REPEATING EARTHQUAKES RECORDED DURING A PERIOD OF SEISMIC UNREST NEAR NAZKO CONE, BRITISH COLUMBIA

The recent 2007-2008 seismic swarm near Nazko cone in central BC, Canada, was the first recorded seismicity in this region. Previous work by the Geological Survey of Canada (GSC) using a regional network indicates that these earthquakes originated ~30 km west of Nazko cone, at a depth of 25-35 km. Over 1,000 earthquakes were observed within three weeks of Oct. 20th, with reported magnitudes between 0 and 3 Because the seismometers were > 25 km from the activity, however, hypocentral locations were poorly resolved. High b-values and the occurrence of spasmodic bursts suggest that the swarm resulted from a magmatic source. It is unresolved whether the quakes are due to brittle failure of rock at the tip of a dike and/or by the activation of nearby faults from changes in the local stress regime by the expansion and movement of nearby magma (i.e. distal VT events). Five temporary broadband seismometers were deployed by the GSC from Sept., 2007 until June, 2008 in response to the seismic swarm near the previously located hypocenters. Examination of this data reveals a significantly larger number of earthquakes; for example, 597 earthquakes were observed within just one six hour period. Repeating earthquakes occur in response to nearly identical stresses and strains within a localized area, bearing similar frequency, amplitude, and form to one another. After picking an initial, well-resolved, earthquake, a search was conducted through the data spanning from Sept., 2007 to June, 2008 to identify event multiplets with a correlation of at least 0.7. Two different sets of repeating earthquakes have been identified based on the above described method, with one set numbering 153 earthquakes, occurring from Oct. 29^thto Nov. 1^st, 2007. The second multiplet has a total of 4428 earthquakes, occurring between Sept. 25^th, 2007 and June 12^th, 2008. The sequences of repeating earthquakes were cross-correlated in order to determine changes in the evolution of the system. Cross-correlation matrices generated for the repeating earthquake sequences show initially similar earthquakes that subtly diverge in similarity over time, decreasing from correlation coefficients near 1 to values nearer 0.5 indicating changes within the system that provide insight into the source mechanism of the Nazko swarm.