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

Report No. 6945
BBN Systems and Technologies Corporation
approximate extrapolation to lower magnitude events. Based on these curves,
M = 6.8 or greater events can be expected to occur in either Shumagin or North
Aleutian Basin once every 50 years (0.02 earthquakes per year). The-curves
predict one to two M = 4 or greater events per year and about ten M = 3 or
greater events per year.
In studying data from many events in.California, Alaska and Japan, Jacob
(1986) demonstrated that the subduction zone thrust events in Alaska and Japan
tend to cause higher acceleration ground motion than similar magnitude events
in California which occur in a strike-slip zone. The trend curves in Fig. 3.6
(adapted from Fig. 6-30 in Jacobs, 1986), which have been added to this
figure, do not represent a regression fit. They have been included to summarize
Jacob's observations and for use in estimating underwater sound levels
which could result from such events. Those estimates are provided below.
3.2.5.3 Seismic Exposure
. Woodward-Clyde Consultants (1982) published a two volume report for the
National Oceanic and Atmospheric Administration in which they developed a
model for predicting ground motion due to earthquakes along the Gulf of Alaska
coast. They developed a seismic exposure software package incorporating three
other programs relating to (1) seismicity .of the region, (2) ground motion
levels for probability of exceedance, and (3) a contour plotting routine. The
results of such an analysis are shown as a contour map of peak acceleration of
ground motion in a selected region which can be expected from seismic events.
Jacob (1986) used their method to compute a seismic-exposure map of the
Shumagin Island region. Figure 3.7, taken from Jacob's report (his Fig. 6-
32), shows peak acceleration ground motion contours having a 67% probability
of non-exceedance within the 40 year period of 1982-2022. That figure
represents a modification to the original Woodward-Clyde model, allowing for
an update of the seismic attenuation law used for subduction zone sources.
3.2.5.4 Estimates of Underwater Sound Due to Earthquakes
Since the only absolute sound pressure level data due to earthquakes
known by the authors are those shown in Fig. 3.2, it is worthwhile to estimate
sound pressure levels based on given ground motion data and a series of
assumptions. Figures 3.6 and 3.7 provide an indication of typical peak
acceleration ground motion which can be expected for M = 6 to 6.8 earthquakes
in the Alaskan subduction zone. Urick (1983) makes a calculation of sound
pressure level in the ocean due to seismic noise vibrating the ocean bottom
using the algorithm:
where p is pressure (dynes/cm2), f = frequency (Hz), p = density of sea water
(g/cm3) and a = ocean bottom displacement amplitude (cm/sec) . He demonstrates
that using this method, typical seismic background noise or microseismic
r.m.s. displacement amplitudes of the ocean bottom can cause sound pressure
levels consistent with those that are observed frequently at frequencies below
1 Hz (microseismic noise peaks at about 1/7 Hz). He assumes that 100% of the
seismic energy is transferred into sound. Figure 3.8 provides overall r.m.s.