Haruyoshi Matsumoto

Monitoring Pacific Ocean seismicity from an autonomous hydrophone array

Since May 1996, an array of autonomous hydrophone moorings has been continuously deployed in the eastern equatorial Pacific to provide long-term monitoring of seismic activity, including low-level volcanic signals, along the East Pacific Rise between 20°N and 20°S and the Galapagos Ridge. The instruments and moorings were designed to continuously record low-frequency acoustic energy in the SOFAR channel for extended periods and produce results comparable to those previously derived by using the U.S. Navy Sound Surveillance System (SOSUS) in the northeast Pacific. The technology and methodology developed for this experiment, including instrument design, mooring configuration, analysis software, location algorithms (with an analysis of errors), and a predicted error field, are described in detail. Volcanic activity is observed throughout the Pacific, along with seismicity along transform faults, subduction zones, and intraplate regions. Comparison data sets indicate detection thresholds and accuracy better than the land networks for open ocean areas and results comparable to, or better than, SOSUS. Volcanic seismicity along the fast spreading East Pacific Rise appears similar to documented examples in the northeast Pacific but with much shorter durations. One example from the intermediate spreading Galapagos Ridge is comparable to northeast Pacific examples, and several episodes of activity were observed in the Wilkes Transform Fault Zone. A site of continuing off-axis seismicity is located near 18°S and 116°W. Isolated intraplate earthquakes are observed throughout the study area. Earthquake information from this experiment and future observations will be provided through the World Wide Web and earthquake data centers.

Acoustic detection of a seafloor spreading episode on the Juan de Fuca Ridge using military hydrophone arrays

Until recently, no practical method has been available to continuously monitor seismicity of seafloor spreading centers. The availability of the U.S. Navys SOund SUrveillance System (SOSUS) for environmental research has allowed the continuous monitoring of low-level seismicity of the Juan de Fuca Ridge in the northeast Pacific. On June 22, 1993, NOAA installed a prototype system at U.S. Naval Facility Whidbey Island to allow real-time acoustic monitoring of the Juan de Fuca Ridge using SOSUS. On June 26, 2145 GMT, a burst of low-level seismic activity, with accompanying harmonic tremor, was observed and subsequently located near 46°15′N, 129°53′W, on the spreading axis of the Juan de Fuca Ridge. Over the following 2 days, the activity migrated to the NNE along the spreading axis with the final locus of activity near 46°31.5′N, 129°35′W. The nature of the seismicity was interpreted to represent a lateral dike injection with the possibility of eruption on the seafloor. Based on this interpretation, a response effort was initiated by U.S. and Canadian research vessels, and both warm water plumes and fresh lavas were subsequently identified at the site.

Seafloor Acoustic Remote Sensing with Multibeam Echo-Sounders and Bathymetric Sidescan Sonar Systems

This paper examines the potential for remote classification of seafloor terrains using a combination of quantitative acoustic backscatter measurements and high resolution bathymetry derived from two classes of sonar systems currently used by the marine research community: multibeam echo-sounders and bathymetric sidescans sonar systems. The high-resolution bathymetry is important, not only to determine the topography of the area surveyed, but to provide accurate bottom slope corrections needed to convert the arrival angles of the seafloor echoes received by the sonars into true angles of incidence. An angular dependence of seafloor acoustic backscatter can then be derived for each region surveyed, making it possible to construct maps of acoustic backscattering strength in geographic coordinates over the areas of interest. Such maps, when combined with the high-resolution bathymetric maps normally compiled from the data output by the above sonar systems, could be very effective tools to quantify bottom types on a regional basis, and to develop automatic seafloor classification routines.

Seasonal presence of cetaceans and ambient noise levels in polar waters of the North Atlantic.

In 2009 two calibrated acoustic recorders were deployed in polar waters of the North Atlantic to study the seasonal occurrence of blue, fin, and sperm whales and to assess current ambient noise levels. Sounds from these cetaceans were recorded at both locations in most months of the year. During the summer months, seismic airguns associated with oil and gas exploration were audible for weeks at a time and dominated low frequency noise levels. Noise levels might further increase in the future as the receding sea ice enables extended human use of the area.

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.

The April 1992 Cape Mendocino Earthquake Sequence: Seismo-Acoustic Analysis Utilizing Fixed Hydrophone Arrays

The oceanic T-waves of earthquakes associated with the 1992 Cape Mendocino earthquake sequence were recorded and analyzed using fixed hydrophone arrays located throughout the north-east Pacific Ocean. The T-waves of these events were well recorded with high S/N ratios and strong acoustic energy present over a 0–64 Hz bandwidth. The smallest event recorded by the hydrophone arrays from the sequence had a local magnitude of 2.4. The hydrophone records of the three largest shocks in the sequence (ML 6.9, 6.2, 6.5) exhibited both T-waves and lithospheric phases from these events. Low-pass filtering (2 Hz) of the lithospheric phases yielded a clear P-wave arrival for epicentral distances of <10°, but no apparent S-wave. A seafloor cable-break was detected immediately after the second M>6 aftershock, possibly the result of a submarine slide. The direct P-wave hydrophone records from the second large aftershock showed a relatively high-amplitude, high-frequency arrival, consistent with seismic analyses which used this information to infer rupture direction. The rupture direction was toward the location of the cable break, thus rupture directivity possibly played a role in initiating the slide event. Modelling of the T-wave propagation path, using the Parabolic Equation model, produced estimates of the acoustic transmission loss from epicenter to receiver. The transmission loss to the most distant phones is typically 10-20 dB , and can be as large as 50–70 dB for acoustic propagation paths that cross the continental margin. The amount of acoustic energy each earthquake released into the ocean at the seafloor–water interface was estimated applying the transmission loss and instrument response to the recorded T-wave signals. This acoustic source power level was calculated for 41 events with magnitudes over a recorded range of 2.4≤ML≤6.9, with 17 of these events having their seismic moment estimates available through the NEIC. Ground displacement spectra were estimated from the acoustic power spectra and showed no indication of a corner frequency. Thus empirical analyses relating source level to magnitude and seismic moment were necessary to quantitatively derive an earthquakes size from hydrophone records. The results of indicator variable regression analyses suggest that T-wave source level increases linearly with the events local magnitude and seismic moment. Furthermore, the source power level versus magnitude relationships for oceanic and continental earthquakes are significantly different, probably illustrating differences in the seismic and acoustic propagation paths from hypocenter to the hydrophone receivers. The results indicate that acoustic measurements provide a reasonable estimate of magnitude and seismic moment of an oceanic earthquake that was not detected by land-based seismic networks.

Estimation of seafloor microtopographic roughness through modeling of acoustic backscatter data recorded by multibeam sonar systems

A method is described for estimating parameters related to the power‐law frequency spectrum of seafloor interface roughness from acoustic backscatter strength measured from hull‐mounted multibeam sonar arrays. Scattering parameters for a sediment‐free seafloor are inversely derived from backscatter data using a Kirchhoff‐based interface scattering model developed by Jackson et al. [J. Acoust. Soc. Am. 79, 1410–1422 (1986)]. A variety of data reduction routines are employed, including slope correction, beam‐pattern and pulse‐width corrections [de Moustier et al., J. Acoust. Soc. Am. 90, 522–531 (1991)], ping cross correlation, and an indirect method to account for the lack of system calibration. The method is applied to well‐studied areas of the Juan de Fuca Ridge and the results compared to geological ground truth. Preliminary results indicate that the method may be valuable as a survey tool for routine mapping of seafloor acoustic properties.

Estimation of seafloor roughness spectral parameters from multi‐beam sonar acoustic backscatter data: Axial Seamount, Juan De Fuca Ridge

A new method to estimate seafloor roughness spectra applies the surface backscatter model of Jackson et al. [1986] to multi-beam sonar backscatter data. The use of principal components analysis, following spectral estimation, derives a single mappable index from correlated spectral parameters. This index can be mapped as a continuous field corresponding to geological observations in the area. This method allows for quantitative delineation of the neovolcanic zone and other zones of volcanic and tectonic activity from hull-mounted sonar systems.

From pole to pole: Soundscapes of the Atlantic Ocean

Between July 2009 and December 2010, two identical calibrated hydrophone packages were deployed and continuously operated (2,000 Hz sampling rate) in the Fram Strait (79°N, 5.5°E) and the Bransfield Strait (62°S, 55.5°W). Analysis of these recordings were combined with a data set collected during the same time period by the Comprehensive Nuclear-Test-Ban Treaty Organization International Monitoring System (CTBTO IMS) hydroacoustic station HA10 (250 Hz sampling rate) at Ascension Island (8°S, 14.4°W). The combination of these datasets allowed a comparison of low-frequency noise levels in polar and tropic areas of the Atlantic Ocean. The recordings were analyzed for major natural (e.g., marine mammals, ice) and anthropogenic (e.g., shipping, seismic) contributors to the ambient sound field and their seasonal variability. Preliminary results indicate (1) a higher seasonal variability of ambient noise levels in polar regions compared to the tropics, (2) the seasonal variability of ambient noise levels in the ...

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 (<1 s) 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.