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

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Report No. 6945 BBN Systems and Technologies Corporation H. Meyers et al. (19761 Figure 3.3. Alaskan Earthquake Epicenters 1786- 1974 (Approximately 10,000 Events). 180 tm 150 . Quaternart volcano . \ \ 8 , \ w , nu - k GJJ OJ A U .52 K. H. Jacob (19861 1Srpum1 Hcrbrrl 180 170 mo 150 140 Figure 3.4. Major Volcanoes and Volcanic Centers in Alaska.
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.
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