Christopher G. Fox

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.

The June-July 1993 seismo-acoustic event at CoAxial segment, Juan de Fuca Ridge: Evidence for a lateral dike injection

The CoAxial segment (Juan de Fuca Ridge) was the site of an intense swarm of earthquakes that began at 21:43 GMT on June 26, 1993 (Julian Day 177). The swarm started near 46°15′N and migrated northward over the next 40 hours to ∼46°36′N, where the majority of 676 events occurred during the following 3 weeks of activity. The earthquakes propagated NNE at a velocity of 0.3±0.1 m s−1 from the southern to northern swarm sites. The activity went undetected by land-based seismic networks along the Oregon and Washington coasts, suggesting that the earthquakes were all M ≤ 4.0. The character of this earthquake swarm is very similar to dike injections observed at Krafla and Kilauea Volcanoes. The earthquake activity and subsequent migration probably represent a lateral dike injection into faults and fissures comprising the CoAxial segment. The reservoir acting as the dikes source likely resides beneath, or to the south of, the initial southern swarm of earthquakes. The large magma supply at Axial Volcano cannot be ruled out as the source for the CoAxial dike, even though Axial Volcano exhibited no earthquake activity related to the CoAxial swarm. The T-wave earthquake swarm reported here is the first deep-ocean observation of volcanic seismicity associated with what most likely was a mid-ocean ridge dike injection event.

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.

The January 1998 Earthquake swarm at Axial Volcano, Juan de Fuca Ridge: Hydroacoustic evidence of seafloor volcanic activity

On January 25, 1998 at 11:33Z, an intense swarm of earthquakes began within the summit caldera of Axial Volcano, central Juan de Fuca Ridge (JdFR). The earthquake swarm was detected using the U.S. Navy hydrophones in the northeast Pacific Ocean. The earthquake swarm lasted 11 days and produced 8247 detected (1037 located) earthquakes. During the first two days of the swarm, earthquake activity migrated from the summit caldera ∼50 km along the south rift zone of Axial Volcano at rates of 0.92±0.13 and 0.23±0.09 m s−1. Earthquake epicenter migration is characteristic of a lateral magma dike injection. The largest three swarm earthquakes occurred within the caldera. Their timing and mechanisms are consistent with adjustment of the caldera floor as magma is removed from beneath the summit. Earthquakes with short T-wave rise times (shallow focal depths) cluster at two spots along the summit and south rift zone, and are considered possible seafloor eruption sites. In situ ground deformation and hydrothermal plume monitors, and later shipboard observations, confirmed the occurrence of a seafloor volcanic eruption at the volcanos summit.

Long-range acoustic detection and localization of blue whale calls in the northeast Pacific Ocean

Analysis of acoustic signals recorded from the U.S. Navys SOund SUrveillance System (SOSUS) was used to detect and locate blue whale (Balaenoptera musculus) calls offshore in the northeast Pacific. The long, low-frequency components of these calls are characteristic of calls recorded in the presence of blue whales elsewhere in the world. Mean values for frequency and time characteristics from field-recorded blue whale calls were used to develop a simple matched filter for detecting such calls in noisy time series. The matched filter was applied to signals from three different SOSUS arrays off the coast of the Pacific Northwest to detect and associate individual calls from the same animal on the different arrays. A U.S. Navy maritime patrol aircraft was directed to an area where blue whale calls had been detected on SOSUS using these methods, and the presence of vocalizing blue whale was confirmed at the site with field recordings from sonobuoys.

Low-frequency whale and seismic airgun sounds recorded in the mid-Atlantic Ocean.

Beginning in February 1999, an array of six autonomous hydrophones was moored near the Mid-Atlantic Ridge (35 degrees N-15 degrees N, 50 degrees W-33 degrees W). Two years of data were reviewed for whale vocalizations by visually examining spectrograms. Four distinct sounds were detected that are believed to be of biological origin: (1) a two-part low-frequency moan at roughly 18 Hz lasting 25 s which has previously been attributed to blue whales (Balaenoptera musculus); (2) series of short pulses approximately 18 s apart centered at 22 Hz, which are likely produced by fin whales (B. physalus); (3) series of short, pulsive sounds at 30 Hz and above and approximately 1 s apart that resemble sounds attributed to minke whales (B. acutorostrata); and (4) downswept, pulsive sounds above 30 Hz that are likely from baleen whales. Vocalizations were detected most often in the winter, and blue- and fin whale sounds were detected most often on the northern hydrophones. Sounds from seismic airguns were recorded frequently, particularly during summer, from locations over 3000 km from this array. Whales were detected by these hydrophones despite its location in a very remote part of the Atlantic Ocean that has traditionally been difficult to survey.

Passive acoustic methods applied to fin whale population density estimation

Assessing the size of cetacean populations in the open ocean has traditionally relied on visual surveys alone. The addition of acoustic monitoring can complement these surveys if reliable protocols can be formulated and calibrated with visual techniques. A study is presented to estimate fin whale population statistics based on near-continuous recording from a single hydrophone. Range to calling animals is estimated by transmission loss and multipath methods to provide a minimum population density estimate. Results are derived from recordings at a hydrophone site north of Oahu, Hawaii that have been the focus of earlier studies. The average calling whale density is 0.027 animals/1000 km2, while the seasonal maximum calling whale density is about three times the average, or 0.081 animals/1000 km2. Over 30 fixed hydrophone sites are available around the Worlds Oceans from which such statistics could be generated.

Aftershock sequences in the mid-ocean ridge environment: an analysis using hydroacoustic data

Abstract Hydroacoustic data from autonomous arrays and the U.S. Navys Sound Surveillance System (SOSUS) provide an opportunity to examine the temporal and spatial properties of seismicity along portions of the slow-spreading Mid-Atlantic Ridge (MAR), intermediate-spreading Juan de Fuca Ridge (JdFR) and fast-spreading East Pacific Rise (EPR). Aftershock and foreshock events are selected from the hydroacoustic earthquake catalog using single-link cluster (SLC) analysis, with a combined space–time metric. In the regions examined, hydroacoustic data improve the completeness level of the earthquake catalog by ∼1.5–2.0 orders of magnitude, allowing the decay constant, p , of the modified Omori law (MOL) to be determined for individual sequences. A non-parametric goodness-of-fit test indicates six of the seven sequences examined are described well by a MOL model. The p -values obtained for individual ridge and transform sequences using hydroacoustic data are larger than that previously estimated from the analysis of a stacked sequence generated from teleseismic data. For three sequences along the Siqueiros, Discovery and western Blanco Transforms, p -values are estimated to be ∼0.94–1.29. The spatial distribution of aftershocks suggests that the mainshock rupture is constrained by intra-transform spreading centers at these locations. An aftershock sequence following a 7.1 M s thrust event near the northern edge of the Easter Microplate exhibits p =1.02±0.11. Within the sequence, aftershocks are located to the north of a large topographic ridge, which may represent the surface expression of the shallow-dipping fault that ruptured during the mainshock. Two aftershock sequences near 24°25′N and 16°35′N on the MAR exhibit higher p -values, 1.74±0.23 and 2.37±1.65, although the latter estimate is not well constrained because of the small number of aftershocks. Larger p -values along the ridge crest might reflect a hotter thermal regime in this setting. Additional monitoring, however, will be needed to determine if p -value differences between the ridge and transform sequences are robust. A 1999 sequence on the Endeavour segment of the JdFR, which has been correlated with changes in the hydrothermal system, is described poorly by the MOL model. The failure of the MOL model, the anomalously large number of earthquakes within the sequence and absence of a clearly dominant mainshock are inconsistent with aftershock activity and the simple tectonic origin that has been proposed previously for this sequence.

SeaBeam depth changes associated with recent lava flows, Coaxial Segment, Juan De Fuca Ridge: Evidence for multiple eruptions between 1981–1993

After a swarm of earthquakes was detected on the CoAxial segment of the Juan de Fuca Ridge in June-July 1993, the area was resurveyed with SeaBeam multibeam sonar to search for depth changes associated with a submarine volcanic eruption. Quantitative comparison of the 1993 SeaBeam survey with surveys in 1981/82 and 1991 shows one area of seafloor depth change (up to 29 m) between 1991–93 exactly where a pristine lava flow was discovered. In addition, two other depth anomalies (up to 37 m and 20 m) are identified between 1981–91, evidence that other recent eruptions have occurred along this spreading ridge segment.

Evidence of a recent magma dike intrusion at the slow spreading Lucky Strike segment, Mid‐Atlantic Ridge

[1] Mid-ocean ridge volcanic activity is the fundamental process for creation of ocean crust, yet the dynamics of magma emplacement along the slow spreading Mid-Atlantic Ridge (MAR) are largely unknown. We present acoustical, seismological, and biological evidence of a magmatic dike intrusion at the Lucky Strike segment, the first detected from the deeper sections (>1500 m) of the MAR. The dike caused the largest teleseismic earthquake swarm recorded at Lucky Strike in >20 years of seismic monitoring, and one of the largest ever recorded on the northern MAR. Hydrophone records indicate that the rate of earthquake activity decays in a nontectonic manner and that the onset of the swarm was accompanied by 30 min of broadband (>3 Hz) intrusion tremor, suggesting a volcanic origin. Two submersible investigations of high-temperature vents located at the summit of Lucky Strike Seamount 3 months and 1 year after the swarm showed a significant increase in microbial activity and diffuse venting. This magmatic episode may represent one form of volcanism along the MAR, where highly focused pockets of magma are intruded sporadically into the shallow ocean crust beneath long-lived, discrete volcanic structures recharging preexisting seafloor hydrothermal vents and ecosystems. INDEX TERMS: 3035 Marine Geology and Geophysics: Midocean ridge processes; 7280 Seismology: Volcano seismology (8419); 8149 Tectonophysics: Planetary tectonics (5475); 4259 Oceanography: General: Ocean acoustics; 9325 Information Related to Geographic Region: Atlantic Ocean; KEYWORDS: Mid-Atlantic Ridge, earthquake, hydroacoustic