Seismic Refraction Measurements in Southeast Alaska and Western British Columbia

Abstract

Crustal thickness determinations were made from seismic refraction measurements along two profiles exLending north-northwest along the Inside Passage of Southeast Alaska and western British Columbia. The source for the seismic waves was a series of 10 explosions detonated by personnel of the Earth Physics Branch. 1)epartment of Energy, Mines and Resources of Canada. Arrivals recorded in Southeast Alaska from explosions in Bird Lake on Graham Island indicate a 26 km thick crust over a mantle with a P,, velocity of 7.9 km/s. Measurements of P,, wave attenuation suggest that the uppermost mantle has a (;! value of 650 z t 120. Arrivals recorded along the mainland coast of British Columbia from explosions in Ripley Bay indicate a 30 km thick crust over a mantle with a PI, velocity of 8.1 km/s. On the basis of compiled gravity and seismic refraction data, the crust of the lnside Passage region from Queen Charlotte Sound to Cross Sound is believed to be anomalously thin. Crustal thickening from ocean to continent appears to occur in a stepwise, rather than continuous, fashion in the vicinity of the Inside Passage. --L'dpaisseur de la croilte a ttd CvaluCe par des mcsures obtenues par rkfraction stismiyue selon deux profiles vers le NNW, suivant Inside Passage au sud-est de ]'Alaska et ri l'ouest de la Colombie-Britannique. Une skrie de 10 explosions prkparkes par le personnel de la section Physique de la 'Terre, dkpartement de l'Energie, des Mines el dcs Kessources du Canada, crbaient les ondes siismiques. Ides arrivkes enregistrees clans le sud-est de 1'Alaska 2 partir d'explosions au lac Bird sur 1'Ile Graham, indiyuelat une kpaisseur de 26 km pour la croi~te recouvrant le manteau, avec une vitesl;e PI, de 7.9 km/s. Les mesures de l'atttnuation des ondes PI, sugg6rent une valeur Q de 650 2 120 pour la partie supCrieure du manteau. Les arrivdes enregistrkes lc long de la cate de la Colombie-Rritannique et provoyilCes par les explosions B la Baie Ripley, montrent une epaisseur de la croilte de 30 km au-dessus du manteau et avec une vitesse PI, de 8.

Reconnaissance engineering geology of Sitka and vicinity, Alaska, with emphasis on evaluation of earthquake and other geologic hazards

“…Microearthquakes are not shown on figure 11 because of a lack of detection owing to their very small size. However, they have been reported from central and southern areas of southeastern Alaska by Tobin and Sykes (1968, p. 3839), Johnson (1972), and Rogers (1972Rogers ( , 1973. Most microearthquakes probably are the result of very small movements along faults .at depth; Rogers (1973) believes that many of them are caused by movements of glaciers near tidewater.…”

“…11). In addition, no local microearthquakes from the Chatham Strait area were recorded during a microearthquake-detection study in July 1970 (Johnson, 1972;Johnson and others, 1972) nor during a total of about a year of intermittent recordings, from 1968 through 1971, by the Canada Geological Survey (Rogers, 1972).…”

Reconnaissance engineering geology of the Metlakatla area, Annette Island, Alaska, with emphasis on evaluation of earthquakes and other geologic hazards

“…7). In addition, no local microearthquakes were recorded (1) during a detection study in July 1970 (Johnson, 1971;Johnson and others, 1972), (2) during a total of about 12 months of intermittent study (1968)(1969)(1970)(1971) by the Seismological Service of Canada (Rogers, 1972(Rogers, , 1973(Rogers, , 1976a, nor (3) during the August 1972 study by Page and Gawthrop (1973). The only known indication of possible Holocene movement is along the south end of the Chatham Strait fault west of Coronation Island ( fig.…”