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Showing papers on "Fault (geology) published in 1972"


Journal ArticleDOI
TL;DR: In this article, the authors attempt to forecast likely locations for large shallow South American earthquakes in the near future by examining the past space-time pattern of occurrence of large (M ≥ 7.7) earthquakes, the lateral extent of their rupture zones, and, where possible, the direction of rupture propagation.
Abstract: This study attempts to forecast likely locations for large shallow South American earthquakes in the near future by examining the past space-time pattern of occurrence of large (M ≥ 7.7) earthquakes, the lateral extent of their rupture zones, and, where possible, the direction of rupture propagation. Rupture zones of large shallow earthquakes generally abut and do not overlap. Patterns of rupture propagation appear to follow certain trends. These facts, plus the nonrandom behavior of the space-time history of seismic activity, present consistencies that may permit prediction, in a gross sense, of future events. By mapping the rupture zones of large earthquakes (in contrast with plotting only epicenters), it is possible to identify segments of the shallow seismic zone that have not ruptured in many decades. Limited experience elsewhere indicates that these gaps between rupture zones tend to be filled by large-magnitude earthquakes. In certain places it is possible to make approximate estimates of the time of occurrence of the next large earthquake. For at least 300 or 400 years, the entire fault segment near the Central Valley province of central and southern Chile (about 32°–46°S) has fractured about once each century from a generally N-S progression of several large (M ≥ 8) earthquakes. Large earthquakes in this region have almost always occurred to the south of a previous large earthquake. In addition, it is possible to infer a direction of rupturing for two large earthquakes in this century (1928 and 1960). Both these earthquakes fractured southward away from the rupture zone of an earlier earthquake. It would be consistent with these observations if a new series started about the end of this century near Valparaiso (33°S) and progressed southward. In other sections of South America there are several extensive segments of the active seismic belt that have not ruptured during this century. Northern Chile and southernmost Peru (about 17°–25°S) have been relatively aseismic for about 100 years. South of Lima (about 12.5°–14°S), between the rupture zones of the 1940 and 1942 Peruvian earthquakes, there is another significant gap in recent activity. Both these regions are probably areas of relatively high earthquake risk. The northern Peru and southern Ecuador region (about 9°–1°S) has also been relatively aseismic during this century. However, this region differs from the two previously mentioned gaps in that this coastal zone was a region of moderate seismicity during historic times. Perhaps aseismic creep is an unusually important factor in relieving tectonic strain along this particular segment of the shallow seismic zone. Another possibility is that large shallow earthquakes in this region have an extremely long recurrence time. Much of the shallow seismic zone of northern Ecuador and southwestern Colombia has ruptured twice during this century. During large earthquakes in this region, the rupturing tends to be directed toward the north or NE. The data for this region suggest that the area to the NE of the 1958 Colombian earthquake may be a region of relatively high earthquake risk.

435 citations


Journal ArticleDOI
TL;DR: The evolution of the Cayman Trough and the history of orogenic activity in Cuba can be explained by assuming that Caribbean lithosphere was transferred to the Americas plate as the trough developed from west to east as discussed by the authors.
Abstract: Convergence of the North and South American blocks and northeastward movement of the East Pacific-Caribbean plate during the Late Cretaceous and early Tertiary led to the Laramide tectonic and igneous activity that has been recorded in the geology of the circum-Caribbean region. Volcanism in Central America and the initiation of major transcurrent faulting along northern South America during the late Eocene suggest that the Caribbean decoupled from the East Pacific plate near the end of the Laramide Orogeny. Lack of post-Eocene structural activity in the Greater Antilles is consistent with the initiation of eastward movement of the Caribbean plate during the Eocene. The evolution of the Cayman Trough and the history of orogenic activity in Cuba can be explained by assuming that Caribbean lithosphere was transferred to the Americas plate as the trough developed from west to east. As each new section was added to the eastward-growing trough, a new transform fault formed in the Caribbean lithosphere to connect the eastern end of the trough with the Cuban Trench. This west to east stepping of the plate boundary transferred “Caribbean” lithosphere to the Americas plate and allowed underthrusting and related tectonic activity to continue longer in eastern than in western Cuba. The presence of intermediate depth earthquakes and high seismicity along the Puerto Rico Trench suggest that the Atlantic lithosphere which underthrust Puerto Rico prior to the Oligocene is beginning to break away from the Americas plate. Fault displacements inferred along the Caribbean-Americas boundary in the Greater Antilles are equivalent to a constant post-Eocene eastward movement of 0.5 cm/yr for the Caribbean plate.

411 citations


Journal ArticleDOI
25 Dec 1972-Nature
TL;DR: The Kuyshu-Palau Ridge as discussed by the authors is probably the remnant of the old transform fault, which was probably connected by a long transform fault with the major ridge system (Kula-Pacific Ridge) which submerged under the Japanese and Kurile Arcs in late Cretaceous.
Abstract: The Central Basin Fault, an extinct mid-oceanic ridge, was probably connected by a long transform fault with the major ridge system (Kula-Pacific Ridge) which submerged under the Japanese and Kurile Arcs in late Cretaceous. When the direction of the motion of the Pacific plate changed from NNW to WNW during Eocene time, the Philippine ridge became extinct and WNW dipping subduction started at the transform fault which turned into an island arc. Then extensional openings of inter-arc basins followed to form basins to the east. The Kuyshu-Palau Ridge is probably the remnant of the old transform fault.

294 citations


Journal ArticleDOI
TL;DR: In this article, the authors determined new focal mechanisms for earthquakes on the belt of seismic activity separating the Pacific and Australian plates, and showed that the pole of rotation for the Australian and Pacific plates is nearby and to the east of this feature.
Abstract: Ninety-six new focal mechanisms were determined for earthquakes on the belt of seismic activity separating the Pacific and Australian plates. The direction of convergence of these plates varies from NE-SW to E-W. The Australian plate underthrusts the Pacific plate to the ENE under the Solomon and New Hebrides islands and overthrusts the Pacific to the east along the Tonga-Kermadec arc and the North Island of New Zealand. The data for the Macquarie ridge concur with the idea that the pole of rotation for the Pacific and Australian plates is nearby and to the east of this feature. The data also suggest a NNE-SSW convergence of the Pacific and Australian plates in northwestern New Guinea. The relative motions of the plates near the Bismarck Archipelago are complex because of the presence of at least three additional small plates. The south Bismarck plate, the best defined, underlies the southern part of the Bismarck Sea. It is bounded on the north by an E-W belt of seismicity at about 3°S defining a left-lateral strike-slip fault. The New Britain arc forms the southern boundary, where the Solomon Sea floor underthrusts the south Bismarck plate to the NNW. There is some evidence for SW convergence of the south Bismarck and Australian plates in northeastern New Guinea. Small plates, less well-defined seismically, are also proposed under the northern part of the Bismarck Sea and under the Solomon Sea. The plate underlying the Solomon Sea floor is bounded by the Solomon and New Britain arcs and by eastern New Guinea. The southern boundary is not sharply defined by seismic data. The Solomon Sea plate is moving approximately NW with respect to the Australian plate and underthrusting the Pacific plate to the NE along the Solomon arc. The consistent pattern of relative motions of these three small plates allows quantitative estimates of relative rates of motion between them. These data demonstrate that plate tectonics is applicable even for regions with dimensions of only a few hundred kilometers. Geologic data from New Guinea are used to speculate about earlier plate motions in that area.

285 citations


Journal ArticleDOI
TL;DR: The East African Rift System as discussed by the authors is a belt of anastomosing faults extending 4,000 km south-southwest from the junction of the Red Sea and Gulf of Aden to the Zambezi River.
Abstract: The active East African Rift System forms a belt of anastomosing faults extending 4,000 km south-southwest from the junction of the Red Sea and Gulf of Aden to the Zambezi River. The presence of certain ancient features, including the Great Dyke of Rhodesia and an alignment of Bushveld-type igneous complexes, indicates that the system formerly extended as an infracrustal lineament another 1,500 km south-southwest down the eastern half of Africa to the Orange River. The rift system is entirely intracontinental, and formed in the ancient African Precambrian platform with whose 3-b.y. geological history it is intimately associated. The Cenozoic rift faults follow mobile belts moulded upon ancient shields and formed during at least seven major orogenic cycles affecting pre-Silurian “assemblages” comprising complex rock groups characterized by structural and metamorphic similarities but including units of greatly differing ages. An analysis of major structural features moulded on the Tanganyika Shield indicates that the belts may have originated at about 2.7 b.y. by dextral transcurrent movement between the ancient shields, but horizontal movement was subsequently impeded by cratonization and replaced by vertical displacement. The nature of the rifted belts indicates control by mantle mechanisms and repeated reactivation, so the term perennial deep lineament is proposed. Comparison with other intra-continental rift systems and preliminary geophysical results in East Africa strongly suggest lithospheric states similar to those underlying the mid-ocean ridges, implying high heat flow restricted to the lineaments in a generally cool continental environment. Geological mapping proves, however, that no formation of new oceanic crust has taken place in spite of the great age of the lineaments. Profuse rift volcanism of continental type affects the northern part of the eastern rift lineament, but this restricted volcanic sector contrasts strongly with the major nonvolcanic portion of the rift system. The lack of spreading of the rifts may be due to the compression of the African Plate between the spreading Mid-Atlantic and Mid-Indian Ocean Ridges. Long fault lineaments are features of the earth9s crust, and a review shows that intra-continental rift valleys of East African type occur where such lineaments must cut through particularly resistant cratonized massifs, forming wide zones of fracturing and shearing with blastomylonites, flaser gneisses, and migmatites in the plastic infracrust. The mobile belts of the system may thus be compared with orogenies in which plastic deformation and migmatization at deep levels are succeeded by isostatic uplift, but the linear Cenozoic arches thus produced in East Africa are affected by typical rift valley faulting at the brittle surface levels and are thus genetically associated with infracrustal Precambrian structures and continued high heat flow while reflecting supracrustal mechanics. The lineaments do not always follow fold belts but may cut obliquely across their grain. Although the most superficial rift faulting is steeply normal and antithetic, observations suggest that the deeper faults are vertical, and it is proposed that subsurface plasticity and expansion due to heat flow is a causative factor in the sinking of strips of cool surface rock in a shield environment.

285 citations


Journal ArticleDOI
TL;DR: In this paper, the authors presented new seismicity data on western Venezuela and northeastern Colombia from 1930 through 1970, which have been relocated by Joint Hypocenter Determination (JHD) or with source-station adjustments calculated by JHD.
Abstract: New seismicity data on western Venezuela and northeastern Colombia are presented. Teleseismically recorded earthquakes from 1930 through 1970 have been relocated by Joint Hypocenter Determination (JHD) or with source-station adjustments calculated by JHD. Additionally, 540 days of recording have been obtained with local seismographs installed near the Bocono Fault. The most intense shallow activity occurred north and south of the Tachira Depression along the eastern flank of the Cordillera Oriental of Colombia. The Bocono Fault Zone is seismically active; small shallow shocks were recorded in it by the local stations. Shallow earthquakes also occur in the Cordillera de Merida away from the Bocono Fault. The new hypocenters for the intermediate-depth Bucaramanga earthquakes define a smaller source volume than defined by previously computed hypocenters. A previously inferred southward-dipping seismic zone near Bucaramanga is probably spurious, a consequence of correlation between errors in latitude and errors in depth. If one assumes that these intermediate-depth earthquakes lie on a single lithospheric slab, that slab strikes approximately north and dips to the east. The distribution of hypocenters and focal mechanisms support the platetectonic hypothesis that the present tectonics of northwestern Venezuela are a result of eastward motion of the Caribbean plate with respect to the South American plate. The principal interface between these two plates may have changed within the last 5 m.y. from a zone of underthrusting west of the Sierra de Perija to the predominantly right-lateral Bocono Fault Zone.

209 citations


Journal ArticleDOI
TL;DR: Focal mechanism solutions for 23 earthquakes that occurred from 1965 to 1970 in Iran, Turkey, Pakistan, and Afghanistan were determined from the long-period first-motion P waves recorded by the World Wide Standardized Seismograph Network stations as mentioned in this paper.
Abstract: Focal mechanism solutions are determined for 23 earthquakes that occurred from 1965 to 1970 in Persia, Turkey, Pakistan, and Afghanistan. These mechanisms were obtained from the long-period first-motion P waves recorded by the World-Wide Standardized Seismograph Network stations. Mechanism solutions for the earthquakes in the folded foothills of the Zagros are predominantly thrust faulting, but depending on the assumed crustal velocity, many have strike-slip components. Cross sections of the seismic zone of the folded foothills of the Zagros show that the earthquakes are confined to a possible lithospheric slab nearly 60 km thick dipping 10° to 20° toward the north. For the earthquakes with focal depth of about 200 km, in the Hindu Kush in Afghanistan, mechanism solutions are characterized by a nearly horizontal compressional axis and a steeper extensional axis. The seismic zone appears as a contorted slab-like structure 20 to 30 km in width. This lithospheric slab may be a remnant of the Tethys Sea floor. Consistent right-lateral strike-slip mechanism solutions are found for earthquakes on the Anatolian fault in Turkey. For earthquakes on the Ferdows fault in eastern Persia, mechanism solutions vary from pure left-lateral strike-slip faulting to pure thrust faulting. Mechanism solutions for earthquakes on the Kirthar-Sulaiman shear zone in West Pakistan show consistent left-lateral strike-slip motion on a fault plane that strikes north-south. This is in agreement with the northward movement of India with respect to Afghanistan. On the basis of seismic and geological evidence, the entire Middle East is divided into eight plates which are moving northward, at different rates, with respect to Eurasia. The differential motion between various plates can explain the sense of offset and other geological features.

194 citations


Journal ArticleDOI
TL;DR: The Red Sea has entered the early stages of continental dispersal, and its structural evolution fundamentally has been the rifting and breaching of continental lithosphere by normal faulting attendant on the process of sea-floor spreading.
Abstract: The Red Sea has entered the early stages of continental dispersal, and its structural evolution fundamentally has been the rifting and breaching of continental lithosphere by normal faulting attendant on the process of sea-floor spreading. In Oligocene time, the continental lithosphere of the Red Sea area was bowed into a large regional arch with normal faults across the crest. Subsequently, rifting by normal faults that propagated upward through the brittle part of the lithosphere caused strong subsidence on horst and graben and tilted blocks; this rifting led to an extensive marine incursion, and a thick evaporite sequence was deposited in the restricted, hot, arid, low-latitude setting of the Miocene Red Sea trough. A second rift west of the earlier central or axial Mi cene rift originated in the southern Red Sea area in Pliocene time, and the two features now are evolving concurrently over a distance of at least 400 km. An evaporite section of very shallow marine origin accumulated in the western rift during the Quaternary; it constitutes a modern example of salt accumulating in a narrow, restricted, rifted trough which is forming as a consequence of continental breakup. Although considerable separation has occurred in the Red Sea, the present opposing coastlines were never in contact, because the fragmentation of continental lithosphere was very largely attained by normal faulting. Only if a vertical fault cuts the entire thickness of lithosphere could points on opposite coastlines ever have been contiguous.

157 citations


Journal ArticleDOI
Yoshio Fukao1
TL;DR: In this paper, a large deep-focus earthquake ( h = 577 km) in the western Brazil region that occurred on November 9, 1963, was followed by a relatively large aftershock.

134 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used the records of the Tottori earthquake of 1943 (M = 7.4, 35.5° N, 134.2° E, depth 10 km) about by a low-magnification seismograph at a relatively short distance to determine dynamics fault parameters such as the particle velocity and the effective tectonic stress.

133 citations


Journal ArticleDOI
01 Sep 1972
TL;DR: In this article, an analysis of the Rhinegraben has been carried out, showing that the majority of the internal and external faults are normal faults, and that the sink and shoulder uplift are controlled by the formation and growth of a laccolithic body.
Abstract: The Rhinegraben is the central segment of a rift system which traverses Western Europe from the North Sea to the Mediterranean. Rifting, which had started in the Middle Eocene, was preceded by early-Teriary nephelinitic eruptions. The center of maximum subsidence shifted along the graben axis from south to north during geologic times. The maximum thickness of sedimentary fill is observed in the northern part of the Rhinegraben to be as much as 3350 m. Most of the internal and external faults are normal faults. By theoretically returning the dip-slip movements and tilted block tectonics back to their original positions a lateral gap can be calculated in the amount of 4.8 km. The graben shoulders began to rise along the marginal fault scarps as outward tilted blocks contemporaneously when fault trough subsidence set in. The graben sinking and shoulder uplift are still in progress. Rates of annual vertical movements up to 0.7 mm, as well as seismic activity, are observed. Below both graben and shoulders a cushion-shaped body has been ascertained, intercalated between crust and mantle, with P-velocities of 7.6–7.7 km s−1. This body is interpreted to be of mantle-derived material; its crest follows the graben axis and ascends up to 24 km beneath the graben floor. It is thought that rifting, like graben subsidence and shoulder elevation, is controlled by the formation and growth of this laccolithic body. Tensional tectonics and sideward plate movements are reduced to gravity slide of the torn and upwarped crustal plates or sub-plates, decoupled from the substratum by heat transfer and consequential partial melting along the cushion surface. The lateral offsets at the transitions from the Rhinegraben to its continual segments are intersected by transversal zones of enhanced seismicity and volcanic action. The observed fault patterns are interpreted as crustal traces of subcrustal transform faulting. Rhine graben tectonism is discussed as to its local interference and temporal interaction with Alpine orogenesis, just as in its causal and spatial connection to the world rift system as a whole, Several arguments pertaining to the understanding of the driving mechanisms are discussed and a new model of rift tectonics is indicated.

Journal ArticleDOI
TL;DR: In this article, six portable seismographs were operated at 82 sites along the Intermountain Seismic Belt from southwestern Utah to northwestern Montana, and 120 micro-earthquakes were located between the surface and a 20 km depth.
Abstract: During the summer of 1969, six portable seismographs were operated at 82 sites along the Intermountain Seismic Belt from southwestern Utah to northwestern Montana. This survey followed a well-known seismic belt along the eastern physiographic boundary of the Basin and Range province, and within the middle and northern Rocky Mountains. In general, the 120 microearthquakes located in this study follow the same spatial trend as the macroseismic earthquakes reported by the NOS (formerly USCGS). Most of the micro-earthquakes clustered in time and space along well-known fault zones on which late Tertiary or younger movements have occurred. All of the accurately located hypocenters occurred between the surface and a 20 km depth. Composite fault plane solutions along the Hurricane and Sevier fault zones (southwestern Utah), Tushar and Sevier fault zones (Marysvale area, Utah), and Wasatch and East Cache fault zones in central and northern Utah indicate vertical-motion on steeply dipping fault planes. These motions may be indicative of differential movements between the Basin and Range province and the Colorado Plateau-Rocky Mountains. Composite fault plane solutions (CFPS) in the Caribou Mountains, southeastern Idaho, and Flathead Lake area, northwestern Montana, show normal faulting on less steeply dipping planes and have west-northwest trending extensional axes. Swarm activity was also observed in the above two regions. Between the above two areas of uplift and extension lies a region of complicated geology and seismicity.

Journal ArticleDOI
TL;DR: In this paper, the focal process of the Rat Island earthquake of March 30, 1965, which occurred beneath the Aleutian trench, is studied on the basis of the long-period surface-wave data and the spatial distribution of the aftershocks.

Journal ArticleDOI
TL;DR: In this article, the accuracy of teleseismic estimates of moment, fault area, dislocation and stress drop was tested for the case of a thrust fault: the San Fernando, California, earthquake of February 9, 1971.
Abstract: The accuracy of teleseismic estimates of moment, fault area, dislocation and stress drop was tested for the case of a thrust fault: the San Fernando, California, earthquake of February 9, 1971. On the basis of P-wave spectra of 25 stations and S-wave spectra of 9 stations, the respective values were found to be 0.7 · 10^(26) dyne-cm, 570 km^2, 45 cm, and 14 bars. They agree well with the same parameters obtained from field observations. It is concluded that Brune's (1970) seismic source model is valid for the area determination of thrust earthquakes. The energy radiated in the form of S wave is estimated to be 5 · 10^(21) ergs.

Book Chapter
01 Jan 1972
TL;DR: The Borrego Mountain earthquake of April 9, 1968, triggered small but consistent surface displacements on three faults far outside the source area and zone of aftershock activity as discussed by the authors.
Abstract: The Borrego Mountain earthquake of April 9, 1968, triggered small but consistent surface displacements on three faults far outside the source area and zone of aftershock activity. Right-lateral displacement of 1-2 1/2 cm occurred along 22, 23, and 30 km of the Imperial, Superstition Hills, and San Andreas (Banning-Mission Creek) faults, respectively, at distances of 70, 45, and 50 km from the epicenter. Although these displacements were not noticed until 4 days after the earthquake, their association with the earthquake is suggested by the freshness of the resultant en echelon cracks at that time and by the absence of creep along most of these faults during the year before or the year after the event. Dynamic strain associated with the shaking is a more likely cause of the distant displacements than is the static strain associated with the faulting at Borrego Mountain because (1) the dynamic strain was much larger and (2) the static strain at the San Andreas fault was in the wrong sense for the observed displacement. The principal surface displacements on the Imperial fault took place within 4 days of the earthquake and may have occurred simultaneously with the passage of the seismic waves, but the possibility of delayed propagation to the surface is indicated by a 1971 event on the Imperial fault in which the surface displacement followed the triggering earthquake by 3-6 days. All three of the distant faults are "active" in that they show evidence of repeated Quaternary movement, and surface displacements occurred only along those segments where the fault trace is well delineated in surface exposures, at least in uncultivated areas. This is the first documented example of fault displacement triggered by seismic shaking far from the source area, although such displacement has probably gone undetected many previous times here and in similar tectonic environments. This phenomenon forces us to be much more conservative in estimating the probabilities of damage from surface displacements along active faults in seismic regions.

Journal ArticleDOI
TL;DR: In this article, the shape of the body-wave spectra in terms of the seismic moment and stress drop was estimated for each of the multiple events recorded on the strong-motion seismograph in El Centro, located about 10 km NW from the instrumentally determined epicenter.
Abstract: The Imperial Valley earthquake was a multiple sequence (Trifunac and Brune, 1970) with at least four events occurring during the main energy release. These four events, recorded on the strong-motion seismograph in El Centro, located about 10 km NW from the instrumentally determined epicenter, and nine aftershocks recorded in the next 5 min are re-examined in this paper to test an approximate source theory (Brune, 1970). This theory predicts the shape of the body-wave spectra in terms of the seismic moment and stress drop. By fitting theoretical spectra to the spectra calculated from the strong-motion accelerogram, moment and stress drop can be estimated for each of the multiple events. Inasmuch as the average displacements at the fault and the source dimensions can be derived from the known moment and stress drop, the pattern of average displacements along the fault was computed from the instrumental records. A test of the theory, then, consists of comparing the fault displacements derived from seismograms with the fault displacements observed at the surface (Buwalda, unpublished field notes). For the Imperial Valley earthquake, agreement between these two independent methods of measurement is good, suggesting that the above theory is an adequate first approximation for the spectra of body waves. The stress drop variations along the fault, inferred also from the above theory, indicate two areas of major stress concentration located near the northwestern and southeastern ends of the dislocation. The stress drops for various events varied from about ten to several hundred bars.

Journal ArticleDOI
TL;DR: In this article, the Gabilan source appears to have moved northwestward along the San Andreas fault during the upper Miocene; together with tentative correlations between parts of the source and part of the clastic deposits, this suggests post-Miocene offset of 145 ± 5 mi (234 ± 8 km), post-early upper Mohnian offset of 158 ± 3 mi (254 ± 5 km), and thus 5 to 21 mi (8 to 34 km) of slip during the later upper miocene.
Abstract: Late Miocene strata occur on both sides of the San Andreas fault for several hundred miles in central California. The unlike facies and contrasting paleogeographic situations which are found across the fault along much of this distance must have been brought together by large-scale strike-slip movement on the fault. Clastic deposits west of the San Andreas need granitic-volcanic sources east of the fault, but the only such sources are to the southeast—in one probable case, more than 110 mi away. Likewise, voluminous coarse conglomerate beds east of the fault in the southern Temblor Range require the presence of a large, highly elevated granitic and metamorphic source immediately west of the fault during the later upper Miocene. The northern Gabilan Range, about 150 mi (240 km) to the northwest, was probably the only such basement area exposed at the time; the granitic and metamorphic bedrock here is very similar to the type of gravel found within the conglomerate of the southern Temblor Range. Thus, the Temblor Range clastic rocks were deposited adjacent to the Gabilan bedrock, and the two areas subsequently have been offset by about 150 mi (240 km) of right-slip on the San Andreas fault. The Gabilan source appears to have moved northwestward along the fault during the upper Miocene; together with tentative correlations between parts of the source and parts of the clastic deposits, this suggests post-Miocene offset of 145 ± 5 mi (234 ± 8 km), post-early upper Mohnian offset of 158 ± 3 mi (254 ± 5 km), and thus 5 to 21 mi (8 to 34 km) of slip during the later upper Miocene. Work in progress by several investigators, including the writer, indicates about 185 to 190 mi (300 km) of displacement for rocks 22 m.y. old. Comparison of the 145- to 158-mi and 185- to 190-mi offsets reveals that movement on the San Andreas in central California included at least one change of rate in the last 22 m.y. Although this conclusion and the indicated offsets restrict the possible histories of displacement somewhat, faulting may have proceeded at widely varying rates, and movement may have been episodic, continuously accelerating, or, if motion on this segment of the fault began only 12 ± 4 m.y. ago, constant.

Journal ArticleDOI
01 May 1972-Nature
TL;DR: In this paper, a continuous east-west reflector has been observed with long range side-scan sonar, extending along the Azores-Gibraltar Ridge from Santa Maria Is. to a point 400 km to the east.
Abstract: A continuous east–west reflector has been observed with long range side-scan sonar, extending along the Azores–Gibraltar Ridge from Santa Maria Is. to a point 400 km to the east. From the continuity, narrowness and straightness of the feature, from the evidence of seismic reflexion profiles crossing it and from other published geophysical evidence about the Azores–Gibraltar Ridge, the reflector is interpreted as a transcurrent fault.

Journal ArticleDOI
TL;DR: The source mechanism of the San Fernando earthquake on February 9, 1971, was studied by utilizing seismic body waves, surface waves, and the static displacement and strain fields in this article.
Abstract: The source mechanism of the San Fernando earthquake on February 9, 1971, was studied by utilizing seismic body waves, surface waves, and the static displacement and strain fields. For computing the static fields due to a fault model with arbitrary dip and slip directions, Maruyama's equations are integrated numerically. Very strong asymmetries in displacement fields and complicated surface strain patterns are produced by the dip of the fault plane and inclination of the dislocation vector. The joint interpretation of the body and surface waves, near-field displacement, and far-field strain fields completely define all important source parameters. The mechanism of the San Fernando earthquake is a reverse fault (strike N70°W, dip 52°NE, slip 225°) with dimensions of 14 by 14 km. The average dislocation on the fault plane is 280 cm. The rupture started near the bottom of the fault plane and propagated toward the surface. Stress drop associated with faulting was Δσ = 70 bars, and the seismic moment was M0 = 16.4 × 1025 dyne cm.

Journal ArticleDOI
31 Jan 1972-Nature
TL;DR: In this paper, plate tectonic considerations are used to interpret various geological data and to construct a speculative model for the evolution of the Tasman Sea, Macquarie Ridge and Alpine Fault.
Abstract: Plate tectonic considerations are used to interpret various geological data and to construct a speculative model for the evolution of the Tasman Sea, Macquarie Ridge and Alpine Fault.

Journal ArticleDOI
TL;DR: In this paper, the authors studied seven moderate earthquakes that occurred in the Delarof-Andreanof Island region during 1969 to 1970 and found that the focal mechanisms of these earthquakes correspond to the motion that would be expected on the basis of plate tectonics.
Abstract: During the period from May 1969 to March 1970, seven moderate earthquakes occurred in the Delarof-Andreanof Island region. The focal mechanisms of these earthquakes correspond to the motion that would be expected on the basis of plate tectonics. Of more particular significance, the motion in one of these shocks, located at intermediate depth in the Benioff zone, indicates horizontal tension parallel to the plate, corresponding to lateral extension as the plate descends under an arcuate structure convex to the plate motion. The spatial and temporal relation of these earthquakes and of their aftershock sequences to the over-all activity of the arc and particularly to the seismicity of the Rat Islands during this period supports the hypothesis that the Aleutian Islands are active by independent blocks and that the boundaries of these blocks are permanent features.

Journal ArticleDOI
TL;DR: In this article, a close-grid gravity survey of the central part of the rift defines more accurately the location of these positive anomalies, and relations are made with the main geological and tectonic features of the Ethiopian rift and crustal models which account for these anomalies.

Journal ArticleDOI
TL;DR: A detailed field study of earthquakes and a determination of principal stress direction for a region within a lithospheric plate was carried out by as mentioned in this paper, where two composite fault-plane solutions were determined.
Abstract: An earthquake swarm was observed in the Blue Mountain Lake area of the southern Adirondacks from early May 1971 until the writing of this paper (late April, 1972). This swarm provided an unusual opportunity for a detailed field study of earthquakes and a determination of principal stress direction for a region within a lithospheric plate. Thousands of events were recorded, the largest of which had a magnitude of 3.6 and was felt as far as 80 km from the epicenter. A number of earthquakes were heard as well as felt—the smaller were heard, but not felt. Thrusting mechanisms were determined from two composite fault-plane solutions. The solution for earthquakes above 2-km depth indicates faulting on a plane striking N12°W and dipping 25°E. The fault plane for the deeper events (between 2 and 3.5 km) strikes N31°E and dips 59°E. The earthquake foci define a surface that dips gently to the east to a depth of 2 km and then steepens, in agreement with the composite fault-plane solutions. There is an indication that the shallower earthquakes may not represent renewed motion on a pre-existing fault, but may mark the generation of a new fault by a regional east-west compressive stress. The deeper events which occurred late in the sequence may represent the extension of the shallow fault and its deflection to an existing weakness. The axis of maximum compressional principal stress for the shallow composite fault-plane solution trends approximately east-west and is nearly horizontal. Geological and geophysical evidence is presented to support the hypothesis that the greatest principal stress in a zone extending from northern New York State to southern Illinois is compressive, large and horizontal, and trends nearly east-west.

Book Chapter
01 Jan 1972
TL;DR: The Borrego Mountain earthquake, magnitude 6.4, occurred at 2:28:59.t. on April 9, 1968 and has been assigned a hypocenter at 33°11.4' N., 116°07.7' W, h=11.1 km as discussed by the authors.
Abstract: The Borrego Mountain earthquake, magnitude 6.4, occurred at 02:28:59.1 G.m.t. on April 9, 1968 and has been assigned a hypocenter at 33°11.4' N., 116°07.7' W., h=11.1 km. The focal-mechanism solution indicates right-lateral slip on a fault striking N. 48° W. and dipping 83° NE., which is consistent with the field observations of faulting and the regional tectonic framework. A single foreshock of magnitude 3.7 preceded the main shock by one minute, but no other precursory activity has been identified. During the year following the event, 135 aftershocks of magnitude 3.0 and greater have been identified and located, outlining a broad zone of activity centered on but displaced 2-3 km northeast of the 33-km-long surface rupture on the Coyote Creek fault. Fracturing at depth during the aftershock period evidently occurred throughout the width of the San Jacinto fault zone, but initial surface faulting was localized along the Coyote Creek fault at the zone's southwestern margin. The area of aftershock activity enlarged progressively with time, and the region of the original epicenter became relatively inactive late in the aftershock period, leading to a doughnut-shaped epicentral distribution of late aftershocks. Inasmuch as the epicenter of the main shock was roughly midway along the zone of aftershock activity, the faulting presumably was bilateral. This kind of faulting is unusual in California.

Journal ArticleDOI
TL;DR: In this article, the authors studied the Hellenide geosyncline and the western foreland in western Greece and found that the latter is a dextrally coupled orocline with minor diapirism.
Abstract: The study area comprises the province of Aitoloakarnania and associated islands in western Greece, and lies within the western sector of the Hellenide geosynclinal belt. It includes the miogeosyncline and western foreland. The sedimentary sequence consists of Permo-Triassic evaporites, a predominantly carbonate section of Late Triassic to Eocene age, and Mio-Pliocene terrigenous clastics, including a thick section of lower Miocene "flysch." Sedimentation within the miogeosyncline occurred in two distinct depositional environments--a western mobile trough which underwent five cycles of subsidence, uplift, and erosion, and an eastern shelf on which deposition occurred at a shallow depth prior to marked subsidence in Miocene time. The foreland on the west remained as a shelf throughout the depositional period. The regional structural trend is NNW-SSE, parallel with the depositional belt. Thrust faulting dominates the structural pattern and reflects transverse compression of the geosyncline. The eugeosynclinal sequence continues westward across the miogeanticlinal ridge in central Greece, on a low-angle dislocation which is interpreted as the sole fault of a major gravitational slide. Orogenic deformation of the miogeosyncline commenced during the early Miocene and failure occurred along thrust faults which, in the mobile trough, flatten at depth to form a decollement at the contact with the evaporites. However, the westerly dipping glide planes required for gravity sliding cannot be reconstructed, and the faults are considered to extend into basement. Continued compression and uplift caused complex faulting and folding within the thrust sheets and above an incompetent substratum. Evaporites crop out in a fault-controlled intrusive belt about 70 km in length along the western margin of the flysch trough and are ascribed to halotectonic breakthrough initiated during the early stages of thrusting. Structural trend directions in the western part of Aitoloakarnania and the offshore islands vary abruptly and are interpreted as a dextrally coupled orocline, possibly with minor diapirism.

Journal ArticleDOI
01 Jun 1972-Nature
TL;DR: According to theories of plate tectonics, a band of concentrated horizontal strain extends from a triplepoint junction near 60° S 158° E through New Zealand to a triple point junction near 2° S 128° E as discussed by the authors.
Abstract: ACCORDING to theories of plate tectonics1, a band of concentrated horizontal strain extends from a triple-point junction near 60° S 158° E through New Zealand to a triple-point junction near 2° S 128° E. The region of the band of strain within New Zealand is known as the Alpine Fault Zone2. From seafloor spreading rates and from the known horizontal strain directions on the Alpine Fault Zone, the rate of horizontal strain is estimated to be about 34 mm per year3,4. In the southern part of New Zealand, the fault zone is represented by the dextral Alpine Fault itself, and in the northern part by several sub-parallel dextral faults that branch off from the Alpine Fault (Fig. 1).

Journal ArticleDOI
TL;DR: In this paper, a radio-telemetry array of eight seismograph stations was used to examine the distribution of micro-earthquakes in the area developed for geothermal power.
Abstract: Microearthquakes in The Geysers area of northern California were recorded for 3 weeks with a radio-telemetry array of eight seismograph stations in order to examine their distribution with respect to the area developed for geothermal power. Locations were determined for 53 earthquakes within about 10 km of The Geysers. Most epicenters lie in a zone about 4 km long and 1 km wide passing through the geothermal field along a principal fault zone. Focal depths in this trend range from near surface to about 4 km. A composite fault-plane solution indicates dextral strike-slip faulting on a NNW-striking plane subparallel with the regional fault pattern. The results of this study suggest that accurate mapping of microearthquakes can be useful in the exploration for geothermal resources, if the earthquakes studied here are not somehow caused by development of the field.

Journal ArticleDOI
TL;DR: A redwood tree near Fort Ross was tilted between 1400 A.D. and 1650 A., and the same tree also records two other tilts, and in 1906 the base of the tree was fractured by faulting as mentioned in this paper.
Abstract: Fault movement accompanied by surface rupture can fracture, twist, or tilt trees that grow on the surface break of the fault. Trees growing near, but not on, the fault may be felled or topped as a result of ground motion. Changes in growth rate of trees may relate to hydrologic and topographic changes or from “release effects” caused by death of neighboring trees. Many other causes such as landslides, wind, and fire can produce similar effects and must be considered when using trees to date past movements on the San Andreas fault. The effects of tilting of trees growing on surface ruptures seem to provide the least ambiguous evidence of past movement on the fault, but few ancient trees that record multiple movements in the past were found. A redwood tree near Fort Ross was tilted between 1400 A.D. and 1650 A.D.; tilting is tentatively attributed to movement on the San Andreas fault. The same tree also records two other tilts, and in 1906 the base of the tree was fractured by faulting. In contrast, a redwood tree possibly 500 to 600 yrs old in Woodside grows only 10 ft (3 m) from the 1906 surface rupture and yet no effects in growth rate or tilt record the event.

Journal ArticleDOI
10 Apr 1972-Nature
TL;DR: The existence of a negative gravity anomaly about 20 mgal below northern Skye was first pointed out by Tuson1, who interpreted it as being due to a sedimentary basin bounded at its NE margin by a large NW-SE fault.
Abstract: THE existence of a negative gravity anomaly of amplitude about 20 mgal below northern Skye was first pointed out by Tuson1, who interpreted it as being due to a sedimentary basin bounded at its NE margin by a large NW-SE fault. Our recent geophysical work confirms the existence of this basin, defines its margins more accurately and provides evidence for the age of the infilling.

Journal ArticleDOI
TL;DR: In this article, a finite dislocation model of a "locked" fault is presented to examine the predicted magnitude of magnetic events which might be associated with creep along the San Andreas fault, and an observable anomaly of 1-2 gammas is obtained for rocks having uniform magnetization I = 10 −3 e.m.u.