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TitleThe 2007 Nazko, British Columbia, earthquake sequence: Injection of magma deep in the crust beneath the Anahim Volcanic Belt
AuthorCassidy, J F; Balfour, N; Hickson, C; Kao, H; White, R; Caplan-Auerbach, J; Mazzotti, S; Rogers, G C; Al-Khoubbi, I; Bird, A L; Esteban, L; Kelman, M; Hutchinson, J; McCormack, D
SourceBulletin of the Seismological Society of America vol. 101, no. 4, 2011 p. 1734-1741,
Alt SeriesEarth Sciences Sector, Contribution Series 20090362
Mediapaper; on-line; digital
File formatpdf
ProvinceBritish Columbia
NTS82L; 82M; 92I; 92J; 92K; 92L; 92M; 92N; 92O; 92P; 93A; 93B; 93C; 93D; 93E; 93F; 93G; 93H; 93I; 93J; 93K; 93L
Lat/Long WENS-128.0000 -118.0000 55.0000 50.0000
Subjectsigneous and metamorphic petrology; tectonics; geophysics; earthquakes; earthquake studies; earthquake risk; earthquake mechanisms; p waves; s waves; seismic waves; crustal studies; crustal movements; seismicity; Anahim volcanic belt; Nechako Basin
Illustrationslocation maps; plots; histograms
ProgramTargeted Hazard Assessments in Western Canada, Public Safety Geoscience
Released2011 08 01
AbstractOn 9 October 2007, an unusual sequence of earthquakes began in central British Columbia about 20 km west of the Nazko cone, the most recent (circa 7200 yr) volcanic center in the Anahim volcanic belt. Within 25 hr, eight earthquakes of magnitude 2.3 - 2.9 occurred in a region where no earthquakes had previously been recorded. During the next three weeks, more than 800 microearthquakes were located (and many more detected), most at a depth of 25 - 31 km and within a radius of about 5 km. After about two months, almost all activity ceased. The clear P- and S-wave arrivals indicated that these were high-frequency (volcanic-tectonic) earthquakes and the b value of 1.9 that we calculated is anomalous for crustal earthquakes but consistent with volcanic-related events. Analysis of receiver functions at a station immediately above the seismicity indicated a Moho near 30 km depth. Precise relocation of the seismicity using a double-difference method suggested a horizontal migration at the rate of about 0:5 km=d, with almost all events within the lowermost crust. Neither harmonic tremor nor long-period events were observed; however, some spasmodic bursts were recorded and determined to be colocated with the earthquake hypocenters. These observations are all very similar to a deep earthquake sequence recorded beneath Lake Tahoe, California, in 2003 - 2004. Based on these remarkable similarities, we interpret the Nazko sequence as an indication of an injection of magma into the lower crust beneath the Anahim volcanic belt. This magma injection fractures rock, producing high-frequency, volcanic-tectonic earthquakes and spasmodic bursts.