State of Stress in Central and Eastern North American Seismic Zones

State of stress in central and eastern North American seismic zones

Stephane Mazzotti1 and John Townend2 1GEOLOGICAL SURVEY OF CANADA, NATURAL RESOURCES CANADA, P.O. BOX 6000, SIDNEY, BC V8L 4B2, CANADA AND SCHOOL OF EARTH AND OCEAN SCIENCES, UNIVERSITY OF VICTORIA, PO BOX 3065 STN CSC, VICTORIA, BC V8W 3V6, CANADA 2SCHOOL OF GEOGRAPHY, ENVIRONMENT, AND EARTH SCIENCES, VICTORIA UNIVERSITY OF WELLINGTON, P.O. BOX 600, WELLINGTON 6140, NEW ZEALAND

ABSTRACT

We use a Bayesian analysis to determine the state of stress from focal mechanisms in ten seismic zones in central and eastern North Amer- ica and compare it with regional stress inferred from borehole measurements. Comparisons of the seismologically determined azimuth of

the maximum horizontal compressive stress (SHS) with that determined from boreholes (SHB) exhibit a bimodal pattern: In four zones, the

SHS and regional SHB orientations are closely parallel, whereas in the Charlevoix, Lower St. Lawrence, and Central Virginia zones, the SHS

azimuth shows a statistically signiﬁ cant 30°–50° clockwise rotation relative to the regional SHB azimuth. This pattern is exempliﬁ ed by the

northwest and southeast seismicity clusters in Charlevoix, which yield SHS orientations strictly parallel and strongly oblique, respectively,

to the regional SHB trend. Similar ~30° clockwise rotations are found for the North Appalachian zone and for the 2003 Bardwell earthquake

sequence north of the New Madrid zone. The SHB/SHS rotations occur over 20–100 km in each seismic zone, but they are observed in zones separated by distances of up to 1500 km. A possible mechanism for the stress rotations may be the interaction between a long-wavelength stress perturbation source, such as postglacial rebound, and local stress concentrators, such as low-friction faults. The latter would allow low-magnitude (<10 MPa) postglacial rebound stresses to locally perturb the preexisting stress ﬁ eld in some seismic zones, whereas postglacial rebound stresses have little effect on the intraplate state of stress in general.

LITHOSPHERE; v. 2; no. 2; p. 76–83, Data Repository 2010020. doi: 10.1130/L65.1

INTRODUCTION Lower St. Lawrence 50° Ontario As in most continental intraplate regions, Quebec stress orientations in central and eastern North Charlevoix America are broadly homogeneous and con- 48° Gatineau sistent over 1000–5000 km spatial scales (e.g., Zoback and Zoback, 1980, 1991; Zoback, 46° Ottawa 1992a). East of the Cordillera deformation front, North the main characteristic of the tectonic stress ﬁ eld Montreal Appalachian is a NE-SW axis of maximum horizontal com- 44°

pressive stress (SH), which is commonly attrib- uted to far-fi eld plate-boundary forces, and in 42° particular to Mid-Atlantic Ridge push (Zoback, 1992a; Richardson, 1992). In a case study of 32 Ohio eastern North American earthquakes, Zoback 40° IL (1992b) noted that most focal mechanisms were km compatible with the regional stress fi eld inferred 38° Central 0 100 200 from borehole data. However, fi ve mechanisms New Virginia suggested local stress perturbations, and two 36° Madrid TN M=6.0 other events implied atypical frictional condi- W M=5.0 tions (e.g., superlithostatic pore-fl uid pressure). W 34° Eastern M=4.0 The spatial distribution of earthquakes in W Tennessee Charleston M=3.0 central and eastern North America is largely W 32° M=2.0 associated with late Proterozoic Iapetus Rift W structures (e.g., Adams and Basham, 1991; 266° 268° 270° 272° 274° 276° 278° 280° 282° 284° 286° 288° 290° 292° 294° 296° 298° Johnston et al., 1994). As shown in Figure 1, Figure 1. Central and eastern North America seismicity and Iapetus Rift structures. Dark- and light- the main concentrations of both small and large gray dots show magnitude M ≥ 3 and M ≥ 5 earthquakes since 1973 (Geological Survey of Canada earthquakes lie along the rifted margin itself or and U.S. Geological Survey catalogs). Focal mechanism (red) sources are given in the supplemen- failed rift arms. The Iapetus structures are typi- tary material (see text footnote 1). Thick gray barbed lines indicate the Iapetus rifted margin and cally reactivated under the present-day stress failed rifts. The ten seismic zones discussed in the text are shown by solid ellipses.

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fi eld as thrust or strike-slip faults. Aside from We focus on ten of the main eastern North which we can compute the median trend and this fi rst-order correlation, rather little is known American seismic zones, which we defi ne as associated confi dence intervals. about the geological and geophysical conditions areas encompassing more than fi ve focal mech- generating intraplate earthquakes and the asso- anisms within a distinct geological and seismo- Borehole Stress Data ciated hazard (cf. Stein and Mazzotti, 2007, and logical area (Fig. 1). The Lower St. Lawrence, references therein). Thus, characterization of the Charlevoix, Montreal, and Eastern Tennessee In order to perform a detailed comparison of regional stress fi eld and local state of stress is zones are associated with the Iapetus rifted mar- our seismological stress estimates with borehole essential for improving our understanding of the gin, and the Ottawa and New Madrid zones are crustal stress measurements on a regional scale, source of intraplate seismicity and better quanti- associated with Iapetus failed rift arms (Adams we extracted stress magnitude and orientation fying seismic hazard. and Basham, 1991; Wheeler, 1995). The Gatin- data from the World Stress Map and the Cana- In this study, we analyze a newly compiled eau seismic zone may be related to the trace dian Crustal Stress databases (Zoback, 1992a; data set of focal mechanisms to estimate stress of the New England Seamount Chain hotspot Adams, 1995; Heidbach et al., 2008). For each parameters and characterize the prevailing state (Adams and Basham, 1991). The North Appala- seismic zone, we calculated a weighted mean

of stress within ten of the main seismic zones chian, Central Virginia, and Charleston seismic SH orientation for borehole breakout and hydro- of central and eastern North America. We com- zones are not afﬁ liated with clear lithosphere- fracture measurements with qualities of A, B, pare our results with regional borehole stress scale structures: in the ﬁ rst two, the seismicity or C made at depths greater than 0.5 km within information from the World Stress Map and occurs within the Appalachian formations that 250 km of each seismic zone. The weighted Canadian Crustal Stress databases to determine overlie rifted Grenville crust (Munsey and Bol- mean and its associated standard deviation were the consistency of seismological and borehole linger, 1985; Adams and Basham, 1991). Other estimated by assigning weights of 3, 2, and 1

estimates of the state of stress near the seismic seismicity clusters can be identiﬁ ed in central to the A, B, and C quality SH measurements,

zones and to elucidate potential stress pertur- and eastern North America, but the paucity of respectively. In most cases, the borehole SH data bations associated with regionally elevated focal mechanisms precludes any stress analysis. are located outside of the seismic zones and earthquake activity. are unevenly distributed. Because of the strong Seismological Stress Estimation regional coherence of the borehole stress orien- DATA SETS AND STRESS ESTIMATION tations over distances of hundreds of kilometers TECHNIQUE We used a Bayesian estimation technique (Zoback, 1992a), we assume that these 250 km

(Arnold and Townend, 2007) to determine the SH averages provide appropriate estimates of the Focal Mechanisms set of stress parameters most consistent with regional stress orientation outside of the seismic the focal mechanisms in a given group. We zones. The earthquake focal mechanism data set is weighted each focal mechanism by convert- shown in Figure 1. (See supplementary mate- ing the aforementioned angular errors to a RESULTS rial for references and focal mechanism octant scalar Matrix Fisher concentration parameter plots.1) It constitutes mostly ﬁ rst-motion solu- (Equation A12 of Arnold and Townend, 2007). Eastern North America tions complemented by a small number of This approach yields a joint probability den- moment tensors (~15% of the total). In most sity function of the three angles specifying the The results are shown in Figures 2 and 3 and

cases, a quality factor was provided in the origi- principal stress axes’ orientations and a single listed in Table 1 in terms of median SHS trend

nal study in the form of a qualitative level (A–D) stress magnitude ratio parameter. Its principal and 90% conﬁ dence interval (90CIS); the sub- or a quantitative angular uncertainty. In order to advantages, in comparison with other stress script “S” is used to denote a seismological esti- obtain a coherent set of inversion results, we estimation algorithms, are the incorporations mate. The 90% conﬁ dence interval is ~20°–40° standardized these quality rankings by assigning of focal mechanism uncertainties, nodal plane in most cases. The Ottawa seismic zone stands

small (±15°), medium (±25°), or large (±35°) ambiguity, and the weak constraint imposed out with a very large 90CIS of 132°, indicating uncertainties to each mechanism’s strike, dip, on the stress tensor by any single focal mecha- that the directions of the principal horizontal and rake angles, which are assumed to be inde- nism. This last point is particularly important stresses are not constrained by this small data pendent (cf. Walsh et al., 2009). In those cases because it allows us to derive conﬁ dence inter- set (eight mechanisms, two of which have large in which the original study did not provide a vals for the stress tensor parameters that take uncertainties). The results for this zone are not quality level, we assigned an uncertainty level into account the degree of similarity between used for further discussion.

on the basis of the data-analysis description. In focal mechanisms within individual seismic In all but one case, the estimated SHS orienta- addition, focal mechanism uncertainties were zones (Arnold and Townend, 2007). As shown tion lies in the NE-SW quadrant, roughly paral-

adjusted on the basis of earthquake magnitudes in the octant plots (supplementary material [see lel to the general SHB trend inferred from bore- to take into account the potentially higher uncer- footnote 1]), the diversity of mechanisms can hole measurements (Fig. 3); the subscript “B” is tainty and lower quality of small-magnitude vary signiﬁ cantly between zones. used to denote a borehole estimate. For the New solutions: all M < 2.5 and 2.5 ≤ M < 3.5 mecha- We computed the trend of the axis of maxi- Madrid, Eastern Tennessee, Montreal, and Gatin-

nisms were assigned large and medium uncer- mum horizontal compressive stress (SH) using eau seismic zones, the median seismological and

tainties (±35° and ± 25°), respectively. the algorithm described by Lund and Townend average borehole SH orientations (SHS and SHB, (2007). Such a transformation is necessary when respectively) are consistent to within 5°–15°, 1GSA Data Repository Item 2010020, focal mecha- none of the three principal stresses is strictly ver- well within the respective 90% conﬁ dence inter- nism data set, octant plots, and Charlevoix stress tical, and it enables all four determinable param- vals. The lack of borehole S data within 250 km analyses, is available at www.geosociety.org/pubs/ HB ft2010.htm, or on request from editing@geosociety eters to be amalgamated into a single, physically of the Charleston seismic zone precludes a direct .org, Documents Secretary, GSA, P.O. Box 9140, Boul- intuitive and readily illustrated parameter. This comparison with the SHS orientation. How-

der, CO 80301-9140, USA. produces a probabilistic description of SH from ever, the nearest SHB measurement (C quality at

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255 km) has an orientation that lies within the

90% conﬁ dence interval of SHS (Fig. 2). For the North Appalachian seismic zone, the median S Lower St. Lawrence Charlevoix Ottawa HS and average SHB values differ by 33°, but the 90% conﬁ dence intervals overlap by 13° (Fig. 2). The three remaining seismic zones (Central Virginia, Lower St. Lawrence, and Charlevoix) show sta-

tistically signiﬁ cant differences between SHS and

SHB azimuths and are discussed in more detail in the following sections.

Central Virginia Seismic Zone

The state of stress in the Central Virginia seismic zone was determined by the focal Gatineau Montreal North Appalachian mechanisms of 11 small earthquakes (M < 3) recorded during 4 yr in the early 1980s (Mun- sey and Bollinger, 1985), complemented by two M ~4 earthquakes that occurred in 2003. These events all occurred at regionally typical depths shallower than ~15 km within the allochtho- nous, thrust-faulted, Appalachian formations that override the Precambrian Grenville basement (Bollinger et al., 1985; Munsey and Bol- linger, 1985).

The SHS orientation is rotated 48° clockwise

with respect to the regional borehole SHB trend,

and the 90% conﬁ dence intervals of SHS and SHB Central Virginia Eastern Tennessee New Madrid are separated by 13° (Table 1; Fig. 2), indicating that the two estimates are signiﬁ cantly distinct. Using the same data set, excluding the 2003 events, Kim and Chapman (2005) inferred

a slightly larger clockwise rotation of SHS rela-

tive to SHB (~68°), with an azimuth and plunge

of N133° and 14°SE for the S1 axis (cf. N086° and 7°W for our results; Fig. 2). The difference

in orientation and plunge of S1 between the two analyses may be in part due to the difference in the stress inversion technique (Bayesian versus grid-search). Alternatively, the difference in results may reﬂ ect the small difference in the Charleston Explanation data sets in relation to the complexity of the seis- S mological state of stress in the Central Virginia 2 seismic zone. The 13 focal mechanisms have P S1 axis orientations in both NE-SW and NW-SE directions, suggesting that the stress regime may change from mainly thrust to mainly strike slip S S HS with depth (Munsey and Bollinger, 1985). SHB 3 Lower St. Lawrence Seismic Zone

The Lower St. Lawrence seismic zone is Figure 2. State of stress in seismic zones. Lower-hemisphere stereonets show the posterior distribu- delineated by pronounced microseismicity located mainly beneath the St. Lawrence River tions of the principal stress axes (S1—red, S2—green, S3—blue). Dashed red lines indicate the median

orientation of the seismologically determined maximum horizontal compressive stress SHS. Dotted (Fig. 1) (Adams and Basham, 1991). The best- gray lines and gray angular sectors indicate the average and 90% conﬁ dence region of the maximum constrained hypocenters place the earthquakes horizontal compressive stress from borehole data S (Table 1). New Madrid: The black dotted line HB between 7 and 25 km depth, within the rifted shows the S orientation for the Bardwell earthquake sequence (Horton et al., 2005). 1 Precambrian Grenville basement and well below the overthrust Appalachian formations (Adams and Basham, 1991; Lamontagne et al., 2003).

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Although tentative correlations have been made between the earthquakes and the Iapetus Rift faults, most of the seismicity appears to occur within fractured volumes of the rift system 50° rather than on the main rift faults themselves (Lamontagne et al., 2003). The S orientation is rotated 44° clock- 48° HS wise compared to the regional borehole stress

orientation SHB (Table 1; Fig. 2). The 90% 46° conﬁ dence intervals for the two estimates are separated by 8°, indicating a statistically sig- niﬁ cant difference. There are two important 44° caveats with regard to this observation: (1) the

SHB orientation and standard error are only 42° based on two B quality data points (individual azimuths of 54° and 65°; Fig. 3); and (2) the

SHS orientation is only deﬁ ned by 12 mecha- 40° nisms, exhibiting a relatively low diversity and

hence yielding a relatively large 90CIS value. 38° Notwithstanding these points, the seismological versus borehole stress rotation appears to

36° be robust, and the borehole SHB orientation is similar to the general direction for eastern Canada (Fig. 3). The seismological S value is 34° HS stable for several tested combinations of eight km or more (out of 12) mechanisms and yields a 32° 0 100 200 SHS orientation between N100° and N120°, in all cases strongly oblique to the regional bore- 272° 274° 266° 268° 270° 276° 278° 280° 282° 284° 286° 288° 290° 292° 294° 296° 298° hole data. Figure 3. Eastern North America maximum horizontal compressive stress. Solid and gray arrows indicate S orientation from borehole observations (A and B–C quality, respectively). Blue arrows HB Charlevoix Seismic Zone indicate borehole observations used in calculating the regional average within 250 km of the seis-

mic zones (solid ellipses). Red arrows and angular sectors indicate SHS orientation from focal mechanism inversion (median and 90% range, Table 1). The Charlevoix seismic zone is one of the historically most active earthquake concentrations in eastern North America, with fi ve M ≥ 6 earthquakes since 1663 (Lamontagne, 1987; Adams and Basham, 1991). Most earthquakes occur between 5 and 25 km depth beneath the St. Lawrence River. The main geological struc- TABLE 1. STRESS INVERSION RESULTS VERSUS LOCAL BOREHOLE STRESS ORIENTATION tures are related to the ca. 1100 Ma Grenville Seismic zone Seismological stress Borehole stress orogeny, ca. 650 Ma Iapetus rifting, ca. 450 Ma S 90CI S 90CI HS S HB B Appalachian orogeny, and a large meteor impact N (°) (°) R N (°) (°) S B ca. 350 Ma. Using remote-sensing, seismic, and Lower St. Lawrence 12 104 (R) 081–124 0.4 2 060 46–73 potential fi eld data, Lamontagne et al. (2000) Charlevoix 60 086 (R) 069–101 0.3 12 054 43–65 identifi ed the Saint-Laurent and Charlevoix Charlevoix NW 25 055 (R) 035–074 0.2 12 054 43–65 Charlevoix SE 35 101 (R) 086–114 0.4 12 054 43–65 faults (Fig. 4) as the main Iapetus Rift faults and Gatineau 19 038 (R) 029–048 0.8 6 043 28–58 the principal structures in the seismic zone. Ottawa 8 078 (R) 001–133 0.3 1 086 — No clear association can be found between Montreal 21 058 (R) 043–073 0.4 7 044 30–59 the seismicity and the main fault structures North Appalachian 12 070 (R) 059–083 0.6 5 038 5–71 (e.g., Lamontagne et al., 2000), but a fi rst-order Central Virginia 13 090 (R) 071–111 0.7 6 042 26–58 NW-SE clustering pattern is apparent in the East Tennessee 26 054 (S) 046–063 0.2 3 058 43–73 New Madrid 18 082 (S) 073–091 0.9 10 076 41–111 earthquakes’ epicenters. As shown in Figure 4, Charleston 11 064 (S) 050–077 1.0 1 040* — a largely aseismic, ~5-km-wide zone separates two distinct microseismicity clusters to the Note: NS—number of focal mechanisms; SHS and 90CIS—median and 90% confi dence interval azimuth of the axis of maximum horizontal compressive stress (R—reverse, S—strike-slip); R—median stress ratio southeast (mostly beneath the river) and the

(S1 – S2)/(S1 – S3); NB—number of borehole data; SHB and 90CIB—weighted average and 90% conﬁ dence northwest (mostly beneath the north shore; Ang- interval azimuth of the borehole axis of maximum horizontal compression within 250 km of the seismic zone. *Based on nearest borehole at 255 km. lin and Buchbinder, 1981). This gap coincides with an elongated high-seismic-velocity body (Vlahovic et al., 2003) and with the location of the Saint-Laurent fault (Fig. 4, cross section).

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NW exhibit clusters of earthquakes conﬁ ned to the km shallow (<15 km) Appalachian formations that override the deeper Grenville basement (Bol- σ 0 10 20 3 linger et al., 1985; Adams and Basham, 1991). 47° 48' 1925, M=6.2 A similar, although slightly smaller, clock- σ 1 σ wise rotation can be deduced for the 2003 M = 2 4.0 Bardwell earthquake sequence north of the New Madrid seismic zone. For the main shock and aftershock sequence, Horton et al. (2005) SE derived a S1 orientation of N104° with an uncer- Impact tainty of approximately ±20°. Assuming that 47° 36' σ 3 Crater SHS = S1, this result indicates a clockwise rotation of 20°–30° compared to our median estimates of S for the New Madrid focal mecha- σ HS 1 nisms (excluding the Bardwell sequence) and σ 2 SHB from nearby borehole data (Table 1; Fig. 2). Although not statistically signiﬁ cant, this appar-

47° 24' ent rotation suggests that large variations in NWSLF CHF SE stress orientation can occur over a distance of 0 ~40 km in the New Madrid–Wabash Valley area (Horton et al., 2005), similar to the ~20 km dis- 10 tance that separates the two seismic clusters in

SLF CHF the Charlevoix seismic zone.

20 It is important to consider whether the appar- 47° 12' ent stress rotations are artifacts stemming from 289° 12' 289° 24' 289° 36' 289° 48' 290° 00' 290° 12' 290° 24' the comparison between relatively shallow 30 010203040borehole data (0.5–2 km) and deeper seismicity (1–30 km). Vertical variations in stress orienta- Figure 4. Charlevoix maximum horizontal compressive stress. Earthquakes and focal mechanisms are tions have been reported over depth intervals as in Figure 1. Solid and dashed lines show Iapetus Rift faults and impact crater structure (Lamon- tagne et al., 2000). SLF—Saint-Laurent fault; CHF—Charlevoix fault. Lower-right insert shows a of 100–1000 m in individual boreholes (e.g., NW-SE depth cross section. The upper-left insert shows stereonets of the state of stress from inver- Hickman and Zoback, 2004), but in general, sion of focal mechanisms to the northwest (blue) and southeast (red) of the Saint-Laurent fault. deep borehole- and earthquake-determined stress orientations are similar and can be jointly accounted for in terms of, for instance,

Analysis of the 60 focal mechanisms in parallel to the regional SH orientation (Bent, dynamical modeling results (e.g., Townend and

the Charlevoix seismic zone yields a SHS ori- 1992; Fig. 4). The rotated SHS observed for the Zoback, 2004). Moreover, the eastern and west- entation of N086°, rotated 32° clockwise rela- southeast side of the Charlevoix seismic zone is ern Charlevoix results are based on earthquakes

tive to the regional SHB from nearby boreholes more compatible than the regional SH direction at comparable depths. Hence, we consider it (Table 1; Fig. 2). The 90% conﬁ dence intervals with the focal mechanism of the 1925 event, as unlikely that the observed rotations are depth- for the two estimates are distinct by 4°. The well as with numerous smaller thrust events that related artifacts. large number of focal mechanisms allows us to have fault planes striking NE-SW. Overall, therefore, we ﬁ nd that the angu-

test for spatial variations in stress orientations lar difference between SHS and SHB exhibits a amongst subsets of events. The most signiﬁ cant DISCUSSION bimodal pattern: four seismic zones exhibit good

difference in SHS orientation exists between the agreement between seismological and borehole

focal mechanism clusters to the southeast and Consistency and Extent of Stress Rotations estimates of the SH orientation (Gatineau, Mon- northwest of the Saint-Laurent fault (Fig. 4). treal, Eastern Tennessee, and New Madrid), and

For the northwest group, SHS is oriented N055°, Out of the ten seismic zones addressed in ﬁ ve imply 30°–50° clockwise rotations of the

parallel to the regional borehole data, whereas this study, the Central Virginia, Lower St. Law- seismological SHS relative to the regional bore-

SHS for the southeast group is oriented N101° rence, and Charlevoix seismic zones each show hole SHB (Lower St. Lawrence, Charlevoix, Cen-

and rotated clockwise by 47° compared to the a 30°–50° clockwise rotation of the seismic SHS tral Virginia, and, to a lesser extent, North Appa-

borehole trend (Table 1; Fig. 4). This rotation is orientation relative to the regional borehole SHB lachian and Bardwell–northern New Madrid). signifi cant at the 90% confi dence level, with a orientation that is signifi cant at the 90% confi - In the case of the Lower St. Lawrence, Central separation of 21° between the seismological and dence level. A similar clockwise rotation of 32° Virginia, and North Appalachian, the rotation

borehole confi dence intervals. Various earth- is observed for the North Appalachian seismic between SHS inside the seismic zone and SHB quake grouping and inversion tests (supplemen- zone, albeit with a 12° overlap of the seismo- outside the seismic zone occurs over distances tary material [see footnote 1]) indicate that the logical and borehole 90% confi dence intervals of 50–100 km (Fig. 3). For Charlevoix and NW-SE clustering is the grouping associated (Fig. 2). Although not statistically signifi cant, Bardwell–northern New Madrid, the rotation with the most signifi cant rotation. that result is worth noting in comparison with occurs over 20–40 km (Fig. 4). In the following The 1925 Charlevoix M = 6.2 earthquake the 48° clockwise rotation observed in the Cen- section, we consider the magnitude and poten- ruptured a steeply dipping thrust fault striking tral Virginia seismic zone: both seismic zones tial causes of the stress perturbation required to

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account for the 30°–50° clockwise rotation of TABLE 2. STRESS PERTURBATION MAGNITUDE AND ORIENTATION the horizontal principal stresses over such short θ γ Structure SHB SHS SL distances. Seismic zone (°) (°) (°) (°) (°) (SH – Sh)/SL (°) Lower St. Lawrence 045 060 104 15 44 0.88 135 Magnitude of the Stress Perturbations Charlevoix 035 054 086 19 32 1.09 125 Central Virginia 027 042 090 15 48 0.81 117 Note: Structure—average azimuth of primary regional structural direction; S and S —azimuth of axis In order to obtain a ﬁ rst-order estimate of the HS HB of maximum horizontal compressive stress from seismic inversion and borehole data (cf. Table 1). θ = S stress perturbation causing the observed rota- γ HB – structure; = SHS – SHB. (SH – Sh)/SL is ratio of local horizontal differential stress to stress perturbation tions, we consider the model of Sonder (1990) (SL, cf. text and Sonder, 1990). SL—azimuth of stress perturbation. and Zoback (1992a). In this model, the orientation of the maximum horizontal compressive stress (represented here by S ) in the vicinity HS 0 Data quality 0 of a major structure can be calculated in terms A of a regional stress orientation (represented 8 B and C 10 here by SHB) and a uniaxial stress perturbation

(SL) orthogonal to the strike of the structure. We assume that, for the Charlevoix, Lower St. 6 Lawrence, and Central Virginia seismic zones, 20 1 the stress responsible for the local rotation is perpendicular to the average local Iapetus Rift 4 μ = 1.0 Number 30 direction (Fig. 1). With this assumption, the Approximate depth (km) magnitude of the uniaxial stress perturbation SL Effective mean stress (MPa) Effective is found using Equation 8 of Zoback (1992a) to 2 40 2 be 80%–110% that of the regional horizontal differential stress (i.e., [S – S ]/S ~ 0.8–1.1; μ = 0.6 H h L μ Table 2). 0 50 = 0.2 Deep intraplate borehole data from a num- 0.0 0.2 0.4 0.6 0.8 1.0 0 1020304050 ber of locations worldwide reveal that at depths Stress ratio, R Differential stress (MPa) of as much as 8 km, the crust is in a state of Figure 5. Stress ratio and differential stress from borehole data. Left: Histogram of stress ratio R stress consistent with incipient frictional failure = (S – S )/(S – S ) from A, B, and C quality borehole stress measurements deeper than 0.5 km in given coeffi cients of friction μ ≈ 0.6–1.0 and 1 3 2 3 eastern North America. Right: Differential stress (S1 – S3) versus effective mean stress (mean stress the observed near-hydrostatic pore-fl uid pres- minus pore fl uid pressure) from borehole stress measurements deeper than 0.1 km of quality A sures (λ ≈ 0.4; Townend and Zoback, 2000). (open circles) and B or C (solid circles) in eastern North America. Similarly, stress magnitude data from boreholes in central and eastern North America (Adams

and Bell, 1991; Adams, 1995; Heidbach et al., tion SL of comparable magnitude is required to similar mechanism or source for seismic zones

2008) show a relationship between differential locally reorient SHB by 30°–50°, as implied by separated by large distances. The calculations

stress (S1 – S3) and effective mean stress that is the Lower St. Lawrence, Charlevoix, and Cen- discussed in the previous section imply a stress consistent with incipient frictional failure for tral Virginia stress orientation results. perturbation at mid-seismogenic depths of coeffi cients of friction μ > 0.5 (Fig. 5). Also Such high stress magnitudes clearly depend 160–250 MPa (in a representative high-friction, illustrated in Figure 5, a histogram of stress ratio on the assumed friction coeffi cient and the fl uid low-fl uid-pressure case) or 20–40 MPa (low measurements from the same boreholes exhibits pressure regime. If we assume that either a low friction or high fl uid pressure) and a maximum ≤ ≤ μ a pronounced peak at 0.5 RB 0.8 and mean friction coeffi cient ( = 0.1) or a near-lithostatic horizontal stress SL trending approximately λ ≈ of RB = 0.6, with few values smaller than 0.4. pore-fl uid pressure ( 0.9) prevails in the seis- N115°–135° (Table 2). The majority of these borehole stress measure- mic zones, the horizontal differential stress and Various mechanisms have been proposed

ments are located in the Canadian Shield, west the corresponding stress perturbation SL would as potential sources of local and regional stress and north of the main seismic zones. Thus, they be of the order of only ~20–40 MPa. However, perturbations, including topography, crustal may be considered as constituting a reference the low ambient stress magnitudes implied density anomalies, fault intersections, ﬂ exure state of stress for the continental crust outside of by those parameters contrast markedly with under local loads (e.g., sedimentary basins), seismically active regions. the values thought to characterize aseismic or and postglacial rebound (e.g., Stein et al., 1989; If we assume that the stress magnitude low-seismicity intraplate continental crust as a Talwani, 1988; Zoback, 1992a; Assameur and parameters in each seismic zone are similar to whole, as outlined herein (Fig. 5). Mareschal, 1995). Most of these mechanisms those in intraplate continental crust as a whole account for stress perturbations of at most a few (μ = 0.8, λ = 0.4, R = 0.6, ρ = 2700 kg m–3; see Potential Sources of the Stress tens of megapascals and would only result in Appendix A for details), the differential stress in Perturbations signiﬁ cant horizontal stress rotations if prevail- the horizontal plane at a mid-seismogenic zone ing horizontal stresses were at the low end of ≈ depth of 8 km is SH – Sh = S1 – S2 250 MPa The consistency of the 30°–50° clockwise stress levels calculated here. Flexural stresses

for a reverse stress state, and SH – Sh = S1 – S3 stress rotations observed from the St. Lawrence may reach several hundred megapascals under ≈ 160 MPa for a strike-slip stress state. This valley to the central Appalachians (and possibly sediment loads of 10 km thickness (Stein et analysis suggests that a local stress perturba- to the New Madrid–Wabash Valley) suggests a al., 1989), but such conditions do not appear

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applicable to continental seismic zones. Simi- CONCLUSIONS This would result in long-wavelength perturba- larly, large crustal density anomalies have been tions affecting only the weak zones. The Lower demonstrated to produce stress perturbations of (1) Comparisons of the axis of maximum St. Lawrence and Charlevoix seismic zones may

as much as ~200 MPa and rotations of up to 90° horizontal compressive stress determined from be examples of such mechanism, where the SHS (Zoback and Richardson, 1996). However, grav- earthquake focal mechanisms in seismic zones orientation in the seismic zones is parallel to

ity and seismic velocity data do not indicate the (SHS) and from borehole data near but outside that predicted by postglacial rebound models

presence of widespread high-density anomalies the seismic zones (SHB) reveal a bimodal pattern: that incorporate a local weak zone (Mazzotti et

associated with the Iapetus Rift structures in the SHS and SHB are closely parallel in four seismic al., 2006). Charlevoix and Lower St. Lawrence seismic zones (Gatineau, Montreal, Eastern Tennessee, zones (Lamontagne et al., 2000, 2003). and New Madrid); in three other zones (Lower APPENDIX A. STRESS MAGNITUDES One possible mechanism by which consis- St. Lawrence, Charlevoix, and Central Virginia), tent local stress perturbations may be gener- S is rotated 30°–50° clockwise compared to To estimate the regional stress magnitudes, we HS adopt the critically stressed crust model of Townend ated over large distances is the concentration of SHB; two other zones (North Appalachian and and Zoback (2000) in which the state of stress in the postglacial rebound stresses by local zones of Bardwell–northern New Madrid) show indica- crust is controlled by optimally oriented Andersonian weakness, perhaps including low-friction faults. tions of similar clockwise rotations but at a low faults on the brink of frictional failure equilibrium. Postglacial rebound stresses show a strong spa- confi dence level. In this case, the maximum and minimum effective stresses, S1 – Pf and S3 – Pf, are related by: tial coherence over distances of thousands of (2) The observed SHS versus SHB rotations are kilometers, but they are typically smaller than similar in both sense and magnitude in seismic SP− 2 1 f =++( μμ2 1 ) ≡ F, (A1) ~10 MPa (e.g., Wu and Hasegawa, 1996). How- zones separated by up to ~1500 km. SP− 3 f ever, the presence of a “weak zone” in the litho- (3) The observed SHS versus SHB rota- μ sphere, such as low upper-mantle viscosity or a tions occur over distances of no more than where Pf is the pore-fl uid pressure, and the coeffi - low-friction fault in the crust, can result in stress 50–100 km. In two cases (SE versus NW Char- cient of friction (see Zoback and Townend, 2001, and references therein). amplifi cations by a factor of 5–10 with respect levoix and Bardwell versus New Madrid), the In normal, strike-slip, and reverse stress states, the to homogeneous lithosphere models (e.g., Grol- rotation appears to occur over a distance of effective vertical stress SV is: limund and Zoback, 2001; Wu and Mazzotti, 20–50 km. 2007). In such cases, the modulation of large- (4) With respect to a simple model of ⎧ S normal ⎪ 1 scale, small-amplitude postglacial rebound stress rotations near structural boundaries, the =−()λ = Sv 1 pgz ⎨S2 strike-slip , (A2) ⎪ stresses by local zones of weakness could pro- observed SHS versus SHB rotations imply a local ⎪S3 reverse duce perturbations that are coherent over large stress perturbation of 160–250 MPa at a repre- ⎩ distances but only affect those areas with locally sentative depth of 8 km. This assumes that the where ρ is the average crustal density, g is the gravi- λ reduced horizontal differential stress levels. In crust in the seismic zones is in a state of stress tational acceleration, z is the depth, and = Pf/Sv is other words, postglacial rebound stresses may similar to that of “typical, low-seismicity” the pore-fl uid factor. In each case, we can express all be high enough to reorient the local stress fi eld continental crust as a whole, namely, incipient three principal stresses in terms of the vertical stress S and the differential stress ΔS = S – S , and solve μ V 1 3 surrounding faults with low friction or other frictional failure with coeffi cients of friction Equation A1 to obtain ΔS as a function of depth and properties that maintain ambient stress at levels ≈ 0.6–1.0 and near-hydrostatic pore-fl uid pres- the frictional parameter F. This yields: lower than in the “typical low-seismicity” conti- sures. If the seismic zones were characterized Δ =−()λ ()− nental intraplate crust. by either a very low coeffi cient of friction (μ S pgz11 F The St. Lawrence Valley may provide pos- = 0.1) or a near-lithostatic pore-fl uid pressure, ⎧1F normal sible examples of this mechanism. In both the the required stress perturbation according to the ⎪ ×+−⎨11()RRF()strike-slip , (A3) Lower St. Lawrence and Charlevoix seismic model used here would be only 20–40 MPa. ⎪ ⎩⎪11 reverse zones, the estimated SHS orientation is strongly (5) Local stress perturbations associated with oblique to the regional S orientation derived fault intersections or large density anomalies HB where the stress ratio from borehole data located 30–100 km outside could be suffi ciently large (hundreds of mega- SS− SS− the seismic zones (Fig. 3). In Charlevoix, the pascals) to account for the observed rotations, R = 12= 12 (A4) − Δ relationships between the seismicity distribution but their short wavelength (~10–100 km) can- SS13 S

and geological structures jointly suggest that the not explain similar SHS rotations over 1500 km appearing in the strike-slip case is used to explicitly main rift faults may have unusually low friction (point 2). relate the intermediate principal stress to the differen- (μ ~ 0.1; Baird et al., 2009). Recent models of (6) Long-wavelength (hundreds of kilo- tial stress. Expressions A2–A4 can be used to compute postglacial rebound stress and strain that incor- meters) stress perturbation mechanisms such all three principal stress magnitudes and hence, given the orientation of the maximum horizontal stress, fully porate a weak zone in the lithosphere beneath the as topography, basin fl exure, and postglacial determine the stress tensor. St. Lawrence Valley show a signifi cant amplifi - rebound are typically associated with stress cation and clockwise rotation of the postglacial magnitudes of the order of tens of megapascals rebound stresses directly above the weak zone and thus cannot straightforwardly account for ACKNOWLEDGMENTS

(Wu and Mazzotti, 2007). The resulting postgla- the observed SHS rotations if midcrustal stress cial rebound horizontal stress and strain orien- magnitudes are of the order of hundreds of We thank John Adams and Mark Zoback for

tations are nearly parallel to our estimated SHS megapascals (point 4). ongoing discussions about eastern North Amer- orientation and to the maximum horizontal con- (7) A possible mechanism for similar stress ica stress and seismicity, John Cassidy and Roy traction rate measured by global positioning sys- perturbations over large distances is the concen- Hyndman for comments on a preliminary ver- tem in the Charlevoix and Lower St. Lawrence tration of postglacial rebound stresses by local sion of the manuscript, and Björn Lund for very seismic zones (Mazzotti et al., 2005, 2006). zones of weakness, such as low-friction faults. detailed input during the review process. Allison

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REVISED MANUSCRIPT RECEIVED 7 OCTOBER 2009 dence for a locally perturbed stress fi eld in the Missis- Townend, J., and Zoback, M.D., 2000, How faulting keeps MANUSCRIPT ACCEPTED 14 OCTOBER 2009 sippi embayment: Bulletin of the Seismological Society the crust strong: Geology, v. 28, p. 399–402, doi: of America, v. 95, p. 431–445, doi: 10.1785/0120040052. 10.1130/0091-7613(2000)28<399:HFKTCS>2.0.CO;2. Printed in the USA

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