abstract

Low-level monitoring in the St. Elias region between September 1978 and March 1981 has identified two principal zones of seismicity that are consistent with the historical pattern of seismicity at higher magnitude levels. The most active zone follows the plate boundary along the coast of southeast Alaska. A recurrence relation based on completeness over varying time intervals is log(NM) = −0.74M + 3.77. A less active zone is observed generally following the Duke River and Dalton segments of the Denali fault system through the southwest Yukon. Instead of continuing into Chatham Strait, this trend of seismicity turns south into the Glacier Bay region to connect with the Fairweather fault north of Cross Sound. The seismicity and high rate of uplift observed in the Glacier Bay region is probably the result of convergence across the Fairweather fault north of Cross Sound. The recurrence relation for the Denali zone is log(NM) = −0.90M + 3.77. A similiar recurrence curve is found for activity with magnitudes down to about 2.0 observed over the 39-month low-level monitoring program. The rate of activity in the Denali zone appeared to decrease for a period of about 1 yr following the 28 February 1979, M 7.2 St. Elias earthquake. The rate of aftershock activity was also found to vary, with the b value decreasing throughout the sequence. Only minor seismicity is observed northeast of the Denali fault system in the southwest Yukon and east of Chatham Strait in northwest British Columbia. A prominent cluster east of Juneau near the British Columbia border may be related to volcanic activity.

A 5-week microearthquake survey over a 40-km segment of the Denali fault system found activity in a highly faulted zone about 15 km wide at depths of less than about 15 km. A composite P-nodal solution indicates motion on either east-west or north-south striking planes, neither of which are consistent with the general trend of the Denali system in the area. An average slip rate of no more than about 1 mm/yr is estimated from the seismicity for the Denali fault system through the southwest Yukon. An estimate of the minimum present accumulated elastic strain suggests the potential for a magnitude 612 earthquake.

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