Following every earthquake felt in eastern North America, journalists ask the eternal question, “Which fault is responsible for this earthquake?” In intraplate environments,such as the Charlevoix Seismic Zone (CSZ), Quebec, Canada, a definite answer seldom exists. Seismologists face major difficulties: most earthquakes occur at mid-crustal depths, the Precambrian basement where most earthquakes occur is geologically complex, and fault locations are poorly known.
Fortunately, seismologists receive help from other geoscientific fields and exploration techniques, most notably remote sensing, seismic methods, and potential fields. The integration of these geophysical tools is helping us to better understand the earthquake-fault connection in the CSZ, which is eastern Canada's most seismically active zone. Most microearthquakes— the type that occur frequently in the CSZ—occur within highly fractured blocks bounded by regional geological faults. Our interpretation is different from the common assumption that earthquakes, independent of their size, occur along regional faults.