Robert P. Dziak

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.

Earthquake-induced changes in a hydrothermal system on the Juan de Fuca mid-ocean ridge

Hydrothermal vents on mid-ocean ridges of the northeast Pacific Ocean are known to respond to seismic disturbances, with observed changes in vent temperature. But these disturbances resulted from submarine volcanic activity; until now, there have been no observations of the response of a vent system to non-magmatic, tectonic events. Here we report measurements of hydrothermal vent temperature from several vents on the Juan de Fuca ridge in June 1999, before, during and after an earthquake swarm of apparent tectonic origin. Vent fluid temperatures began to rise 4–11 days after the first earthquake. Following this initial increase, the vent temperatures oscillated for about a month before settling down to higher values. We also observed a tenfold increase in fluid output from the hydrothermal system over a period of at least 80 days, extending along the entire ridge segment. Such a large, segment-wide thermal response to relatively modest tectonic activity is surprising, and raises questions about the sources of excess heat and fluid, and the possible effect on vent biological communities.

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.

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.

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.

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

Hydroacoustic detection of volcanic activity on the Gorda Ridge, February–March 1996

Abstract Beginning at 0700 GMT on 28 February 1996, intense seismicity was detected in the northeast Pacific Ocean using the T-phase Monitoring System developed by NOAA/PMEL to access the U.S. Navy’s SOund SUrveillance System (SOSUS) in the North Pacific. The event was preliminarily located on the northernmost segment of the Gorda Ridge near 42.67°N and 126.8°W, in the vicinity of the ridge segment high (“narrowgate”). The nature of the seismicity was similar to that observed in June 1993 at the CoAxial segment of the Juan de Fuca Ridge, which was later documented to be a lateral magma injection with subsequent eruption. Due to several gaps in the data, the detection information was not as comprehensive as during the CoAxial event, but an initial migration of epicenters from the narrowgate area down rift is inferred based on arrival bearings from a single array; there is evidence for an additional diking event on the second and third day of activity. There is also indication of a concentration of epicenters located near 42.6°N, as occurred during the CoAxial episode at what was later determined to be an eruption site. Examination of T-wave rise times generally supports this interpretation. Based on the nature and duration of the activity, a response effort was initiated, which later confirmed hot-water plumes and fresh lava flows at the site. Based on both hydroacoustic information and field observations, it is proposed that the episode began with a lateral dike injection, possibly with eruptive activity in the summit region, followed by multiple magma pulses and eventual focusing of the seismic activity and extrusion near 42.6′N.

Sounds from airguns and fin whales recorded in the mid-Atlantic Ocean, 1999–2009

Between 1999 and 2009, autonomous hydrophones were deployed to monitor seismic activity from 16° N to 50° N along the Mid-Atlantic Ridge. These data were examined for airgun sounds produced during offshore surveys for oil and gas deposits, as well as the 20 Hz pulse sounds from fin whales, which may be masked by airgun noise. An automatic detection algorithm was used to identify airgun sound patterns, and fin whale calling levels were summarized via long-term spectral analysis. Both airgun and fin whale sounds were recorded at all sites. Fin whale calling rates were higher at sites north of 32° N, increased during the late summer and fall months at all sites, and peaked during the winter months, a time when airgun noise was often prevalent. Seismic survey vessels were acoustically located off the coasts of three major areas: Newfoundland, northeast Brazil, and Senegal and Mauritania in West Africa. In some cases, airgun sounds were recorded almost 4000 km from the survey vessel in areas that are likely occupied by fin whales, and at some locations airgun sounds were recorded more than 80% days/month for more than 12 consecutive months.

Spectra and magnitudes of T‐waves from the 1993 earthquake swarm on the Juan de Fuca Ridge

A swarm of earthquakes on the crest of the Juan de Fuca Ridge was detected in June and July 1993 by a network of hydrophones. The activity migrated 60 km along the crest, suggesting a lateral dike injection and the possibility of a volcanic eruption. Subsequent geologic and oceanographic investigations confirmed that an eruption had taken place. Examination of the individual acoustic arrivals shows changes in the character of the signal that are consistent with an injection of magma. A reduction in the rise time of the wave packet and a proportional increase in high frequency energy was observed and is interpreted to result from a shoaling of the earthquake source region. Second, the source magnitudes were largest at the onset of the swarm and became smaller over time, also consistent with shoaling of the dike. The appearance of the T-wave arrivals changed significantly 5 days after the beginning of the swarm, potentially indicating the onset of a surface eruption.