Analysis and Ranking of the Acoustic Disturbance Potential of ...

Report No. 6945
BBN Systems and Technologies Corporation
small Magnitude 1.0 (or less) event in the Arctic that was measured under the
ice at a distance of 300 La (Keenan and Dyer, 1984). These spectra are the
only calibrated data (i.e., absolute levels) which could be located in this
brief study. However, as will be seen, the sound pressure levels shown can be
considered to be representative. Insofar as we know, all other published
underwater sound data related to earthquakes and volcanic activity were
uncalibrated and therefore could not be included in this comparison of sound
levels due to natural causes. Spectrum shape and bandwidth shown here are
consistent with those for the uncalibrated spectra, however.
The Milne curves for a T-Phase arrival lasting about 30 sec were obtained
simultaneously on two hydrophones, one located in the deep sound channel at
365 m depth and one at 45 m. One-third octave band sound levels of 134 dB and
130 dB re 1 uPa, respectively, were reported. An earthquake T-phase (tertiary
wave) is compressional wave energy which can propagate many thousands of
kilometers in the deep sound channel and usually originates at the continental
slope or mid-ocean ridge nearest an earthquake. The signature recorded at 45m
by Milne was, in comparison to the 365 m signature, lower in level and peaked
at a higher frequency (20 Hz vs. 10 Hz) since it represented acoustic energy
that had "leaked" out of the deep sound channel.
The under-ice event (also T-phase) was one of a series of small earttiquakes
located by Keenan and Dyer through triangulation and correlation
analysis to have occurred along the mid-Arctic ridge. This event was measured
300 km west of the ridge and about 320 km north of Greenland with an under-ice
hydrophone array. The duration of the Arctic event was about 1-minute and it
generated 1/3 octave band levels of 112-115 dB at about 10 Hz. These curves
demonstrate that earthquakes can cause high levels of low frequency sound in
the ocean.
The following discussion provides information on the seismicity, earthquake
magnitudes, estimates of frequency of occurrence and estimates of
overall sound pressure level which can be expected for typical earthquakes in
the Gulf of Alaska and Bering Sea regions.
3.2.5.1 Seismicity
Earthquake and Volcanic Activity
The seismicity of Alaska has been reported in detail by many authors,
notably Meyers ( 1976), Meyers et al. (1976), Biswas et al. ( 1986), Jacob
(1986), Sykes (1971), Davies et al. (1981), and Taber and Beavan (1986). In a
concise treatment of seismicity, tectonics and geohazards of'the Gulf of
Alaska region, Jacob ( 1986) states that the Pacific Plate moves against and
under the North American Plate in a north-northwest direction at a rate of
about 5 cm/yr. In Alaska, the plate moves under the Chugach-St. Elias
mountains, Prince-William Sound, Cook Inlet and the Alaska Peninsula generating
subduction forces and heat which result in plate fracturing and volcanic
activity. A high rate of occurrence of earthquakes results along the subduction
zone at the rim of the Gulf of Alaska and out along the Aleutians as well
as in some inland areas and in the Bering Sea coastal regions. Major fault
zones have been generated: the Aleutian Trough, ~hugach-st. Elias and