scispace - formally typeset
Search or ask a question

Showing papers on "Fault (geology) published in 1978"


Journal ArticleDOI
TL;DR: A double-planed structure of deep seismic zone has been found over a wide area more then 300 km × 200 km in the Tohoku District, the northeastern part of Honshu, Japan.

417 citations


Journal ArticleDOI
TL;DR: In this article, the authors analyzed surface wave and body wave data from the Guatemala earthquake of February 4, 1976 and found that the seismic moment was 2.6 × 10^27 dyn cm and the rupture velocity was 3 km/s.
Abstract: Detailed analyses of teleseismic surface waves and body waves from the Guatemala earthquake of February 4, 1976, show the following: (1) Left lateral displacement along a vertical fault with a strike varying from N66°E to N98°E is consistent with the teleseismic data. (2) The seismic moment was 2.6 × 10^27 dyn cm. The directivity of the surface wave radiation indicates an asymmetric (1:2.3) bilateral faulting with a total length of 250 km. In modeling the displacement a rupture velocity of 3 km/s was used, and the fault curvature was included. (3) If a fault width of 15 km is assumed, the average offset is estimated to be about 2 m. This value is about twice as large as the average surface offset. (4) Although the observed directivity suggests a uniform overall displacement along the fault, the body wave analysis suggests that the earthquake consists of as many as 10 independent events, each having a seismic moment of 1.3–5.3 × 10^26 dyn cm and a fault length of about 10 km. The spatial separation of these events varies from 14 to 40 km. This multiple-shock sequence suggests that the rupture propagation is jagged and partially incoherent with an average velocity of 2 km/s. (5) The average stress drop estimated from surface waves is about 30 bars, but the local stress drop for the individual events may be significantly higher than this. (6) The complex multiple event is a manifestation of a heterogeneous distribution of the mechanical properties along the fault, which may be caused by either asperities, differences in strength, differences in pore pressure, differences in slip characteristics (stable sliding versus stick slip), or combinations of these factors. (7) This complexity has important bearing on the state of stress along transform faults and is important in assessing the effect of large earthquakes along other transform faults like the San Andreas.

345 citations


ReportDOI
TL;DR: In this paper, a dyke tip propagated with the velocity of 0.4-0.5 m/sec during the first 9 hours, but the velocity decreased as the length of the dyke increased.
Abstract: The July 1978 deflation of the Krafla volcano in the volcanic rift zone of NE-Iceland was in most respects typical of the many deflation events that have occurred at Krafla since December 1975. Separated by periods of slow inflation, the deflation events are characterized by rapid subsidence in the caldera region, volcanic tremor and extensive rifting in the fault swarm that transects the volcano. Earthquakes increase in the caldera region shortly after deflation starts and propagate along the fault swarm away from the central part of the volcano, sometimes as far as 65 km. The deflation events are interpreted as the result of subsurface magmatic movements, when magma from the Krafla reservoir is injected laterally into the fault swarm to form a dyke. In the July 1978 event magma was injected a total distance of 30 km into the northern fault swarm. The dyke tip propagated with the velocity of 0.4-0.5 m/sec during the first 9 hours, but the velocity decreased as the length of the dyke increased. Combined with surface deformation data, these data can be used to estimate the cross sectional area of the dyke and the driving pressure of the magma. The cross sectional area is variablemore » along the dyke and is largest in the regions of maximum earthquake activity. The average value is about 1200 m{sup 2}. The pressure difference between the magma reservoir and the dyke tip was of the order of 10-40 bars and did not change much during the injection.« less

298 citations


Journal ArticleDOI
TL;DR: In this paper, an elementary analysis of faulting in three-dimensional strain field is presented, which considers the deformation of an idealized model due to slip along sets of faults; the model is subjected to strain boundary conditions.

267 citations


Journal ArticleDOI
Isamu Aida1
TL;DR: In this article, a finite difference hydrodynamical method with finer grids in shallower water was used to estimate the vertical displacement field of the sea bottom derived from the seismic fault model for each earthquake.
Abstract: Numerical experiments of tsunami generation and propagation are carried out for five earthquakes which occurred off the Pacific coast of the Tohoku and Hokkaido districts. The tsunami sources used in the experiments are the vertical displacement field of the sea bottom derived from the seismic fault model for each earthquake. Water surface disturbances are computed by a finite difference hydrodynamical method with finer grids in shallower water. The comparison of the computed tsunami behavior with available tsunami records along the coast shows that the distribution of observed tsunami heights can be explained in the first approximation by seismic fault models while the observed heights are 1.2 to 1.6 times larger than the computed heights. An example of a fault model inferred from seismic data (June 12, 1968) which is not suitable for a tsunami source is presented.

233 citations


Journal ArticleDOI
TL;DR: In this paper, two-dimensional half-space models with surface loading are used to study the change in strength produced by these mechanisms for thrust, normal, and strike slip faulting.
Abstract: The association of a significant increase in seismic activity with the filling of some large water reservoirs is well documented. One possible such ase is the earthquake sequence at Oroville, California, in 1975. It has been suggested that this activity may be triggered on faults close to failure by the weight of the water, the increase in pore pressure, or both. Two-dimensional half-space models with surface loading are used to study the change in strength produced by these mechanisms for thrust, normal, and strike slip faulting. For the case of the water weight alone the strength increased across fault planes dipping as in the Oroville sequence. It siconcluded that the water load alone is an unlikely mechanism. The inclusion of pore pressure effects in these models, however, is shown to be very encouraging. Using the Biot linearized quasi-static elasticity theory for fluid-infiltrated porous materials, two types of loaded half-space models were studied. The first was homogeneous with respect to permeability and found to produce broad zones of weakening. The second type included a fault zone of different permeability. This model intensifies the magnitude and broadens the zone of strength drop if the fault zone is highly permeable in relation tomore » the rest of the half space. For fault dips roughly corresponding to the Oroville situation it was found that although weakening occurs for ambient stresses favoring either thrust or normal faulting, the magnitude and breadth of strength drop were significantly greater for normal faulting. Similar effects were observed for strike slip faulting. It was also found that all types of models developed significant zones of weakening very rapidly, permitting reasonable values of permeability to be estimated. The case of the impervious reservoir bottom was examined, with significantly differing results.ater strength drops produced by the heterogeneous model, both of which are reasonable rock values.« less

230 citations


Journal ArticleDOI
TL;DR: In this article, the Sagami trough deformation was modeled as a low-angle right-lateral faulting with a thrust component at the plate boundary, and the 1703 and 1923 earthquakes were interpreted as the result of lowangle, right-angle, and thrust component, respectively.
Abstract: The southern Kanto region has had two shocks of magnitude 8 or greater during the past 1,000 years. They were the 1703 and 1923 earthquakes, which occurred along the Sagami trough, a northeastern boundary of the Philippine Sea crustal plate in contact with the Asian plate. Although they occurred in nearly the same region, the 1703 earthquake was significantly different from the 1923 earthquake in the distribution of coastal uplift and tsunami height. The 1703 earthquake deformation is described on the basis of the height of the marine terraces along the coast of the southern Kanto region. The 1703 earthquake is interpreted, as is the 1923 earthquake, as the result of low-angle right-lateral faulting with a thrust component at the plate boundary. However, the fault surface in 1703 was longer (about 200 km) and was located farther east than that of the 1923 earthquake. On the basis of the pattern of coastal uplift and the trend of the Sagami trough, the fault surface of the 1703 earthquake can be divided into three planes, which involve the eastern part of the source region of the 1923 earthquake to the west (plane A), the Kamogawa submarine cliff in the middle (plane B), and a segment near the source region of the 1953 Boso-Oki earthquake (M equals 8.0) to the east (plane C). The Boso and Miura Peninsulas in the sourthern Kanto region have been uplifted during at least the last 6,000 years, and major uplifts have been accompained by earthquakes like those of 1703 and 1923 many times. The recurrence time of similar uplifts is estimated at 800 to 1,500 years on the basis of the numbers of the uplifted Holocene terraces in the Boso Peninsula, the rate of upheaval during the last 6,000 years, and the present geodetic data. Thus, it is unlikely that major earthquakes such as the 1703 and 1923 earthquakes will occur in the same segments in the near future. The Oiso area, however, which is located west of the western end of the 1703 faulting, seems higher in seismic risk than the other parts of the Sagami trough fault, because the sum of the recent uplift in the 1703 and 1923 earthquakes in that area is significantly less than the average rate of uplift there during the past 6,000 years. /Author/

198 citations


Journal ArticleDOI
TL;DR: The Arakapas fault belt is an elongate east-west fracture zone where an intensely brecciated basement of ocean crust is overlain by a variety of mafic volcanic rocks and clastic sediments.
Abstract: It is widely accepted that the Troodos massif of Cyprus is a fragment of oceanic lithosphere formed at a constructive margin beneath a small marginal sea some 85 m.y. ago. The Arakapas fault belt is an elongate east-west fracture zone where an intensely brecciated basement of ocean crust is overlain by a variety of mafic volcanic rocks and clastic sediments. While constructive margin processes were still active elsewhere on the massif, the Arakapas fault belt existed as a trough with a rugged bathymetry formed of numerous fracture zones between which were relatively undeformed blocks. The north-south–trending dikes of the main massif swing progressively westward into an east-west alignment as the fault is approached. This deviation could be due to horizontal drag along the fault, but dike injection into a sygmoidal stress field prior to the development of the fault is considered. Onto the rugged bathymetry, in which fault scarps and associated scree deposits are still identifiable, mafic lavas were extruded, and a variety of sediments, produced by the submarine erosion of bathymetric highs within and on the flank of the trough, were deposited. Some of the volcanic rocks within the fault belt are more primitive than those of the main massif, and this is interpreted as owing to a higher percentage of melt in the underlying mantle and easier egress for the basaltic magmas. The metamorphic imprint indicates that the thermal gradient in the fault zone was steeper than elsewhere on the massif and that quantities of circulating sea water were greater. Later, once this part of the fault zone had moved well outside the constructive margin offset, serpentinite masses were emplaced and normal and reverse faulting occurred. Structural, petrochemical, and sedimentary features combine to suggest that the Arakapas fault belt is a fossil transform fault.

178 citations


Journal ArticleDOI
TL;DR: In this article, a quasithreedimensional faulting model with non-uniform distributions of static frictions or the fracture strength under a finite shearing pre-stress was investigated, and the displacement and stress time functions were obtained by solving numerically the equations of motion with a finite stress-fracture criterion, using the finite difference method.
Abstract: Summary Dynamical rupture process on the fault is investigated in a quasithreedimensional faulting model with non-uniform distributions of static frictions or the fracture strength under a finite shearing pre-stress The displacement and stress time functions on the fault are obtained by solving numerically the equations of motion with a finite stress-fracture criterion, using the finite difference method If static frictions are homogeneous or weakly non-uniform, the rupture propagates nearly elliptically with a velocity close to that of P waves along the direction of pre-stress and with a nearly S wave velocity in the direction perpendicular to it The rise time of the source function and the final displacements are larger around the centre of the fault In the case when the static frictions are heavily non-uniform and depend on the location, the rupture propagation becomes quite irregular with appreciably decreased velocities, indicating remarkable stick-slip phenomena In some cases, there remain unruptured regions where fault slip does not take place, and high stresses remain concentrated up to the final stage These regions could be the source of aftershocks at a next stage The stick-slip faulting and irregular rupture propagation radiate highfrequency seismic waves, and the near-field spectral amplitudes tend to show an inversely linear frequency dependence over high frequencies for heavily non-uniform frictional faults

158 citations



Journal ArticleDOI
TL;DR: In this paper, the problem of frictional heating on a fault of finite thickness is considered and the temperature distribution during and after faulting is obtained for faults of various thicknesses.
Abstract: Summary. The problem of frictional heating on a fault of finite thickness is considered. Temperature distributions during and after faulting are obtained for faults of various thicknesses. A thick fault approximation is shown to be valid if a fault has a thickness greater than about 1 cm. The thickness of the melted zone is predicted for various frictional stress levels and displacements. The predicted thicknesses are shown to be in reasonable agreement with field observations.

Journal ArticleDOI
TL;DR: In this paper, the Indo-Australian/Pacific plate boundary crosses the New Zealand continental block as the Alpine fault system, an oblique continental transform, and the sedimentary history is consistent with the development of the plate boundary from a zone of slow oblique extension, to a through-going continental transform.
Abstract: The Indo-Australian/Pacific plate boundary crosses the New Zealand continental block as the Alpine fault system, an oblique continental transform. Plate tectonic calculations for the SW Pacific imply the inception of this plate boundary in the latest Eocene. In western South Island, rapidly subsiding flysch basins were established at this time. Southeast of the Alpine Fault, basins developed along an active tectonic trend, the Moonlight Tectonic Zone. Early Oligocene infilling by thick submarine fan sequences was accompanied by regional subsidence, so that by late Oligocene, deep marine mudstone was dominant in basins and limestone on adjacent shelf areas. In contrast, early to mid Miocene time was characterized by contemporaneous uplift and subsidence, with tectonic activity on basin margins and development of new source areas adjacent to the Alpine Fault. Reverse faulting, folding and uplift developed at about 12 Ma and continues today. The sedimentary history is consistent with the development of the plate boundary from a zone of slow oblique extension, to a through-going continental transform, to a compressive transform system. Continental transform boundaries appear to be characterized by small, tectonically active basins, and by a complex tectonic history, particularly when close to the pole of relative rotation. Although difficult to recognise, they may be important in the geologic record.


Journal ArticleDOI
TL;DR: In this article, an attempt to correlate synchronous tectonic movements over huge areas of the continent is made, which is appropriate to recognize in such correlation mobile and stable zones of different scales.
Abstract: Late Pleistocene and Holocene activity on major faults of western and central Asia are inferred from offsets of young topography. Large active strike-slip faults bound the northward-moving Arabian and Indian plates. The Indian plate moves more rapidly with respect to Eurasia than does the Arabian plate, with the highest rate of young lateral motion (1.2 to 1.4 cm/yr) along the Darvaz fault on its northwestern side. Convergence of these plates on the Eurasian plate produces the north-trending compression of the latter. It results in the creation of northwest-trending dextral wrench faults, smaller northeast-trending sinistral faults, east-west–trending thrust faults, and rare north-trending normal and extension faults. The rates of relative motion are slower here than on the borders of the southern plates, being higher to the north of the Indian plate than to the north of the Arabian. Not all relative motion of the southern plates is accommodated by their boundary deformation and by the compression of the Eurasian plate. This motion and resistance of the Eurasian plate produce squeezing of the rock masses on both sides of the southern plates that results in dextral west-trending wrench faulting on the western side and sinistral on the eastern side of each plate. This pattern can be recognized throughout the Neogene-Quaternary tectonic evolution of the Asian segment of the Alpine–central Asian orogenic belt. This paper is an attempt to correlate synchronous tectonic movements over huge areas of the continent. The author believes it is appropriate to recognize in such correlation mobile and stable zones of different scales. The depth and character of motion in the mobile zones define important features of the structure and evolution of the Earth9s crust.

Journal ArticleDOI
TL;DR: The Taranaki Basin contains the only commercial gas and condensate fields in New Zealand as mentioned in this paper, including the Kapuni and Maui fields, and has been extensively studied.
Abstract: The Taranaki Basin contains the only commercial gas and condensate fields in New Zealand. Thirteen offshore wells have been drilled, three of which delineated the Maui Field while six deep tests have been drilled onshore, one of which discovered the Kapuni Field. The geology of the Taranaki Basin is synthesised into a transgressive stratigraphic framework which was modified by two tectonic phases, initial rifting and foundering, followed by wrench faulting. The basin consists of the Western Platform and Taranaki Graben Complex. The former was a relatively stable block throughout most of the Tertiary, only affected during Late Cretaceous to Eocene times by normal block faulting. The latter is bounded to the east by the Taranaki Fault Zone which was mainly active during the Miocene. To the west, the Graben Complex generally shallows across a series of en-echelon steep normal to reverse faults which often show drastic changes in throw over short distances. Upper Cretaceous coal measures were deposited in fault angle depressions. Marine sediments were deposited in western areas by Paleocene times. A regressive phase occurred during Eocene times when coal measures were deposited in southern and eastern areas. Quartzose sandstones of these coal measures are the reservoirs in the Kapuni and Maui Fields. In Late Eocene to Oligocene times, regional submergence recommenced and mainly calcareous sediments were deposited: pelagic -rich sediments in the west, neritic limestones, sandstones and mudstones in the south and east. With the development of the Taranaki Graben Complex from the Miocene onwards, sedimentary sequences consist of graben -fill mudstones and flysch, and prograding wedges comprising the continental shelf.

Journal ArticleDOI
TL;DR: In this article, the authors studied the seismicity of western Canada for the period 1899-1975 and showed that most seismic energy is released along the Queen Charlotte- Fairweather fault system and that at present a significant accumulation of strain may be available for release as earthquakes in the Vancouver Island- Puget Sound area.
Abstract: The seismicity of western Canada has been studied for the period 1899–1975. The quality of the data collected improved through this period as the number of recording stations increased and the location and analysis methods developed, but significant uncertainties and biases remain. Although these restrictions limit detailed correlation of seismic events with specific tectonic features, in general the most active earthquake areas correspond to the boundaries between the major lithospheric plates. These are the Queen Charlotte – Fairweather fault system (Pacific–America plates), the offshore ridge-fracture zone system (Pacific – Juan de Fuca plates), and the Vancouver Island – Puget Sound region (Juan de Fuca – America plates). Strain release calculations show that most seismic energy is released along the Queen Charlotte – Fairweather fault system and that at present a significant accumulation of strain may be available for release as earthquakes in the Vancouver Island – Puget Sound area. Except for the a...

Journal ArticleDOI
TL;DR: Fault plane solutions of 67 earthquakes along the Middle America arc were determined and focal mechanisms delineate tectonic activity characteristics of a subduction zone were determined as discussed by the authors, indicating that the subducted portion of the plate is under extensional stress.
Abstract: Fault plane solutions of 67 earthquakes along the Middle America arc were determined. These focal mechanisms delineate tectonic activity characteristics of a subduction zone. Focal mechanisms of shallow events beneath the trench axis indicate normal faulting resulting from initial flexure of the downgoing plate. Shallow focus earthquakes along the inclined seismic zone yield focal mechanisms which indicate underthrusting on fault planes dipping beneath the coast. The tensional axes of the focal mechanism solutions of the intermediate focus earthquakes are aligned down the dip of the seismic zone and indicate that the subducted portion of the plate is under extensional stress. The strike-slip solutions of nine events along the arc possess one nodal plane which strikes approximately perpendicular to the trend of the arc. The locations of these nine events correlate very well with transverse features mapped by Stoiber and Carr (1973) from discontinuities and offsets in the volcanic chain and dip changes in t...

Journal ArticleDOI
TL;DR: The Denali Fault system, a major crustal break in Alaska and northwestern Canada, is characterized by horizontal movement although vertical separation has been suggested for parts of the system as mentioned in this paper, which is not the case here.
Abstract: The Denali Fault system, a major crustal break in Alaska and northwestern Canada, is characterized by horizontal movement although vertical separation has been suggested for parts of the system. Es...

Journal ArticleDOI
TL;DR: In this paper, the authors emphasize the important role of the large scale strike-slip faults in the tectonic history of the circum-Japan Sea region including the origin of the Japan Sea Basins.
Abstract: The authors emphasize the important role of the large scale strike-slip faults in the tectonic history of the circum-Japan Sea region including the origin of the Japan Sea Basins. NE-SW to NNE-SSW trending faults in the region were formed by the intense compression from the south which corresponds to the "pulse" suggested by LARSON and PITMAN (1972). NS to NNW-SSE trending faults in Northeast Japan were formed one after another from east to west by the left-lateral simple shear force induced by the subduction of the peculiar transform fault between the Kula and Tethys plates. Subduction of the seamount chain on the peculiar transform fault formed NW-SE trending faults and the cusp structure in the Kanto region. The subduction of the Kula-Pacific and Tethys ridges produced the acidic igneous activities of the Cretaceous to Paleogene in the Asiatic continental margin. Japan Sea Basins were formed in the Paleogene time by the southward drift of the western part of Japan bounded on the east by the Tanakura shear zone and on the west by the Tsushima fault, both of which had already been formed as left-lateral faults in the Cretaceous time.

Journal ArticleDOI
TL;DR: A 9 km length of the fast-slipping Quebrada transform fault zone near 4°S on the East Pacific Rise was examined with deeply towed sonars and cameras as mentioned in this paper.
Abstract: A 9 km length of the fast-slipping (152 mm/yr) Quebrada transform fault zone near 4°S on the East Pacific Rise was examined with deeply towed sonars and cameras. The fault zone occupies a 7 km-wide transform valley, which is a graben with step-faulted walls thickly mantled by pillow-basalt talus. Left-lateral strike-slip faulting is concentrated in a single narrow (< 200 m-wide) crush belt near the foot of the younger wall. The fault outcrops within a 20-50 m deep furrow that parallels the direction of plate separation and is similar to the topographic "rifts'* along some continental strike-slip faults. The older, deeper, part of the valley floor, south of the active strike-slip fault, has been broken into secondary horsts and grabens by tensional faults that are 45° oblique to the strike-slip trend. The valley floor has experienced differential sediment deposition from an eastward bottom current, and part of the deepest graben has been flooded with young pahoehoe flows. There is increased volcanism in fa...

Journal ArticleDOI
TL;DR: In this article, the authors focused on the determination of fault parameters and source processes of the Caracas (Venezuela) earthquake of July 29, 1967 (mb = 6.5, Ms =6.7).
Abstract: A general study of Caribbean plate tectonics is first focused on the determination of fault parameters and source processes of the Caracas (Venezuela) earthquake of July 29, 1967 (mb = 6.5, Ms = 6.7). Synthetic seismograms which closely reproduce the observed P, SH, and Love wave seismograms were generated using generalized ray and mode theories. The results indicate a complicated faulting process, consisting of at least three separated sources aligned along a Nl0°W trending ‘en echelon’ vertical left lateral strike slip system of three faults that ruptured from north to south, at three discrete places with an extreme separation of 90 km. The process of rupture progressed southward with a mean velocity of 3 km/s. The focal depths of the individual sources varied between 8 and 27.5 km. The total dislocation was calculated as 120 cm along the direction N 10°W, and the total average moment as 4 × 1026 dyn cm. The multiple character of the event severely constrains the number of suitable source models that can be inferred, thus facilitating the process of inversion. Tectonic implications are briefly discussed, and local geology is successfully invoked to support the source model.


Journal ArticleDOI
TL;DR: The main Ethiopian and Stefanie rifts are relatively symmetrical and lie at the crests of basement arches; gneisses are exposed at 2750 m a.s.l..

Book ChapterDOI
Amos Nur1
TL;DR: In this article, a review of simple models and observations suggests that the main first-order features of active faulting may be explained by a single characteristic of crustal faults: the spatial variation of the effective frictional stress, which resists slippage on faults.
Abstract: A review of simple models and observations suggests that the main first-order features of active faulting — mechanical instability, the frequency—magnitude relations, seismic and aseismic slip, seismic radiation, incoherency and rupture stoppage — may be explained by a single characteristic of crustal faults: the spatial variation of the effective frictional stress, which resists slippage on faults. Fault offset data suggest that rupture propagation ceases in regions of high resistance which act as barriers. In these regions slippage is associated with negative stress drop. The spacing λ and the amplitude A(λ) of the barriers, as inferred from the frequency—magnitude and moment relation for earthquakes, obeys a simple statistical relation A(λ) ~ λρ. On the scale of particle motion, this variability of frictional stress provides a mechanical instability which may be associated with the concept of dynamic friction. Invariably, the rapid particle motion in the model is always preceded by accelerated creep. The particle acceleration is highly irregular, giving rise to an almost random acceleration record on the fault. The particle displacement is relatively smooth, giving rise to simple displacement time function in the far field. Rupture propagation time is approximately proportional to the gradient of frictional stress along the fault. Consequently sharp changes of this stress may cause multiple events and other long period irregularities in the fault motion.

Journal ArticleDOI
TL;DR: A revised faultplane solution indicating strike-slip faulting (probably right lateral on a northwest striking plane) is the most consistent with observed intensities, water disturbances, and calculated ground deformation.
Abstract: Available near- and far-field data have been used to reassess and reevaluate the focal parameters of the June 23, 1946 British Columbia earthquake. The preferred epicenter (49.76°N, 125.34°W) is located on Vancouver Island, inland from the population centers along the east coast. This location is consistent with observed intensities, water disturbances, and calculated ground deformation. The hypocentral depth is near 30 km, making surface rupture a distinct possibility. A revised fault-plane solution indicating strike-slip faulting (probably right lateral on a northwest striking plane) though not a unique interpretation, is the most consistent with observed intensities, water disturbances, and calculated ground deformation. A new surface-wave magnitude calculation of 7.2 ± 0.1 agrees with the previously published value of 7.3. Calculated source parameters are as follows: seismic energy release of 5.6 × 10 22 ergs; seismic moment of 2.5 × 10 27 dyne-cm for the preferred (strike-slip) solution; an apparent stress of 10 bars for the preferred solution. The lack and relatively small size of aftershocks may be indicative of a high stress drop but a reliable evaluation of stress drop is not possible because of uncertainties in estimates of fault dimensions. The epicentral location favors an intraplate setting because it is away from the continental-oceanic boundary and appears to lie within the continental crust of Vancouver Island, which overlies subducted oceanic lithosphere. However, tectonic forces causing the earthquake probably result from the interplate dynamics of the subduction zone.

Journal ArticleDOI
21 Sep 1978-Nature
TL;DR: In this paper, the utility of ratios of He/Ar and N2/Ar in fault airs is examined in this context, where the utility is based on the anomalous change of fluid phases of the ground, which are expelled by the stress at depth before earthquakes.
Abstract: THE geochemical method for the prediction of earthquakes depends on the ability to find the anomalous change of fluid phases of the ground, which are expelled by the stress at depth before earthquakes. Here the utility of ratios of He/Ar and N2/Ar in fault airs is examined in this context.

Journal ArticleDOI
TL;DR: In this article, the authors study the near-field generated by Haskell9s rectangular fault model used extensively to interpret seismic data and find an exact solution for the near field particle velocities in the case of a step function source slip history.
Abstract: We study the near-field generated by Haskell9s rectangular fault model used extensively to interpret seismic data. By means of the Cagniard-de Hoop method we have been able to find an exact solution for the near-field particle velocities in the case of a step-function source slip history. The results show that there are two distinct regions of radiation. One is a cylindrical region in front of the fault where two-dimensional approximations are valid and result in a substantial reduction of computation. In the rest of space the field is dominated by spherical waves radiated from the corners of the dislocation. These waves are much more complicated to calculate. First motion approximations demonstrate that the cylindrical waves are stronger than the spherical waves; in particular, infinite accelerations at the cylindrical wave fronts are predicted even for ramp sourcetime functions. The stress field on the fault plane generated by this dislocation is also calculated. Strong stress singularities of type r −1 are found all around the edges of the fault. These singularities are a consequence of the assumption of constant dislocation across the fault width. They may only be eliminated by smoothing the dislocation near the fault edges. These singularities are so strong that an infinite averaged stress drop on the fault is predicted independently of any source parameters. As a consequence, the Haskell model is essentially a long-period model of faulting.


Journal ArticleDOI
TL;DR: In this paper, the authors show that the late Precambrian sedimentary rocks of the Varanger Peninsula, north Norway, occur in the Tanafjord-Varangerfjord and Barents Sea (allochthonous) regions.
Abstract: The late Precambrian sedimentary rocks of the Varanger Peninsula, north Norway, occur in the Tanafjord–Varangerfjord (autochthonous) and Barents Sea (allochthonous) regions. The autochthonous region includes 4-5 km of shallow water clastic sediments with subordinate carbonates and also tillites, of late Precambrian (Upper Riphean) to Tremadocian age. The allochthonous region includes about 14 km of deep and shallow water clastic sediments with subordinate carbonates, all of Precambrian age. On Varanger Peninsula these two regions are always separated by a major strike-slip tectonic disturbance, the Trollfjord–Komagelv fault. This is a single, high-angle fault in Inner Varanger Peninsula, but in the Trollfjordalen–Lille Molvik area a braided fault pattern occurs in which an early trace of the Trollfjord-Komagelv fault has been later folded and faulted. Some structural observations support strike-slip movement along the Trollfjord-Komagelv fault, while comparison with recent strike-slip fault zones suggests that the folded fault in the Trollf jordalen-Lille Molvik area represents an inactive, curved fault trace which was abandoned and subsequently tightened by (? Caledonian) folding. Stratigraphic and structural data support recent geochronological and palaeomagnetic results which suggest the geological separation of the two regions during their deposition and perhaps during their principal deformations.

Book ChapterDOI
01 Jan 1978
TL;DR: Arthaud and Matte as discussed by the authors showed that transform motion on the Proto-Bay of Biscay fault opens the Rockall Trough and the eastern Norwegian Sea during the Permian.
Abstract: The Greenland-Svalbard and Proto-Bay of Biscay fault zones are small circles to a common pole, and it can be shown that transform motion on these faults opens the Rockall Trough and the eastern Norwegian Sea. Arthaud and Matte calculate a dextral movement along the Proto-Bay of Biscay fault of more than 100 km. This movement took place between the emplacement of the Variscan granites and the beginning of the Triassic, and we argue here that this was one of the results of ocean floor spreading in Rockall Trough and the eastern Norwegian Sea. Widespread intrusions of dolerite in Sweden, Norway, England and Scotland c. 290 Ma imply that the lithosphere had reached its limit of strength in relative tension and it is surmised that this is the time that the lithosphere began to separate between Greenland and Northwest Europe. Magmatism continued in the Early Permian as evidenced by the Oslo extrusives and fluorite deposits of magmatic derivation in England. Rosemary Bank is an extinct volcano in the northern Rockall Trough, and judging from the remanent magnetic vector, this seamount was probably active during the Permian.