J. H. Haxel

P- and T-Wave Detection Thresholds, Pn Velocity Estimate, and Detection of Lower Mantle and Core P-Waves on Ocean Sound-Channel Hydrophones at the Mid-Atlantic Ridge

Since 1999 six Sound Fixing and Ranging (SOFAR) hydrophones have been moored along the Mid-Atlantic Ridge (MAR) (15-35 N). These hydrophones (8-bit data resolution) are designed for long-term monitoring of MAR seismicity using the acoustic T waves of seafloor earthquakes. The completeness level of the MAR T- wave earthquake catalog estimated from size-frequency constraints is mb 3.0, a significant improvement in detection compared to the mb 4.6 completeness level estimated from National Earthquake Information Center magnitude-frequency data. The hydrophones also detect the acoustic phase of converted upper mantle P arrivals from regional earthquakes at epicentral distances of 374-1771 km and from events as small as mb 3.6. These regional P waves are used to estimate a Pn velocity of 8.0 0.1 km sec 1 along the east and west MAR flanks. An unexpected result was the identification of P arrivals from earthquakes outside the Atlantic Ocean basin. The hydrophones detected P waves from global earthquakes with magnitudes of 5.8-8.3 at epicentral distances ranging from 29.6 to 167.2. Examination of travel times suggests these teleseismic P waves constitute the suite of body-wave arrivals from direct mantle P to outer- and inner-core reflected/refracted phases. The amplitudes of the teleseismic P waves also exhibit the typical solid-earth wave field phenomena of a P shadow zone and caustic at D 144. These instruments offer a long-term, relatively low-cost alternative to ocean-bottom seismometers that allows for obser- vation of Pn velocities and mantle/core phases arriving at normally inaccessible deep- sea locations.

Submarine landslide triggered by volcanic eruption recorded by in situ hydrophone

NW Rota-1 is a submarine volcano in the Mariana volcanic arc that is notable as the site where underwater explosive eruptions were first witnessed in A.D. 2004. After years of continuous low-level eruptive activity, a major landslide occurred at NW Rota-1 in August 2009, triggered by an unusually large eruption that produced 10 times the acoustic energy of the background level of activity. An anomalous earthquake swarm preceded the eruption, suggesting that the sequence started with a magmatic intrusion and associated faulting beneath the volcano. We quantify the size and extent of the landslide using bathymetric resurveys and interpret the timing of events using data from an in situ hydrophone. This is the first instrumental documentation of an earthquake-eruption-landslide sequence at a submarine volcano, and illustrates the close interaction between magmatic activity and mass wasting events in the growth of undersea arc volcanoes.

Antarctic icebergs: A significant natural ocean sound source in the Southern Hemisphere

In late 2007, two massive icebergs, C19a and B15a, drifted into open water and slowly disintegrated in the southernmost Pacific Ocean. Archived acoustic records show that the high-intensity underwater sounds accompanying this breakup increased ocean noise levels at mid-to-equatorial latitudes over a period of ∼1.5 years. More typically, seasonal variations in ocean noise, which are characterized by austral summer-highs and winter-lows, appear to be modulated by the annual cycle of Antarctic iceberg drift and subsequent disintegration. This seasonal pattern is observed in all three Oceans of the Southern Hemisphere. The life cycle of Antarctic icebergs affects not only marine ecosystem but also the sound environment in far-reaching areas and must be accounted for in any effort to isolate anthropogenic or climate-induced noise contributions to the ocean soundscape.

Observations of shallow water marine ambient sound: the low frequency underwater soundscape of the central Oregon coast.

A year-long experiment (March 2010 to April 2011) measuring ambient sound at a shallow water site (50 m) on the central OR coast near the Port of Newport provides important baseline information for comparisons with future measurements associated with resource development along the inner continental shelf of the Pacific Northwest. Ambient levels in frequencies affected by surf-generated noise (f < 100 Hz) characterize the site as a high-energy end member within the spectrum of shallow water coastal areas influenced by breaking waves. Dominant sound sources include locally generated ship noise (66% of total hours contain local ship noise), breaking surf, wind induced wave breaking and baleen whale vocalizations. Additionally, an increase in spectral levels for frequencies ranging from 35 to 100 Hz is attributed to noise radiated from distant commercial ship commerce. One-second root mean square (rms) sound pressure level (SPLrms) estimates calculated across the 10-840 Hz frequency band for the entire year long deployment show minimum, mean, and maximum values of 84 dB, 101 dB, and 152 dB re 1 μPa.

Mapping the sound field of an erupting submarine volcano using an acoustic glider

An underwater glider with an acoustic data logger flew toward a recently discovered erupting submarine volcano in the northern Lau basin. With the volcano providing a wide-band sound source, recordings from the two-day survey produced a two-dimensional sound level map spanning 1 km (depth) × 40 km(distance). The observed sound field shows depth- and range-dependence, with the first-order spatial pattern being consistent with the predictions of a range-dependent propagation model. The results allow constraining the acoustic source level of the volcanic activity and suggest that the glider provides an effective platform for monitoring natural and anthropogenic ocean sounds.

Eruptive modes and hiatus of volcanism at West Mata seamount, NE Lau basin : 1996–2012

We present multiple lines of evidence for years to decade-long changes in the location and character of volcanic activity at West Mata seamount in the NE Lau basin over a 16 year period, and a hiatus in summit eruptions from early 2011 to at least September 2012. Boninite lava and pyroclasts were observed erupting from its summit in 2009, and hydroacoustic data from a succession of hydrophones moored nearby show near-continuous eruptive activity from January 2009 to early 2011. Successive differencing of seven multibeam bathymetric surveys of the volcano made in the 1996–2012 period reveals a pattern of extended constructional volcanism on the summit and northwest flank punctuated by eruptions along the volcanos WSW rift zone (WSWRZ). Away from the summit, the volumetrically largest eruption during the observational period occurred between May 2010 and November 2011 at ∼2920 m depth near the base of the WSWRZ. The (nearly) equally long ENE rift zone did not experience any volcanic activity during the 1996–2012 period. The cessation of summit volcanism recorded on the moored hydrophone was accompanied or followed by the formation of a small summit crater and a landslide on the eastern flank. Water column sensors, analysis of gas samples in the overlying hydrothermal plume and dives with a remotely operated vehicle in September 2012 confirmed that the summit eruption had ceased. Based on the historical eruption rates calculated using the bathymetric differencing technique, the volcano could be as young as several thousand years.

Flux measurements of explosive degassing using a yearlong hydroacoustic record at an erupting submarine volcano

at NW Rota-1 are primarily H2O, SO2, and CO2. Instantaneous fluxes varied by a factor of � 100 over the deployment. Using melt inclusion information to estimate the concentration of CO2 in the explosive gases as 6.9 � 0.7 wt %, we calculate an annual CO2 eruption flux of 0.4 � 0.1 Tg a � 1 . This result is within the range of measured CO2 fluxes at continuously erupting subaerial volcanoes, and represents � 0.2–0.6% of the annual estimated output of CO2 from all subaerial arc volcanoes, and � 0.4–0.6% of the mid-ocean ridge flux. The multiyear eruptive history of NW Rota-1 demonstrates that submarine volcanoes can be significant and sustained sources of CO2 to the shallow ocean.

Sources and Levels of Ambient Ocean Sound near the Antarctic Peninsula

Arrays of hydrophones were deployed within the Bransfield Strait and Scotia Sea (Antarctic Peninsula region) from 2005 to 2009 to record ambient ocean sound at frequencies of up to 125 and 500 Hz. Icequakes, which are broadband, short duration signals derived from fracturing of large free-floating icebergs, are a prominent feature of the ocean soundscape. Icequake activity peaks during austral summer and is minimum during winter, likely following freeze-thaw cycles. Iceberg grounding and rapid disintegration also releases significant acoustic energy, equivalent to large-scale geophysical events. Overall ambient sound levels can be as much as ~10–20 dB higher in the open, deep ocean of the Scotia Sea compared to the relatively shallow Bransfield Strait. Noise levels become lowest during the austral winter, as sea-ice cover suppresses wind and wave noise. Ambient noise levels are highest during austral spring and summer, as surface noise, ice cracking and biological activity intensifies. Vocalizations of blue (Balaenoptera musculus) and fin (B. physalus) whales also dominate the long-term spectra records in the 15–28 and 89 Hz bands. Blue whale call energy is a maximum during austral summer-fall in the Drake Passage and Bransfield Strait when ambient noise levels are a maximum and sea-ice cover is a minimum. Fin whale vocalizations were also most common during austral summer-early fall months in both the Bransfield Strait and Scotia Sea. The hydrophone data overall do not show sustained anthropogenic sources (ships and airguns), likely due to low coastal traffic and the typically rough weather and sea conditions of the Southern Ocean.

Autonomous Hydrophones at NOAA/OSU and a New Seafloor Sentry System for Real-time Detection of Acoustic Events

Development is under way of a new intelligent autonomous hydrophone system, the Quasi-Eulerian hydrophone, or QUEphone. A tether-free portable hydrophone float with built-in data processor, it is capable of repeated vertical ascents and descents from seabed to surface using an internal buoyancy controller. While on the seafloor, it runs an acoustic event detection algorithm continuously; upon detection of a significant acoustic event, or at regular intervals, it surfaces to transmit its position at the surface and a small data file to shore via satellite. Issues of detecting acoustic events at low false alarm rate and applications of this new technology to monitoring other geochemical, biological, and oceanographic properties associated with the worlds ecosystem are discussed. Float payload, depth limitation, and vertical travel speed are also discussed

Obtaining baseline measurements of ocean ambient sound at a mobile test berth site for wave energy conversion off the central Oregon coast

A year-long experiment to monitor underwater ambient noise measurements in shallow, (∼50m) open water along an energetic coastline in the Pacific Northwest (USA) was conducted. Bottom mounted passive acoustic recorders were deployed in March 2010 by Oregon State University (OSU) and NOAA/Pacific Marine Environmental Laboratorys Marine Acoustics Research Group in an area designated by the Northwest National Marine Renewable Energy Center (NNMREC) as a mobile ocean test berth (MOTB) site for wave energy conversion (WEC) platforms off the central coast of Oregon. Acoustic recording packages recorded continuous (1 Hz–2 kHz) sampling at two offshore locations near and within the MOTB. Maximum and minimum total sound pressure levels recorded during the experiment reached 136 dB re 1 µPa and 95 dB re 1 µPa respectively. Meanwhile, the time averaged sound pressure levels for the year long deployment were 113 dB re 1µPa. These data provide the initial baseline recordings required for a meaningful assessment of the shallow water ambient noise levels in the Oregon nearshore coastal environment prior to the installation and operation of wave energy conversion devices.