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I
-1977, RelImlnary documentat!on lor a FORTRAN
pragram to compute gravity terrain corrections
based on topography digitized an a geogrephio
grid: U.S. Geologtcal Survey Open-File Report
77-535, 45 p.
?hM, Edward, Bonini, W. E., Ostenso, N. A., and
Woollard, G. P., 1958, Gravity measurements in
Alaska: Woods Hole Oceanographic Institution
Reports, Reference 58-54, 104 p.
Bg -he D. S&@WIS, C. hhr, and Robert
A. Pase
During the past year, analysh of seismio data
from the network in southern Alaska has focused on
ahallow seismicity in three areas: the Yakataga
selsmic gap along the northeastern Gulf of Alaska, the
southern Kenai Peninsula, and the active volcanoes
west of Cook Inlet. A summary of the results for each
of these areas Is presented below.
Y.AKATAG.4 SEISMIC GAP
Continued monitoring of the seismicity In and
near the Yakataga seismic gap has revealed some
interesting variations in the rates of ectivity during
the past 2 years. Figure 3 shows the epicenters of
earthquakes that occurred between Ootober 1, 1978,
and September 14, 1981. As noted in earlier reports
(for example, Stephens and others, 198l), the spatial
pattern of the seismicity is remarkably stable. Most
of the activity occurs at or near the perimeter of the
Yakataga gap, as defined by McCann and others (1980)
and Lahr and others (19801, and is dominated by after
shmks from the 1979 St. Elias earthquake. Figure 4
plots the rate of activity as a function of time for
various subregions of figure 3. The most striking
featme in these curves is a twofold to threefold in-
crease in the montNy number of located events that
began about October 1980 for the Waxell Ridge,
Copper River delta, St. Elias, and Wrangell sub-
regions. These elevated levels of activlty continued
for about 6 to 8 months and then returned to new
hose observed during the earlier period. The in-
reased level of activlty within the aftershock zone of
he 1979 St. EUas earthquakes appears to be superim-
posed on a long-term decay in the rate of aftershock
activity that approximates the expected inverse time
decay for aftershwks. The ofhhore subregion under-
went a gradual increase beginning about June lQ8O and
returned to normal in November 1980. A review of the
data-processing procedures and station-operation his-
tory for the period since October 1979 suggests that
although these factors may introduce some apparent
changes In activity rate, they are not likely sources of
systematic errors wNch could account for the ob-
served long-term variations in seismidty rates. The
seismicity rates for the subregions of northern Prloce
WUam Sound and Yakutat Bay were also reviewed,
but actual changes in the rates of sebmialty cannot be
reliably established owing to the systematic biases
known to exist. For Prince Willtam Sound, a change in
the timing criteria to reduce the number of small
events to be located coincides with an apparent sharp
decrease in activity beginning In October 1980. Jn the
Yakutat Bay subregion, a reduction in the number of
operating stattons may have (ntroduced the apparent
decrease in seismicity beglnnlng In August 1980.
These observations suggest that a perturbation
occurred in the reglonal stress field in and mound the
Yakataga seismic gap. The possible significance of
thia change in seismicity as a precursor to a gap-fllllng
earthquake la being weighed.
The Waxell Ridge subregion Is the most sehrnie
ally active area within the Yaketaga seismic gap, as
defined in figure 3. Whether this shallow seismicity
occum at the thrust interface between the Paclflc
plate and the overrfding North American plate, which
is thought to underlle this subregion at a depth of 10 to
20 km, or on faults within the overlying plate is uncer-
tain, prharlly because the focal depths of events
located in thts mea are poorly constrained. Orellmi-
nary results from a study of selected events in the
Waxell Ridge subregion suggest that the earthquakes
occur at depths shallower than 20 fnn. Although the
coverage of the focal sphere for Fwave flrst motions
is inadequate to distinguish between di-p or strke
slip motion for the earthquake meohanisms, the orIen-
tations of the axe3 of maximum compressive stress are
constrained to be northwest-soutbeastw&d. This
result is compatble with the direction of convergence
between the North American and Pacific plates
inferred from adjacent arm along this part of the
plate boundary (for example, Peree and Jacob, 1980).
SOUTHERN KBNAI PENINSULA
Using data from the Bradley Lake network
(northeast of Homer) for December 1980 through July
1981, a preliminary review was made of the firstmotion
data for cmatal earthquakes that occurred
beneath the southern Kenai Peninsula. This review
was made in conjunction with Woodward-Clyde Consultants,
who are under contract with the U.S. Army
Corps of Engineers to assefcs the seismic hazards In the
region of a prop3ed hydroelectrio project. Compwlte
focal mechdsrna were made for a few of the clusters
of earthquakes that occurred near the mapped surface
traces of maJor faults. nese focal mechanisms ware
clearly consistent with normal faulting, notwithstnnding
the ordnary systematic errors In properly locating
the h t arrivals on the focal sphere due to uncertdnties
in the velocity structure. The orientation of the
principal tension axls was constrained to be within
about 20' of east-west. This result, in a region of
northwestdlreoted subduction, was nclt expected. One
possible explanation Is that the crust is 1- in
tension now, since the 1964 earthquake, but that the
stress pattern wU1 change to northwestsoutheastwardoriented
comprersion In the future as stresses build up
before another large Wst earthquake.
WESTERN COOK INLET VOLCANOES
With partid support from the U.S. Geological
Surveyls Volcano Hazards Program,' sehlaity in the
vicinity of three actlve volcanoes-spurr, Redoubt and
IUamaa--weat af Cook Met waa examined in detall for