A Mw 3.1 earthquake occurred in Lake Ontario along the United States–Canada border, about 30 km south from Port Hope, Ontario, Canada, on 4 August 2004. Despite its small size, the shock was very well recorded by broadband seismographic stations deployed in recent years in Ontario, Canada, and in New York State. More than 40 broadband stations at local and regional ranges provided high-quality digital data. Waveform data analysis constrained the source at a depth of 4 (±2) km, which places the shock in the shallow Precambrian basement beneath Paleozoic platform deposits. Th e source mechanism from the regional waveform inversion for the double-couple moment tensor is predominantly strike-slip faulting. A NS striking (8°) nodal plane dipping to the east (dip = 59°) is the likely fault plane which represents right-lateral strike-slip motion. Th e subhorizontal P-axis orientation (trend = 234° and plunge = 12°) is consistent with the maximum horizontal compressional stress (SHmax) direction in eastern North America. Although the 4 August 2004 event is a small shock and has the seismic moment of M0 = 4.45 (±2.30) × 1013 Nm, it is the largest instrumentally recorded earthquake that has occurred in Lake Ontario. This and other significant earthquakes in the region suggest a broad-scale strike-slip faulting stress regime with a shallow seismogenic layer in the Erie–Ontario Lowlands region. The shallow focal depths of earthquakes in the region increase the risk of higher ground shaking compared to other seismic zones in northeastern North America with a deeper seismogenic layer.