An unusual series of seismic events has been observed routinely on networks operated by the Geological Survey of Canada in Sidney, British Columbia, and the Alaska Earthquake Information Center, Fairbanks, Alaska. Events are concentrated in a cluster near Mt. Ogden, about 50 km southeast of Juneau, and have a seasonal cycle of recurrence. A study of earthquake activity indicates that the annual cycle has two pulses, one beginning in May and increasing into June and July, and the second beginning in late August and increasing into October. Events are largely absent during winter months. In the 17-year period between 1975 and 1991, a cumulative total of 164 events with ML ∼ 3 has occurred in the month of October, contrasted with none in March or April.
Epicenters for the events appear to lie near the Speel River, a northeast-southwest-oriented stream that drains glaciated areas of Mt. Ogden. Although the regional tectonic setting holds many candidates for seismic sources, including an active transform plate margin and recent volcanic activity, the periodic behavior of the cluster is not easily reconciled with these phenomena. In addition, the frequency of occurrence and magnitudes of the events are not consistent with patterns of glacier-generated seismicity. Examination of recent seismic activity yields a b-value of 1.0.
Statistical comparisons of earthquake activity near Mt. Ogden with cycles of seasonal phenomena support a hydrologically related causal mechanism. These apparent correlations raise the possibility that the earthquakes are triggered either by transient changes in pore pressures due to the influx of meteoric water within faulted rock near Mt. Ogden or by increased pore pressures due to surface loading of glaciated areas.