Daniel A. Walker

High-Frequency Pn Phases Observed in the Pacific at Great Distances

Earlier observations of a seismic waveguide in the northwestern Pacific with a velocity of 8.3 kilometers per second to distances of approximately 30� are complemented by suggestions of a possible waveguide with a velocity of 7.8 kilometers per second to distances well in excess of 30�.

Ridge event detection: T-phase signals from the Juan de Fuca spreading center

An analysis of T-phase source locations determined in the mid-1960s for an area of the northeast Pacific Ocean encompassing the Juan de Fuca spreading center reveals that most of the source locations are associated with regions where seamount chains intersect the spreading center and with edifices both along and near the spreading center. The T-phase source locations also tend to cluster on, or near, areas of the most concentrated and vigorous hydrothermal venting along the Juan de Fuca Ridge. Of the 58 T-phase source locations determined for a period from October 1964 through December 1966, only one was found to be associated with an earthquake detected by the National Geophysical Data Center/National Earthquake Information Service because of the characteristic small magnitude of spreading-center seismic events. Monitoring T-phase activity originating along the 80 000 km-long global seafloor spreading-center system offers a practical and unique opportunity to better understand the dynamics and oceanic effects of episodic spreading-center tectonic, volcanic, and hydrothermal processes.

Seismicity of the interiors of plates in the Pacific Basin

Historical listings of instrumentally recorded earthquakes raise serious questions about the validity and usefulness of the long-held presumption that the interiors of ocean plates are aseismic stable masses. Unusual distributions of epicenters within these regions may indicate stress patterns resulting from plate motions, or may indicate nascent subduction zones, ridge systems, or hot spots. Data recorded on ocean-bottom hydrophones and seismometers in an earlier investigation [Walker and McCreery, 1988] revealed 28 intraplate earthquakes in the deep interior of the Northwestern Pacific Basin that were unreported by the worldwide network of land-based seismic stations. The data sources included drum recordings of hydrophones near Wake and Enewetok islands in operation for different time intervals from 1963 through 1969, tape recordings from a 9-element 1500-km-long linear array of hydrophones in operation for 2 months in 1981, and tape recordings from September 1982 through 1985 of an 8-element hydrophone array near Wake Island.

Dynamic interaction of seismic activity along rising and sinking edges of plate boundaries

Abstract Cumulative and differential cumulative seismic strain release of shallow earthquakes during 1964 through 1972 show significant time-delayed correlation for many of the rises and/or sinks of tectonic plate boundaries for the Atlantic, Indian and Pacific oceans. For the Pacific, time delays are almost exclusively from areas of higher to areas of lower strain release, and could imply viscous stress relaxation along or near the lithosphere—asthenosphere boundary. Seismically determined slip rates for many of the subduction zones during 1964–1972 are lower than those of the preceding part of the century by a factor of 5. The significant correlation between many rises and sinks suggests worldwide interaction and coupling of plate motion.

Hydroacoustics detect submarine volcanism

On January 19, an unusual clustering of impulsive hydroacoustic signals (T-phases) was observed on recordings from SOFAR (Sound Fixing and Ranging) channel hydrophones located near the island of Oahu, Hawaii. Accompanying these randomly distributed T-phases was a continuously elevated level of background noise. The most intense activity began around 10 hours UT and lasted for about 1 hour (Figure 1). Since these characteristics are typical of submarine volcanism (see “Submarine Volcano,” Eos, November 7, 1989), a message was sent to the National Oceanic and Atmospheric Administrations VENTS Program (Pacific Marine Environmental Laboratory, Newport, Oreg.) advising of possible submarine volcanic activity.

Wake Island hydrophone seismic array

The hydrophone array installed near Wake Island (WHA) was reactivated as a seismic station in June 1979 by the Hawaii Institute of Geophysics. WHA consists of six bottom phones, at 5.5‐km depth, in an array 40 km across and an additional four pairs of phones, at large spacing, suspended at SOFAR depth. The bottom array lies in a flat area on high Q lithosphere > 108 years old. All of the deep and five of the SOFAR phones are operational. Until September 1982, generally data were recorded from only three phones, on a slow speed analog cassette recorder. The station is now being upgraded for digital recording of all 11 phones. Many circumpacific earthquakes and nuclear tests have been well recorded by WHA, its location being ideally suited for monitoring such events. Nearly pure oceanic paths between WHA and circumpacific sources facilitate studies of the ocean‐lithosphere waveguide such as propagation and attenuation of high‐frequency, teleseismic Pn and Sn. For frequencies above 3 Hz, WHA is quieter than ...

Yearly Seismic Energy Release: World Totals Versus Ridge System Totals

Yearly seismic energy totals for many different regions of the earth show highs in 1965 and lows in 1967. Correlations found between totals for ridge systems and for the world are attributed to ambient stresses, which are close to those needed for failure in the lithosphere underlying those ridge systems. Energy highs for many different plate edges are thought to be the result of triggering by the large Alaskan earthquake of 1964. Other suggestions of triggering by major earthquakes are found in 1969 and 1971.

Historical Gorda Ridge t-phase swarms : relationships to ridge structure and the tectonic and volcanic state of the ridge during 1964-1966

Abstract The U.S. Navy’s Sound Surveillance System (SOSUS) hydrophone arrays are extemely efficient receptors of a high-frequency earthquake energy phase known as the t(ertiary)-wave, or t-phase (Fox et al., 1994). After a nearly 30-year hiatus in such studies, SOSUS arrays are again being utilized to detect t-phases and to locate seismic and volcanic events occurring along the Gorda seafloor spreading center (Fox et al., 1995; Fox and Dziak, 1998). Earlier, Northrop et al. (1968) also used other military arrays to infer tectonic structure along the Gorda Ridge. From October 1964 through December 1966, over 600 low-magnitude earthquakes occurred along the Gorda Ridge. Nearly all of these events had magnitudes below the detection thresholds of land-based seismic networks. Northrop et al. (1968) interpreted the geographic distribution of these events as evidence for a nascent fracture zone near the midpoint of the ridge. In the present study, the spatial distributions of these older data and, for the first time, their temporal distributions as well, were examined with respect to detailed bathymetry of the ridge that was acquired in the early 1980s. This analysis, of 570 on-axis and 74 off-axis events, led to the following observations: (1) nearly all of the Gorda Ridge t-phase events occurred in discreet swarms centered about the ridge axis, (2) most of the events within each of 8 (of 9) observed swarms occurred mainly along single ridge segments, and, (3) reconfirming the earlier Northrop et al. (1968) conclusion, most of the events originated in the region of a major change in the strike of the ridge axis. During the 27-month interval that the ridge was observed, relatively few t-phase events took place along the northernmost segment of the Gorda Ridge where the 1996 eruption occurred. However, a unique sequence of small events which visually resemble the events associated with a Juan de Fuca Ridge eruption in 1993 (Fox et al., 1995) and a Gorda Ridge eruption in 1996 (Fox and Dziak, 1998) may have been associated with an eruption on the ridge during 1965.

Repeat of thirty‐year‐old experiment could prove or disprove global warming

Thirty years ago an important investigation was conducted that could be used today as a benchmark experiment for measuring long-term global temperature changes. In fact, such an approach could be the most cost-effective method for providing the scientific community with rapid and reliable evidence for or against global warming.

Long‐term 0.05–5 Hz ambient deep‐ocean noise, wind, and ocean waves.

Long‐term ambient ocean noise data from the Wake Island hydrophone array in the northwest Pacific are compared to wind measured at Wake and to ocean wave estimates near Wake from U.S. Navy models. Between 0.05 and 0.1 Hz, the hydrophone data are limited by system noise, although long‐period signals from moderate to large earthquakes often exceed that limit. From 0.1 to 0.2 Hz, the ambient noise correlates well with the estimated long‐period ocean swell, with about a 2.5:1 correspondence in their respective frequencies−close to the 2:1 correspondence predicted by nonlinear wave interaction theory. Between 0.2 and 0.3 Hz, the most energetic part of the noise spectrum, the correlation with ocean waves is weaker, suggesting that this noise may have its origin at a more distant location. Between 0.3 and 1 Hz, the noise again correlates strongly with the estimated ocean waves, although the frequency correspondence is 5:1 or greater. And from 1 to 5 Hz, the ambient noise correlates very strongly with windspeed a...