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Earthquakes in the Eastern Great Lakes Basin from a regional perspective
Tectonophysics 353 (2002) 17 – 30 www.elsevier.com/locate/tecto
Earthquakes in the Eastern Great Lakes Basin from a regional perspective John E. Ebel a,*, Martitia Tuttle b a
Department of Geology and Geophysics, Boston College, Weston Observatory, 381 Concord Road, Weston, MA 02493, USA b M. Tuttle and Associates, 128 Tibbetts Lane, Georgetown, ME 04548, USA Received 13 October 1999; accepted 15 February 2002
Abstract This paper presents a summary of the seismicity and its relation to stress and geologic structures in the Eastern Great Lakes Basin (EGLB) and compares it with that of other regions in the central and eastern North America (CENA). The earthquakes scattered throughout the EGLB are occurring at a rate somewhat less than that of the Appalachians and along the Atlantic Seaboard. Paleoseismology studies suggest that the lower seismicity rate may be characteristic of the EGLB since the Late Wisconsin. North of the EGLB, earthquakes have primarily thrust mechanisms, while to the south of the EGLB, most earthquakes are strike-slip. Throughout the region, including the EGLB, the average P axes of the earthquakes are oriented NE – SW and are aligned with the direction of the current plate driving stress. On a regional basis, earthquakes are centered primarily in the Precambrian basement beneath the Paleozoic cover. Many of the earthquakes in the EGLB have occurred in areas of preexisting faults, at least some of which may have been active during past episodes of continental rifting. For individual faults that have been studied in some detail, however, it is not clear whether earthquakes represent reactivations of local preexisting structures or nucleation of new ruptures in or near the old fault zones. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Regional seismicity; Stress; Geologic structures; Paleoseismology
1. Introduction The Eastern Great Lakes Basin (EGLB) of eastern North America is a region of low but steady seismicity at the western edge of the broad zone of intraplate earthquake activity that is occurring along the Appalachian Mountains and Atlantic Seaboard. Because it has not been the center of a major earthquake (magnitude above 6.0) in historic time, the EGLB is less *
Corresponding author. E-mail addresses: [email protected] (J.E. Ebel), [email protected] (M. Tuttle).
well known as a region of seismic hazard than such places as the Charlevoix and the western Quebec seismic zones to the north, the New Madrid seismic zone to the southwest, the earthquakes of eastern Massachusetts to the east, or the seismicity in the Charleston, SC, area to the south, all of which have included the epicenters of major earthquakes. The largest documented earthquakes in the EGLB itself have been less than about magnitude 5 1/2, large enough to have cracked or toppled chimneys and alarmed people, but not large enough to have caused much serious damage. Nevertheless, with important population and industrial centers such as Toronto, St.
0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 0 2 ) 0 0 2 7 7 - 9
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Catherines and Hamilton in Ontario, Rochester, Syracuse and Buffalo in New York, Cleveland in Ohio, Erie in Pennsylvania and Detroit in Michigan, there is substantial risk of damage and casualties should a large earthquake be centered in the EGLB. For that reason, studies of the seismicity, paleoseismicity, and neotectonics of the EGLB are important to refine our understanding of the regional seismic hazard. On a regional basis, the earthquake activity of the EGLB displays characteristics similar to those of other parts of central and eastern North America (CENA). Observed earthquake epicenters in the regional catalog are scattered with varying levels of activity throughout the EGLB, with the greatest activity occurring east of Buffalo, NY, around the western end of Lake Ontario, and northeast of Cleveland, OH, along the southern shore of Lake Erie. The regional geology, a sequence of relatively undeformed Paleozoic sedimentary rocks overlying the Precambrian basement, does not reveal any large-scale geologic structure to explain the earthquake activity spread throughout the area. The relatively low density of modern seismic stations monitoring the current earthquake activity in the region limits the amount of important source information (like depth of focus and focal mechanism) that can be determined for most EGLB earthquakes that are detected today. For this reason, inferences about the neotectonics and seismic hazard of the EGLB must necessarily be made from relatively few (and sometimes poor) earthquake recordings. Four paleoseismology studies covering small areas have been conducted in the EGLB. These studies did not find evidence of large, moment-magnitude, M>6, earthquakes during the Late Wisconsin or Holocene. This contrasts with other portions of the CENA, such as the seismic zones near Charleston, SC, and New Madrid, MO, where evidence has been discovered for several large earthquakes during the past 2000– 4000 years (e.g., Talwani et al., 1999; Tuttle et al., 2002b). The purpose of this paper is to examine the EGLB seismicity, past and present, in the broader context of the earthquake activity throughout the CENA. The regional spatial patterns of the seismicity are described, the stress patterns associated with the earthquakes are analyzed, possible relations of the earthquakes to regional geological structures are discussed, and the search for paleoseismological evi-
dence of past strong earthquakes is summarized. This paper provides a CENA framework for the other papers in this volume.
2. Spatial distribution of EGLB earthquakes As Fig. 1 illustrates, earthquake epicenters occur throughout most of the CENA, while Fig. 2 shows the smoothed rates of body-wave magnitude mb z 3 for this same region (Frankel, 1995). Two areas stand out as being concentrations of unusually intense seismicity, the New Madrid seismic zone in the central US and the Charlevoix seismic zone along the St. Lawrence River in Quebec. Other zones of more diffuse but fairly high earthquake activity are in western Quebec province and in the lower St. Lawrence River region of Canada, in the southern Appalachians of the US, in the Adirondack Mountains of New York State and broadly through the coastal and lowland areas east of the Appalachians from the Carolinas through New England and into New Brunswick. The EGLB is characterized by a somewhat lower rate of earthquake activity than most of those regions already mentioned, comparable to that in southern Oklahoma and around the Nebraska – South Dakota border. Just west of the EGLB in Michigan, the seismicity rate is very low (Fujita and Sleep, 1991). A large majority of the earthquakes of mb z 5 since 1700 has taken place in the more seismically active parts of the CENA. Like the smaller events, the mb z 5 earthquakes are spread throughout the region. The EGLB and surrounding regions have experienced a few earthquakes approaching or exceeding magnitude 5: the 1929 Attica, NY, earthquake (mb 5.2 or M 4.9); the 1939 Anna, OH, earthquake (estimated M 5.3); the 1944 Cornwall, ON – Massena, NY, earthquake (M 5.8); the 1986 Leroy, OH, earthquake (M 5.0); and the 1998 Pymatuning, PA, earthquake (M 5.2). Focal depths of earthquakes in the CENA show some systematic local variations. In New England, most of the earthquakes take place at depths of 10 km or less, while in southeastern Canada they can be as deep as 30 km (Ebel, 1999). In the southeastern US, earthquakes under the coastal plain are predominantly less than 10 km deep, while they are deeper on average under the Valley and Ridge and the Blue Ridge provinces of the southern Appalachians (Bol-
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Fig. 1. Seismicity of central and eastern North America. Earthquakes to 1985 are from Seeber and Armbruster (1991), from 1985 to 1995 are from Frankel et al. (1996) and after 1996 are from the Canadian Geological Survey. Indicated are: CH—Charlevoix seismic zone; EGLB—the eastern Great Lakes Basin region; NM—the New Madrid seismic zone; SA—the southern Appalachians seismic zone; and WQ—the western Quebec seismic zone.
linger et al., 1991). In the active part of the New Madrid seismic zone itself, most of the hypocenters are at depths of less than 10 km, although some events have taken place below 20 km depth (Mitchell et al., 1991). Wheeler and Johnston (1992) argued that earthquake focal depths systematically vary throughout different parts of the CENA. They delineate three general regions with different focal depth distributions: the Precambrian central craton with depths generally between 15 km and the surface (most events within a few km of the surface); the late Proterozoic
rifted margin with depths between 5 and 15 km but some events as deep as 30 km; and the Appalachian orogen, with depths between 5 and 10 km. Most of the earthquakes in the EGLB appear to have very shallow focal depths, generally within a few kilometers of the earth’s surface. At Attica, NY, Herrmann (1978) reported the focal depth of the 1 January 1966 mb 4.6 earthquake to be about 2 km, while the 12 June 1967 mb 4.4 earthquake was centered at 3 km depth. Seeber and Armbruster (1993) argued that the rapid change of earthquake intensity
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Fig. 2. Map of the 100-year seismicity rate of earthquakes of magnitude 0.0 (Frankel, 1995) from Frankel et al. (1996). The values of the contours are the number of events in 11-km square grid cells, for 60 years, that have a magnitude between 0.0 and 0.1.
with epicentral distance for historic earthquakes between lakes Ontario and Erie suggests shallow hypocentral depths. At Anna, OH, Wheeler and Johnston (1992) reported that the 9 March 1937 earthquake had a depth of 3 F 5 km, while Nicholson et al. (1988) found a depth of 5 F 1 km for the 31 January 1986 earthquake at Leroy, OH. Cranswick et al. (1999) reported that the 1998 Pymatuning, PA, earthquake had a focal depth of 4 – 5 km (Cranswick et al., 1999). In the Wheeler and Johnston (1992) scheme, the depths of the EGLB seismicity are most similar to those from earthquakes of the Precambrian central craton. Wheeler and Johnston (1992) conclude that
the depths and dips of the focal planes of the earthquakes in the central craton explain the lack of geologic evidence of recent surface ruptures but that rare, large, shallow earthquakes that break the surface can take place in the craton, as has been observed in northern Quebec and in Australia.
3. Regional stresses and EGLB earthquakes The broad scatter of earthquake epicenters across much of CENA suggests that regional or plate-scale processes are the source of the stresses that are being
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released in a large majority of earthquakes (Zoback, 1992b). In early studies, the earthquakes in northeastern North America were suspected to be caused by the retreat of the continental glacial ice sheet (e.g., Collins, 1937). Subsequent studies of the principal stress directions ( P, T and B axes) from earthquakes throughout the CENA along with measurement of stress directions in boreholes have revealed that the regional stress field is generally characterized by a maximum compressive stress direction (SHmax) that closely parallels that expected from the absolute motion of the North American plate (Zoback, 1992a,b; Richardson, 1992). Zoback (1992a) notes that the mean SHmax (N63jE F 28j) for CENA is well-defined, although there is considerable variation around that mean. Examples of localities where there are significant variations in earthquake focal mechanisms, and yet with an average SHmax roughly consistent with the plate driving stress direction, are the Charlevoix and Lower St. Lawrence seismic zones in Quebec. Adams and Bell (1991) reported that for each of these zones the average P-axis directions are approximately E –W, even though there is a very wide range of P-axis directions for the individual earthquakes. Variations in the structural grain of the Paleozoic Appalachian orogenic belt of eastern North American do not seem to lead to local changes in the SHmax direction (Zoback, 1992b; Richardson, 1992), indicating that the maximum stress direction is not being significantly controlled by the upper crustal geology. Adams and Bell (1991) reported that the preexisting faults in the Charlevoix seismic zone may only be causing a slight local rotation in the regional stress field. Engelder (1982) determined that regional joint orientations on the Appalachian Plateau and adjacent areas are also consistent with mode I cracks opening parallel to the modern SHmax direction from borehole and earthquake focal mechanism studies. Later work by Gross and Engelder (1991), Hancock and Engelder (1989) and Engelder and Gross (1993) has added to the observational evidence for this relation in the EGLB, while Meng and Jacobi (1997) have provided analyses for the relation between joints and modern tectonic stresses. While S Hmax is quite stable throughout the CENA, there is less consistency in the other two principal stresses. Hasagawa et al. (1985) and
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Zoback (1992b) found that the region of CENA north of roughly 42 –41jN has predominantly thrust earthquakes, while south of that boundary most of the earthquakes have strike-slip mechanisms. Thus, in the northern part of the CENA, the least principal stress is approximately vertical; while in the southern part, the least principal stress is approximately horizontal. The EGLB straddles this transition zone from thrust to strike-slip faulting regimes, and this is seen in the few earthquake focal mechanisms from the region (Fig. 3). Throughout the CENA there is a very strong correlation between the SHmax direction and the orientation of the stresses caused by the ridge push force acting on the North American plate (Richardson, 1992; Zoback, 1992a; Coblentz and Richardson, 1995). However, the change from thrust faulting in the north to strike-slip faulting in the south necessitates some laterally varying stress field that is not due to plate driving forces (Zoback, 1992b). Zoback (1992b) showed that stresses caused by continental postglacial rebound have the correct orientation to cause this change in focal mechanisms. However, the postglacial rebound stresses are very small in magnitude, making it problematic to use this as an explanation for the southward change in earthquake focal mechanisms (Zoback, 1992b). Zoback (1992b) suggested that perhaps compressive stresses related to the support of a dense lower crustal structure beneath the St. Lawrence rift may contribute to the observed change from thrust to strike-slip earthquakes in the EGLB region. In a theoretical analysis of postglacial strain rates using a simple Laurentide glacial loading model, James and Bent (1994) determined that the postglacial strain rates are one to three orders of magnitude greater than the strain rates from the observed seismicity. They also argued that for the northeastern US and southeastern Canada the predicted glacial rebound strain rates are not oriented so as to augment the observed deviatoric earthquake stresses except for the St. Lawrence Valley region. James and Bent (1994) noted that the discrepancy between the observed seismic strain rates compared to the predicted postglacial strain rates could be due to the misorientation of the principal stress directions of the postglacial rebound relative to those of the earthquakes or it might indicate that the lithosphere can undergo sig-
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Fig. 3. Map of major structural features of the CENA. The New Madrid rift and proposed extensions are from Braile et al. (1982). The St. Lawrence rift, the Ottawa rift and the possible extension of the St. Lawrence rift are from Adams and Basham (1991). The eastern boundary of the Grenville craton is from Wheeler (1996). The focal mechanisms of the earthquakes in the EGLB are from the following sources: 1966 and 1967 earthquakes at Attica, NY, from Herrmann (1979); 1986 Leroy, Ohio earthquake from Nicholson et al. (1988); the 1986 Anna, Ohio earthquake from Schwartz and Christensen (1988); and the 1998 Pymatuning, Pennsylvania earthquake from the USGS.
nificant deformation without a concomitant level of seismicity.
4. Relations between earthquakes and structures Perhaps the most enigmatic aspect of the CENA seismicity is the lack of clearcut evidence that identifies which are the seismically active faults in the region, especially those capable of major earthquakes. While most of the CENA earthquakes occur on or in the vicinity of large-scale structures that are fractured and faulted, evidence for neotectonic movement on
individual faults is almost invariably lacking. In the New Madrid seismic zone, there are some faults on which neotectonic deformations have been found (e.g., Van Arsdale et al., 1998), but in the northeastern US (Ebel and Kafka, 1991), southeastern US (Bollinger et al., 1991) and southeastern Canada (Adams and Basham, 1991), confirmed evidence of active tectonic faulting has yet to be documented. Curiously, the Ungava peninsula earthquake in 1989 (Adams et al., 1992) caused surface rupture along surface lineaments that had not had tectonic displacement since Paleozoic time. Thus, the lack of evidence of modern surface faulting in the CENA apparently says little about the
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potential for future large (M > 6) earthquakes in the region. If Precambrian lineaments or faults with no previous Phanerozoic tectonic movement can suddenly reactivate in modern times with a large earthquake, then the Paleozoic and later cover rocks may be hiding potentially seismogenic basement faults throughout the EGLB. One way in which seismically active faults might be identified is to find faults upon which recorded earthquake activity has been concentrated. For the EGLB, Seeber and Armbruster (1993) reported that there is historic and recent earthquake activity concentrated along the Akron aeromagnetic lineament in Ohio, along the Clarendon– Linden Fault (CLF) in western New York, parallel to basement structures at the western end of Lake Ontario, and along the Niagara gorge between New York and Ontario. Of these seismic zones, the Clarendon – Linden Fault has received the most attention. Seeber and Armbruster (1993) report that there have been several instrumentally recorded earthquakes that have been concentrated near the main strand of the CLF. Their analyses of the confidence limits of the locations of the 1929 mb 5.2 Attica earthquake and of an M3.5 earthquake near Attica in 1844 indicate that both of these events could also have been associated with the CLF. Both Jacobi et al. (1996) and Wallach et al. (1998) regard the CLF as seismically active. However, Brennan (1998) notes that relatively few epicenters are known from the vicinity of the CLF and that these epicenters do not define curvilinear trends along the fault. These observations, combined with the orientation of the fault at 70j from the ambient stress direction and the small amount of displacement since lower Silurian, led Brennan (1998) to conclude that the CLF is only a minor contributor to the natural seismicity of the region. Wallach et al. (1998) note that the western Lake Ontario area has a higher level of earthquake activity than adjacent regions. This locally elevated earthquake activity is occurring in an area in which three geologic structural trends intersect. The first is the Niagara – Pickering zone, which Wallach et al. (1998) infer to be a northeastern extension of the Akron lineament of Seeber and Armbruster (1993). This interpretation of Wallach et al. (1998) connects the Niagara gorge seismicity with that in Lake Ontario, although Seeber and Armbruster (1993) explain the seismicity near
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Niagara Falls as due to the rapid erosion of the river bed by the waterfall. The second intersecting geologic trend is a zone of faults and lineaments that extends from Georgian Bay to western Lake Ontario and into New York State. The third zone is a lineament along a possible fault from Hamilton to Lake Erie. In all three cases, Wallach et al. (1998) do not feel that there is sufficient evidence at the present time to regard any of these features as necessarily seismogenic. Even so, they do regard the western Lake Ontario area as one where a major earthquake could be possible. The existence of a few earthquakes hypocenters in the vicinity of a basement fault or lineament is not convincing evidence that the fault or lineament is currently capable of generating a large earthquake. Both Ebel (1983) and Seeber and Armbruster (1993) observe that the spatial association of modern earthquakes and preexisting faults could be due to the reactivation of the old faults or to the formation of new fractures that nucleate on or near the preexisting faults. In the latter case, the seismic events may nucleate on the preexisting fault, but the ruptures associated with the earthquakes may be new cracks that form along some new direction rather than following the existing basement fault. This means that modern earthquakes might show an apparent spatial association with preexisting faults without those faults themselves being favorably oriented with the modern stress field or themselves being capable of slipping in a large earthquake. If indeed the modern earthquakes represent the formation of new basement faults, then the sizes of preexisting faults do not give any indication of how large an earthquake might occur in any modern seismically active area. The question of whether the earthquakes are reactivations of old faults or are new fractures could be addressed by better seismic monitoring of the major faults in the region to provide high-quality earthquake hypocenters and focal mechanisms. On a regional basis, the structures with which most of the CENA earthquakes are spatially associated include the buried rift structures of the Mississippi embayment, the St. Lawrence rift, the Saguenay graben, the Ottawa – Bonnechere graben, part of the Adirondack uplift, and the Appalachian orogen, especially that part involved in Triassic rifting (Fig. 3). In general, these active regional structures are features left over from previous continental rifting episodes.
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Johnston et al. (1994) documented that most of the great (M >7) earthquakes in the stable cratons occur in previously rifted crust, and especially in crust that has rifted in Mesozoic and Cenozoic time. Much of the CENA seismicity outside the EGLB appears to fit this pattern, and this may be true of the EGLB as well. Wheeler (1995, 1996) argued that many of the CENA earthquakes are concentrated on late Proterozoic or early Paleozoic normal faults of the cratonic Iapetan passive margin, which formed during a continental rifting episode in the early Paleozoic along the eastern Grenville margin. While much of this boundary is buried today by thrust sheets and nappes of the Paleozoic Appalachian orogen, Wheeler (1995, 1996) traces the Iapetan normal faults in a broad zone from central Alabama through eastern Quebec to Labrador. In the EGLB, Wheeler (1995, 1996) indicates that Iapetan faults occur beneath all but the westernmost and southeasternmost edges of Pennsylvania and New York and that they extend into northwestern Vermont and southeastern Quebec. Evidence for Iapetan normalfault activation of the Clarendon –Linden Fault zone in western New York State (Jacobi and Fountain, 1998a,b) supports Wheeler’s (1995) argument that the western Iapetan faults must extend to at least this area. Also, Harper and Jacobi (2000) inferred from a detailed joint study that fractures related to the St. Lawrence rift system may extend as far to the southwest as southwestern Lake Ontario. Thus, there is much geological and geophysical evidence that the basement of at least the eastern half and perhaps much of the ELGB contains crust that experienced normal faulting as late as Upper Devonian time. The possible existence of rifted crust beneath western Lake Ontario and into Lake Erie was postulated by Adams and Basham (1991), who noted that this could explain the highly speculative idea of Woollard (1969) that the seismicity along the St. Lawrence rift system in Quebec might be connected southwest through Lakes Ontario and Erie and from there southwest through Ohio and Indiana to the New Madrid seismic zone. Buried rift structures beneath the EGLB would explain the EGLB seismicity consonant with the ideas of Johnston et al. (1994) and Wheeler (1996) that much of the seismicity of the stable craton is associated with preexisting continental rift faults. However, why some parts of the continental rifts (e.g., New Madrid and Charlevoix) would be highly
active while other sections are relatively inactive is not understood. Perhaps different parts of the rift systems are active at different times. Thus, New Madrid and Charlevoix are active now, but a few tens of thousands of years from now those areas might have much lower seismicity while other parts of the preexisting rift structures might be more active. Such an idea could explain the lack of evidence of significant neotectonic topography at Charlevoix and New Madrid (Schweig and Ellis, 1994).
5. Paleoseismological studies in CENA and implications for the EGLB In the CENA, paleoseismology studies have employed liquefaction features (e.g., Obermeier et al., 1985), fault-related deformation (e.g., Kelson et al., 1996), silt layers in lake deposits (Doig, 1990) and subaqueous slides (Shilts, 1984) to identify prehistoric earthquakes in the geologic record and have been conducted in highly active, moderately active and relatively inactive regions. Although its length varies from 1000 to 12,000 years, and rarely up to 20,000 years, in different regions of the CENA, the paleoearthquake record, in general, appears to reflect historic and modern seismicity (Fig. 4). In those regions that have been studied, seismicity appears to have been relatively stable for at least the past few thousand years. In the New Madrid seismic zone (NMSZ), one of the most seismically active regions of the CENA, liquefaction features indicate that M > 7.5 earthquakes have occurred every 200 –800 years during the past 1200 years for an average recurrence interval of 500 years (e.g., Tuttle et al., 2002b; Cramer, 2001). Similarly, deformation along the Reelfoot scarp, thought to reflect faulting at depth, suggests an average repeat time of 400 –500 years during the past 1200 years (Kelson et al., 1996). These recurrence estimates for large earthquakes are consistent, albeit on the shorter side, with the frequency – magnitude relation developed by Johnston and Nava (1985) from historical and instrumentally recorded seismicity in the New Madrid region. A diffuse concentration of moderate earthquakes extends north from the NMSZ into southeastern Missouri, southern Illinois and southwestern Indiana. Along the northern boundary of the Mississippi
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Fig. 4. Map showing locations and results (M—magnitude estimate; RI—recurrence interval) of paleoseismological studies in the CENA. NM—New Madrid seismic zone; EF—English Hills fault; SL—St. Louis region; SI—Springfield, IL; LW—Lower Wabash valley; CH— Charlevoix seismic zone; LL—Lower St. Lawrence region; SR—Saguenay region; CS—Charleston, SC; MN—Massena, NY; MA— northeastern Massachusetts; AN—Attica, NY; RR—Rouge River, Ontario. The recurrence interval times are based on calibrated radiocarbon ages and are in years bp.
embayment, north – northeast and northeast oriented strike-slip faults are mapped above the northwestern margin of the buried Reelfoot rift (Harrison and Schultz, 1994). One of these, the English Hill fault, juxtaposes Tertiary and Cretaceous deposits against Late Wisconsin Peoria loess and provides the first direct evidence of Quaternary age faulting in the embayment and outside the NMSZ (e.g., Palmer et al., 1997). Investigations of several other geologic structures located northwest of the Mississippi embayment with which earthquakes are spatially associated found no evidence of Holocene faulting (Tuttle et al., 1999). Paleoliquefaction studies, however, have found
evidence for at least two episodes of strong ground shaking near St. Louis, MO, during the past 7000 years (e.g., McNulty and Obermeier, 1997; Tuttle et al., 1999). McNulty and Obermeier (1997) attribute paleoliquefaction features in Illinois southeast of St. Louis to a local event of M > 6 during the midHolocene. Tuttle et al. (1999), who have found additional liquefaction features in Illinois and Missouri, think that this and other possible interpretations of the data warrant additional testing due to poor age constraints of liquefaction features in the region. North-central Illinois has experienced only small earthquakes during the historic period. Even so,
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paleoliquefaction features have been found near Springfield and are interpreted as evidence for a local M >6 earthquake about 6500 radiocarbon years ago (e.g., Hajic and Wiant, 1997; McNulty and Obermeier, 1997). The largest historic earthquake in the lower Wabash valley of southern Indiana and Illinois was of M 5.5. Paleoliquefaction studies in this area indicate that at least eight prehistoric earthquakes of M >5.5 have occurred there since the Late Pleistocene (e.g., Munson et al., 1997). Two of the events are estimated to be of M >7 and to have struck the area about 6000 and 12,000 radiocarbon years ago. It is interesting that such a large region north of the NMSZ appears to have been affected by several large earthquakes during the mid-Holocene. As suggested by its timing, perhaps this seismicity was triggered by glacial unloading of the crust. Alternatively, the large mid-Holocene earthquakes in Indiana and Illinois may reflect an active period along the postulated extensions of the Reelfoot and St. Lawrence rift systems. A paleoseismology study was conducted in the Western Quebec seismic zone and the meizoseismal area of the 1944 M 5.8 Massena, NY – Cornwall, Ontario, earthquake. The study involved relocation and excavation of a site of earthquake-induced liquefaction in 1944 and reconnaissance for other liquefaction features (Tuttle, 1996). The study was hampered by disturbance of the liquefaction site during construction of the Eisenhower Locks and by poor exposure in the area. Although a few, very small liquefaction features were found, there was no evidence for large earthquakes during the past 12,000 years. In the Charlevoix seismic zone, a 2300-year record of ground shaking was established by studying lake sediments. The paleoseismic record suggests an average repeat time of 75 years for M >6 earthquakes (Doig, 1990). In the less seismically active Saguenay region to the north, lake sediments indicate that M >6 events have occurred every 350 –1000 years during the past 3000 years (Doig, 1998). This is supported by earthquake-induced liquefaction features that suggest that a M > 6 earthquake struck the region between 370 and 770 years before the 1988 M 5.9 Saguenay earthquake (Tuttle, 1994). The source of the large earthquake could not be determined without further study.
Along the Atlantic Seaboard, liquefaction features have been found in Massachusetts and from North Carolina to Georgia. In northeastern Massachusetts, where two magnitude 5.5 – 6.0 earthquakes occurred in 1727 and 1755, liquefaction features suggest only one similar-size event during the preceding 4000 years (Tuttle and Seeber, 1991). In the vicinity of Charleston, South Carolina, liquefaction studies indicate that M > 7 earthquakes have occurred every 500– 600 years during the past 1000 years (e.g., Talwani et al., 1999). To the north and south of Charleston, the earthquake potential appears to be considerably less, with repeat times for M > 6 earthquakes of more than 2000 years. Four paleoseismology studies, covering relatively small areas, have been conducted in the EGLB. These studies include searches for earthquake-induced liquefaction features along the Clarendon – Linden fault system in western New York State and in the St. Lawrence Lowlands east of Lake Ontario, and evaluation of faults exposed along the Rouge River near Toronto, Ontario. No unequivocal evidence of earthquake-induced liquefaction was found in Late Wisconsin and Holocene sediments along the Clarendon– Linden fault system, including the epicentral area of the 1929 M 4.9 Attica earthquake (Tuttle et al., 2002a). Coates (1975) conducted a search for earthquake-related deformation structures in 200 sand and gravel pits in the St. Lawrence Lowlands. Although abundant, most of the deformation stuctures found in Quaternary sediments were attributed to glaciation. Along the Rouge River, E – W to W– NW oriented normal faults displace the Scarborough Formation (ca. 70 ka) up to 1.25 m, but do not affect the overlying Halton till ( < 13 ka). Mohajer et al. (1992) attribute the Pleistocene-age faulting to either extension related to glacial rebound or reactivation of bedrock joints in response to the W –NW-oriented maximum-principal stress. In contrast, Adams et al. (1993) argue that the Rouge River faults can be explained by glaciotectonics. These four studies found no unequivocal evidence for large earthquakes in western New York, the St. Lawrence Lowlands, and southernmost Ontario, since the Late Wisconsin. These results suggest that the modern, low seismicity rate may be characteristic of the long-term behavior of the region. If the EGLB has the potential to generate M >6 earthquakes, the recurrence interval of these large events may be longer than at least 12,000 years. At this time, however, too little
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of the EGLB region has been studied to draw definitive conclusions.
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natural event (i.e., not induced) and was located in the vicinity of the western branch of the Clarendon – Linden fault system.
6. Induced seismicity in the CENA 7. Summary and conclusions While it is clear from early historical records that many natural earthquakes take place in CENA (e.g., Ebel, 1996), some of the earthquakes in the area may not be natural seismicity but rather may be induced by man-made activities. Several types of human activity can induce earthquakes. Fluid injection and extraction have been found to induce earthquakes in several localities: at the Gobles oil field in southwestern Ontario (Mereu et al., 1986); along the Clarendon – Linden Fault in western New York State due to salt brine recovery operations (Fletcher and Sykes, 1977); and near Ashtabula, OH, due to waste injection (Seeber and Armbruster, 1993). In eastern New York, quarrying at Wappingers Falls appears to have induced a nearby earthquake (Pomeroy et al., 1976), and in eastern Pennsylvania, changes in the pumping of the abandoned Wyomissing Hills quarry were documented around the time of a pair of earthquakes centered at the quarry (Armbruster et al., 1994). In the southeastern US, reservoir-induced seismicity has been documented at several localities (Bollinger et al., 1991). Thus, some of the earthquakes shown in Fig. 1 may have been induced events and are not indicative of natural seismicity. In some situations, there is debate about earthquakes being induced or natural. Nicholson et al. (1988) argued that the 1986 M 5.0 Leroy, OH, earthquake was probably not induced, even though there were waste injection wells within 15 km of the epicenter. Ammon et al. (1998) think that the recent moderate earthquakes at the Wyomissing Hills quarry in Pennsylvania would have taken place even in the absence of the quarry. Seeber and Armbruster (1993) report that salt brine extraction was already under way at Dale, NY, at the time of the nearby Attica earthquake in 1929 but that the epicenter of the 1929 event is too poorly determined to firmly attribute it to mining activities. However, Tuttle et al. (2002a) point out that ground failure related to the mainshock as well as felt reports of a possible aftershock in the same area 3– 4 km west of Dale suggest that the 1929 earthquake may have been a
Over all, EGLB seismicity is typical of that found throughout much of the CENA. The EGLB seismicity rate is less than that in much of the Appalachians and the Atlantic Seaboard but is greater than the earthquake rates detected further northwest toward the center of the craton. The EGLB earthquakes seem to be occurring in the Grenville basement, similar to other parts of the CENA. The source of the stress driving the seismicity is predominantly plate tectonics, especially ridge push from mid-Atlantic spreading. However, some other source of stress, perhaps postglacial rebound, must contribute to the change in average focal mechanism from thrust in the north to strike-slip in the south of the EGLB. On a regional basis, many of the earthquakes in the EGLB have occurred in areas of preexisting faults, at least some of which may have been active during past episodes of continental rifting. On a global scale, most major earthquakes in stable cratonic crust, like that in the EGLB, have occurred in previously rifted crust. However, for individual faults that have been studied in some detail, the relation between the local earthquakes and the preexisting faults has not been clearly deciphered. It is not clear whether the local earthquakes represent reactivations of the old faults or the nucleation of new ruptures in or near the old fault zones. Most evidence of large prehistoric earthquakes has been found in more seismically active parts of the CENA, with no unequivocal evidence of strong ground shaking uncovered so far in the EGLB. Some of the earthquakes in the EGLB may be induced by fluid injection, mining and quarrying activities and, therefore, are not indicative of the natural seismicity of the region.
Acknowledgements We are grateful for the constructive reviews of this manuscript by Robert Fakundiny, Robert Jacobi, Mike Lewis, Roy Van Arsdale and an anonymous reviewer.
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Ohio earthquake of 31 January 1986: was it induced? Bull. Seismol. Soc. Am. 78, 188 – 217. Obermeier, S.F., Gohn, G.S., Weems, R.E., Gelinas, R.L., Rubin, M., 1985. Geologic evidence for recurrent moderate to large earthquakes near Charleston, South Carolina. Science 227, 408 – 411. Palmer, J.R., Shoemaker, M., Hoffman, D., Anderson, N.L., Vaughn, J.D., Harrison, R.W., 1997. Seismic evidence of Quaternary faulting in the Benton Hills area, southeast Missouri. Seismol. Res. Lett. 68, 650 – 661. Pomeroy, P.W., Simpson, D.W., Sbar, M.L., 1976. Earthquakes triggered by surface quarrying—the Wapingers Falls, New York sequence of June, 1974. Bull. Seismol. Soc. Am. 66, 685 – 700. Richardson, R.M., 1992. Ridge forces, absolute plate motions, and the intraplate stress field. J. Geophys. Res. 97, 11739 – 11748. Schwartz, S.Y., Christensen, D.H., 1988. The 12 July 1986 St. Marys, Ohio earthquake and recent seismicity in the Anna, Ohio seismogenic zone. Seismol. Res. Lett. 59, 57 – 62. Schweig, E.S., Ellis, M.A., 1994. Reconciling short recurrence intervals with minor deformation in the New Madrid seismic zone. Science 264, 1308 – 1311. Seeber, L., Armbruster, J.G., 1991. The NCEER-91 earthquake catalog: Improved intensity-based magnitudes and recurrence relations for US earthquakes east of New Madrid, National Center for Earthquake Engineering. Technical Report 91-0021, 98 pp. Seeber, L., Armbruster, J.G., 1993. Natural and induced seismicity in the Lake Erie – Lake Ontario region: reactivation of ancient faults with little neotectonic displacement. Geogr. Phys. Quat. 47, 363 – 378. Shilts, W.W., 1984. Sonar evidence for postglacial tectonic instability of the Canadian Shield and Appalachians. Geol. Surv. Can, Pap. 84-1A, 567 – 579. Talwani, P., Amick, D.C., Schaeffer, W.T., 1999. Paleoliquefaction studies in the South Carolina Coastal Plain. Nuclear Regulatory Commission, NUREG/CR-6610, 109 pp. Tuttle, M.P., 1994. The liquefaction method for assessing paleoseismicity. Nuclear Regulatory Commission, NUREG/CR-6258, 38 pp. Tuttle, M.P., 1996. Case study of liquefaction induced by the Massena, New York – Cornwall, Ontario earthquake. Nuclear Regulatory Commission, NUREG/CR-6495, 23 pp. Tuttle, M.P., Seeber, L., 1991. Historic and prehistoric earthquakeinduced liquefaction in Newbury, Massachusetts. Geology 19, 594 – 597. Tuttle, M.P., Chester, J., Lafferty, R., Dyer-Williams, K., Cande, R., 1999. Paleoseismology study northwest of the New Madrid seismic zone. Nuclear Regulatory Commission, NUREG/CR5730. Tuttle, M., Dyer-Williams, K., Barstow, N., 2002a. Paleoliquefaction study of the Clarendon – Linden fault system, western New York State, this issue. Tuttle, M.P., Schweig, E.S., Sims, J.D., Lafferty, R.H., Wolf, L.W., Haynes, M.L., 2002b. The earthquake potential of the New Madrid seismic zone. Bull. Seismol. Soc. Am. 92, in press. Van Arsdale, R., Purser, J., Stephenson, W., Odum, J., 1998. Faulting along the southern margin of Reelfoot Lake, Tennessee. Bull. Seismol. Soc. Am. 88, 131 – 139.
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Earthquakes in the Eastern Great Lakes Basin from a regional perspective John E. Ebel a,*, Martitia Tuttle b a
Department of Geology and Geophysics, Boston College, Weston Observatory, 381 Concord Road, Weston, MA 02493, USA b M. Tuttle and Associates, 128 Tibbetts Lane, Georgetown, ME 04548, USA Received 13 October 1999; accepted 15 February 2002
Abstract This paper presents a summary of the seismicity and its relation to stress and geologic structures in the Eastern Great Lakes Basin (EGLB) and compares it with that of other regions in the central and eastern North America (CENA). The earthquakes scattered throughout the EGLB are occurring at a rate somewhat less than that of the Appalachians and along the Atlantic Seaboard. Paleoseismology studies suggest that the lower seismicity rate may be characteristic of the EGLB since the Late Wisconsin. North of the EGLB, earthquakes have primarily thrust mechanisms, while to the south of the EGLB, most earthquakes are strike-slip. Throughout the region, including the EGLB, the average P axes of the earthquakes are oriented NE – SW and are aligned with the direction of the current plate driving stress. On a regional basis, earthquakes are centered primarily in the Precambrian basement beneath the Paleozoic cover. Many of the earthquakes in the EGLB have occurred in areas of preexisting faults, at least some of which may have been active during past episodes of continental rifting. For individual faults that have been studied in some detail, however, it is not clear whether earthquakes represent reactivations of local preexisting structures or nucleation of new ruptures in or near the old fault zones. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Regional seismicity; Stress; Geologic structures; Paleoseismology
1. Introduction The Eastern Great Lakes Basin (EGLB) of eastern North America is a region of low but steady seismicity at the western edge of the broad zone of intraplate earthquake activity that is occurring along the Appalachian Mountains and Atlantic Seaboard. Because it has not been the center of a major earthquake (magnitude above 6.0) in historic time, the EGLB is less *
Corresponding author. E-mail addresses: [email protected] (J.E. Ebel), [email protected] (M. Tuttle).
well known as a region of seismic hazard than such places as the Charlevoix and the western Quebec seismic zones to the north, the New Madrid seismic zone to the southwest, the earthquakes of eastern Massachusetts to the east, or the seismicity in the Charleston, SC, area to the south, all of which have included the epicenters of major earthquakes. The largest documented earthquakes in the EGLB itself have been less than about magnitude 5 1/2, large enough to have cracked or toppled chimneys and alarmed people, but not large enough to have caused much serious damage. Nevertheless, with important population and industrial centers such as Toronto, St.
0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 0 2 ) 0 0 2 7 7 - 9
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Catherines and Hamilton in Ontario, Rochester, Syracuse and Buffalo in New York, Cleveland in Ohio, Erie in Pennsylvania and Detroit in Michigan, there is substantial risk of damage and casualties should a large earthquake be centered in the EGLB. For that reason, studies of the seismicity, paleoseismicity, and neotectonics of the EGLB are important to refine our understanding of the regional seismic hazard. On a regional basis, the earthquake activity of the EGLB displays characteristics similar to those of other parts of central and eastern North America (CENA). Observed earthquake epicenters in the regional catalog are scattered with varying levels of activity throughout the EGLB, with the greatest activity occurring east of Buffalo, NY, around the western end of Lake Ontario, and northeast of Cleveland, OH, along the southern shore of Lake Erie. The regional geology, a sequence of relatively undeformed Paleozoic sedimentary rocks overlying the Precambrian basement, does not reveal any large-scale geologic structure to explain the earthquake activity spread throughout the area. The relatively low density of modern seismic stations monitoring the current earthquake activity in the region limits the amount of important source information (like depth of focus and focal mechanism) that can be determined for most EGLB earthquakes that are detected today. For this reason, inferences about the neotectonics and seismic hazard of the EGLB must necessarily be made from relatively few (and sometimes poor) earthquake recordings. Four paleoseismology studies covering small areas have been conducted in the EGLB. These studies did not find evidence of large, moment-magnitude, M>6, earthquakes during the Late Wisconsin or Holocene. This contrasts with other portions of the CENA, such as the seismic zones near Charleston, SC, and New Madrid, MO, where evidence has been discovered for several large earthquakes during the past 2000– 4000 years (e.g., Talwani et al., 1999; Tuttle et al., 2002b). The purpose of this paper is to examine the EGLB seismicity, past and present, in the broader context of the earthquake activity throughout the CENA. The regional spatial patterns of the seismicity are described, the stress patterns associated with the earthquakes are analyzed, possible relations of the earthquakes to regional geological structures are discussed, and the search for paleoseismological evi-
dence of past strong earthquakes is summarized. This paper provides a CENA framework for the other papers in this volume.
2. Spatial distribution of EGLB earthquakes As Fig. 1 illustrates, earthquake epicenters occur throughout most of the CENA, while Fig. 2 shows the smoothed rates of body-wave magnitude mb z 3 for this same region (Frankel, 1995). Two areas stand out as being concentrations of unusually intense seismicity, the New Madrid seismic zone in the central US and the Charlevoix seismic zone along the St. Lawrence River in Quebec. Other zones of more diffuse but fairly high earthquake activity are in western Quebec province and in the lower St. Lawrence River region of Canada, in the southern Appalachians of the US, in the Adirondack Mountains of New York State and broadly through the coastal and lowland areas east of the Appalachians from the Carolinas through New England and into New Brunswick. The EGLB is characterized by a somewhat lower rate of earthquake activity than most of those regions already mentioned, comparable to that in southern Oklahoma and around the Nebraska – South Dakota border. Just west of the EGLB in Michigan, the seismicity rate is very low (Fujita and Sleep, 1991). A large majority of the earthquakes of mb z 5 since 1700 has taken place in the more seismically active parts of the CENA. Like the smaller events, the mb z 5 earthquakes are spread throughout the region. The EGLB and surrounding regions have experienced a few earthquakes approaching or exceeding magnitude 5: the 1929 Attica, NY, earthquake (mb 5.2 or M 4.9); the 1939 Anna, OH, earthquake (estimated M 5.3); the 1944 Cornwall, ON – Massena, NY, earthquake (M 5.8); the 1986 Leroy, OH, earthquake (M 5.0); and the 1998 Pymatuning, PA, earthquake (M 5.2). Focal depths of earthquakes in the CENA show some systematic local variations. In New England, most of the earthquakes take place at depths of 10 km or less, while in southeastern Canada they can be as deep as 30 km (Ebel, 1999). In the southeastern US, earthquakes under the coastal plain are predominantly less than 10 km deep, while they are deeper on average under the Valley and Ridge and the Blue Ridge provinces of the southern Appalachians (Bol-
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Fig. 1. Seismicity of central and eastern North America. Earthquakes to 1985 are from Seeber and Armbruster (1991), from 1985 to 1995 are from Frankel et al. (1996) and after 1996 are from the Canadian Geological Survey. Indicated are: CH—Charlevoix seismic zone; EGLB—the eastern Great Lakes Basin region; NM—the New Madrid seismic zone; SA—the southern Appalachians seismic zone; and WQ—the western Quebec seismic zone.
linger et al., 1991). In the active part of the New Madrid seismic zone itself, most of the hypocenters are at depths of less than 10 km, although some events have taken place below 20 km depth (Mitchell et al., 1991). Wheeler and Johnston (1992) argued that earthquake focal depths systematically vary throughout different parts of the CENA. They delineate three general regions with different focal depth distributions: the Precambrian central craton with depths generally between 15 km and the surface (most events within a few km of the surface); the late Proterozoic
rifted margin with depths between 5 and 15 km but some events as deep as 30 km; and the Appalachian orogen, with depths between 5 and 10 km. Most of the earthquakes in the EGLB appear to have very shallow focal depths, generally within a few kilometers of the earth’s surface. At Attica, NY, Herrmann (1978) reported the focal depth of the 1 January 1966 mb 4.6 earthquake to be about 2 km, while the 12 June 1967 mb 4.4 earthquake was centered at 3 km depth. Seeber and Armbruster (1993) argued that the rapid change of earthquake intensity
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Fig. 2. Map of the 100-year seismicity rate of earthquakes of magnitude 0.0 (Frankel, 1995) from Frankel et al. (1996). The values of the contours are the number of events in 11-km square grid cells, for 60 years, that have a magnitude between 0.0 and 0.1.
with epicentral distance for historic earthquakes between lakes Ontario and Erie suggests shallow hypocentral depths. At Anna, OH, Wheeler and Johnston (1992) reported that the 9 March 1937 earthquake had a depth of 3 F 5 km, while Nicholson et al. (1988) found a depth of 5 F 1 km for the 31 January 1986 earthquake at Leroy, OH. Cranswick et al. (1999) reported that the 1998 Pymatuning, PA, earthquake had a focal depth of 4 – 5 km (Cranswick et al., 1999). In the Wheeler and Johnston (1992) scheme, the depths of the EGLB seismicity are most similar to those from earthquakes of the Precambrian central craton. Wheeler and Johnston (1992) conclude that
the depths and dips of the focal planes of the earthquakes in the central craton explain the lack of geologic evidence of recent surface ruptures but that rare, large, shallow earthquakes that break the surface can take place in the craton, as has been observed in northern Quebec and in Australia.
3. Regional stresses and EGLB earthquakes The broad scatter of earthquake epicenters across much of CENA suggests that regional or plate-scale processes are the source of the stresses that are being
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released in a large majority of earthquakes (Zoback, 1992b). In early studies, the earthquakes in northeastern North America were suspected to be caused by the retreat of the continental glacial ice sheet (e.g., Collins, 1937). Subsequent studies of the principal stress directions ( P, T and B axes) from earthquakes throughout the CENA along with measurement of stress directions in boreholes have revealed that the regional stress field is generally characterized by a maximum compressive stress direction (SHmax) that closely parallels that expected from the absolute motion of the North American plate (Zoback, 1992a,b; Richardson, 1992). Zoback (1992a) notes that the mean SHmax (N63jE F 28j) for CENA is well-defined, although there is considerable variation around that mean. Examples of localities where there are significant variations in earthquake focal mechanisms, and yet with an average SHmax roughly consistent with the plate driving stress direction, are the Charlevoix and Lower St. Lawrence seismic zones in Quebec. Adams and Bell (1991) reported that for each of these zones the average P-axis directions are approximately E –W, even though there is a very wide range of P-axis directions for the individual earthquakes. Variations in the structural grain of the Paleozoic Appalachian orogenic belt of eastern North American do not seem to lead to local changes in the SHmax direction (Zoback, 1992b; Richardson, 1992), indicating that the maximum stress direction is not being significantly controlled by the upper crustal geology. Adams and Bell (1991) reported that the preexisting faults in the Charlevoix seismic zone may only be causing a slight local rotation in the regional stress field. Engelder (1982) determined that regional joint orientations on the Appalachian Plateau and adjacent areas are also consistent with mode I cracks opening parallel to the modern SHmax direction from borehole and earthquake focal mechanism studies. Later work by Gross and Engelder (1991), Hancock and Engelder (1989) and Engelder and Gross (1993) has added to the observational evidence for this relation in the EGLB, while Meng and Jacobi (1997) have provided analyses for the relation between joints and modern tectonic stresses. While S Hmax is quite stable throughout the CENA, there is less consistency in the other two principal stresses. Hasagawa et al. (1985) and
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Zoback (1992b) found that the region of CENA north of roughly 42 –41jN has predominantly thrust earthquakes, while south of that boundary most of the earthquakes have strike-slip mechanisms. Thus, in the northern part of the CENA, the least principal stress is approximately vertical; while in the southern part, the least principal stress is approximately horizontal. The EGLB straddles this transition zone from thrust to strike-slip faulting regimes, and this is seen in the few earthquake focal mechanisms from the region (Fig. 3). Throughout the CENA there is a very strong correlation between the SHmax direction and the orientation of the stresses caused by the ridge push force acting on the North American plate (Richardson, 1992; Zoback, 1992a; Coblentz and Richardson, 1995). However, the change from thrust faulting in the north to strike-slip faulting in the south necessitates some laterally varying stress field that is not due to plate driving forces (Zoback, 1992b). Zoback (1992b) showed that stresses caused by continental postglacial rebound have the correct orientation to cause this change in focal mechanisms. However, the postglacial rebound stresses are very small in magnitude, making it problematic to use this as an explanation for the southward change in earthquake focal mechanisms (Zoback, 1992b). Zoback (1992b) suggested that perhaps compressive stresses related to the support of a dense lower crustal structure beneath the St. Lawrence rift may contribute to the observed change from thrust to strike-slip earthquakes in the EGLB region. In a theoretical analysis of postglacial strain rates using a simple Laurentide glacial loading model, James and Bent (1994) determined that the postglacial strain rates are one to three orders of magnitude greater than the strain rates from the observed seismicity. They also argued that for the northeastern US and southeastern Canada the predicted glacial rebound strain rates are not oriented so as to augment the observed deviatoric earthquake stresses except for the St. Lawrence Valley region. James and Bent (1994) noted that the discrepancy between the observed seismic strain rates compared to the predicted postglacial strain rates could be due to the misorientation of the principal stress directions of the postglacial rebound relative to those of the earthquakes or it might indicate that the lithosphere can undergo sig-
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Fig. 3. Map of major structural features of the CENA. The New Madrid rift and proposed extensions are from Braile et al. (1982). The St. Lawrence rift, the Ottawa rift and the possible extension of the St. Lawrence rift are from Adams and Basham (1991). The eastern boundary of the Grenville craton is from Wheeler (1996). The focal mechanisms of the earthquakes in the EGLB are from the following sources: 1966 and 1967 earthquakes at Attica, NY, from Herrmann (1979); 1986 Leroy, Ohio earthquake from Nicholson et al. (1988); the 1986 Anna, Ohio earthquake from Schwartz and Christensen (1988); and the 1998 Pymatuning, Pennsylvania earthquake from the USGS.
nificant deformation without a concomitant level of seismicity.
4. Relations between earthquakes and structures Perhaps the most enigmatic aspect of the CENA seismicity is the lack of clearcut evidence that identifies which are the seismically active faults in the region, especially those capable of major earthquakes. While most of the CENA earthquakes occur on or in the vicinity of large-scale structures that are fractured and faulted, evidence for neotectonic movement on
individual faults is almost invariably lacking. In the New Madrid seismic zone, there are some faults on which neotectonic deformations have been found (e.g., Van Arsdale et al., 1998), but in the northeastern US (Ebel and Kafka, 1991), southeastern US (Bollinger et al., 1991) and southeastern Canada (Adams and Basham, 1991), confirmed evidence of active tectonic faulting has yet to be documented. Curiously, the Ungava peninsula earthquake in 1989 (Adams et al., 1992) caused surface rupture along surface lineaments that had not had tectonic displacement since Paleozoic time. Thus, the lack of evidence of modern surface faulting in the CENA apparently says little about the
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potential for future large (M > 6) earthquakes in the region. If Precambrian lineaments or faults with no previous Phanerozoic tectonic movement can suddenly reactivate in modern times with a large earthquake, then the Paleozoic and later cover rocks may be hiding potentially seismogenic basement faults throughout the EGLB. One way in which seismically active faults might be identified is to find faults upon which recorded earthquake activity has been concentrated. For the EGLB, Seeber and Armbruster (1993) reported that there is historic and recent earthquake activity concentrated along the Akron aeromagnetic lineament in Ohio, along the Clarendon– Linden Fault (CLF) in western New York, parallel to basement structures at the western end of Lake Ontario, and along the Niagara gorge between New York and Ontario. Of these seismic zones, the Clarendon – Linden Fault has received the most attention. Seeber and Armbruster (1993) report that there have been several instrumentally recorded earthquakes that have been concentrated near the main strand of the CLF. Their analyses of the confidence limits of the locations of the 1929 mb 5.2 Attica earthquake and of an M3.5 earthquake near Attica in 1844 indicate that both of these events could also have been associated with the CLF. Both Jacobi et al. (1996) and Wallach et al. (1998) regard the CLF as seismically active. However, Brennan (1998) notes that relatively few epicenters are known from the vicinity of the CLF and that these epicenters do not define curvilinear trends along the fault. These observations, combined with the orientation of the fault at 70j from the ambient stress direction and the small amount of displacement since lower Silurian, led Brennan (1998) to conclude that the CLF is only a minor contributor to the natural seismicity of the region. Wallach et al. (1998) note that the western Lake Ontario area has a higher level of earthquake activity than adjacent regions. This locally elevated earthquake activity is occurring in an area in which three geologic structural trends intersect. The first is the Niagara – Pickering zone, which Wallach et al. (1998) infer to be a northeastern extension of the Akron lineament of Seeber and Armbruster (1993). This interpretation of Wallach et al. (1998) connects the Niagara gorge seismicity with that in Lake Ontario, although Seeber and Armbruster (1993) explain the seismicity near
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Niagara Falls as due to the rapid erosion of the river bed by the waterfall. The second intersecting geologic trend is a zone of faults and lineaments that extends from Georgian Bay to western Lake Ontario and into New York State. The third zone is a lineament along a possible fault from Hamilton to Lake Erie. In all three cases, Wallach et al. (1998) do not feel that there is sufficient evidence at the present time to regard any of these features as necessarily seismogenic. Even so, they do regard the western Lake Ontario area as one where a major earthquake could be possible. The existence of a few earthquakes hypocenters in the vicinity of a basement fault or lineament is not convincing evidence that the fault or lineament is currently capable of generating a large earthquake. Both Ebel (1983) and Seeber and Armbruster (1993) observe that the spatial association of modern earthquakes and preexisting faults could be due to the reactivation of the old faults or to the formation of new fractures that nucleate on or near the preexisting faults. In the latter case, the seismic events may nucleate on the preexisting fault, but the ruptures associated with the earthquakes may be new cracks that form along some new direction rather than following the existing basement fault. This means that modern earthquakes might show an apparent spatial association with preexisting faults without those faults themselves being favorably oriented with the modern stress field or themselves being capable of slipping in a large earthquake. If indeed the modern earthquakes represent the formation of new basement faults, then the sizes of preexisting faults do not give any indication of how large an earthquake might occur in any modern seismically active area. The question of whether the earthquakes are reactivations of old faults or are new fractures could be addressed by better seismic monitoring of the major faults in the region to provide high-quality earthquake hypocenters and focal mechanisms. On a regional basis, the structures with which most of the CENA earthquakes are spatially associated include the buried rift structures of the Mississippi embayment, the St. Lawrence rift, the Saguenay graben, the Ottawa – Bonnechere graben, part of the Adirondack uplift, and the Appalachian orogen, especially that part involved in Triassic rifting (Fig. 3). In general, these active regional structures are features left over from previous continental rifting episodes.
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Johnston et al. (1994) documented that most of the great (M >7) earthquakes in the stable cratons occur in previously rifted crust, and especially in crust that has rifted in Mesozoic and Cenozoic time. Much of the CENA seismicity outside the EGLB appears to fit this pattern, and this may be true of the EGLB as well. Wheeler (1995, 1996) argued that many of the CENA earthquakes are concentrated on late Proterozoic or early Paleozoic normal faults of the cratonic Iapetan passive margin, which formed during a continental rifting episode in the early Paleozoic along the eastern Grenville margin. While much of this boundary is buried today by thrust sheets and nappes of the Paleozoic Appalachian orogen, Wheeler (1995, 1996) traces the Iapetan normal faults in a broad zone from central Alabama through eastern Quebec to Labrador. In the EGLB, Wheeler (1995, 1996) indicates that Iapetan faults occur beneath all but the westernmost and southeasternmost edges of Pennsylvania and New York and that they extend into northwestern Vermont and southeastern Quebec. Evidence for Iapetan normalfault activation of the Clarendon –Linden Fault zone in western New York State (Jacobi and Fountain, 1998a,b) supports Wheeler’s (1995) argument that the western Iapetan faults must extend to at least this area. Also, Harper and Jacobi (2000) inferred from a detailed joint study that fractures related to the St. Lawrence rift system may extend as far to the southwest as southwestern Lake Ontario. Thus, there is much geological and geophysical evidence that the basement of at least the eastern half and perhaps much of the ELGB contains crust that experienced normal faulting as late as Upper Devonian time. The possible existence of rifted crust beneath western Lake Ontario and into Lake Erie was postulated by Adams and Basham (1991), who noted that this could explain the highly speculative idea of Woollard (1969) that the seismicity along the St. Lawrence rift system in Quebec might be connected southwest through Lakes Ontario and Erie and from there southwest through Ohio and Indiana to the New Madrid seismic zone. Buried rift structures beneath the EGLB would explain the EGLB seismicity consonant with the ideas of Johnston et al. (1994) and Wheeler (1996) that much of the seismicity of the stable craton is associated with preexisting continental rift faults. However, why some parts of the continental rifts (e.g., New Madrid and Charlevoix) would be highly
active while other sections are relatively inactive is not understood. Perhaps different parts of the rift systems are active at different times. Thus, New Madrid and Charlevoix are active now, but a few tens of thousands of years from now those areas might have much lower seismicity while other parts of the preexisting rift structures might be more active. Such an idea could explain the lack of evidence of significant neotectonic topography at Charlevoix and New Madrid (Schweig and Ellis, 1994).
5. Paleoseismological studies in CENA and implications for the EGLB In the CENA, paleoseismology studies have employed liquefaction features (e.g., Obermeier et al., 1985), fault-related deformation (e.g., Kelson et al., 1996), silt layers in lake deposits (Doig, 1990) and subaqueous slides (Shilts, 1984) to identify prehistoric earthquakes in the geologic record and have been conducted in highly active, moderately active and relatively inactive regions. Although its length varies from 1000 to 12,000 years, and rarely up to 20,000 years, in different regions of the CENA, the paleoearthquake record, in general, appears to reflect historic and modern seismicity (Fig. 4). In those regions that have been studied, seismicity appears to have been relatively stable for at least the past few thousand years. In the New Madrid seismic zone (NMSZ), one of the most seismically active regions of the CENA, liquefaction features indicate that M > 7.5 earthquakes have occurred every 200 –800 years during the past 1200 years for an average recurrence interval of 500 years (e.g., Tuttle et al., 2002b; Cramer, 2001). Similarly, deformation along the Reelfoot scarp, thought to reflect faulting at depth, suggests an average repeat time of 400 –500 years during the past 1200 years (Kelson et al., 1996). These recurrence estimates for large earthquakes are consistent, albeit on the shorter side, with the frequency – magnitude relation developed by Johnston and Nava (1985) from historical and instrumentally recorded seismicity in the New Madrid region. A diffuse concentration of moderate earthquakes extends north from the NMSZ into southeastern Missouri, southern Illinois and southwestern Indiana. Along the northern boundary of the Mississippi
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Fig. 4. Map showing locations and results (M—magnitude estimate; RI—recurrence interval) of paleoseismological studies in the CENA. NM—New Madrid seismic zone; EF—English Hills fault; SL—St. Louis region; SI—Springfield, IL; LW—Lower Wabash valley; CH— Charlevoix seismic zone; LL—Lower St. Lawrence region; SR—Saguenay region; CS—Charleston, SC; MN—Massena, NY; MA— northeastern Massachusetts; AN—Attica, NY; RR—Rouge River, Ontario. The recurrence interval times are based on calibrated radiocarbon ages and are in years bp.
embayment, north – northeast and northeast oriented strike-slip faults are mapped above the northwestern margin of the buried Reelfoot rift (Harrison and Schultz, 1994). One of these, the English Hill fault, juxtaposes Tertiary and Cretaceous deposits against Late Wisconsin Peoria loess and provides the first direct evidence of Quaternary age faulting in the embayment and outside the NMSZ (e.g., Palmer et al., 1997). Investigations of several other geologic structures located northwest of the Mississippi embayment with which earthquakes are spatially associated found no evidence of Holocene faulting (Tuttle et al., 1999). Paleoliquefaction studies, however, have found
evidence for at least two episodes of strong ground shaking near St. Louis, MO, during the past 7000 years (e.g., McNulty and Obermeier, 1997; Tuttle et al., 1999). McNulty and Obermeier (1997) attribute paleoliquefaction features in Illinois southeast of St. Louis to a local event of M > 6 during the midHolocene. Tuttle et al. (1999), who have found additional liquefaction features in Illinois and Missouri, think that this and other possible interpretations of the data warrant additional testing due to poor age constraints of liquefaction features in the region. North-central Illinois has experienced only small earthquakes during the historic period. Even so,
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paleoliquefaction features have been found near Springfield and are interpreted as evidence for a local M >6 earthquake about 6500 radiocarbon years ago (e.g., Hajic and Wiant, 1997; McNulty and Obermeier, 1997). The largest historic earthquake in the lower Wabash valley of southern Indiana and Illinois was of M 5.5. Paleoliquefaction studies in this area indicate that at least eight prehistoric earthquakes of M >5.5 have occurred there since the Late Pleistocene (e.g., Munson et al., 1997). Two of the events are estimated to be of M >7 and to have struck the area about 6000 and 12,000 radiocarbon years ago. It is interesting that such a large region north of the NMSZ appears to have been affected by several large earthquakes during the mid-Holocene. As suggested by its timing, perhaps this seismicity was triggered by glacial unloading of the crust. Alternatively, the large mid-Holocene earthquakes in Indiana and Illinois may reflect an active period along the postulated extensions of the Reelfoot and St. Lawrence rift systems. A paleoseismology study was conducted in the Western Quebec seismic zone and the meizoseismal area of the 1944 M 5.8 Massena, NY – Cornwall, Ontario, earthquake. The study involved relocation and excavation of a site of earthquake-induced liquefaction in 1944 and reconnaissance for other liquefaction features (Tuttle, 1996). The study was hampered by disturbance of the liquefaction site during construction of the Eisenhower Locks and by poor exposure in the area. Although a few, very small liquefaction features were found, there was no evidence for large earthquakes during the past 12,000 years. In the Charlevoix seismic zone, a 2300-year record of ground shaking was established by studying lake sediments. The paleoseismic record suggests an average repeat time of 75 years for M >6 earthquakes (Doig, 1990). In the less seismically active Saguenay region to the north, lake sediments indicate that M >6 events have occurred every 350 –1000 years during the past 3000 years (Doig, 1998). This is supported by earthquake-induced liquefaction features that suggest that a M > 6 earthquake struck the region between 370 and 770 years before the 1988 M 5.9 Saguenay earthquake (Tuttle, 1994). The source of the large earthquake could not be determined without further study.
Along the Atlantic Seaboard, liquefaction features have been found in Massachusetts and from North Carolina to Georgia. In northeastern Massachusetts, where two magnitude 5.5 – 6.0 earthquakes occurred in 1727 and 1755, liquefaction features suggest only one similar-size event during the preceding 4000 years (Tuttle and Seeber, 1991). In the vicinity of Charleston, South Carolina, liquefaction studies indicate that M > 7 earthquakes have occurred every 500– 600 years during the past 1000 years (e.g., Talwani et al., 1999). To the north and south of Charleston, the earthquake potential appears to be considerably less, with repeat times for M > 6 earthquakes of more than 2000 years. Four paleoseismology studies, covering relatively small areas, have been conducted in the EGLB. These studies include searches for earthquake-induced liquefaction features along the Clarendon – Linden fault system in western New York State and in the St. Lawrence Lowlands east of Lake Ontario, and evaluation of faults exposed along the Rouge River near Toronto, Ontario. No unequivocal evidence of earthquake-induced liquefaction was found in Late Wisconsin and Holocene sediments along the Clarendon– Linden fault system, including the epicentral area of the 1929 M 4.9 Attica earthquake (Tuttle et al., 2002a). Coates (1975) conducted a search for earthquake-related deformation structures in 200 sand and gravel pits in the St. Lawrence Lowlands. Although abundant, most of the deformation stuctures found in Quaternary sediments were attributed to glaciation. Along the Rouge River, E – W to W– NW oriented normal faults displace the Scarborough Formation (ca. 70 ka) up to 1.25 m, but do not affect the overlying Halton till ( < 13 ka). Mohajer et al. (1992) attribute the Pleistocene-age faulting to either extension related to glacial rebound or reactivation of bedrock joints in response to the W –NW-oriented maximum-principal stress. In contrast, Adams et al. (1993) argue that the Rouge River faults can be explained by glaciotectonics. These four studies found no unequivocal evidence for large earthquakes in western New York, the St. Lawrence Lowlands, and southernmost Ontario, since the Late Wisconsin. These results suggest that the modern, low seismicity rate may be characteristic of the long-term behavior of the region. If the EGLB has the potential to generate M >6 earthquakes, the recurrence interval of these large events may be longer than at least 12,000 years. At this time, however, too little
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of the EGLB region has been studied to draw definitive conclusions.
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natural event (i.e., not induced) and was located in the vicinity of the western branch of the Clarendon – Linden fault system.
6. Induced seismicity in the CENA 7. Summary and conclusions While it is clear from early historical records that many natural earthquakes take place in CENA (e.g., Ebel, 1996), some of the earthquakes in the area may not be natural seismicity but rather may be induced by man-made activities. Several types of human activity can induce earthquakes. Fluid injection and extraction have been found to induce earthquakes in several localities: at the Gobles oil field in southwestern Ontario (Mereu et al., 1986); along the Clarendon – Linden Fault in western New York State due to salt brine recovery operations (Fletcher and Sykes, 1977); and near Ashtabula, OH, due to waste injection (Seeber and Armbruster, 1993). In eastern New York, quarrying at Wappingers Falls appears to have induced a nearby earthquake (Pomeroy et al., 1976), and in eastern Pennsylvania, changes in the pumping of the abandoned Wyomissing Hills quarry were documented around the time of a pair of earthquakes centered at the quarry (Armbruster et al., 1994). In the southeastern US, reservoir-induced seismicity has been documented at several localities (Bollinger et al., 1991). Thus, some of the earthquakes shown in Fig. 1 may have been induced events and are not indicative of natural seismicity. In some situations, there is debate about earthquakes being induced or natural. Nicholson et al. (1988) argued that the 1986 M 5.0 Leroy, OH, earthquake was probably not induced, even though there were waste injection wells within 15 km of the epicenter. Ammon et al. (1998) think that the recent moderate earthquakes at the Wyomissing Hills quarry in Pennsylvania would have taken place even in the absence of the quarry. Seeber and Armbruster (1993) report that salt brine extraction was already under way at Dale, NY, at the time of the nearby Attica earthquake in 1929 but that the epicenter of the 1929 event is too poorly determined to firmly attribute it to mining activities. However, Tuttle et al. (2002a) point out that ground failure related to the mainshock as well as felt reports of a possible aftershock in the same area 3– 4 km west of Dale suggest that the 1929 earthquake may have been a
Over all, EGLB seismicity is typical of that found throughout much of the CENA. The EGLB seismicity rate is less than that in much of the Appalachians and the Atlantic Seaboard but is greater than the earthquake rates detected further northwest toward the center of the craton. The EGLB earthquakes seem to be occurring in the Grenville basement, similar to other parts of the CENA. The source of the stress driving the seismicity is predominantly plate tectonics, especially ridge push from mid-Atlantic spreading. However, some other source of stress, perhaps postglacial rebound, must contribute to the change in average focal mechanism from thrust in the north to strike-slip in the south of the EGLB. On a regional basis, many of the earthquakes in the EGLB have occurred in areas of preexisting faults, at least some of which may have been active during past episodes of continental rifting. On a global scale, most major earthquakes in stable cratonic crust, like that in the EGLB, have occurred in previously rifted crust. However, for individual faults that have been studied in some detail, the relation between the local earthquakes and the preexisting faults has not been clearly deciphered. It is not clear whether the local earthquakes represent reactivations of the old faults or the nucleation of new ruptures in or near the old fault zones. Most evidence of large prehistoric earthquakes has been found in more seismically active parts of the CENA, with no unequivocal evidence of strong ground shaking uncovered so far in the EGLB. Some of the earthquakes in the EGLB may be induced by fluid injection, mining and quarrying activities and, therefore, are not indicative of the natural seismicity of the region.
Acknowledgements We are grateful for the constructive reviews of this manuscript by Robert Fakundiny, Robert Jacobi, Mike Lewis, Roy Van Arsdale and an anonymous reviewer.
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