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

Report No. 6945
BBN Systems and Technologies Corporation
Fairweather faults, Lake Clark Fault (passing through Anchorage) and the
Denali Fault which is further to the north and trending westerly to the Bering
Sea. Figures 3.3 and 3.4, taken from Meyers et al. (1976) and Jacob (1986)
respectively, provide an indication of earthquake epicenter and important
volcano locations in Alaska. Earthquakes of Magnitudes 4-8.9 occurring
between 1899 to 1974 have been plotted, where each earthquake is represented
by a dot. The events are so numerous that the epicentral locations overlap on
the scale map. Biswas et al. (1986) performed -a seismicity study of Western
Alaska concentrating on the Northern Bering Sea and Chukchi Sea areas (Norton
Sound, Seward Peninsula and Kotzebue Sound); they demonstrated that many M > 4
events occur in that region as well. Twelve M = 5.6-7.3 earthquakes have
occurred there between 1928 and 1965.
Most of the Alaskan volcanoes or volcanic areas are shown in Fig. 3.4.
Many of these are or have been active in recent time. Coats (1950) stated
that at least 76 major volcanoes had been identified in the Aleutian Arc by
the time of his paper. Thirty-six of those had been active since 1760. There
appears to be about a 20-yr periodicity of volcanic activity in the Aleutians.
Eruptions are frequently explosive in nature. One of the most recent major
events involved the St. Augustine volcano in Cook Inlet, which had a ventclearing
explosive phase in March 1986. That volcano erupted previously in
1976. Pavlov Volcano near the Shumagin Islands has a past history of
activity, sometimes explosive, about every 10-15 years (Coats, 1950). In
1912, Novarupta on the Alaska Peninsula near Kodiak Island had the largest
volcanic eruption ever witnessed in the Gulf of Alaska region. As noted by
Jacob (1986), it was the world's largest eruption in this century and included
frequent explosive activity. In terms of volume of ejecta, the Mt. St. Helens
explosion in 1980 was ten times smaller than Novarupta. Seismic noise and, in
the case of coastal events, underwater sound, results from volcanic eruptions,
particularly those which are explosive. However, even without explosions,
broadband high level underwater sound results when lava flows are emitted from
the ocean floor or when they reach the ocean from land events are emitted from
the ocean floor or when they reach the ocean from land vents. Snodgrass and
Richards ( 1956) monitored sounds near a volcano in Mexico, where a lava flow
entered the ocean from the coast. About 600 m from this lava flow, high level
hissing and rumbling sounds dominated all other natural background, including
high surf noise, with most energy in the 100 to 700 Hz band. For comparison,
see Fig. 3.2 for typical surf noise sound level data.
3.2.5.2 Earthquake Magnitude and Ground Motion
Meyers et al. (1976) performed a detailed historical analysis of
earthquake activity in Alaska and concentrated on the boxed region shown in
Fig. 3.3. They tabulated earthquakes as a function of magnitude and
epicentral location and noted frequency of occurrence of events within a 75-km
radius of 1-degree latitude/longitude intersection intervals throughout the
boxed area and then plotted data to demonstrate trends. Figure 3.5 (from
their Figs. 12 and 13) shows cumulative magnitude-frequency curves for the
Shumagin and North Aleutian Basin areas. These data cover events in the
Magnitude (M) = 4 to 6.8 range with a regression fit curve allowing for