[1] Univ Alberta, Dept Phys, Edmonton, AB T6G 2E1, Canada
[2] Univ Alberta, Dept Earth & Atmospher Sci, Edmonton, AB T6G 2E3, Canada
[3] Univ Washington, Dept Mech Engn, Seattle, WA 98195 USA
[4] Univ Calif Berkeley, Dept Mech Engn, Berkeley, CA 94720 USA
[5] Delft Univ Technol, Fac Civil Engn & Geosci, NL-2628 CD Delft, Netherlands
[6] Univ Oxford, Dept Engn Sci, Oxford OX1 3PJ, England