • Northern Mid-Atlantic Ridge (NMAR), 25 mm yr–1 • Southern Mid-Atlantic Ridge (SMAR), ~ 30 mm yr–1 • , ~ 35 mm yr–1 • , ~ 15 mm yr–1 • , ~ 70 mm yr–1 • Mohns Ridges, ~ 15–20 mm yr–1 • Reykjanes Ridge, ~ 20 mm yr–1 • Juan de Fuca and Gorda Ridges (inset), ~ 60 mm yr–1 Earthquake data from http://earthquake.usgs.gov/eqcenter.

Figure 2. Diagram showing how a hydrophone deployed in the ocean sound channel is used to record acoustic waves from seismic and volcanic activity originating at a mid-ocean ridge. Red circles represent hydroacoustic wave fronts produced by seafloor seismic events that eventually become trapped and laterally propagate along the sound channel. A float keeps the hydrophone moored vertically in the water column. The entire package can be retrieved via acoustic release near the seafloor.

118 Oceanography | Vol. 25, No. 1 Historical Hydroacoustic detect earthquakes from the Mohns and abyssal T-phases was later introduced Monitoring (pre-1990) Knipovich Ridges, establishing the exis- by Johnson et al. (1968), who proposed The Discovery of T-phases tence of long-range acoustic propagation the coupling of acoustic energy into the The first published report of a teleseismic in the Arctic, despite the sound channel sound channel via sea surface scattering T-phase wavetrain is attributed to Jagger being surface limited there. directly above the earthquake epicenter. (1930), who described a high-frequency Northrop et al. (1968) and Northrop arrival recorded on Hawai’i Volcano The Hawai’i T-phase Group (1970) presented analysis of MILS Observatory seismometers within the (1960s to Early 1970s) hydrophone records of T-phases from coda of a large 1927 Alaskan earthquake The study of T-phases and the effec- the , a spreading center (Okal, 2008). Several years later, Collins tiveness of hydroacoustic methods for in the Northeast . These (1936) noted that the seismogram of volcano monitoring were advanced studies focused on the discrepancy a Caribbean earthquake featured a in the 1960s by use of a Pacific-wide between seismic and T-phase locations third arrival following the primary (P) network of sound channel hydrophones, of ridge earthquakes (with T-phase loca- and secondary (S) phases recorded on the Air Force’s Missile Impact Location tions being more accurate). Hammond short-period channels. Linehan (1940) System (MILS; Figure 3), whose data and Walker (1991) used MILS data first coined the term “T-phase” when were analyzed by researchers at the to show that the Juan de Fuca Ridge he identified this third or “tertiary” Hawai’i Institute of Geophysics. Johnson produced 58 T-phase events during a arriving phase in the West Indian et al. (1963) introduced the concept of two-year period in the mid-1960s, all region, although he could only speculate down-continental-slope conversion for of which were located either at areas as to its source. the generation of T-phases by subduc- of vigorous hydrothermal venting or The effort to develop antisubma- tion earthquakes. A potential generation volcanic edifices near the spreading rine warfare techniques during World mechanism for mid-ocean ridge and axis. Moreover, Walker and Hammond War II brought considerable progress in . Ewing et al. (1946) were the first to suggest that under- water sounds recorded during an ocean 60°E 120°E 180° 120°W 60°W 0° acoustic experiment were generated 60°N by submarine volcanic activity, and - 60°E 120°E 180° 120°W 60°W 0° they proposed that a network of sound 60°N 30°N - channel hydrophones could be used to monitor seafloor volcanism. Ewing and 30°N Worzel (1948) provided the theoretical 0° basis for this phenomenon by describing 0° the basics of long-range propagation in the ocean sound channel. Two years 30°S later, Tolstoy and Ewing (1950) provided 30°S the first identification of T-phases as the 60°S 60°S - water-borne phases of an earthquake - source resulting from the conversion Figure 3. Map of major hydro-U.S. Navy SOSUSU.S. Hydrophones Navy SOSUS Hydrophones (real time) (real time) of seismic energy to acoustic energy at acoustic arrays and systemsNOAA - AutonomousNOAA -Hydrophones Autonomous Hydrophones (delayed time) (delayed time) the seafloor-ocean interface. The first discussed in text. Legend at right Spinnaker Array and other Arctic hydrophone deployments shows the system names associatedSpinnaker Array and other Arctic hydrophone deployments detection of T-phases from MOR earth- U. Hawai’i MILS Hydrophones (1960s) with each icon. SOSUS = SoundU. Hawai’i MILS Hydrophones (1960s) quakes was reported by Bath (1954), who International Monitoring System Hydrophones (real time) Surveillance System. MILS =International Missile Monitoring System Hydrophones (real time) used a land-based seismic network to Impact Location System. Ocean-Bottom Hydrophone and Sonobuoy Studies Ocean-Bottom Hydrophone and Sonobuoy Studies

Oceanography | March 2012 119 (1998) showed that of the 644 T-phase or slip on graben faults. Sonobuoy sonobuoys was air deployed to record Gorda Ridge events detected, nearly all experiments also were performed acoustic arrivals and refine earthquake occurred in discreet swarms centered on along the Mid-Atlantic Ridge (MAR) locations. These data showed the earth- the ridge axis, and the swarms remained at 36.5°–37°N (Reid and Macdonald, quakes were centered on a volcanic confined to individual ridge segments. 1973; Spindel et al., 1974), the Galápagos complex on the Reykjanes axis. Subsequent side-scan and submersible surveys showed the earthquake source region was an area of high acoustic back- scatter and relatively recent lava flows. Detection of [mid-ocean ridge] magma A hydrophone array suspended through the Arctic sea ice was used intrusion and rifting events is critical to our to record the T-phases generated by understanding of the global pace of volcanism 32 mid-Arctic Ridge earthquakes and the ephemeral physical and chemical (Keenan and Dyer, 1984). The sea “ surface ice scattered the earthquake impacts these events have on the ocean acoustic rays, enabling long range and seafloor ecosystems. T-phase propagation despite the absence of a sound channel in the polar ocean. Keenan and Merriam (1991) provided further spectral analysis of Arctic T-phases and their long-range propaga- Global Mid-Ocean Ridge Rift” (MacDonald and Mudie,1974), and tion characteristics, as well as giving esti- Hydroacoustic Observations the Nansen Ridge in the Arctic Basin mates of their acoustic magnitudes. The (1960s to Early 1990s) (Kristoffersen et al., 1982). earthquake locations published in these There was increasing awareness that Brocher (1983) recorded T-phases studies indicate the events were near the ocean T-phases were being recorded from MAR (31.6°N) earthquakes on Spitzbergen (transform at coastal and island seismic stations. seafloor seismometers off Nova Scotia. fault) and not the , which Cooke (1967) reported that T-phases A total of 16 T-phase events were was not discernible from the were recorded by seismometers in detected over 30 hours, indicating the available at the time. New Zealand from earthquakes occur- swarm was possibly of volcanic origin. ring on the Pacific-Antarctic Ridge. Toomey et al. (1985) used an array of Recent Past and Present Bath and Shahidi (1971) again reported ocean-bottom hydrophones (OBHs) Day Hydroacoustic T-phases from earthquakes on the to record microearthquakes from the Monitoring (1990–2010) Mohns and Knipovich Ridges, showing MAR at 23°N. During three weeks of The US Navy Sound acoustic rays propagated by sea surface monitoring, they detected an average Surveillance System and seafloor reflections. Sonobuoys of 15 earthquakes per day, inter- Following the end of the Cold War, the were first employed during this time as preted to reflect the extensional brittle US Navy sought environmental appli- an inexpensive method to detect mid- failure of the crust. cations for many military assets and ocean ridge seismicity at the sea surface. In May 1989, an earthquake swarm developed a dual-use program to share Reid et al. (1973) used sonobuoys to of 45 teleseismic events (M= 4–5.5) technology with civilian researchers. detect multiple large earthquake swarms was detected from the Reykjanes Ridge, Two projects were initiated to use the along the Guaymas basin rift in the 500 km southwest of Iceland. The swarm Navy’s networks of bottom-mounted Gulf of California. They attributed these was interpreted as a seafloor eruption hydrophones, referred to as the Sound swarms to either magma movement (Crane et al., 1997), and an array of Surveillance System (SOSUS), one in

120 Oceanography | Vol. 25, No. 1 the Atlantic (Nishimura and Conlon, (NOAA) Pacific Marine Environmental model of the ocean, yielded significant 1994) and one in the Northeast Pacific Laboratory’s T-phase monitoring improvements in the accuracy of derived (Fox and Hammond, 1994; Figure 3). project began the first systematic effort earthquake locations (Slack et al., 1999). The Atlantic effort focused on cetacean to use hydrophone data to produce a This much enhanced earthquake research; however, one notable exception continuous catalog of mid-ocean ridge detection capability led to the first real- was the use of northern Atlantic SOSUS seismicity by screening SOSUS data for time observations of a seafloor spreading arrays to detect a submarine eruption earthquakes from the Juan de Fuca and event on the Juan de Fuca Ridge (Fox on the Mohns Ridge (Blackman et al., Gorda Ridges. Availability of SOSUS to et al., 1995). Nearly 700 earthquakes 2000). T-phase earthquake swarms the civilian research community enabled were tracked during a 23-day period concentrated at various locations along real-time detection of T-phases from following the intrusion of a magma a 50–70 km length of the ridge, which seafloor earthquakes, reducing the detec- dike 60 km along the rift axis of the Blackman et al. (2000) interpreted as tion threshold of seismicity by almost CoAxial ridge segment (Dziak et al., representing focused eruptions at a few two orders of magnitude (Fox et al., 1995; Figure 4). The remote detection discrete areas rather than the intrusion 1994). The excellent array geometry of of this volcanic episode resulted in and propagation of a magma dike. the SOSUS network relative to Northeast several in situ multidisciplinary studies In 1991, the National Oceanic Pacific spreading centers, combined with that led to the discovery of a hydro- and Atmospheric Administration the existence of a well-defined velocity thermal plume, a still cooling lava flow,

a 46°42’N46°42’N 6666 46°40’N46°40’N 46°42’Nb –1 –1 66

46°40’N –1

46°30’N46°30’N 4444 m) m)

46°30’N 44 m)

mama speed speed = 0.3 = 0.3 m sm s

ma speed = 0.3 m s 46°18’N46°18’N MagMag 2222 Distance (k Distance (k

46°18’N Mag 22 Distance (k 46°20’N46°20’N 46°20’N 46°06’N46°06’N 00 46°06’N2525 2727 2929 131357570 JuneJune25 27 29 JulyJuly 199313 1993 57 June July 1993

1212 c 46°00’N46°00’N 12 46°00’N ime (sec) ime (sec)

T T 88 ime (sec) T

ise ise 8 ise 44 wave R wave R

T- T- 4 wave R T- 45°40’N45°40’N 00 2727 2929 1313575799 1111 45°40’N 130°00’130°00’WW 129°40’129°40’WW 129°20’129°20’WW 0 JuneJune 27 29JulyJuly 199313 1993 579 11 130°00’W 129°40’W 129°20’W June July 1993 Figure 4. (a) Bathymetric map of the CoAxial segment of the Juan de Fuca Ridge. Circles show earthquake loca- tions recorded during a 23-day period. (b) Along-segment position of earthquake locations vs. time during the swarm. (c) T-wave rise time vs. time during the swarm. Events show a decrease in rise time consistent with shoaling of earthquakes as the magma approaches the seafloor, erupting fresh lava along the northern part of the segment. Reproduced from Dziak et al. (1995)

Oceanography | March 2012 121 and microbial communities living in a amounts of hydrothermal fluid during Autonomous Portable subseafloor ecosystem (e.g., Baker et al., MOR spreading events. Hydrophones 1995; Embley et al., 1995; Holden et al., The SOSUS earthquake locations Early successes using SOSUS facilitated 1998). Estimates of relative earthquake also have been used in several studies of the development of moored autonomous depths were also obtained using the Northeast Pacific transform faults. Dziak underwater hydrophone systems (AUHs) T-phase rise time, defined as the time et al. (1996) presented evidence of earth- that could be used to monitor global between the onset of the signal and its quake and volcanic-tremor activity from ridge segments. In this design, the hydro- amplitude peak (Schreiner et al., 1995). an extensional basin within the western phone sensor and instrument package are suspended within the SOFAR channel using a seafloor tether and foam flotation (Figure 2). These instruments have been deployed successfully along mid-ocean ridge spreading centers in the Atlantic Given the expected improvements in global, (Smith et al., 2002, 2003; Goslin et al., deep-ocean monitoring technologies during 2005; Simao, 2010), eastern Pacific (Fox the next century, we foresee a time when et al., 2001), and Indian (Royer et al., 2009) , as well as along the back- even a segment-scale magmatic or seafloor “ arc spreading systems of the western spreading event will be detected as it happens Pacific (Dziak et al., 2005) and Antarctic anywhere in the deep ocean. Peninsula (Dziak et al., 2010). A typical deployment consists of only six to seven instruments that can monitor more than 2,000 km along axis and provide a record of earthquakes with M > 2.5–3. ” On the fast-spreading East Pacific Two other major eruptive episodes have Blanco Transform. Subsequent submers- Rise (EPR), seismicity was observed been documented using SOSUS, one at ible dives in the basin confirmed the to concentrate along active transform the northern Gorda Ridge in 1996 (Fox presence of recently formed construc- faults, with the exception of a few swarm and Dziak, 1998) and another at Axial tional mounds as well as sites on the ridge axis (Fox et al., 2001). Volcano in 1998 (Dziak and Fox, 1999). diffuse venting. Dziak et al. (2003) The likely volcanic nature of these

In addition, three dike injection/seafloor showed that an Mw 6.2 earthquake on EPR swarms was confirmed in 2006 spreading episodes were detected at the western Blanco Transform caused when an in situ seismic array observed Endeavour Segment in 1999, 2000, and precursory and coseismic temperature an eruption as it occurred (Tolstoy 2005 (Bohnenstiehl et al., 2004b; Hooft changes at sites et al., 2006). Thousands of events were et al., 2010). Prolonged, intrusion-style on the southern Juan de Fuca Ridge, recorded by the seismometers, whereas earthquake swarms also were observed at ~ 39 km distant from the main shock. only 23 T-phase events were detected the Gorda Ridge (Jackson segment) and Merle et al. (2008) described the detec- by the regional hydrophone array Middle Valley segment in 2001, and the tion of a 2008 earthquake sequence (Dziak et al., 2009). northern Gorda Ridge segment in 2008. on SOSUS that began within the Juan In contrast to the EPR, hydrophone- Using this T-phase earthquake record, de Fuca Plate. After a week, the earth- recorded seismicity on the slow- Dziak et al. (2007) were able to infer quakes moved to the eastern Blanco spreading MAR is common along both that a rapid intrusion of magma within Transform and ended 30 days later the rift axis and its transform offsets a rift zone typically leads to seafloor with a seafloor-spreading event on the (Smith et al., 2002). Smith et al. (2003) eruptions and expulsion of massive northern Gorda Ridge. showed ridge-crest seismicity correlates

122 Oceanography | Vol. 25, No. 1 with axial thermal structure, where The results of this monitoring effort are et al. (2005) deployed seismometers regions of low and high numbers of expected to provide new insight into on an Arctic iceflow to record the events correspond to thinner (hotter) volcano-tectonic processes along this acoustic phases of volcanic explo- and thicker (colder) . poorly understood section of the ridge. sions from the Gakkel Ridge. During Bohnenstiehl et al. (2002, 2003) and Royer et al. (2008) located more than 11 days, a total of 200 explosions were Dziak et al. (2004a) used hydrophone 2,000 T-phase earthquakes during a located at a large volcanic center, and a seismicity at the MAR to quantify the 16-month deployment of autonomous recent lava flow was discovered in 1999 completeness level of the hydrophone hydrophones in the Indian Ocean. (Edwards et al., 2001). earthquake catalog (magnitude ≥ 2.5) Southeast Indian Ridge seismicity occurs OHBs also have been used to study and the temporal distribution of ridge- predominantly along transform faults, ridge-crest seismicity. Kong et al. (1992) crest aftershock sequences. Location the Southwest Indian Ridge exhibits employed seven OBHs to detect micro- comparisons again demonstrated signifi- some periodicity in earthquake activity earthquakes over a three-week period cant improvements relative to more between adjacent ridge segments, and from the TAG segment of the MAR distant land-based seismic monitoring two large tectono-volcanic earthquake at 26°N. The high seismicity levels at (Bohnenstiehl and Tolstoy, 2003; Pan swarms were observed along the Central 26°N have recently been interpreted and Dziewonski, 2005). Indian Ridge near the . as due to slip on the local detachment Dziak et al. (2004b) showed that a Autonomous hydrophone arrays also fault (deMartin et al., 2007). Sohn et al. 2001 earthquake swarm at Lucky Strike have been deployed at two back-arc (1999) recorded microseismicity using Seamount (MAR at 37°N) was likely spreading centers, the Bransfield Strait OBHs following a large eruption at Axial caused by magma intrusion. Subsequent in Antarctica (Dziak et al., 2010) and Volcano on the Juan de Fuca Ridge in submersible observations confirmed the Lau Basin in the western Pacific 1998. These local earthquakes were inter- increased venting and microbial activity (Bohnenstiehl et al., 2010). The Bransfield preted as either slip along the caldera at the summit. Goslin et al. (2005) Strait array detected 3,900 earthquakes rim fault or shear along the volcano’s documented several large earthquake during a two-year deployment, including southeast flank. Haxel et al. (2010) have sequences along the Reykjanes Ridge eight earthquake swarms located on the maintained an array of four OBHs within south of Iceland. These sequences exhib- 400 km long central rift zone. Only five Axial’s summit caldera since 2006. The ited spatio-temporal patterns consistent months of the Lau Basin data have been OBHs have recorded thousands of earth- with involvement of magmatic or hydro- analyzed to date; however, preliminary quakes annually, which have steadily thermal processes. Escartín et al. (2008) results indicate many of the 26,900 earth- increased through time, consistent with showed that high rates of hydroacoustic quakes detected so far are focused on the geodetic observations of caldera floor seismicity at the northern MAR correlate main transform (Peggy Ridge) and the uplift caused by a renewed influx of with the locations of detachment faults large (~ 50 km) overlapping spreading magma (Nooner and Chadwick, 2009) that bring lower crust and upper center in the region. and leading to discovery of a summit rocks to the seafloor and are typically eruption in April 2011. associated with hydrothermal activity. Additional Cabled and Deployed During 2010, NEPTUNE Canada, Simao et al. (2010) showed that T-phase Hydroacoustic Arrays a fiber-optic cabled node of deep-sea earthquakes along the MAR north Sohn and Hildebrand (2001) used the sensors deployed along the northern and south of the Azores tend to cluster Spinnaker hydrophone array (Figure 3) Juan de Fuca Ridge, became operational on mantle Bouguer gravity anomaly in the Arctic Ocean to detect tectonic (Barnes and Tunnicliffe, 2008). The maxima. Most of these clusters seemed earthquakes from the Gakkel Ridge node’s seismometers and hydrophones to be caused by magma intrusion and and further established the effective- are deployed on and off the ridge axis. propagation along the ridge axis. An ness of using of T-phases in the Arctic The acoustic phases of hundreds of array of eight hydrophones is currently for long-range earthquake detection earthquakes from the ridge and nearby deployed along the equatorial MAR. beneath the ice canopy. Schlindwein transforms have been recorded to date.

Oceanography | March 2012 123 International Monitoring System be determined and therefore enhancing The Future of Mid-Ocean During the late 1990s, a global real-time the location capabilities afforded by the Ridge Monitoring system of radionuclide, seismic, infra- relatively sparse network. It is interesting to speculate upon what sound, and hydroacoustic sensors was Hanson and Bowman (2005) used developments will occur in deep-ocean constructed to support a Comprehensive T-phases recorded on the IMS stations in acoustic monitoring. Recent improve- Nuclear Test Ban. This infrastructure is the Indian Ocean to locate 1,146 earth- ments in autonomous underwater collectively known as the International quakes from the Central and Southeast vehicle (AUV) technology will lead to Monitoring System (IMS; Figure 3). Indian Ridges during a 10-month period the next significant advancement in The hydroacoustic component consists in 2003. The Indian Ocean T-phase seis- hydroacoustic monitoring. A recent of five island-based seismic stations micity clustered at ridge-transform inter- example occurred when an ocean glider and six cabled hydrophone installa- sections, with several gaps in earthquake capable of satellite data transmission was tions at Diego Garcia, Cape Lueewin, activity occurring within ridge segments. flown around an erupting volcano with a and Crozet Island in the Indian Ocean; Other projects have used T-phase seis- hydrophone in its payload (Matsumoto Juan Fernandez and Wake Islands in micity to study the diffuse nature of the et al., 2011). One can envision a constel- the Pacific; and Ascension Island in the plate boundary system along the Indian lation of gliders or autonomous floats Atlantic. Each hydrophone station hosts Ocean spreading centers and the orga- (e.g., Simons et al., 2009) circling large a set of three sound-channel moored nization of transform faults within the regions of the world’s MORs, screening sensors deployed as a small-aperture basin (e.g., Bohnenstiehl et al., 2004b; acoustic signals for volcano-tectonic (~ 2 km) horizontal array, allowing the Yun et al., 2009). seismicity and reporting on the latest direction of incoming acoustic energy to eruption or seafloor spreading event.

Figure 5. (Left) Image of quasi-Eulerian autonomous hydrophone (Que-phone) float. The Que-phone self controls buoyancy and can perform several ascent/descent cycles to survey the ocean sound field from seafloor to sea surface. (Right) Image of an ocean glider (Webb Research Inc.) with a hydrophone and recording package mounted on the plat- form (Matsumoto et al., 2011). The glider is capable of a more structured survey methodology and can vertically and later- ally survey the water column over a several tens of square kilometers. Haru Matsumoto is shown for scale.

124 Oceanography | Vol. 25, No. 1 Within the next few years, the from mid-ocean ridges and how this and volcanic activity within the Lau Basin. Eos, Transactions, American Geophysical US Regional Scale Nodes, a counterpart information was used to provide insight Union 90(52):Fall Meeting Abstract T11E-02. to the NEPTUNE Canada initiative, into spreading center processes. Given Bohnenstiehl, D.R., R.P. Dziak, M. Tolstoy, will instrument portions of the Juan de the expected improvements in global, C. Fox, and M. Fowler. 2004. Temporal and spatial history of the 1999–2000 Endeavour Fuca Ridge. The Axial Volcano node deep-ocean monitoring technologies Segment seismic series, Juan de Fuca Ridge. will include an array of seafloor seis- during the next century, we foresee Geochemistry Geophysics Geosystems 5, Q09003, http://dx.doi.org/10.1029/2004GC000735. mometers and at least one hydrophone a time when even a segment-scale Bohnenstiehl, D.R., and M. Tolstoy. 2003. moored in the water column. These magmatic or seafloor spreading event Comparison of teleseismically and hydroacous- systems will enable real-time, in situ, will be detected as it happens anywhere tically derived earthquake locations along the north-central Mid-Atlantic Ridge and equato- seismo-acoustic monitoring of ridge- in the deep ocean. rial East-Pacific Rise. Seismological Research crest volcanic activity, albeit a spatially Letters 74:790–801, http://dx.doi.org/10.1785/ limited view of Northeast Pacific Acknowledgements gssrl.74.6.791. Bohnenstiehl, D.R., M. Tolstoy, and E. Chapp. spreading center dynamics. The authors wish to thank M. Tolstoy, 2004. Breaking into the plate: A 7.6 Mw frac- Ocean glider and AUV technology J. Perrot, E. Hooft, and E. Kappel for very ture-zone earthquake adjacent to the Central Indian Ridge. Geophysical Research Letters 31, will continue to improve in both physical insightful comments that improved the http://dx.doi.org/10.1029/2003GL018981. maneuverability and the quality and manuscript. SOSUS studies discussed in Bohnenstiehl, D.R., M. Tolstoy, R.P. Dziak, amount of data collected (Figure 5). this paper were supported by the NOAA C.G. Fox and D.K. Smith. 2002. Aftershock sequences in the mid-ocean ridge environ- Perhaps future developments will allow Vents Program and during 2006–2009 by ment: An analysis using hydroacoustic for deployment of a shoal of platforms the National Science Foundation, Grant data. 354:49–70, http:// dx.doi.org/10.1016/S0040-1951(02)00289-5. with multiple hydrophones, which can OCE-0623649. This paper is PMEL Brocher, T.M. 1983. T-phases from an earthquake beamform and localize acoustic sources contribution number 3746. swarm on the Mid-Atlantic Ridge at 31.6°N. while at sea. 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