Rifle-like sounds produced by ice-wedge cracking, although seldom heard, have been reported in the permafrost literature for a century. The sounds, similar to those produced by the rapid cracking of lake ice, indicate a high speed of crack propagation. The speed of ice-wedge crack propagation has been measured at two field sites for the 1966-1989 period. No evidence of rapid crack propagation has been detected at these sites. The fracture patterns of ribs, as seen on vertical crack faces, and the numerous short interacting cracks, as seen in plan view, suggest slow rates of cracking. Enquiries made of many northern residents indicate that rifle-like sounds are uncommon along the mainland coast from the Alaska–Yukon boundary eastward to Coppermine, N.W.T. but are heard more frequently in the Arctic islands. High speeds of crack propagation appear associated with sharp drops in temperature, windblown sites, and long, straight ice wedges. As a preliminary estimate, crack propagation rates exceeding about 200 m/s are necessary to produce rifle-like sounds and accompanying ground tremors.

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