Earthquake-triggered slumps (1935 Timiskaming M6.2) in Lake Kipawa, Western Quebec Seismic Zone, Canada

Abstract

The Western Quebec Seismic Zone (WQSZ) of eastern North America is characterised by frequent moderate magnitude intracratonic earthquakes (e.g., 1732, M5.8; 1935, M6.2 and 1944, M5.2). The WQSZ is centered along the Timiskaming and Ottawa-Bonnechere grabens, which form part of a complex aulacogen (St.Lawrence Rift) within the Canadian Shield. The WQSZ includes the urban areas of Montreal and Ottawa but seismic risk analysis is challenged by short instrumental records and long recurrence intervals. The M6.2 1935 Timiskaming Earthquake is the largest recorded to date and was felt over some 1.3 million km 2 of eastern North America with many aftershocks of magnitude 4 to 5. Its epicenter lies below the western margin of Lake Kipawa, Quebec in the area where a major Proterozoic crustal boundary (the Grenville Front Tectonic Zone) crosses the Timiskaming Graben. A high-resolution ‘chirp’ seismic reflection survey of the lateglacial and postglacial sediment infill of Lake Kipawa reveals a clear record of recent ground shaking that is attributed to the 1935 earthquake. Widespread large slumps record down slope failure of the lateglacial and postglacial sediment fill indicating that the 1935 temblor was the largest in this area since deglaciation some 8000 years ago. Systematic mapping of landslides shows that they extend across an area of 600 km 2 around the earthquake's epicenter. Lakes cover a large area of eastern Canada; a regional-scale survey of lake floors could constrain historic epicenters and postglacial seismic history of the heavily populated WQSZ.