Robert F. Mereu

Crustal Structure Of The Kapuskasing Uplift From Lithoprobe Near-vertical/wide-angle Seismic Reflection Data

Reprocessing of one LITHOPROBE Kapuskasing deep seismic reflection profile discloses significant new information on the structure of the Kapuskasing Uplift (KU). The shallow structure of the Ivanhoe Lake fault zone (ILFZ), along which the high-grade granulites of the KU were thrust to the surface, is conspicuously imaged on a seismic section as a series of prominent northwest-dipping reflections with listric geometry. The new images show that the ILFZ is a steep fault (∼50°) near the surface which quickly flattens out at shallow depths. These reflections coincide with bright positive aeromagnetic anomalies over the fault zone. Direct correlation with geological observations indicates that the high reflectivity and high magnetism associated with the fault zone likely originate from mylonites. The reprocessing also reveals a pronounced midcrustal reflector within the Abitibi greenstone belt (AGB), dipping northwest and plunging under the KU. The existence of such a reflector is independently confirmed by wide-angle reflection data acquired from a cross-profile. This reflector is apparently also detected on two other reflection profiles crossing the ILFZ about 80 km to the southwest. Its concave-down shape and broad lateral extent suggest that it represents underthrusting of the AGB beneath the KU. With these new results, a more complete structural cross-section can be constructed. It shows that, on the northwest side, the KU was overthrust along the northwest-dipping ILFZ and, on the southeast side, the AGB was underthrust northwesterly. It seems evident that the KU is a product of intraplate collision during the early Proterozoic and the underthrusting of the AGB is primarily responsible for the emplacement of the KU.

The Grenville Front Tectonic Zone: Results from the 1986 Great Lakes Onshore Seismic Wide-Angle Reflection and Refraction Experiment

The Grenville Front, which marks the orogenic boundary between the Archean Superior Structural Province and the much younger Grenville Province to the southeast, is one of the major tectonic features of the Canadian Shield. Within Canada, it is approximately 1900 km in length extending from the north shore of Lake Huron across Ontario and Quebec to Labrador. In 1986, a major coincident onship near-vertical reflection and onshore wide-angle reflection/refraction experiment (GLIMPCE–Great Lakes International Multidisciplinary Program on Crustal Evolution) was conducted along a series of lines across the Great lakes. One of the lines, line J, ran across Georgian Bay and Lake Huron for a distance of 350 km and crossed the Grenville Front Tectonic Zone (GFTZ). The seismic signals from the air gun array source were well recorded by the onshore stations up to distances of 250 km with a seismic trace spacing of 50–62.5 m. The GFTZ had a profound effect on the nature of the reflector patterns observed on the onshore seismic sections. Data recorded by the stations on the east end of the line indicate that the crustal P phases are very complex and form a “shinglelike” pattern of reflected waves. Data recorded by stations at the center and at the western end of the line show that the Pg phases are normal and lack the shinglelike appearance. This character of arrivals was also observed on the corresponding S wave sections. A combined P and S wave forward modeling analysis shows that the GFTZ is composed of bands of reflectors dipping at angles of 20°–35° extending to the lower crust. These reflectors were also well imaged on the coincident near-vertical reflection data. Reflectors under the Britt domain to the east of the GFTZ have a shallower dip than those along the zone. The structure of the crust under the Manitoulin terrane to the west of the GFTZ is laterally homogeneous with a major intracrustal reflector at a depth of 17–20 km below the surface. Poissons ratio is slightly higher to the east of the Grenville Front compared to the region to the west. The travel times of the PmP signals indicate that the Moho may be deeper under the GFTZ than under the surrounding regions. Our results give added support to tectonic theories that the Grenville Front owes its origin to a continental collision process.

Pg shingles: preliminary results from the onshore GLIMPCE refraction experiment

Abstract In 1986, a series of near vertical Seismic reflection lines and coincident Seismic refraction lines were recorded across the Great Lakes in the GLIMPCE experiment. From a refraction point of view this experiment was unique as this was the first time in North America that long range refraction lines up to 350 km were recorded with a trace spacing of less than 100 metres. The source of energy was a large air gun. Some preliminary results of the nature of the onshore recordings made by the University of Western Ontario are presented in this paper. Observations of many of the in-line profiles show that the character of the Pg crustal phase is not simple but has a “shingle-like” pattern which may be caused by wide-angle reflection effects from numerous long and short reflectors within the crust. Theoretical studies indicate that the amplitudes of the wide-angle reflected signals from thin high-velocity layers are in general much larger than those from thin low-velocity layers. Supercritical reflected waves are not possible from the top of a low-velocity layer. The closely spaced traces show in minute detail the manner in which one travel-time branch fades and the next one arrives. The results seem to indicate that the Pg phase in our data set has little direct wave component but, instead, is made up of a whole series of “shingled or fish-scaled” wide-angle reflection components, each with successively larger apparent velocities. These results are in agreement with the findings of the near-vertical reflection component of the experiment.

The October 2005 Georgian Bay, Canada, Earthquake Sequence: Mafic Dykes and Their Role in the Mechanical Heterogeneity of Precambrian Crust

On 20 October 2005 at 21:16 UTC, a moderate earthquake (mN 4.3) occurred in an area of low seismicity within Georgian Bay, approximately 12 km north of Thornbury, Ontario (44.67 N, 80.46 W). Despite its moderate magnitude, it was exceptionally well recorded and is of particular interest because of its location 90 km from a proposed long-term storage facility for low- and medium-level nuclear waste. No damage was reported, but ground shaking was felt to a distance of 100 km. Within 24 hours after the mainshock, four portable seismograph systems were in- stalled in the epicentral region. In total, eight events were recorded over a 4-day period, including a foreshock and six aftershocks. The unusually rich dataset from this moderate earthquake sequence enabled robust determination of hypocentral pa- rameters, including well-constrained focal depths for most events. For the mainshock, we estimated a seismic moment of M0 4.5 10 14 N m and corner frequency of 3.7 Hz, based on a spectral fit using Brunes source model. Least-squares waveform inversion of P and S phases yielded a double-couple focal mechanism with a reverse- sense of slip and northwest-striking nodal planes. The reverse mechanism and mid- crustal focal depths (10-12 km) are characteristic, in general, of more abundant seismicity located 200 km northeast of this event in the western Quebec seismic zone. These parameters differ, however, from shallow (2-6 km) earthquakes, with predominantly strike-slip mechanisms, observed near Lake Erie 200 km to the south. We attribute this north-south change in rupture mechanism to variations in crustal stress induced by postglacial isostatic rebound. Aeromagnetic data in and around the epicentral region reveal prominent northwest-striking lineations caused by Precam- brian mafic dykes. Under midcrustal conditions, the dyke material is mechanically stronger than generally more felsic upper-crustal host rocks. We propose that where large dykes are favorably oriented with respect to the stress field, they may strongly influence the locations of intraplate earthquake rupture in Shield regions.

Seismicity of the Southern Great Lakes: Revised Earthquake Hypocenters and Possible Tectonic Controls

Using data from 27 seismograph stations for the period 1990-2001, we have relocated 106 hypocenters of earthquakes with magnitudes from 0.9 to 5.4 in the region of the southern Great Lakes. Two complementary methods were used for relocation: a conventional least-squares approach (Lienert and Havskov, 1995) and joint hypocentral determination (Pujol, 2000). These two methods yielded mutually consistent spatial patterns of seismicity with an average difference of 3.7 km in epicentral locations and 1.1 km in focal depths. We show that the hypocenter locations are not very sensitive to realistic uncertainties in 1D crustal velocity. Our sharpened definition of zones of seismicity delineates several clusters beneath Lake Ontario, around Niagara Falls, and near the south shore of Lake Erie. These seismicity zones appear to correlate with areas where the regional magnetic data exhibit prominent short-wavelength (<5 km) linear anomalies. The magnetic anomalies are associated with basement structures that formed during the Precambrian (Mesoproterozoic) Grenville orogen. Both the seismicity and magnetic anomalies exhibit statistically significant preferred orientations at N40°E-N45°E, but the correlation of the earthquake clusters with specific aeromagnetic lineaments remains uncertain. Three preliminary focal mechanisms of earthquakes with magnitudes m N 3.1 to 3.8 show unusual normal faulting, with nodal planes in almost the same direction as the magnetic trends, N42°E-N52°E. Proximity of the earthquake clusters to large bodies of water, coupled with colinearity with magnetic anomaly trends, suggests that both surface water and pre-existing basement structures may play significant roles in controlling intraplate seismicity in the southern Great Lakes region. Manuscript received 3 January 2003.

The Nature of the Kapuskasing Structural Zone: Results from the 1984 Seismic Refraction Experiment

A few years ago, the Canadian Consortium for Crustal Reconnaissance using Seismic Techniques (COCRUST) conducted a major long-range seismic refraction and wide-angle reflection experiment across the Kapuskasing structural zone (KSZ) in Northern Ontario. The main purpose of this experiment was to determine the structure of the crust below this zone and to help clarify geological theories on its origin. The interpretation of this data set made use of data processing which involved conventional travel-time procedures coupled with synthetic seismogram analysis using programs that were written to handle laterally heterogeneous structures. Two-dimensional P wave velocity models show that the velocities of the upper portion of the crust in the region varies from 5.9 to 6.5 km/s. The highest velocities were found to lie along the axis of the KSZ. Wide-angle reflection observations show that the Moho discontinuity in the survey area is not well defined and is transitional in nature. A combined P and S wave analysis shows that over most of the region Poisson’s ratio does not differ very significantly from 0.25. There is some evidence that its value increases to 0.26–0.27 in the upper crust under the axis of the KSZ. A gravity inversion was also performed by using the seismic models as constraints. The calculations indicate that the largest densities lie along the axis of the KSZ and agree qualitatively well with the high 6.5 km/s P wave velocity found just below the surface along the axis. The results of our analyses give added support to the theory that the KSZ may contain rocks which were uplifted from the middle crust.

Wind‐induced microseisms from Lake Ontario

Abstract The characteristics of microseisms measured in four vaults of the Southern Ontario Seismic Network within 30 km of the shore of Lake Ontario are analysed. It is shown that the rms values in the 1–3 Hz band are coherent between the stations, indicating a common generative mechanism. A distinct onshore intermittent flux of Rayleigh‐like wave energy was detected at a site near the shore. Microseismic energy in this band is distinctly correlated with the wind speed. The incremental microseismic energy above an absolute minimum activity as a function of wind direction, for a given fixed wind speed, correlates with the average fetch of the wind over the lake, indicating that the source of microseisms is the lake itself. The sensitivity to fetch effects is similar for both onshore and offshore stations indicating that shoaling is probably not a source. Niagara Falls, which also can have a wind‐dependent flow from Lake Erie, causes a measurable effect to at least 25 km but does not significantly affect s...

A seismic investigation of the crust and Moho underlying the Peace River Arch, Canada

Abstract The Peace River Arch is a structural uplift in northwestern Alberta which has experienced an anomalous history of movement with respect to the surrounding Western Canada Sedimentary Basin. The supracrustal geology of the region provides a detailed record of the vertical motion of the Arch in the Phanerozoic era. In an attempt to gain information on the underlying crystalline crust and upper mantle in the region, the Peace River Arch Seismic Experiment (PRASE) was carried out by the Geological Survey of Canada and several Canadian universities in the summer of 1985. Four reversed seismic refraction lines, each 300–350 km in length, were shot. The two-dimensional, laterally homogeneous models developed from the in-line data suggest that there is little lateral variation in velocity in the region. The upper 2–4 km of each section was modelled by sedimentary layers. The upper 20 km of the models are composed of horizontal, laterally homogeneous layers with velocities increasing from 6.0 to 6.5 km/s. The data from three of the lines indicate an intracrustal discontinuity at a depth of 16–24 km. The data from a line running along the axis of the Arch, suggests a second discontinuity at 30 km. The lower half of the crust increases in velocity to 6.6–7.2 km/s. The upper mantle has an average velocity of 8.13 ± 0.08 km/s and has a much lower velocity gradient than the overlying crust. There is some variation in the nature of the Moho, which is generally well defined, nearly horizontal, and occurs at a depth between 36 and 43 km. There is little correlation of the arch uplift structure with the structure in the underlying crust. There appears to be some local thickening of the crust immediately below the axis of the Arch. Upward warping of the velocity contours below the axis of the Arch in the lower half of the crust may also be an indication of the uplift at depth. No definite driving mechanism can be established for the origin of the Peace River Arch.

A Note on the Ratio of the Moment Magnitude Scale to Other Magnitude Scales: Theory and Applications

ABSTRACT The relationship between the moment magnitude scale and other magnitude scales is a subject of continuing research ever since the moment magnitude ( M w ) was first proposed. Empirical results show that in western North America, the moment magnitude is greater than, equal to, and less than the Richter ( M L ) and Nuttli magnitudes ( m b Lg ), whereas over 97% of the earthquakes in eastern North America have an M w value that is less than M L , m b Lg , or M e . To explain the large differences that exist between the two regions, first it is shown theoretically that the magnitudes that are based on a peak amplitude measurement are approximately equal to the energy magnitude ( M e ) that is based on the square root of the observed seismic trace Lg coda energy. If we define p as the ratio M w / M e and α as the seismic moment scaling constant, then it is shown theoretically that p =(4/9) α . Most of the major fault‐parameter relations such as stress drop, corner frequency, fault area, etc., can be obtained from the observed ratio of M w / M e . The value of this ratio is not unique but depends on the nature of faulting and the tectonic environment. For self‐similar earthquakes α =3, M w =(4/3)   M e . For most earthquakes in western North America, α varies from 2 to 3, leading to a range of values for the ratio M w / M e , whereas for most earthquakes in eastern North America ( α ≈2 and M w ≈(8/9)   M e ). This type of scaling for small‐to‐intermediate earthquakes may be due to earthquake fault zones being shaped by the geometry of sloping rock layers within the crust.

The Heterogeneity of the Crust and its Effect on Seismic Wide-Angle Reflection Fields

During the past 90 years, our conceptual image of the Earth’s crust has gradually evolved from that of a simple one or two layered crust (e.g., Mohorovicic, 1910; Conrad, 1925; Jeffereys, 1926; Hodgson, 1953), to multi-layered crusts (e.g., McCamy and Meyer, 1966; Mueller, 1977; Sandmeier and Wenzel, 1986; Stephenson et al., 1989), to highly complex morphologies with fractal characteristics (e.g., Nikolajev and Tregub, 1970; Mereu and Ojo 1981; Frankel and Clayton, 1986; Levander and Holliger, 1992; Goff and Levander, 1996). This change has occurred as a result of a revolution in both seismic data acquisition technology (e.g., Berry and Mair, 1977; Brown et al., 1980; Meissner, 1986; Mooney and Brocher, 1987) and computational technology (e.g., Fuchs and Muller, 1971; Kelly et al.,1976; McMechan and Mooney, 1980; Cervený and Psencik, 1984; White, 1989; Zelt and Smith, 1992). Much of the current information about crustal structure has come from numerous controlled-source seismic refraction and reflection experiments. These have been described in many published works over the past 40 years (e.g., Steinhart and Smith, 1966; Barazangi and Brown, 1986a,b; Mathews and Smith, 1987; Meissner et al.,1991; Klemperer and Mooney, 1998).