Regional depth phase modeling is a useful technique to constrain earthquake focal depths for events within sparse networks. We demonstrate how regional depth phases can be used to obtain focal depths for hundreds of earthquakes with mN ≥2.8 that occurred from 1980 to 2004 in southern Ontario, western Quebec, and northern New York State. We discuss the development of regional depth phases with distance, their use in focal depth determinations by matching waveforms to synthetics, and possible sources of error. In general, focal depths can be determined to within about 3 km when there is at least one record at regional distances with clearly identifiable depth phases such as sPmP or sPg.

In southwestern Ontario and northern New York, focal depths range from 2 to 15 km. In areas of western Quebec and along the Ottawa River valley in Ontario, focal depths range from 2 to 25 km. More than half of the earthquakes in a cluster of activity near Maniwaki in western Quebec are deeper than 20 km. All earthquakes deeper than 20 km occurred north of about 45.3° N.

There is an intriguing tendency for earthquake depths to cluster near 8 km, 12 km, 15 km, and 22 km. Another interesting observation is that there are several earthquake pairs or small groups with very similar epicenters but quite different depths. The most active regions, in western Quebec and along the Ottawa River valley, tend to exhibit a relatively wide range of focal depths along broad zones that trend roughly northwest, suggesting that seismicity is occurring along diffuse northwest-trending faults throughout the crust. The wide range of focal depths may indicate the potential for a larger maximum scale of fault rupture and serve as a contemporary marker of locations where large events have occurred in the recent geologic past.

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