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OceThe OFFiciaaL MaganZineog OF the Oceanographyra Spocietyhy CITATION Dziak, R.P., D.R. Bohnenstiehl, and D.K. Smith. 2012. Hydroacoustic monitoring of oceanic spreading centers: Past, present, and future. Oceanography 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. USAGE Permission is granted to copy this article for use in teaching and research. Republication, systematic reproduction, or collective redistribution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The Oceanography Society. Send all correspondence to: [email protected] or The Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. 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 Seamount, Juan de Fuca Ridge. Photo courtesy of William Chadwick, Oregon State University/NOAA 116 Oceanography | Vol. 25, No. 1 AbsTRACT. Mid-ocean ridge volcanism and extensional faulting are the magmatic explosions from degassing. fundamental processes that lead to the creation and rifting of oceanic crust, yet these All three volcano-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 oceanic crust. 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 sea 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 water column. 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, Geology and Geophysics 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 Arctic Ocean Greenland Middle 75°N Valley Mohns Juan de Axial Fuca Plate Cobb JuanBlanco de Fuca Pacic N 60°N Plate Gorda Gorda Reykjanes 40° 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 Lau Basin 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: • East Pacific Rise (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 • Central Indian Ridge, ~ 35 mm yr–1 • Southwest Indian Ridge, ~ 15 mm yr–1 • Southeast Indian Ridge, ~ 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 mooring 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 Gorda Ridge, a spreading center (Okal, 2008). Several years later, Collins tiveness of hydroacoustic methods for in the Northeast Pacific Ocean. 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.