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P
1 57 KILOMETERS
Figure 3.-Epicenters of 7,298 earth~akes in the Gulf stippled curve, Yekataga seismlc gap. Lettered afees
of Alaska and southern AIaskaregion between October correspond to subreglong in figure 4, as foLlows: 0,
1, 2979, and September 30, 1981. Three symbol sizes offshore; CRD, Copper River delta; WR, Waxell Ridge;
(small to large1 correspond to earthquakes of mag- SE, St. Elias; W, Wrangell; YB, Yakutat Bay; PWS,
nitudes less then 2 (dot), 2.0-3.9 (small cross), and northern Prince William Sound. Stars denote locations
greater than 3.9 barge crass). Solid curve rupture of Quaternary volcanoes. Depth contours in fathoms.
zone of 1979 St. EUe earthq~ae (&3.1fi dashed
the interval October l9gO through June lQ8l. Loca-
tions were determined for aU shallow-focus earth-
quakes with four or more g and S phases recosded at
three or more stations. During iJlis interval, no pto-
nounced swarms of shallow earthquakes were observed
within 20 km of any of Ute three volcanic centers.
Epicenters of five shallow shocks, rartging in depth
from 0 to 10 km and in magnitude from O to 1.0, were
located within 3 km of the summit of Spurr. In addi-
tion, a cluster of 14 events scattered over time was
located 10 to 20 km south of the summit. Spurr, which
Is the closest volcano to the Anahorage metroQolitm
area, lest erupted in 1958 and deposited about 5 mm of
ash in Anchorage. On September 9,1979, a swarm of
47 events with magnitudes &s high as 0.7 occurred near
the summit of Spurr in an 11-how interval; no similar
activity was observed during the recent 9-month inter-
vd. A total of 10 earthquakes shailower than 20 km
were bcated in a diffuse pattern within 20 krn of
Redoubt, the largest af which was of magnitude 2.0.
Ihe most recent eruptions of Redoubt consisted of a
series of explosive ash eruptions in 1966-69. Fewer
shceks were located in the vicinity of Iliamns
Volcano--two events within 5 km of the volcano
summit but no others within 20 km. lliamna has been
quiescent over the past 3 decades.
REFERENCES CITED
Lfllrr, J. C., Stephens, C. D., Hasegawa, d. S., Boat-
wright, John, and Plafker, George, 1980, Alaska
seismic gap only partially filled by 28 February
1979 earthquake; Science, v. 207, no. 4437, p.
1351-1353,