Long‐runout rockslide on glacier at Tsar Mountain, Canadian Rocky Mountains: potential triggers, seismic and glaciological implications

A rhomboidal shaped rockslide with a length of 2·2 km and an area of 1·35 km2 covers 30% of a small Clemenceau Icefield Group cirque glacier. By means of remote sensing, airborne photography, climate and seismic data analysis, the extent, timing, possible trigger mechanisms, and rockslide effects on the glacier are discussed. An ML 2·0 seismic event on 14 September 2000 represents the rockslide and has a spindle‐shaped waveform and high energy in the low frequencies. Based on a strong signal duration of 56–108 seconds the rockslide had an average runout velocity of 22–45 m s−1. Stress fatigue and a combination of rain and snow followed by rapid melt and night‐time freezing could have co‐triggered the slide. Additional seismic events with epicentres <40 km from the cirque were recorded 26 times since 1985 and range between ML 0·9–3·3, but none were caused by, or triggered, large mass movement. With a rock source volume of 1·6 × 106 m3 and a deposited volume of 2–4 × 106 m3 the sum of bulking and entrainment is between 25% and 150%. Five melt seasons post‐event, levees 20–30 m wide tower 30 m over the surrounding glacier. Rockslide characteristics, and its Fahrböschung of 15° [height/length (H/L) = 0·28], indicate that this rockslide is quite typical for long‐runout rockslides on glaciers. However, the basal friction coefficient was higher than found on other glaciers and could relate to the presence of medial moraines transverse to the flowline and to pre‐slide debris‐cover. The difference between the coefficient of friction in the main flowline (0·11–0·14) and that in a lobe confined between two medial moraines (0·065) suggests that confinement increases mobility by 70–115%. Apart from reduced ablation under the rock debris, effects of the rockslide on the glacier were minimal.