Charlevoix Seismic Zone Research Articles

Assessing earthquake rates and b-value given spatiotemporal variation in catalog completeness: Application to Atlantic Canada

Spatiotemporal variations in the magnitude of completeness Mc make it challenging to confidently assess seismic hazard or even to simply compare earthquake rates between regions. In this study, we introduce new techniques to correct for heterogeneous Mc in a treatment of the eastern and Atlantic Canada earthquake catalog (1985--2022). We first introduce new methodology to predict Mc(x,t) based on the distribution of seismometers. Second, we introduce a modified maximum-likelihood estimator (MLE) for b (the b-value) that accounts for spatiotemporal Mc variation, allowing the inclusion of more earthquakes. Third, we compute the ratio of detected/predicted M>1 earthquakes as a function of Mc and apply it to create a calibrated M>1 event-rate map. The resulting map has advantages over a moment-rate map, which is effectively sensitive only to the very largest earthquakes in the dataset. The new MLE results in a modestly more precise b when applied to the Charlevoix Seismic Zone, and a substantial increase in precision when applied to the full Atlantic Canada region. It may prove useful in future hazard assessments, particularly of regions with highly heterogeneous Mc and relatively sparse catalogs.

Depth‐Dependent Crustal Stress Rotation and Strength Variation in the Charlevoix Seismic Zone (CSZ), Québec, Canada

AbstractIntraplate tectonic stress fields are complex due to the imprint of a long geological history. Here we use a new data set of earthquake focal mechanism solutions and relocated events to investigate the relationship between regional stress, crustal strength, and seismicity in the Charlevoix Seismic Zone (CSZ), the most active seismic zone in eastern Canada. Our stress inversion shows that SHmax gradually rotates clockwise from approximately St. Lawrence River‐parallel (∼54°) near the surface to river‐perpendicular (∼120°) in the lower crust, as postglacial rebound stress becomes increasingly dominant at greater depth. The stress rotation occurs primarily between ∼13 and ∼26 km depth, where glacial rebound induced stress perturbation is further amplified by a “weaker” middle crust. Finally, depth‐dependent b‐values confirm the rheological difference between upper and middle crust in the CSZ.

Crustal Velocity Variations and Constraints on Material Properties in the Charlevoix Seismic Zone, Eastern Canada

AbstractCrustal velocity variation within impact‐related seismic zones is commonly attributed to factors such as the presence of dense fracture networks, fluid pressure change, and compositional variations. In this study, we combine seismic tomography, rock physics analysis, and gravity anomaly modeling to quantitatively investigate the mechanisms that influence crustal velocity heterogeneities in the Charlevoix Seismic Zone (CSZ), the most active seismic zone in eastern Canada. Earthquakes in the CSZ align in two broad NE‐SW trending clusters, possibly due to the reactivation of St. Lawrence paleorift faults. Within the meteorite impact structure, earthquakes are diffusely distributed, and ubiquitous lower velocity volumes have been attributed to crustal damage from tectonic inheritance exacerbated by the meteorite impact. The Bouguer gravity anomaly contrast across the St. Lawrence River is due to density disparities between the Grenville Province on the north shore and the Appalachians on the south shore. We find a higher velocity region northeast of the impact structure that does not exhibit an observable gravity anomaly, which suggests the co‐existence of rock types of comparable densities but different elastic moduli (e.g., anorthosite and charnockite). Outside the impact structure, compositional variations control velocity changes, whereas inside the impact structure, velocity variations can be explained by porosity enhancement of up to 10% by low (0.1) aspect‐ratio cracks. Our results suggest that intense fracturing and compositional alteration, rather than pore pressure change, have the primary control on seismic velocity variations, and consequently, earthquake processes in the CSZ.

Earthquake Depths, Focal Mechanisms, and Stress in the Lower St. Lawrence Seismic Zone

AbstractWe examine earthquake hypocenters, focal mechanisms, and the state of tectonic stress in the Lower St. Lawrence Seismic Zone (LSZ), a paleorift zone in eastern Canada. The largest earthquake recorded in the region is the 1999 Côte–Nord MN 5.1, which was followed by ∼80 aftershocks of MN>1. It is not known if the region is capable of producing hazardous Mw>6 earthquakes, similar to the Charlevoix Seismic Zone ∼250 km upriver. Focusing on 2015–2020, we apply a machine-learning-based phase picker to detect 72 earthquakes in addition to the 150 catalog earthquakes in the same region over this time span. We produce an updated 1D, gradient velocity model via a Monte Carlo search using a uniform VP/VS=1.77, which we computed with the Wadati method. We refine hypocenter estimates using the triple-difference method, with sP depth phases as additional constraints on earthquake depth. We estimate focal mechanisms for >100 earthquakes with automatically picked P-wave first motions and absolute value P-SV-SH amplitude ratios, and we use the focal mechanisms to invert for the state of tectonic stress. Grid searches and Bayesian analysis allow for robust uncertainty estimates of focal mechanisms, which in turn allow for uncertainty estimates of the stress tensor. The recovered west-northwest–east-southeast σ1 is consistent with previous estimates and with a stress tensor controlled by glacial isostatic adjustment, although a contrast between deep and shallow focal mechanisms suggests that these stresses may be concentrated in the lower crust. Epicenter lineations up to ∼40 km long may be indicative of sizable faults in the LSZ capable of generating Mw>6 earthquakes, but hypocenter and focal mechanism uncertainties are too high to say so definitively, thus pointing to a need for denser station coverage, including ocean-bottom seismometers.

Hunting for Quaternary Faults in Eastern Canada: A Critical Appraisal of Two Potential Candidates

Abstract This study documents two potential neotectonic features in the seismically active St. Lawrence estuary and western part of the Gulf of St. Lawrence of Quebec, Canada. Historically, the region is the locus of series of damaging earthquakes, including the 1663 M 7 earthquake, which suggests the occurrence of coseismic surface ruptures beneath the St. Lawrence River. In the western Gulf of St. Lawrence (Lower St. Lawrence seismic zone), a potential fault scarp identified on a vintage seismic profile has been investigated through high-resolution seismic and multibeam bathymetry data. On the seafloor, the scarp corresponds to an ∼1.8 m high (maximum) feature that is located above a buried escarpment of the Paleozoic bedrock. Holocene units are draping over the escarpment on one profile, but are possibly cut on two others. The scarp meets several of the criteria generally associated with neotectonic features. However, a close look at the data indicates that the staircase geometry of the top of the bedrock and its expression at the surface is linked, at least partially, with the presence of an erosion-resistant unit. This makes a neotectonic reactivation possible but not proven. In the Tadoussac area, ∼40 km north of the Charlevoix seismic zone, the offshore extension of the St. Laurent fault corresponds to an ∼110 m high bathymetric escarpment with well-preserved triangular facets. Such “fresh” morphology is unique in the St. Lawrence River Estuary and may attest to Quaternary displacements, yet other interpretations may also explain the unusual preservation of the escarpment. These two case studies illustrate the difficulty to unambiguously document Holocene fault scarps, even in the marine domain in which the sedimentary succession is generally continuous.

Effects of Preexisting Structures on the Seismicity of the Charlevoix Seismic Zone

AbstractThe Charlevoix Seismic Zone (CSZ) is located along the early Paleozoic St. Lawrence rift zone in southeastern Quebec at the location of a major Devonian impact structure. The impact structure superimposed major, steeply dipping basement faults trending approximately N35°E. Approximately 250 earthquakes are recorded each year and are concentrated within and beneath the impact structure. Most M4+ earthquakes associated with the rift faults occurred outside the impact structure. Apart from the unique distribution of earthquakes, stress inversion of focal mechanisms shows stress rotations within the CSZ, and in the CSZ relative to the stress orientation determined from borehole breakouts. The primary goal of this research is to investigate the combined effects of the preexisting structures and regional stresses on earthquake activity and stress rotations in the CSZ. We approach this using PyLith, a finite‐element code for simulations of crustal deformation. Adopting the results from recent hypocenter relocation and 3‐D tomography studies, we modify the locations and dips of the rift faults and assess the effect of the new fault geometries on stress distributions. We also discuss the effects of resolved velocity anomalies. We find that the observed stress rotation is due to the combined effect of the rift faults and the impact structure. One‐dimensional velocity models of the CSZ with an embedded impact structure and a combination of 65°‐40°‐40° and constant 70° fault dip models with a very low friction coefficient of 0.3 and cohesion of 0 MPa can explain the observed seismicity and more than 50% of the stress rotations.

Earthquake Stress Drop in the Charlevoix Seismic Zone, Eastern Canada

AbstractStress drop scaling relations of earthquakes in intraplate seismic zones are less well constrained, partly due to less dense instrumentation and lower seismicity rate. Here we use new data to estimate the static stress drop values of earthquakes in the Charlevoix Seismic Zone in eastern Canada (MN < 4.5), June 2012 to July 2017. We first perform double‐difference relocation to obtain the hypocentral distribution of 518 events, which highlights a diffuse distribution within the Charlevoix Seismic Zone, probably related to the highly fractured crust by a Devonian meteorite impact. Using spectral ratios, we obtain stress drop values of 47 events ranging from ~2 to 200 MPa, typical of intraplate earthquakes, and observe an invariant stress drop scaling in the magnitude range Mw 2.2–3.8. Events within the impact structure have higher stress drops than those outside, which may indicate differences of fault maturity of the St. Lawrence paleorift system and the presence of a distributed fracture network.

Velocity models and hypocenter relocations for the Charlevoix Seismic Zone

AbstractThree‐dimensional P and S wave velocity (Vp and Vs) models and hypocenter relocations are determined for the Charlevoix Seismic Zone (CSZ) using local arrival time tomography. The CSZ is located along the St. Lawrence River about 100 km downstream from Quebec City, Canada, and is one of the most active seismic zones in eastern North America. Earthquakes occur in Grenvillian basement rocks affected by Iapetan Ocean rifting and a large Devonian meteor impact. Low Vp and Vs are associated with the impact structure to a depth of 12 km. Relocated hypocenters form a semicircle delineating the eastern margin of the impact structure. Northeast of the impact, hypocenters cluster into two groups separated by a region of high Vp and Vs velocity. Hypocenters within each group align along planar surfaces, suggesting the presence of distinct seismogenic faults. The planes trend NE, in the same direction as the Iapetan rift faults, and dip to the SE. One plane is located beneath the north shore of the St. Lawrence River and extends to a depth of at least 30 km. Two other planes with shallower dips are located beneath the river and extend to depths of 12 and 15 km. All three planes are disrupted within the impact zone but can be followed, suggesting partial preservation of rift‐parallel fault fabric.

Seismicity along St. Lawrence Paleorift Faults Overprinted by a Meteorite Impact Structure in Charlevoix, Québec, Eastern Canada

Abstract The Charlevoix seismic zone (CSZ) in Quebec has the highest number of earthquakes in eastern Canada, with cataloged events diffusely distributed between the St. Lawrence paleorift faults. In this study, we relocate 1242 out of 1639 earthquakes in the CSZ from 1988 to 2010 using double‐difference relocations with phase arrivals from manual picks and waveform cross correlations and requiring a strict set of quality control criteria. Our results show a diffuse distribution of earthquakes beneath the St. Lawrence River within a meteorite impact structure and clear lineations of event hypocenters outside the inner boundary of the impact structure. The diffuse seismicity distribution coupled with a lower cumulative seismic moment within the impact structure, despite a higher number of events, suggests the presence of highly fractured crust and weakened pre‐existing faults. A lack of M N ≥4.0 earthquakes and a wide variation of focal mechanism P‐axis orientations also provide evidence for weakened pre‐existing faults. Relocated earthquakes beneath the north shore of the St. Lawrence River illuminate a broad southeast‐trending linear structure, suggesting the geometry of the Gouffre northwest River fault. A paucity of seismicity between the north shore and the river may correspond to fault‐controlled basins of unconsolidated sediments at the surface and volumes of high‐seismic‐velocity material at midcrustal depths. Online Material: Earthquake catalogs.

Isolated cases of remote dynamic triggering in Canada detected using cataloged earthquakes combined with a matched‐filter approach

AbstractHere we search for dynamically triggered earthquakes in Canada following global main shocks between 2004 and 2014 with MS > 6, depth < 100 km, and estimated peak ground velocity > 0.2 cm/s. We use the Natural Resources Canada (NRCan) earthquake catalog to calculate β statistical values in 1° × 1° bins in 10 day windows before and after the main shocks. The statistical analysis suggests that triggering may occur near Vancouver Island, along the border of the Yukon and Northwest Territories, in western Alberta, western Ontario, and the Charlevoix seismic zone. We also search for triggering in Alberta where denser seismic station coverage renders regional earthquake catalogs with lower completeness thresholds. We find remote triggering in Alberta associated with three main shocks using a matched‐filter approach on continuous waveform data. The increased number of local earthquakes following the passage of main shock surface waves suggests local faults may be in a critically stressed state.

A Closed‐Form Solution for Earthquake Location in a Homogeneous Half‐Space Based on the Bancroft GPS Location Algorithm

The traditional approach to both earthquake and Global Positioning System (GPS) location problems in a homogeneous half-space produces a nonlinear relationship between a set of known positions, seismic stations or GPS satellites, and an unknown point, an earthquake hypocenter or GPS receiver. Linearization, followed by an iterative inversion, is typically used to solve both problems. Although sources and receivers are swapped in the earthquake and GPS location problems, the obser- vation equation is the same for both, due to the principle of reciprocity. Consequently, the mechanical part of the solution of the equations is the same and single-step closed- form solutions for the GPS location problem, such as the Bancroft algorithm, can also be used to solve for earthquake hypocenters in a homogeneous half-space. This article applies the Bancroft algorithm to synthetic and real data for the Charlevoix seismic zone and compares the location of ∼1200 events estimated with both the Bancroft algorithm and HYPOINVERSE. The Bancroft algorithm shows quantifiable improve- ments in accuracy compared with traditional methods. We also show how tools com- monly used by the GPS community, such as the geometric dilution of precision, can be used to better estimate the precision of the results obtained by a seismic network.

Correlation between Coulomb Stress Changes Imparted by Historic Earthquakes and Current Seismicity in Charlevoix Seismic Zone, Eastern Canada

ABSTRACT The Charlevoix seismic zone (CSZ) is the most seismically active region in eastern Canada and has experienced several large historic events, such as the 1663 M 7 event (in which M is moment magnitude), as well as ongoing low‐level activity. Recent statistical studies (Fereidoni and Atkinson, 2012, 2014) suggest the contemporary seismicity in Charlevoix cannot be reconciled as part of a long aftershock sequence from the 1663 event. However, this does not eliminate the possibility that current seismicity in Charlevoix might still be influenced by stress changes related to the 1663 event. In this article, we investigate the correlation between the location of contemporary seismicity in the region and the static stress changes imparted by the 1663 earthquake. We model the 1663 earthquake as a primarily thrust event on a southeast‐dipping fault, which would have produced regions of increased stress that coincide with areas where current microseismicity is concentrated. With our assumed rupture model, we find that ∼75%–80% of the current seismicity (from 1978 to the present) is located in the area of positive Coulomb stress changes. The relatively good correlation between the models of static stress changes and the seismicity pattern observed in Charlevoix may suggest the background seismic activity in the region is still influenced by the stress perturbations due to the 1663 shock.

Geomorphological characteristics and variability of Holocene mass-transport complexes, St. Lawrence River Estuary, Canada

Recently acquired multibeam bathymetry data are used to investigate seafloor instability features along a 310km-long segment of the St. Lawrence River Estuary. The analysis of this dataset indicates that submarine slides occur over a much larger area than previously recognized and that Holocene sediments are reworked by mass-transport along significant portions of both the northwest and southeast margins of the Laurentian Channel. In the surveyed area, 96 individual mass-transport complexes (MTCs) were identified representing 13% of the seabed. MTCs vary in area from less than 1km2 to more than 40km2 and exhibit various geomorphological signatures. Qualitative observation reveals an apparent disparity between MTCs that remain coherent and those that disintegrate during downslope transport evolving into a blocky morphological signature. For all MTCs, morphological parameters have been measured (area, length, and height) or calculated (slope and roughness). This quantitative analysis provides a unique opportunity to study these parameters in a statistically significant and homogeneous dataset located in a relatively small area that experienced a similar Quaternary history.In many cases, mass transport events appear to initiate in the vicinity of steep bedrock walls located along some segments of the estuary. The timing of mass-transport events was not constrained during this study. However, the fact that the region hosts the Charlevoix seismic zone, the most tectonically active area in eastern Canada, strongly suggests that earthquakes acted as a trigger for submarine landsliding.

Research Activities in Precise Positioning at Laval University

This review paper summarises the recent, on-going, and future research activities in precise positioning at Laval University. The projects undertaken by the GPS and Geodesy Research Group are presented following three main research topics. The first one deals with the use of precise positioning for deformation monitoring of engineering structures. The study of ice forces on dams using a robotic total station and the improvement of GPS height determination using multiple GPS antennas linked to a single receiver with calibrated fiber optics are presented. A second topic of research is the crustal deformation of the Charlevoix seismic zone in Quebec. This study uses GPS and levelling techniques and involves an investigation of the temporal stability of the GPS permanent stations available in the surrounding area. Finally, research on single GPS receiver positioning techniques is presented, namely, Precise Point Positioning (PPP) and Time Relative Positioning (TRP).

A New Analysis of the Magnitude of the February 1663 Earthquake at Charlevoix, Quebec

Abstract This paper presents a new and comprehensive analysis of the magnitude of the 1663 Charlevoix, Quebec, earthquake. Based on a modified Mercalli intensity scale (MMI) of about VI from reports of damage to chimneys and a masonry wall in Roxbury and Boston, Massachusetts, the best estimate of the moment magnitude of this earthquake is M 7.3 to 7.9 from MMI attenuation relations. Using ground-motion attenuation relations and the threshold for chimney and masonry damage from fragility curves for seventeenth- and eighteenth-century dwellings in Boston, the magnitude of the 1663 earthquake is at least M 6.9 if the damage occurred on very soft soil conditions and is M 7.3 to M 7.7 if the damage took place on firm soil or bedrock. Both the best estimate of a length of 73 km for the most active section of the Charlevoix Seismic Zone and an estimated fault area of 73 km×25 km for the 1663 earthquake are consistent with an earthquake of M 7.3 to 7.6 based on scaling relationships. Also, in the 1811–1812 New Madrid earthquake sequence only the 7 February 1812 shock, the largest of the sequence, caused chimney damage beyond about 600 km. The 7 February New Madrid event is thought to have taken place on the Reelfoot fault, for which the length of the recent earthquake activity and of the suspected 1812 rupture is less than that of the active seismicity zone at Charlevoix. These observations suggest that the 1663 Charlevoix earthquake was approximately comparable in magnitude to or even larger than the largest of the 1811–1812 New Madrid earthquakes and therefore was at least M 6.8. When put together, the several lines of analysis in this study indicate that the best estimate of the size of the 1663 earthquake is M 7.5±0.45.

Relationship between structures, stress and seismicity in the Charlevoix seismic zone revealed by 3‐D geomechanical models: Implications for the seismotectonics of continental interiors

The Charlevoix seismic zone in the St. Lawrence valley of Québec is the most active in eastern Canada. The structurally complex region comprises a series of subparallel steeply dipping Iapetan rift faults, superimposed by a 350 Ma meteorite impact structure, resulting in a heavily faulted volume. The elongate seismic zone runs through the crater parallel to the rift. Most large events localize outside the crater and are consistent with slip along the rift faults, whereas background seismicity primarily occurs within the volume of rock bounded by the rift faults within and beneath the crater. The interaction between rift and crater faults is explored using the three‐dimensional stress analysis code FLAC3D. The rift faults are represented by frictional discontinuities, and the crater is represented by a bowl‐shaped elastic volume of reduced modulus. Differential stresses are slowly built up from boundary displacements similar to tectonic loading. Results indicate that weakening the rift faults produces a stress increase in the region of the crater bounded by the faults. This causes a decrease in stability of optimally oriented faults and may explain the localization of low‐level seismicity. Additionally, slip distribution along the rift faults shows that large events localize at the perimeter of the crater and produce focal mechanisms with P axes oblique to the applied stress field, consistent with historic large earthquakes. It is speculated that similar systematic rotation of focal mechanism P axes may be expected along other intraplate rift zones, raising a potential caveat for the use of focal mechanisms for stress estimation in continental interiors.

Localization of Large Earthquakes in the Charlevoix Seismic Zone, Quebec, Canada, during the Past 10,000 Years

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 episodes approximately 5,000 and 10,000 years ago. The 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. The apparent 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. The geologic record of earthquakes may be incomplete in both areas due to fluctuations in Holocene sea level. Nevertheless, the rate of large earthquakes has apparently been much higher in the Charlevoix seismic zone than in adjacent areas of the St. Lawrence for thousands of years. These findings suggest that seismicity is localized in Charlevoix and that the presence of Iapetan rift faults that underlie the St. Lawrence Valley of southeastern Canada may not, in itself, indicate earthquake potential. These results may have important implications for other Iapetan rift faults in the eastern United States, as well as seismic source zone characterization and hazard assessment throughout eastern North America.

Historical Earthquake Damage in the Ottawa-Gatineau Region, Canada

Other| January 01, 2010 Historical Earthquake Damage in the Ottawa-Gatineau Region, Canada Maurice Lamontagne Maurice Lamontagne Natural Resources Canada 615 Booth Street, Room 216 Ottawa, Ontario K1A 0E9 Canada malamont@nrcan.gc.ca Search for other works by this author on: GSW Google Scholar Author and Article Information Maurice Lamontagne Natural Resources Canada 615 Booth Street, Room 216 Ottawa, Ontario K1A 0E9 Canada malamont@nrcan.gc.ca Publisher: Seismological Society of America First Online: 09 Mar 2017 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © 2010 by the Seismological Society of America Seismological Research Letters (2010) 81 (1): 129–139. https://doi.org/10.1785/gssrl.81.1.129 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Maurice Lamontagne; Historical Earthquake Damage in the Ottawa-Gatineau Region, Canada. Seismological Research Letters 2010;; 81 (1): 129–139. doi: https://doi.org/10.1785/gssrl.81.1.129 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search Abstract Based on earthquake hazard and population, the seismic risk of the Ottawa-Gatineau region ranks third in Canadian urban areas. As part of a seismic microzonation project, the impact of earthquakes was documented for the 18 strongest events that affected the region. These events were 12 moderate earthquakes (magnitude between 5.0 and 6.1) at less than 300 km epicentral distance and five more-distant earthquakes (350 to 525 km epicentral distance) with epicenters in the Charlevoix seismic zone and in the Saguenay region. It was found that between 1830 and 2008, 13 of these earthquakes had an impact in Ottawa of at least Modified Mercalli Intensity (MMI) level V. Most earthquakes occurred in the first half of the 20th century when Ottawa occupied only a small proportion of its current urban area. Consequently, most damages occurred in the city's historical downtown, where most of the buildings were located. At a very local level, certain city districts (wards) were affected more than others, suggesting higher levels of ground motions due to unconsolidated deposits. Historically, a 10–50-m thick basin filled with unconsolidated deposits in downtown Ottawa had most instances of MMI VI and VII. In contrast, other areas with a thin veneer of unconsolidated material had very little damage. Considering the rapid development of the Ottawa region during the past few decades, other areas that were weakly populated during past earthquakes could suffer damage in future earthquakes. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Stress channelling and partitioning of seismicity in the Charlevoix seismic zone, Québec, Canada

SUMMARY The Charlevoix seismic zone (CSZ) in the St Lawrence valley of Quebec is historically the most active in eastern Canada. The structurally complex region comprises rift faults formed during the opening of the Iapetus Ocean, superimposed by a 350 Ma meteorite impact structure, resulting in a circular highly fractured zone. Although seismicity is localized along two steeply dipping planar rift-parallel zones, previous work indicates that most of the large-scale rift faults bound the low magnitude background seismicity rather than generate earthquakes themselves. In order to gain insight into the mechanics of the partitioning of this seismicity, a 2-D model of the CSZ was built using the stress analysis code FLAC. The rift faults are represented by frictional discontinuities. The heavily fractured impact structure is represented by an elastic continuum of reduced modulus. Boundary displacements are used to generate a regional stress field with the major horizontal component in the direction of tectonic loading. Given a high strength, the rift faults have little effect on the stress patterns. Stress trajectories naturally flow around the crater of reduced elastic modulus, leaving the fractured area with lower stresses than the background level. However, when the rift faults have low strength, they are unable to support stress trajectories inclined to them, due to the resolved shear stress exceeding their strength. This prevents trajectories from flowing out of the rift, effectively channelling higher magnitude stresses into the region of the impact structure between the faults. Low-strength bounding faults can thus explain the localization of seismicity into linear bands, rather than distributed seismicity throughout the impact structure. It also explains how the rift faults act as boundaries to regions of low magnitude seismicity. These results indicate that the interplay between faults of varying strength and zones of differing elastic modulus can give rise to complicated stress patterns, and can explain many of the seismicity patterns observed in the CSZ. This has implications for other intraplate seismic zones, as it shows an example of how regional weak faults can modify stress conditions around local structures and drive seismicity. The results are particularly relevant for other regions located within rifted crust, such as the New Madrid seismic zone, which possibly display evidence of stress channelling.

Description and Analysis of the Earthquake Damage in the Quebec City Region between 1608 and 2008

Quebec City is rated sixth for earthquake risk among Canadian cities but ranks first or second in terms of earthquake damage over the course of its 400-year history. The examples of earthquake damage in the Quebec City region between 1608 and 2008 were collected in a single database. For each significant earthquake, the database provides the description of damage together with the geographic position, the corresponding intensity on the Modified Mercalli Intensity (MMI) scale, and the supporting document. Historically, Quebec City was subject to several moderate to strong earthquakes with epicenters in the Charlevoix seismic zone (80–180 km epicentral distance) in 1663, 1791, 1860, 1870, and 1925; and in the Saguenay region (150 km epicentral distance) in 1988. M ∼ 5 earthquakes at shorter epicentral distances were also felt but caused only minor damage. These facts bring to light that certain wards, mostly located in the lower part of the city, are more likely to sustain higher levels of ground motion in future moderate to strong earthquakes. This information correlates fairly well with known near-surface shear-wave velocities, natural periods, and geotechnical properties. Knowledge of these more-hazardous areas is important for land-use planning and emergency preparedness.