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

Report No. 6945
BBN Systems and Technologies Corporation
sound pressure level estimates for local earthquake events in the M = 6-7
magnitude range using that equation for a frequency of 10 Hz and the "trendff
curve for ground motion in Alaska given in Fig. 3.6. The top curve in Fig.
3.8 assumes 100% coupling of seismic energy into water and the lower curve of
the M = 6-7 envelope assumes 25% coupling. For 100% coupling, M = 6-7 earthquakes
in the Alaska subduction zone should generate r.m.s. overall sound
pressure levels from 240 dB down to 218 dB as distance from the event varies
from 10 to 100 krn, For 25% coupling efficiency, the sound levels will be
about 12 dB lower. The data points for the M = 6.7 Coalinga, California
earthquake were computed using the California ground motion trend curve in
Fig. 3.6. In the context of the seismic exposure contours shown in Fig. 3.7,
2
500 cm/sec peak acceleration at 10 Hz yields an r.m.s. sound pressure level
of 'about 239 dB for 100% coup1 'ng of ocean bottom acceleration to ocean
acoustic energy and 250 cm/sec2 would generate about 233 dB.
Sound pressure levels for lower magnitude events (e.g., M = 4) have been
estimated using an equation of Gutenberg's and reported by Richter (1958) for
equating seismic energy (E) of body waves to local earthquake'magnitude (ML):
ffLocal" earthquakes are those detected less than 9 degrees away, or less than
about 1000 km from the earthquake epicenter. The M = 6-7 event curves have
been scaled as 10 log of the ratio of the energy for M = 6.5 and M = 4 events.
Assuming that the Gutenberg scaling is valid for lower magnitudes, sound pressure
level for M = 2 events would be 35. dB below the M = 4 curves. For M = 4
events, the overall r.m.s. sound pressure level should be about 199 dB at 10
km from the source and about 177 dB at a distance of 100 km.
Based on the curves in Fig. 3.8 (and accepting the assumptions used in
deriving them) it is clear that local earthquake events occurring in Alaskan
coastal regions have the potential to cause very high level sounds at low
frequencies (e.g., 10 to 50 Hz). These sound levels would exceed those shown
for earthquakes in Fig. 3.2 (e.g., 40 dB or more higher when a M = 4 event is
about 50 km away from the receiver; compare the Milne 45-m curve in Fig. 3.2
with the Fig. 3.8 estimates. Recall, though, that these are short term events
(-30 seconds) which are relatively infrequent except during the few days
following a large earthquake when aftershocks can be expected.
3.2.5.5 Possible Gray Whale Response to Earthquake Noise
While the following account is anecdotal, it is included here as a
limited observation of implied cetacean behavior during earthquake events.
During the latter part of April and early-May 1983, BBN was performing a
field study regarding potential behavioral response of migrating gray whales
(the mother/calf pair phase of migration) to controlled playback of underwater
sound near Monterey, California. Details of that study were reported by Malme
et al. (1983). Shore-based observation of gray whale mother/calf pairs
migrating northward near and in the surf zone commenced on 16 April and
continued for 20 days until 5 May. The experiments were performed near the
beginning of the migration pulse and through the period of maximum passage of