The Prince William Sound cluster is named for the sound on the eastern
side of the Kenai Peninsula of southern Alaska, but it extends well
north of the sound, through the Chugach Mountains and south into the
Gulf of Alaska. The epicenter of the 9.2 Mw “Good Friday” earthquake on
March 28 (March 27 local time), 1964 is on the northern shore of the
sound. The cluster includes the mainshock and most of the significant
aftershocks of the eastern part of the 1964 sequence, east of ~148°W.
The full aftershock zone extends westward to ~154°W. Until the mid-1980s
there were very few seismograph stations in the area; the nearest
station to the 1964 mainshock was station COL at a distance of more than
300 km. Since the 1990s the density of stations has increased
dramatically. Selection of earthquakes for the cluster reflected the
time-dependence in station density. For all dates, events were only
selected if they had a minimum of 30 arrival time observations in the
ISC Bulletin, but for events after 1989 a minimum magnitude of 3.5 was
also imposed to keep the number of events within a manageable limit.
Events were only retained if some form of depth constraint was
available. For events prior to late 1966 this was almost exclusively
from teleseismic depth phases. For subsequent events the preferred
source of depth constraint for most events is near-source and
local-distance readings but teleseismic depth phases are available for
many of the more recent events and they were analyzed for validation.
Approximately 1/3 of the events were observed only to regional
distances; they were retained partly to provide stronger statistical
power for the location calibration, and also because of their
significance for seismotectonic investigations. In addition to
containing more earthquakes (596) than any GCCEL cluster to date, the
Prince William Sound cluster is one of the largest in geographical
terms, approximately 180 x 280 km. Moreover it extends over the
transition from inland Alaska, through the Prince William Sound, to the
continental shelf. The use of a single crustal model for the entire
region is certainly optimistic and it is likely that epicenters near the
perimeter are subject to greater uncertainty than the formal error
estimate.
