Relocation and analysis of the 2007 Nechako, B.C., seismic swarm: evidence for magmatic intrusion in the lower crust

On October 9th, 2007, a seismic swarm, known as the Nechako swarm, began in south-central British Columbia, approximately 20 kilometers west of the Nazko polygenetic cinder cone. After lasting for well over a month, seismic activity tapered off by November 21st, 2007. This study analyzes data from several temporary broadband seismometers deployed by the Geological Survey of Canada near the epicentral locations of initial events from the swarm. Over 4400 events were observed during this period, from which 1048 absolute locations were calculated, with depths ranging from 26-35 kilometers. All of the events recorded by the temporary seismometers were high frequency, volcano-tectonic earthquakes. A previous study by members of the Geological Survey of Canada reported a b-value (the slope of the magnitude-frequency relationship) of 1.9, indicating magmatic activity as the source for seismic unrest. Algorithmic double-difference programs HypoDD and TomoDD allowed for precise relocations of earthquake multiplets (earthquakes with similar waveforms) from the swarm, suggesting two distinct spatial and temporal pulses of seismic activity. The first pulse recorded by the temporary seismometers began on Oct. 21st, migrating southeast at a rate of 0.44 km/day from 26.5-28.3 km deep, until Oct. 29th. The second pulse began on Oct. 29th at a depth of 29-31 km, approximately 3 km to the southeast of the first pulse. No clear migration of events between the areas could be observed. On Nov. 2nd the first region of activity resumed seismic unrest. Both regions remained active for the remainder of the swarm. Distinct waveforms and hypocenters from spasmodic bursts (rapidly occurring events with overlapping waveforms) and earthquake multiplet clusters lend further credence to the simultaneous rupturing of the two regions. The proximity of high-frequency volcano-tectonic events to the crust-mantle boundary (approximately 30 km deep), the presence of spasmodic bursts, high b-value, and two distinct regions of simultaneous seismic activity provide strong evidence that the Nechako swarm was generated by the expansion and propagation of magma in the lower crust. Inverted, nearly identical waveforms are interpreted as originating from the brittle fracturing of solidified magma plugs, driven by the force of magma injection along a dike. From the evidence provided, I have concluded that the two spatially distinct regions of activity are representative of two large sills at the base of the crust, emplaced by crustal underplating, with branching dikes. The swarm was initiated by the brittle failure and fracturing of rock in the lower crust around these regions by either buoyantly rising magma in preexisting sills/dikes, or an injection of new magma from a mantle source