Natural Resources Canada Ressources naturelles Canada EARTHQUAKE REPORT GEOLOGICAL SURVEY OF CANADA SIDNEY, B.C. Tel. (250) 363-6500 Fax (250) 363-6565 Date: Wed, 12 Mar 2003 15:32:10 -0800 (PST) EARTHQUAKE RESEARCH NOTE: SCIENTISTS MONITOR FAULT SLIP DEEP BENEATH JUAN DE FUCA STRAIT Starting at 1:00 am PST on Wednesday February 26, the initiation of deep seismic tremor activity was detected beneath Juan de Fuca Strait on seismographs of both the Geological Survey of Canada and the University of Washington seismic networks, signalling the onset of a slip event on a portion of the Cascadia subduction fault. The daily analysis of Global Position System (GPS) data from regional continuously operating sites of the Western Canada Deformation Array (WCDA) and the Pacific Northwest Geodetic Array (PANGA)confirmed the initiation of small systematic surface displacements which characterize these deep slips. Twenty temporary seismographs and three temporary GPS installations have been deployed on southern Vancouver Island to augment existing network coverage and improve our monitoring of both seismic and geodetic aspects of this newly discovered phenomena. Our present understanding of these slip events suggests that each slip incrementally adds stress across the locked portion of the subduction thrust fault, bringing it closer to failure. Further research is required to determine whether these very small stress increments play a significant role in triggering great earthquakes, and to determine the effectiveness of tremors in identifying, in real time, the occurrence of slip. Information on deep slip events: http://www.pgc.nrcan.gc.ca/geodyn/docs/slip/slip_info_1.html contact Dr. Herb Dragert or Dr. Garry Rogers Government of Canada / Gouvernement du Canada