Ⓔ Intraplate Seismicity of a Recently Deglaciated Shield Terrane: A Case Study from Northern Ontario, Canada by Shutian Ma, David W. Eaton, and John Adams Abstract The economy of northern Ontario, Canada, is heavily dependent on mining, so accurate knowledge of seismicity is important for the safe design and op- eration of mines and other critical facilities, including a proposed underground repos- itory for nuclear waste. In this study, we analyzed 537 cataloged earthquakes that occurred from 1980 to 2006. Seismicity is mainly concentrated in topographically elevated Archean terranes northwest of Lake Superior and in the James Bay and Ka- puskasing regions. We analyzed waveforms to determine the focal depth for 331 re- corded events, using the regional depth-phase modeling (RDPM) method coupled with surface-wave relative-amplitude analysis. The majority of events are shallow (< 6 km) and concentrated in areas of relatively high elevation (> 350 m), although in the east- ern part this pattern breaks down and some deeper earthquakes (> 12 km) are ob- served. Based on a moving-window event-counting technique, we show that distinct spatial clusters of seismicity can be delineated that are statistically significant relative to background seismicity levels. A particularly active cluster is located within James Bay, where focal depths range from a few kilometers to more than 20 km. Another cluster near Kapuskasing contains deep-focus events and may occur along a hot spot track that runs through western Quebec. Near Dryden, a shallow (∼1 km) earthquake swarm concentrated in a 1 × 1 km region commenced in May 2002, faded, and then started up again in February 2003. Shallow mining-induced events are also common around Sudbury, a major world center for nickel mining. The overall pattern of seis- micity appears to correlate with upper-mantle P-wave velocity anomalies, suggesting that lateral variations in mantle rheology may play a significant role in controlling intraplate seismicity of shield areas. It is also likely that crustal stresses caused by glacial isostatic adjustment are an important factor, although the correlation of seis- micity with uplift rate is not as clear. Online Material: Focal depth solutions for earthquakes in northern Ontario, Canada. Introduction Northern Ontario (Canada), a region extending north- ward from the Great Lakes to Hudson Bay, covers an area of ∼1:0 × 10 6 km 2 in the continental interior of North Amer- ica. The physiography of most of northern Ontario exem- plifies a recently deglaciated shield terrane, with numerous lakes, extensive exposures of crystalline bedrock, and glacial till deposits of variable thickness. Although not as seismi- cally active as other parts of eastern North America, there is, nevertheless, a significant level of intraplate seismicity (Fig. 1). Accurate knowledge of the distribution and rates of regional seismicity are important for the safe design and operation of numerous underground mines, dams, and other critical facilities, including a proposed underground reposi- tory for nuclear waste (Fenton et al., 2006). Wetmiller and Cajka (1989) described the seismic activ- ity of northern Ontario, mainly for the period from 1983 to 1987 subsequent to installation of the northern Ontario seis- mograph network for the Canadian Nuclear Fuel Waste Man- agement Program. The majority of the earthquakes that they located in northern Ontario (1983–1988) fell within a broad zone in the eastern part of our study region (Fig. 1), suggest- ing an extension into northeastern Ontario of more active tectonics from the western Quebec seismic zone (WQSZ). Near-surface stress measurements (mostly in mines) and earthquake focal mechanisms show that much of the region is subject to high horizontal stress, oriented predominantly in the northeast–southwest azimuth (Wetmiller and Cajka, 1989; Adams and Bell, 1991; Adams, 1995). 2828 Bulletin of the Seismological Society of America, Vol. 98, No. 6, pp. 2828–2848, December 2008, doi: 10.1785/0120080134