140 Seismological Research Letters Volume 81, Number 1 January/February 2010 doi: 10.1785/gssrl.81.1.140 ABSTRACT Earthquake-induced liquefaction features have been found in Holocene and Late Wisconsin sediments exposed along three rivers in the Charlevoix seismic zone. On the basis of their stratigraphic position and radiocarbon age constraints, the liquefaction features are thought to have formed during three or more earthquake episodes centered in Charlevoix during the past 10,000 years, including at least two prehistoric epi- sodes approximately 5,000 and 10,000 years ago. he spatial distribution of liquefaction features coupled with liquefaction potential analysis suggests that the Charlevoix earthquakes were of moment magnitude (M) ≥ 6.2. Liquefaction features have not been found in similar sediments exposed along eight rivers in the Quebec City–Trois Rivieres area, 70 to 150 km from Charlevoix in the St. Lawrence River Valley. he appar- ent absence of liquefaction features in the Quebec City–Trois Rivieres area suggests that few, if any, large earthquakes have occurred here during the same time period. he geologic record of earthquakes may be incomplete in both areas due to luctua- tions in Holocene sea level. Nevertheless, the rate of large earth- quakes has apparently been much higher in the Charlevoix seismic zone than in adjacent areas of the St. Lawrence for thousands of years. hese indings suggest that seismicity is localized in Charlevoix and that the presence of Iapetan ri t faults that underlie the St. Lawrence Valley of southeastern Canada may not, in itself, indicate earthquake potential. hese results may have important implications for other Iapetan ri t faults in the eastern United States, as well as seismic source zone characterization and hazard assessment throughout east- ern North America. INTRODUCTION he Charlevoix seismic zone was the source of three histori- cal earthquakes of M (moment magnitude) > 6 dating back to the 1660s and is spatially associated with Iapetan rit faults and with the Devonian-age meteor impact crater (Figures 1 and 2). Charlevoix seismicity has been attributed to reactiva- tion of regional Iapetan faults in a uniform continent-wide stress ield ( e.g., Adams and Basham 1989) and, alternatively, to stress concentration by the impact crater ( e.g., Lamontagne et al. 2000). Recent numerical modeling suggests that ri t faults might channel stress into the weak impact structure, increasing the potential for earthquakes (Baird et al. 2008). A key issue in the assessment of seismic hazard is the extent to which large earthquakes are localized by certain types of geologic structures. Most modern seismic hazard assessments assume that the likelihood of future events may be smoothed over groups of similar geologic structures, which are used to delineate seismic source zones (EERI Committee on Seismic Risk 1989; McGuire 1995, 2004). Within the source zones, earthquakes are generally assumed to occur randomly in space and time (though the condition of a uniform spatial distribu- tion is not required). It is thus important to identify and char- acterize the geologic structures that make up these zones of uniform hazard. A large body of work suggests that large earthquakes in stable cratonic regions around the globe occur preferentially within extended margin areas ( e.g., Sykes 1978; Johnston and Kanter 1990) that were faulted and weakened by previous major tectonic events. In ENA, this includes Iapetan ri ted margin areas ( e.g., Adams and Basham 1989; Wheeler and Johnston 1992; Wheeler 1995), which experienced deep-seated ri t- related faulting during tectonic opening of the Iapetus Ocean, about 500 million years ago. Most of the damaging histori- cal earthquakes of ENA have occurred within Iapetan ri ted areas, including the M ≥ 7.5 1811–12 New Madrid, Missouri, earthquakes; the M 7.3 1886 Charleston, South Carolina, earthquake; the M 6.2 1925 Charlevoix, Quebec, earthquake; the 1935 M 6.2 Timiskaming, Quebec, earthquake; the M 5.8 1944 Cornwall, Ontario, earthquake; and the 1988 M 5.8 Saguenay, Quebec, earthquake (Adams and Basham 1989; Lamontagne et al. 2000; Atkinson and Boore 2006). Given Localization of Large Earthquakes in the Charlevoix Seismic Zone, Quebec, Canada, during the Past 10,000 Years Martitia P. Tuttle and Gail M. Atkinson Martitia P. Tuttle M. Tuttle & Associates Gail M. Atkinson University of Western Ontario