Broadband Seismographs Research Articles

Rayleigh wave phase velocity measurements across the Philippine sea from a broad-band OBS array

SUMMARY We obtained phase velocities of fundamental Rayleigh waves in the northwestern Pacific ocean and the northern Philippine sea from seismograms retrieved from a long-term broad-band ocean bottom seismometer (LT-OBS) array deployed in 1999‐2000 and from those recorded in Japan, Guam and Ponape islands. This array observation was one of the first attempts to record continuous seismograms on the seafloor using broad-band seismographs on a long-term basis. The geographical distribution of the LT-OBS stations and the surrounding land stations enabled us to determine pure-path phase velocities in a frequency range from 0.01 to 0.05 Hz across the Mariana trough, the Parece Vela basin, the Shikoku basin and the Minami Daito basin, which were found to be systematically lower than those of the Pacific seafloor at comparable ages. This result, along with other geophysical and geological observations in the Philippine sea, may be explained by an uppermost mantle that is relatively Fe rich.

Shear‐wave splitting observations in the lower Great Lakes region: Evidence for regional anisotropic domains and keel‐modified asthenospheric flow

Fast‐axis directions obtained from shear‐wave splitting analysis of core‐refracted shear waves reveal several distinct anisotropic domains in the lower Great Lakes region. We used data from 27 broadband seismograph stations extending across a low‐velocity anomaly in the lithospheric mantle. Observed splitting times vary from 0.4 to 1.4 s, with a mean of 0.7 s. A subset of the splitting vectors across a failed rift are oriented parallel to one arm of the rift zone; most others show a coherent pattern of fast directions close to the direction of plate motion, but oblique to surface tectonic belts. Within the area of the low‐velocity anomaly, our observations are most simply explained by single‐layer anisotropy induced by asthenospheric flow. Spatial variability in the direction and magnitude of splitting are consistent with a flow regime influenced by basal topology of the lithospheric keel.

Observing the historic eruption of northern Mariana Islands volcano

Anatahan volcano erupted for the first time in recorded history at about 7:30 GMT on 10 May 2003, covering the island of Anatahan, in the Commonwealth of the Northern Mariana Islands (CNMI), with ash, and providing scientists with important opportunities to study this volcano. The eruption was first reported by the National Oceanic and Atmospheric Administration's Volcanic Ash Advisory Center at 12:32 GMT, based on satellite images of the ash cloud. At about the same time, unusual light flares were observed from an approaching small ship, the Super Emerald, which was carrying a group of seismologists from Washington University in St. Louis, Scripps Institution of Oceanography, and the CNMI Emergency Management Office. As morning broke, the ship was approximately 10 km from the island, and those on board witnessed billowing ash and gas rise from the volcano's caldera to form a great cloud exceeding 6 km in altitude (Figure 1).The scientists were in the region installing land seismographs for the Mariana Subduction Factory Imaging Experiment, a joint U.S. Japanese deployment of 20 land broadband seismographs and 58 ocean bottom seismographs funded (on the U.S. side) by the Margins program of the National Science Foundation. The experiment has the goal of imaging the magma production regions and mantle flow patterns within the upper mantle beneath the Mariana arc and backarc (see http://epsc.wustl.edu/seismology/MARIANA).

Receiver function study of the Hellenic subduction zone: imaging crustal thickness variations and the oceanic Moho of the descending African lithosphere

SUMMARY We use data from recently installed broad-band seismographs on the islands of Crete, Gavdos, Santorini, Naxos and Samos in the Hellenic subduction zone to construct receiver function images of the crust and upper mantle from south of Crete into the Aegean Sea. The stations are equipped with STS-2 seismometers and they are operated by GFZ Potsdam, University of Chania and ETH Zurich. Teleseismic earthquakes recorded by these stations at epicentral distances between 35° and 95° have been used to calculate receiver functions. The receiver function method is a routinely used tool to detect crustal and upper-mantle discontinuities beneath a seismic station by isolating the P–S converted waves from the coda of the P wave. Converted P–S energy from the oceanic Moho of the subducted African Plate is clearly observed beneath Gavdos and Crete at a depth ranging from 44 to 69 km. This boundary continues to the north to nearly 100 km depth beneath Santorini island. Because of a lack of data the correlation of this phase is uncertain north of Santorini beneath the Aegean Sea. Moho depths were calculated from primary converted waves and multiply reflected waves between the Moho and the Earth's surface. Beneath southern and eastern Crete the Moho lies between 31 and 34 km depth. Beneath western and northern Crete the Moho is located at 32 and 39 km depth, respectively, and behaves as a reversed crust–mantle velocity contrast, possibly caused by hydration and serpentinization of the forearc mantle peridotite. The Moho beneath Gavdos island located south of Crete in the Libyan Sea is at 26 km depth, indicating that the crust south of the Crete microcontinent is also thinning towards the Mediterranean ridge. This makes it unlikely that part of the crust in Crete consists of accreted sediments transported there during the present-day subduction process which began approximately 15 Ma because the backstop, i.e. the boundary between the current accretionary wedge of the Mediterranean ridge and the Crete microcontinent, is located approximately 100 km south of Gavdos. A seismic boundary at 32 km depth beneath Santorini island probably marks the crustal base of the Crete microcontinent. A shallower seismic interface beneath Santorini at 20–25 km depth may mark the depth of the detachment between the Crete microcontinent and the overlying Aegean subplate. The Moho in the central and northern Aegean, at Naxos and Samos, is observed at 25 and 28 km depth, respectively. Assuming a stretching factor of 1.2–1.3, crustal thickness in the Aegean was 30–35 km at the inception of the extensional regime in the Middle Miocene.

Seismicity and tectonics of the South Shetland Islands and Bransfield Strait from a regional broadband seismograph deployment

We investigate the tectonics of the South Shetland Trench and Bransfield Strait by performing a detailed study of local seismicity. During 1997–1999 we deployed seven land seismometers and 14 ocean bottom seismometers in the South Shetland Island region. The data we obtained indicate a high level of local seismicity (mb 2–5), and we accurately located ∼150 earthquakes. Many of the earthquakes occur at locations and depths indicative of ongoing subduction in the South Shetland trench. A focal mechanism for the largest event in the forearc indicates shallow angle thrusting. The maximum depth of seismicity is ∼65 km, but the majority of the events are shallower than 30 km. These seismic results are consistent with recent magnetic, GPS, and multichannel seismic reflection data that suggest continued subduction at a very slow rate. The South Shetland trench thus represents an extreme end‐member of hot subduction resulting from slow convergence of young lithosphere, and the absence of intermediate depth earthquakes is consistent with thermal assimilation of the slab at shallow depths. We have located many earthquakes associated with volcanism and rifting in Bransfield Strait. A swarm of events near a submarine volcano suggests current magmatic activity. A normal faulting focal mechanism in the northeastern part of the strait gives evidence of extension. Earthquakes associated with rifting in the northeastern portion of the strait are clustered along well‐established rifts, but the seismicity is much more diffuse to the southwest. This observation is consistent with other evidence that extension has propagated from northeast to southwest.

Structure of the crust and upper mantle of the Great Slave Lake shear zone, northwestern Canada, from teleseismic analysis and gravity modelling

The Great Slave Lake shear zone (GSLsz) exposes lower crustal rocks analogous to deep-seated segments of modern strike-slip fault zones, such as the San Andreas fault. Extending for 1300 km beneath the Western Canada Sedimentary Basin to the southern margin of the Slave Province, the GSLsz produces one of the most prominent linear magnetic anomalies in Canada. From May to October 1999, 13 three-component portable broadband seismograph stations were deployed in a 150-km profile across a buried segment of the shear zone to investigate its lithospheric structure. Splitting analysis of core-refracted teleseismic shear waves reveals an average fast-polarization direction (N49°E ± 19°) that is approximately parallel to the shear zone. Individual stations near the axis of the shear zone show more northerly splitting directions, which we attribute to interference between regional anisotropy in the upper mantle (fast axis ~N60°E) and crustal anisotropy within the shear zone (fast axis ~N30°E). At the location of our profile, the shear zone is characterized by a 10-mGal axial gravity high with a wavelength of 30 km, superimposed on a longer wavelength 12-mGal low. This gravity signature is consistent with the basic features of the crustal model derived from receiver-function analysis: a Moho that dips inward toward the shear-zone axis and a mid-crustal zone with high S-wave velocity (ΔVs = 0.6 ± 0.2 km/s). The axial gravity high may be related to uplift of deeper crustal material within the shear zone, or protolith-dependent compositional differences between the shear zone and surrounding wall rocks.

MC1 Micro‐Power Consuming Broadband Digital Seismograph

AbstractA new generation of digital seismic data acquisition device—MC1 Micro‐Power Cosuming and Broad‐Band Digital Seismograph was designed and built in our Lab. Its power dissipation is less than 300mW. It has some merits including miniature volume, light mass and low costs. It is an ideal device for the acquisition of high‐resolution natural seismic data, as well as for the fields of seismic sounding and engineering seismology.

USArray design implications for wavefield imaging in the lithosphere and upper mantle

Although seismic tomography is currently the mainstay of global seismic imaging, the large-scale deployment of portable instruments to be conducted as the USArray component of Earthscope over the next decade will allow much higher resolution imaging of the solid earth using migration/inversion methods similar to those used in petroleum exploration. Direct wavefield imaging in solid earth structural seismology has been uncommon in comparison to practices in exploration seismology, generally due to the large separation between seismic stations. With the development of portable observatory quality broadband (0.01–20 Hz) seismographs and acquisition of modest numbers of them for the PASSCAL instrument pool over the past decade, a number of academic field programs have had receiver spacing dense enough for wavefield back-projection and imaging. Recordings from these experiments have been used in several different imaging schemes, all of which are outgrowths of and bear resemblance to either post- or prestack d...

Crustal shear velocity structure of the south Indian shield

The south Indian shield is a collage of Precambrian terrains gathered around and in part derived from the Archean‐age Dharwar craton. We operated seven broadband seismographs on the shield along a N‐S corridor from Nanded (NND) to Bangalore (BGL) and used data from these to determine the seismic characteristics of this part of the shield. Surface wave dispersion and receiver function data from these sites and the Geoscope station at Hyderabad (HYB) give the shear wave velocity structure of the crust along this 600 km long transect. Inversion of Rayleigh wave phase velocity measured along the profile shows that the crust has an average thickness of 35 km and consists of a 3.66 km s−1, 12 km thick layer overlying a 3.81 km s−1, 23 km thick lower crust. At all sites, the receiver functions are extremely simple, indicating that the crust beneath each site is also simple with no significant intracrustal discontinuities. Joint inversion of the receiver function and surface wave phase velocity data shows the seismic characteristics of this part of the Dharwar crust to be remarkably uniform throughout and that it varies within fairly narrow bounds: crustal thickness (35 ± 2 km), average shear wave speed (3.79 ± 0.09 km s−1), and Vp/Vs ratio (1.746 ± 0.014). There is no evidence for a high velocity basal layer in the receiver function crustal images of the central Dharwar craton, suggesting that there is no seismically distinct layer of mafic cumulates overlying the Moho and implying that the base of the Dharwar crust has remained fairly refractory since its cratonization.

Crustal structure across the Canadian High Arctic region from teleseismic receiver function analysis

SUMMARY Results from an investigation of the crustal structure beneath the Canadian High Arctic and western Greenland are presented, using teleseismic data recorded by a network of 12 threecomponent broad-band seismograph stations deployed across the region. Of these stations, eight were deployed in summer 2000, in order to improve the spatial density of seismographs in this tectonically active but little studied region, in order to provide more information on the patterns of seismicity and the structure of the area. The typical station separation is of the order of 400‐600 km, a substantial improvement on the previous >1000 km spacing afforded by the permanent broad-band network. Teleseismic receiver functions are used to model the shear wave velocity structure of the crust beneath the stations, using both 1-D and 3-D analysis techniques. The velocity‐depth models show significant variation in the thickness and nature of the crust across the region. In the south and east, where the seismic stations lie on the Canadian Shield, the crustal structure is relatively simple, with Moho depths ranging from 35 km on the Boothia Peninsula to 45 km beneath southern Baffin Island. Further to the north and west, where the lithosphere has been deformed by orogenic events and basin formation, the patterns of azimuthal variation in the radial and tangential receiver functions suggest 3-D structure throughout the crust. The Moho depths around the margins of the Sverdrup Basin lie in the range 33‐37 km. At Mould Bay, on the western margin of the basin, a 5 km thick low-velocity layer overlies the main crustal sequence. The base of this layer can be modelled as a planar dipping structure with a southeasterly strike. The thinnest crust (27‐32 km) is modelled at Alert, on the northern coast of Ellesmere Island. In the southwest of the Arctic archipelago, tangential receiver functions at Holman show a pattern consistent with crustal anisotropy. The Moho depth in the region is undetermined; a layer of intermediate shear wave velocity, consistent with mafic lower-crustal material, is modelled at depths of 35‐55 km. The region around Holman lies at the focal point of the Mackenzie dyke swarm, believed to result from plume activity; therefore the intermediate-velocity layer may result from mafic underplating caused by the presence of the Mackenzie plume.

POLARIS Update: Fall 2002

In this note we report on the progress of the Canadian geophysics research project known as POLARIS ( P ortable O bservatories for L ithospheric A nalysis and R esearch I nvestigating S eismicity). POLARIS is a multi-institutional infrastructure composed of mobile geophysical observatories that transmit continuous real-time data to research centers using a satellite-telemetry communications system. The major components of POLARIS, when fully installed in 2004, will be 90 seismograph and 30 magnetotelluric (MT) mobile field systems, complementary data-acquisition and satellite-communications equipment, and satellite downlink facilities. The seismograph network is three subarrays of 30 three-component broadband seismographs, sampling at 40 samples/second or 100 samples/second, to be deployed in Ontario, southern British Columbia, and the Northwest Territories. Data acquisition and telemetry are provided by the Nanometrics, Inc. Libra system, while the seismometers are broadband Guralp CMG-3ESP's. This system is ideal for remote installations, requiring no existing infrastructure. All that is needed is a bit of sunshine and a view of the satellite. The POLARIS network complements and works in tandem with the Canadian National Seismographic Network (visit http://www.seismo.nrcan.gc.ca for CNSN locations and data downloads). POLARIS follows an open-data policy, and we invite …

Depth to Moho in Greenland: receiver-function analysis suggests two Proterozoic blocks in Greenland

The GLATIS project (Greenland Lithosphere Analysed Teleseismically on the Ice Sheet) with collaborators has operated a total of 16 temporary broadband seismographs for periods from 3 months to 2 years distributed over much of Greenland from late 1999 to the present. The very first results are presented in this paper, where receiver-function analysis has been used to map the depth to Moho in a large region where crustal thicknesses were previously completely unknown. The results suggest that the Proterozoic part of central Greenland consists of two distinct blocks with different depths to Moho. North of the Archean core in southern Greenland is a zone of very thick Proterozoic crust with an average depth to Moho close to 48 km. Further to the north the Proterozoic crust thins to 37–42 km. We suggest that the boundary between thick and thin crust forms the boundary between the geologically defined Nagssugtoqidian and Rinkian mobile belts, which thus can be viewed as two blocks, based on the large difference in depth to Moho (over 6 km). Depth to Moho on the Archean crust is around 40 km. Four of the stations are placed in the interior of Greenland on the ice sheet, where we find the data quality excellent, but receiver-function analyses are complicated by strong converted phases generated at the base of the ice sheet, which in some places is more than 3 km thick.

Source mechanism of very‐long‐period signals accompanying dome growth activity at Merapi volcano, Indonesia

Very‐long‐period (VLP) pulses with period of 6–7s, displaying similar waveforms, were identified in 1998 from broadband seismographs around the summit crater. These pulses accompanied most of multiphase (MP) earthquakes, a type of long‐period event locally defined at Merapi Volcano. Source mechanisms for several VLP pulses were examined by applying moment tensor inversion to the waveform data. Solutions were consistent with a crack striking ∼70° and dipping ∼50° SW, 100m under the active dome, suggest pressurized gas transport involving accumulation and sudden release of 10–60 m3 of gas in the crack over a 6s interval.

Fault Slip, Seismicity, and Deformation in Mexicali Valley, Baja California, Mexico, after the M 7.1 1999 Hector Mine Earthquake

A local small deformation network in Mexicali Valley (Baja California, Mexico) recorded fault slip, seismicity, and deformation produced by the 16 October 1999 M 7.1 Hector Mine earthquake 260 km away. The network consists of two crackmeters, two biaxial surface tiltmeters, and one broadband seismograph, all recording continuously; these instruments are complemented by other seismological instruments within the valley. Local microearthquakes with clear P and S phases began 2 hours after arrival of the Hector Mine P wave, and earthquakes with M >2.5 (large enough to be located) started about 33 hours after the earthquake. Local seismicity continued, with decreasing frequency, for about 2 weeks after the Hector Mine earthquake. Vertical and horizontal slip and tilt began on the Imperial fault simultaneously with the Hector Mine earthquake seismic wave arrival, which is clearly recorded by the tiltmeters. Deformation recorded by a tiltmeter on the Cerro Prieto geothermal field began about 30 hours after the main event and ended 13 hours later after the occurrence of the largest local earthquake ( M 4.1) in this episode. The temporal pattern of seismicity and deformation suggests that deformation is the main process triggered by the Hector Mine earthquake waves in the study area and the transient local seismicity increase is a response to the triggered deformation.

Integrated teleseismic studies of the southern Alberta upper mantle

This paper presents results from a teleseismic experiment conducted across the Hearne Province in south-central Alberta. Data from an array of nine portable broad-band seismographs deployed along a 500 km NW–SE array have been supplemented with recordings from two Canadian National Seismograph Network stations. P-wave delay times from 293 earthquakes have been inverted for upper-mantle velocity structure below the array. The recovered model reveals high velocities beneath much of the southern Hearne Province to depths of 200–250 km, which are interpreted as deep-seated lithospheric structure. Contrary to recent tectonic models, these results suggest that the Hearne lithosphere has remained intact. In particular, it appears unlikely that evidence for extensive, lower crustal melting derives from lithospheric delamination. However, the results admit the possibility that high mantle conductivity, as revealed in magnetotelluric studies, originates through small volumes of connected hydrous minerals or other conductive species introduced during subduction. Decreased upper-mantle velocities at the northern end of the Medicine Hat block also pose challenges for the interpretation of differential subsidence across the region which may manifest distant forcing due to more recent subduction. Multievent SKS-splitting analysis yields an average polarization direction that is broadly consistent with both the orientation of fossil strain fields, related to ~ 1.8 Ga NW–SE shortening, and North American absolute plate motion. Moho depth estimates from receiver functions are fairly uniform (~ 38 km) beneath northern stations but show crustal thickening (>40 km) within the Medicine Hat block to the south and are consistent with values from active-source profiling.

三宅島２０００年噴火活動に伴う長周期地震のメカニズムとその解釈

A sudden collapse of the summit of Miyake-jima occurred on July 8, 2000, together with intermittent eruptions. This collapse generated long-period seismic waves with a dominant period of about 10s. Following this event, very long-period seismic pulses (VLP pulses) with a duration of about 50s were observed a few times a day until they ceased at the largest summit eruption on August 18. We analyzed these seismic pulses using waveform data recorded at several domestic stations for broadband seismographs and strong motion seismometers on Miyake-jima. The July 8 event is well characterized by a single-force directed initially upward and later downward during 12 sec. The single-force is interpreted as an abrupt collapse of massive rock. The total mass is estimated to be about 5 × 1010 kg with fall of about 300 m. On the other hand, VLP pulses are modeled by moment-tensors with an isotropic component. They are located about 1 km southwest from the summit and 2 to 3 km deep. All three principal values are positive. The largest one is horizontal and the smallest one is near vertical. The total volume change due to 39 VLP pulses is 2.6× 108m3, amounting to nearly one half of the total volume of the summit collapse. Based on theresults, we propose a buried geyser model. A large reservoir of hot water was formed just after the summit collapse on July 8. The ground water poured into the reservoir, being rapidly heated by hot rock underneath, and evaporated to form a highly pressurized steam, which pushed a lower conduit piston into the magma reservoir to generate VLP pulses. Non-isotropic expansion of the VLP pulses may be ascribed to the shape of the magma reservoir.

Attenuation structure beneath the volcanic front in northeastern Japan from broad-band seismograms

Anelastic structure in the asthenosphere beneath the volcanic front in northeastern Japan arc is estimated by using the spectral amplitude ratio data of P and S waves from about 100 events which occurred in the subducting Pacific slab below Japan. These earthquakes occurred within a 90 km radius centered about the station Sawauchi (SWU), with focal depths ranging from 60 to 200 km. Waveforms were recorded by the Carnegie broad-band three-component seismograph and were corrected for instrument responses, crustal reverberations, corner frequencies, and superimposed noise. Ray paths and travel times of P and S waves are calculated using a three-dimensional velocity model [Zhao, D., Hasegawa, A., Horiuchi, S., 1992. J. Geophys. Res. 97, 19909–19928]. We find a low- Q region ( Q S∼70) extending down to 55 km depth from the lower crust beneath the volcanic front. Using Q-temperature laboratory results [Sato, H., Sacks, I.S., Murase, T., Muncill, G., Fukushima, H., 1989. J. Geophys. Res. 94, 10647–10661], this implies a temperature of about 130°C higher than the eastern forearc region and about 30°C higher than the western backarc region, in good agreement with the tomographic results of Zhao et al. [Zhao, D., Hasegawa, A., Horiuchi, S., 1992. J. Geophys. Res. 97, 19909–19928]. This suggests that low velocities in the crust and uppermost mantle beneath SWU may be explained by a subsolidus temperature increase without partial melting.

International Geodynamics Research Center opens in Kyrgyz Republic

On June 10, 2000, the International Research Center‐Geodynamic Proving Ground was inaugurated near Bishkek, the capital city of the Kyrgyz Republic.The centers purpose will be to facilitate collaborative research in geodynamics, not only between Kyrgyz and Russian scientists, but involving scientists from other countries as well.The new center takes advantage of existing capabilities in geophysical research at both the Scientific Station of the Associated Institute of High Temperatures, Russian Academy of Sciences, and at Kyrgyz institutes. For instance, the Kyrgyz Seismograph Network, operated by the Kyrgyz Institute of Seismology (KIS) and the Scientific Station, with support from the U.S. National Science Foundation (NSF),consists of 10 state‐of‐the‐art broadband seismographs whose data are transmitted in near‐real time to the Institute of Geophysics and Planetary Physics at the University of California in San Diego.

Broadband seismic experiment at Merapi Volcano, Java, Indonesia: very-long-period pulses embedded in multiphase earthquakes

Multiphase (MP) and low frequency (LF) earthquakes with spectral peak amplitudes at 3–4 and 1 Hz, respectively, are two common types of shallow volcanic earthquakes previously recognized at Merapi Volcano. Their mechanisms are poorly understood but MPs have been temporally associated with lava dome growth. We conducted a seismic experiment in January–February 1998, using four broadband seismographs to investigate the nature of seismic activity associated with dome growth. During our experiment, Merapi experienced mild dome growth with low-level seismic activity. We compare our data to that recorded on a local short-period (SP) network, with the following preliminary results. MP and LF events as recorded and classified on the short-period network instruments were recognized on the broadband network. Frequency spectrograms revealed similar patterns in the near summit region at widely separated broadband stations. Higher frequency spectra than previously recognized were identified for both MP and LF events, and were strongly attenuated as a function of radial distance from the source. Thus the spectral characteristics of these events as recorded on far-field stations are not fully indicative of the source processes. In particular, many events classified as LF-type appear to have much high frequency energy near the source. This aspect of these so-called LF earthquakes, and their association with very-long-period (VLP) pulses, suggests that many events identified in the far-field as LF events are in actuality a variety of the MP event and involve similar source processes. Broadband records indicated that simple large-amplitude VLP pulses were embedded in MP and LF wavetrains. From event to event these pulses were similar in their waveforms and had periods of 4 s. VLP events embedded in LF and MP earthquakes were located using particle motions. The epicenters were clustered in a central region of the dome complex, and preliminary source depths were within about 100 m of the dome surface, suggesting a source region deep within the dome or the uppermost conduit. A similar source location was established by study of MP high-frequency onsets. Our broadband data suggests that we have recorded both elastic seismic waves and a simple embedded pulse that is interpreted to represent a surface tilt at the seismometer site. The inferred tilt indicates an inflation and subsequent deflation, possibly caused by a gas pressure pulse or episodic shallow magma transport with stick-slip movement of the conduit wall.

Lithospheric assembly and modification of the SE Canadian Shield: Abitibi‐Grenville teleseismic experiment

This paper presents the results of a joint Lithoprobe‐Incorporated Research Institutions for Seismology (IRIS)/Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) teleseismic experiment that investigates portions of the Grenville and Superior Provinces of the Canadian Shield along the Québec‐Ontario border. Data from a 600‐km‐long, N‐S array of 28 broadband seismographs deployed between May and October 1996 have been supplemented with additional recordings from an earlier 1994 deployment and from stations of the Canadian National Seismograph Network and the Southern Ontario Seismic Network. Relative delay times of P and S waves from 123 and 40 teleseismic events, respectively, have been inverted for velocity perturbations in the upper mantle and reveal a low‐velocity, NW‐SE striking corridor that crosses the southern portion of the line at latitude 46°N and lies between 50 and 300 km depth. Multievent S K S‐splitting results yield an average delay time of 0.57±0.22 s and a direction of fast polarization of N93°E±18°, which is consistent with an earlier interpretation as being due to fossil strain fields related to the last major regional tectonic event. Subtle variations in splitting parameters over the low‐velocity corridor may suggest an associated disruption in mantle fabric. Profiling of radial receiver functions reveals large and abrupt variations in Moho topography, specifically, a gradual thickening in crust from 40 to 45 km between latitudes 45°N and 46°N, which is followed by an abrupt thinning to 35 km at 46.6°N, some 65 km southeast of the Grenville Front. This structure is interpreted as a subduction suture extending the full length of the Front and punctuating a major pre‐Grenvillian (Archean‐Proterozoic) episode of lithospheric assembly in the southeast Canadian Shield. The low‐velocity mantle corridor, by contrast, is better explained as the extension of the Monteregian‐White Mountain‐New England seamount hotspot track below the craton and is here postulated to represent interaction of the Great Meteor plume with zones of weakness within the craton developed during earlier rifting episodes.