The Queen Charlotte Islands refraction project. Part I. The Queen Charlotte Fault Zone

1988 ◽  
Vol 25 (11) ◽  
pp. 1857-1870 ◽  
Author(s):  
Sonya A. Dehler ◽  
Ron M. Clowes
Keyword(s):  
Fault Zone ◽  
Structural Model ◽  
Seismic Refraction ◽  
Oceanic Lithosphere ◽  
North American Plate ◽  
Ocean Bottom ◽  
Queen Charlotte Islands ◽  
Lower Terrace ◽  
Vertical Fault ◽  
Ocean Bottom Seismographs

The active margin between the continental North American plate and oceanic Pacific plate west of the Queen Charlotte Islands was the site of an extensive onshore–offshore seismic refraction project in 1983. An airgun line shot over two ocean-bottom seismographs (OBS's) and a 32-charge explosion line recorded on the two OBS's and eight land-based seismographs (LBS's) deployed across northern Moresby Island were selected to study the structure of the predominantly transform Queen Charlotte Fault Zone and the associated offshore terrace. Two-dimensional ray tracing and synthetic seismogram modelling produced a pronounced laterally varying velocity structural model showing three major crustal components (oceanic, terrace, and continental) separated by an outer, crustally pervasive fault and active Queen Charlotte Fault, respectively. The 3 km thick block-faulted upper terrace unit, overlain by deformed sediments, is indistinguishable from adjacent oceanic sediments and upper crustal basalts located to the west. The upper part of the 10–17 km thick lower terrace unit has anomalously low velocities relative to the adjacent oceanic and continental crustal units. A high gradient increases terrace velocity rapidly with depth until the contrast becomes negligible at approximately 17 km depth. Changes in depth to Moho beneath the terrace suggest an increase in eastward Moho dip from 2–5 °observed west of the terrace to 19 °below it. Tectonic mechanisms proposed to explain the anomalous terrace structure involve sediment accretion during subduction of oceanic lithosphere, alternating or combined with compressive upthrusting of material along near-vertical fault planes during periods of active transform motion.

Queen Charlotte fault zone: microearthquakes from a temporary array of land stations and ocean bottom seismographs

1981 ◽  
Vol 18 (4) ◽  
pp. 776-788 ◽  
Author(s):  
R. D. Hyndman ◽  
R. M. Ellis
Keyword(s):  
Fault Zone ◽  
Strike Slip ◽  
Ocean Bottom ◽  
Small Component ◽  
Queen Charlotte Islands ◽  
The North ◽  
Vertical Fault ◽  
Linear Sections ◽  
Ocean Bottom Seismographs ◽  
Steep Slopes

A temporary array of land and ocean bottom seismograph stations was used to accurately locate microearthquakes on the Queen Charlotte fault zone, which occurs along the continental margin of western Canada. The continental slope has two steep linear sections separated by a 25 km wide irregular terrace at a depth of 2 km. Eleven events were located with magnitudes from 0.5 to 2.0, 10 of them beneath the landward one of the two steep slopes, some 5 km off the coast of the southern Queen Charlotte Islands. No events were located beneath the seaward and deeper steep slope. The depths of seven of these events were constrained by the data to between 9 and 21 km with most near 20 km. The earthquake and other geophysical data are consistent with a near vertical fault zone having mainly strike-slip motion. A model including a small component of underthrusting in addition to strike-slip faulting is suggested to account for the some 15° difference between the relative motion of the North America and Pacific plates from plate tectonic models and the strike of the margin. One event was located about 50 km inland of the main active zone and probably occurred on the Sandspit fault. The rate of seismicity on the Queen Charlotte fault zone during the period of the survey was similar to that predicted by the recurrence relation for the region from the long-term earthquake record.

The Queen Charlotte Islands refraction project. Part II. Structural model for transition from Pacific plate to North American plate

1989 ◽  
Vol 26 (9) ◽  
pp. 1713-1725 ◽  
Author(s):  
D. J. Mackie ◽  
R. M. Clowes ◽  
S. A. Dehler ◽  
R. M. Ellis ◽  
P. Morel-À-l'Huissier
Keyword(s):  
Fault Zone ◽  
North American ◽  
Velocity Structure ◽  
Pacific Plate ◽  
North American Plate ◽  
Lithospheric Structure ◽  
Gravity Modelling ◽  
Oblique Subduction ◽  
Queen Charlotte Islands ◽  
Thin Crust

The oceanic-continental boundary west of the Queen Charlotte Islands is marked by the active Queen Charlotte Fault Zone. Motion along the fault is predominantly dextral strike slip, but relative plate motion and other studies indicate that a component of convergence between the oceanic Pacific plate and the continental North American plate presently exists. This convergence could be manifest through different types of deformation: oblique subduction, crustal thickening, or lateral distortion of the plates. In 1983, a 330 km offshore–onshore seismic refraction profile extending from the deep ocean across the islands to the mainland of British Columbia was recorded to investigate (i) structure of the fault zone and associated oceanic–continental boundary and (ii) lithospheric structure beneath the islands and Hecate Strait to define the regional transition from Pacific plate to North American plate and thus the nature of the convergence. Two-dimensional ray tracing and synthetic seismogram modelling of many record sections enabled the derivation of a composite velocity structural section along the profile. The structural section also was tested with two-dimensional gravity modelling. Part I of the study addressed the structure of the fault zone; part II addresses lithospheric structure extending eastward to the mainland.The derived velocity structure has some important and well-constrained features: (i) anomalously low crustal velocities (5.3 km/s with a 0.2 km/s per km gradient) underlain by a steep, 19 °eastward-dipping boundary above the mantle in the terrace region west of the main fault; (ii) a thin crust of 21–27 km beneath the Queen Charlotte Islands; and (iii) a gentle 4 °eastward dip of the Moho below Hecate Strait as crustal thickness increases from 27 km to 32 km. The gravity modelling requires that mantle material extend upwards to a depth of about 30 km below the mainland and indicates that an underlying subducted slab, if it exists, extends eastward no farther than the mainland.Unfortunately, the velocity structure delineated by this study could not unambiguously determine the mode of deformation, because the lowermost crustal block beneath Queen Charlotte Islands and Hecate Strait can be interpreted as subducted oceanic crust or middle to lower continental crust. Thus, two different tectonic models for the transition from Pacific plate to North American plate are discussed: in one, oblique subduction is the principal characteristic; in the other, oceanic lithosphere juxtaposed against continental lithosphere across a narrow boundary zone along which only transcurrent motion occurs is the dominant feature. Based on the thin crust beneath the Queen Charlotte Islands, the lack of a wide zone of deformation along the plate boundary region, and other geological and geophysical characteristics, oblique subduction is the more plausible model.

A microseismicity study of the Queen Charlotte Islands region

1989 ◽  
Vol 26 (12) ◽  
pp. 2556-2566 ◽  
Author(s):  
Joane Bérubé ◽  
Garry C. Rogers ◽  
Robert M. Ellis ◽  
Elizabeth O. Hasselgren
Keyword(s):  
Seismic Events ◽  
Seismic Gap ◽  
Ocean Bottom ◽  
Stress Regime ◽  
Focal Mechanism Solutions ◽  
Thrust Faulting ◽  
Queen Charlotte Islands ◽  
Composite Focal Mechanism ◽  
Predominant Orientation ◽  
Ocean Bottom Seismographs

Nineteen land and three ocean-bottom seismographs were operated in the Queen Charlotte Islands region for periods of up to 9 weeks and 5 days, respectively, during the summer of 1983. Three hundred and seventeen seismic events were detected. One hundred and nine earthquakes ranging in size from magnitude −0.5 to 5.1 were well recorded at three or more stations and could be accurately located. Of these, 84 lie on or close to the Queen Charlotte Fault, most within the rupture zone of the great 1949 earthquake (MS = 8.1). The seismic gap, between the rupture zones of the 1949 event and the 1970 earthquake (MS = 7.4) that occurred just south of the Queen Charlotte Islands, exhibited little activity. Eighteen earthquakes, the largest with ML = 3.8, were located east of the fault on northern Graham Island or in adjacent Hecate Strait. Focal depths were generally less than 20 km, and none could be associated with known faults. Composite focal mechanism solutions were obtained for four suites of earthquakes along the Queen Charlotte Fault and for a group east of the fault zone on northern Graham Island. In all cases the solutions indicate thrust mechanisms with the predominant orientation of pressure axes northeast–southwest. The presence of thrust faulting close to the Queen Charlotte Fault suggests that the microseismicity is not occurring on the main transcurrent fault but on subsidiary faults that are moving due to the regional stress regime. Thrust faulting on northern Graham Island can best be interpreted as reflecting the stress field from a locked Pacific and North American boundary.

scholarly journals The marine activities performed within the TOMO-ETNA experiment

2016 ◽  
Vol 59 (4) ◽  
Author(s):  
Mauro Coltelli ◽  
Danilo Cavallaro ◽  
Marco Firetto Carlino ◽  
Luca Cocchi ◽  
Filippo Muccini ◽  
...  
Keyword(s):  
High Resolution ◽  
Structural Model ◽  
Seismic Refraction ◽  
Velocity Model ◽  
Ocean Bottom ◽  
Etna Volcano ◽  
Remotely Operated Vehicle ◽  
Reflection Seismic ◽  
Air Gun ◽  
Mt Etna

<p>The TOMO-ETNA experiment was planned in order to obtain a detailed geological and structural model of the continental and oceanic crust beneath Mt. Etna volcano and northeastern Sicily up to the Aeolian Islands (southern Italy), by integrating data from active and passive refraction and reflection seismic methodologies, magnetic and gravity surveys. This paper focuses on the marine activities performed within the experiment, which have been carried out in the Ionian and Tyrrhenian Seas, during three multidisciplinary oceanographic cruises, involving three research vessels (“Sarmiento de Gamboa”, “Galatea” and “Aegaeo”) belonging to different countries and institutions. During the offshore surveys about 9700 air-gun shots were produced to achieve a high-resolution seismic tomography through the wide-angle seismic refraction method, covering a total of nearly 2650 km of shooting tracks. To register ground motion, 27 ocean bottom seismometers were deployed, extending the inland seismic permanent network of the Istituto Nazionale di Geofisica e Vulcanologia and a temporary network installed for the experiment. A total of 1410 km of multi-channel seismic reflection profiles were acquired to image the subsurface of the area and to achieve a 2D velocity model for each profile. Multibeam sonar and sub bottom profiler data were also collected. Moreover, a total of 2020 km of magnetic and 680 km of gravity track lines were acquired to compile magnetic and gravity anomaly maps offshore Mt. Etna volcano. Here, high-resolution images of the seafloor, as well as sediment and rock samples, were also collected using a remotely operated vehicle.</p>

Large‐offset seismic surveying using ocean‐bottom seismographs and air guns: Instrumentation and field technique

Geophysics ◽  
1987 ◽  
Vol 52 (12) ◽  
pp. 1601-1611 ◽  
Author(s):  
Yosio Nakamura ◽  
Paul L. Donoho ◽  
Phillip H. Roper ◽  
Paul M. McPherson
Keyword(s):  
Seismic Refraction ◽  
Ocean Bottom ◽  
Large Capacity ◽  
Seismic Reflection Data ◽  
Reflection Data ◽  
Seismic Surveying ◽  
Ocean Bottom Seismographs ◽  
Marine Seismic ◽  
Processing Techniques ◽  
Signal Sources

Repeatable, closely spaced signal sources from large‐capacity air guns and detection and recording of signals using highly flexible, microprocessor‐controlled, digital ocean‐bottom seismographs allow us to acquire high‐quality, large‐offset, marine seismic refraction and reflection data. The acquired data are readily adaptable to various processing techniques originally developed for seismic reflection data. There are several requirements and problems specific to the technique. For example, bubbly signals from one or two large‐capacity air guns are often preferable to bubble‐suppressed signals from tuned arrays in identifying weak arrivals at large offset distances. Recorded water‐wave signals at near ranges provide precise locations of detectors relative to shots.

scholarly journals Cambridge radio sonobuoys and the seismic structure of oceanic crust

2019 ◽  
Vol 74 (1) ◽  
pp. 55-72 ◽  
Author(s):  
Melvyn Mason ◽  
Robert S. White
Keyword(s):  
Oceanic Crust ◽  
Plate Tectonics ◽  
Seismic Refraction ◽  
Continental Margins ◽  
Ocean Bottom ◽  
Seismic Structure ◽  
Cambridge University ◽  
The Usa ◽  
Ocean Bottom Seismographs ◽  
Marine Seismic

The Cambridge University Department of Geodesy and Geophysics pioneered the development of radio sonobuoys which could be used from a single ship to study the structure of the submarine crust. By contrast, contemporaneous marine seismic research, mainly in the USA, used more expensive techniques requiring the use of two ships. For nearly three decades from the early 1950s several generations of Cambridge sonobuoys were used as the primary tool to study the structure of the oceanic crust and the adjacent continental margins by seismic refraction methods, until superseded by ocean-bottom seismographs. An early result was to confirm the ubiquity across the world of relatively thin (compared with continental crust), probably volcanic, oceanic crust. This in turn underpinned the subsequent recognition of seafloor spreading and plate tectonics.

Crustal thickness variations between the Greenland and Ellesmere Island margins determined from seismic refraction

1994 ◽  
Vol 31 (9) ◽  
pp. 1407-1418 ◽  
Author(s):  
H. Ruth Jackson ◽  
I. Reid
Keyword(s):  
Sedimentary Basin ◽  
Seismic Refraction ◽  
Plate Boundary ◽  
Ellesmere Island ◽  
North American Plate ◽  
Ocean Bottom ◽  
Transform Faults ◽  
Ocean Bottom Seismometers ◽  
Thickness Variations ◽  
Thin Crust

Two densely sampled marine refraction lines were shot in northern Baffin Bay on the shelves of Devon and Ellesmere islands (North American plate) and Greenland (Greenland plate). A total of 11 ocean-bottom seismometers recorded the airgun signals. The processed data were analyzed by the use of ray tracing and amplitude modelling. Two-dimensional models were derived that reproduce the characteristics of the observed data. A 5 km deep sedimentary basin was identified on the south end of line 3. On both lines the crustal velocity has a range of 5.7–6.6 km/s. Midway along the line on the shelf of Devon and Ellesmere islands, the Moho shallows abruptly northward from 27 to 20 km. The thinned crust is not overlain by a sedimentary basin to compensate for the elevated Moho, suggesting this is not an extensional feature. The thickness of the crust adjacent to northwest Greenland increases from south (22 km) to north (37 km). The thickening occurs in two stages: a sharp increase in the depth to Moho northwest of the sedimentary basin followed by a gradual deepening to the end of the line. The thin crust on the shelf of Ellesmere Island is located adjacent to the thick crust of Greenland. Plate reconstructions based on regional magnetic anomalies and transform faults indicate that Greenland is a separate plate. The crustal structure revealed by seismic refraction and reflection profiles and the variations in the depth to Moho are consistent with the plate boundary occurring between the refraction lines.

scholarly journals Electrical characterization of the North Anatolian Fault Zone underneath the Marmara Sea, Turkey by ocean bottom magnetotellurics

2013 ◽  
Vol 193 (2) ◽  
pp. 664-677 ◽  
Author(s):  
T. Kaya ◽  
T. Kasaya ◽  
S. B. Tank ◽  
Y. Ogawa ◽  
M. K. Tuncer ◽  
...  
Keyword(s):  
Fault Zone ◽  
Electrical Characterization ◽  
North Anatolian Fault ◽  
Marmara Sea ◽  
Ocean Bottom ◽  
North Anatolian Fault Zone ◽  
The North
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