Group A, Poster #109, Earthquake Geology

Possible recent fault scarp near Montréal - candidate fault for the 1732 M5.8 Earthquake

Aube Gourdeau, Veronica B. Prush, & Christie D. Rowe
Poster Image: 

Poster Presentation

2022 SCEC Annual Meeting, Poster #109, SCEC Contribution #12505 VIEW PDF
Montréal falls within the Western Quebec Seismic Zone (WQSZ). The WQSZ is a region of elevated but poorly defined earthquake risk with several historic, damaging earthquakes. In 1732, a M5.8 earthquake caused significant damage in the Montréal area (Leblanc, 1981). Given the relatively large magnitude of this earthquake and other historic, larger events, we anticipate that there should be surficial fault scarps in southern Quebec (Leblanc, 1981, Brooks and Adams, 2002). However, despite these large earthquakes, no studies to date have identified active faults in the region. Due to the lack of identified surface ruptures, there are currently no earthquake hazard scenarios in use in the WQSZ.< />
In 2019, the Québec government released lidar-derived digital elevation models (DEMs) that cover most of the WQSZ and two major urban centers (Montréal and Québec City). Using remote mapping techniques, we have identified several potential fault scarps offsetting ~12 ka glaciomarine sediments (Globensky, 1987). We identified an outstanding possible fault scarp ~45 km NE of Montreal near the town of St. Liguori. Given the proximity of this scarp to the city of Montreal, it is a priority structure for further investigation. The expression of the scarp was assessed in the field. The scarp observed in remote sensing imagery delineates a contact that juxtaposes upthrown Ordovician limestone bedrock on the south side of the scarp against unconsolidated clayey sands to the north. We dug a 1 m-deep pit into the sediment on the north side of the scarp. The pit reveals apparent drag folds consistent with south-up dip-slip motion. A ground penetrating radar survey in February 2022 revealed a steeply north dipping contact between the limestone and sediments, consistent with north-side down normal faulting.

Continental stress field models predict subhorizontal maximum compressive stress following ice sheet retreat (Wu, Johnston and Lambeck, 1999). In order to test the viability of this model and to inform future regional deformation/seismicity studies, we are planning a paleoseismic trench excavation at the St-Liguori site to confirm throw, dip direction, characterize deformation patterns around the fault, and establish fault activity rates.