Showing papers on "Fault (geology) published in 1977"

Fault rocks and fault mechanisms

Abstract: Physical factors likely to affect the genesis of the various fault rocks—frictional properties, temperature, effective stress normal to the fault and differential stress—are examined in relation to the energy budget of fault zones, the main velocity modes of faulting and the type of faulting, whether thrust, wrench, or normal. In a conceptual model of a major fault zone cutting crystalline quartzo-feldspathic crust, a zone of elastico-frictional (EF) behaviour generating random-fabric fault rocks (gouge—breccia—cataclasite series—pseudotachylyte) overlies a region where quasi-plastic (QP) processes of rock deformation operate in ductile shear zones with the production of mylonite series rocks possessing strong tectonite fabrics. In some cases, fault rocks developed by transient seismic faulting can be distinguished from those generated by slow aseismic shear. Random-fabric fault rocks may form as a result of seismic faulting within the ductile shear zones from time to time, but tend to be obliterated by continued shearing. Resistance to shear within the fault zone reaches a peak value (greatest for thrusts and least for normal faults) around the EF/QP transition level, which for normal geothermal gradients and an adequate supply of water, occurs at depths of 10–15 km.

A model for earthquake swarms

Abstract: A model for earthquake swarms in volcanic regions consists of the following concepts: (1) clusters of magma-filled dikes exist within brittle volumes of the crust, (2) dikes within a cluster are systematically oriented with their long dimension in the direction of the regional greatest principal stress, and (3) a sequence of shear failures (an earthquake swarm) occurs along a system of conjugate fault planes joining en echelon offset dike tips at oblique angles. This model accounts for commonly observed geometric relations between surface faulting patterns, the hypocentral distribution of swarm earthquakes, and fault plane solutions in a variety of situations. Swarm areas dominated by strike-slip faulting, however, provide the most compelling examples of the utility of the model. Specific examples considered here include a swarm on the east rift zone of Kilauea volcano, Hawaii, and swarms in the Imperial Valley, California, and the Reykjanes Peninsula, Iceland, which represent transitional zones between spreading centers and transform faults.

Experimental deformation of dry westerly granite

Abstract: The deformation behavior of quartz and feldspar has been studied in Westerly granite deformed dry at a constant strain rate of 10−6/s, confining pressures of 1.5–15 kbar, and temperatures of 25°–1000°C. Samples deformed at lower temperatures and pressures show throughgoing faults; those deformed at intermediate conditions show a combination of grain-scale faults and plastic deformation; and those deformed at higher temperatures and pressures show plastic deformation with no faults of any scale. On a grain scale the deformation is inhomogeneous at all conditions because of the polyphase nature of the material. Detailed petrographic and transmission electron microscope (TEM) observations have been made of the deformed specimens. The fault gouge consists of very fine grained material which verges on being amorphous, but no evidence of melt was seen. In the regions away from fault zones, there is a transition from dominantly microcracking to dominantly dislocation glide and climb; this transition is primarily a function of temperature. Dislocation motion is thermally activated and is probably almost unaffected by pressure over the range investigated. Thus at low temperature the strain rate that can be produced by dislocation motion is limited, and the difference between this and the imposed strain rate must occur by microcracking. The way in which the microcracking accomplishes the deformation depends on pressure. At low pressures (<5 kbar) the microcracks link up to form a throughgoing fault after very low strain; at higher pressures (7.5–15 kbar) they produce only grain-scale faults, ‘deformation bands,’ and undulatory extinction, and no throughgoing faults are formed after 15–20% shortening. At the laboratory strain rate of 10−6/s the transition from dominantly microcracking to dominantly dislocation motion occurs at approximately 300°–400°C for quartz and 550°–650°C for feldspar. When one allows for the slower natural strain rates and the presence of water, this grain-scale brittle-ductile transition may correspond to the limiting depth of earthquakes on strike slip faults.

Profiles and ages of young fault scarps, north-central Nevada

Abstract: The geomorphic characteristics of young fault scarps can be used as a key to the ages of fault displacements. The principal features of scarps younger than a few thousand years are a steep free face, a debris slope standing at about 35°, and a sharp break in slope at the crest of the scarp. The principal slope of older scarps declines with age, so that scarps of about 12,000 yr of age have maximum slope angles of 20° to 25°, and slopes as low as 8° to 9° represent ages much older than about 12,000 yr. The crestal break in slope broadens with age. The material in the scarp face, whether loose fanglomerate or indurated bedrock, controls to a large extent the rate of scarp degradation. Where more than one displacement has occurred along a fault, a composite or multiple scarp develops. Composite or multiple scarps suggest mean recurrence intervals on individual faults measured in thousands of years.

High-frequency radiation from crack (stress drop) models of earthquake faulting

Abstract: Summary. We present a theory for the radiation of high-frequency waves by earthquake faults. We model the fault as a planar region in which the stress drops to the kinematic friction during slip. This model is entirely equivalent to a shear crack. For twodimensional fault models we show that the high frequencies originate from the stress and slip velocity concentrations in the vicinity of the fault’s edges. These stress concentrations radiate when the crack expands with accelerated motion. The most efficient generation of high-frequency waves occurs when the rupture velocity changes abruptly. In this case, the displacement spectrum has an u-’ behaviour at high frequencies. The excitation is proportional to the intensity of the stress concentration near the crack tips and to the change in the focusing factor due to rupture velocity. We extend these two-dimensional results to more general three-dimensional fault models in the case when the rupture velocity changes simultaneously on the rupture front. Results are similar to those described for twodimensional faults. We apply the theory to the case of a circular fault that grows at constant velocity and stops suddenly. The present theory is in excellent agreement with a numerical solution of the same problem. Our results provide upper bounds to the high-frequency radiation from more realistic models in which rupture velocity does not change suddenly. The u-’ is the minimum possible decay at high frequencies for any crack model of the source.

Seismic moments of major earthquakes and the average rate of slip in central Asia

Abstract: Seismic moments for 12 major earthquakes (M ≥ 7.6) in central Asia from 1911 to 1967 were calculated from long-period Rayleigh and Love wave spectral densities. With fault lengths estimated from geological field observations of surface faulting, intensity distributions, or master event relocations of aftershocks, the calculated moments place bounds on the average slip and fault widths. The following table summarizes the calculated moments, estimated fault lengths, and inferred possible average displacements.

Deep-tow studies of the structure of the Mid-Atlantic Ridge crest near lat 37°N

Abstract: A detailed study of the structure of the Mid-Atlantic Ridge median valley and rift mountains near lat 37°N (FAMOUS) was conducted using a deep-tow instrument package. The median valley may have either a very narrow inner floor (1 to 4 km) and well-developed terraces or a wide inner floor (10 to 14 km) and narrow or no terraces. The terraces appear to be non–steady-state features of the rift valley. The entire depth and gross morphology of the median valley may be accounted for by normal faulting, while volcanic relief contributes to the short-wavelength topography (<2 km). Most faults dip toward the valley axis an average of 50°, and the blocks are tilted back 2° to 3°. Fault dip is asymmetric about the valley axis. Active crustal extension in the inner floor and inner walls has the same sense of asymmetry as the local spreading rates, reaching a maximum of 18 percent. Thus, asymmetric spreading appears to be accomplished by asymmetric crustal extension on a fine scale as well as by asymmetric crustal accretion. Spreading is 17° oblique to the transform faults and shows no indication of readjusting to an orthogonal system, even on a fine scale. Eighty percent of the decay or transformation of median-valley relief into rift-mountain topography is accomplished by normal faults that dip away from the valley axis. Most of the outward-facing faulting occurs near the median-valley–rift-mountain boundary. Tilting of crustal blocks accounts for only 20 percent of the decay of median-valley relief. Most long-wavelength topography in the rift mountains has a faulted origin. As in the median valley, volcanic relief is short wavelength (<2 km) and appears to be fossil, originating in the median-valley inner floor. Bending of large faulted blocks toward nearby fracture zones suggests that spreading-center tectonics is affected by fracture-zone tectonics throughout the length of the rift in the FAMOUS area. Both the crustal accretion zone and transform fault zone are narrow, only 1 to 2 km wide, over short periods of time. In the course of millions of years, however, they apparently migrate over a zone 10 to 20 km wide.

Structural geomorphology of a fast-spreading rise crest: The East Pacific Rise near 3°25′S

Abstract: A deeply-towed instrument package was used in a detailed survey of the crest of the East Pacific Rise (EPR) near 3°25′S, where the Pacific and Nazca plates are separating at 152 mm/yr. A single 90 km-long traverse of the rise crest extends near-bottom observations onto the rise flanks. A ridge at the spreading axis is defined by its steep regional slopes, probably caused by rapid crustal contraction as the spreading magma chamber freezes, rather than by outward-facing fault scarps. It can be divided into a marginal horst-and-graben zone with low (<50 m), symmetric fault blocks, and a 2 km-wide elongate axial shield volcano that is unfaulted except for a narrow crestal rift zone. This has a summit graben (10–35 m deep) probably formed by caldera collapse, and narrow pillow basalt walls built over vent fissures. Small, low (<50 m) volcanic peaks occur on the shield volcano and the horst-and-graben zone, and some may have been built away from the spreading axis. Major plate-building lava flows issue from the crestal rift zone and flow an average of 500 m down the sides of the volcano. The marginal horst-and-graben zone results from tensional faulting of a thin crust of lava, and evolves by progressive shearing on inclined fault planes. Crustal extension continues at least as far as 20 km from the axis, and the large, long fault blocks formed in thicker crust beyond the subaxial magma chamber develop into abyssal hills. Pelagic sedimentation, at a maximum rate of 22 m/106 years, gradually infills open fissures and smooths the small-scale roughness of the fault blocks. Off-axis volcanism has also resulted in smoother crust, and built seamounts. Comparison of the EPR at 3°25′S with the Famous Rift and Galapagos Rift reveals a similarity in the processes and small-scale landforms at fast, medium and slow-spreading ridges. There are significant differences in the medium-scale landforms, probably because plate-boundary volcanic and tectonic processes act on crust of very different strength, thickness, and age.

A fault-rupture model for seismic risk analysis

Abstract: A model for seismic risk analysis consistent with existing theories of earthquake mechanism and characteristics is developed. This model is based on the assumption that an earthquake originates as an intermittent series of fault ruptures in the Earth9s crust, and that the intensity of motion at a site is mainly contributed by the segment of the ruptured fault that is closest to the site. Since active faults in a region may be well-defined, partially defined, or completely unknown, various idealized source models are introduced in order to permit the modeling of all conceivable seismic sources. The significance of model parameters on the calculated seismic risk is studied: certain previous conclusions in this regard are critically reexamined. In particular, it is pointed out that previous seismic risk models, which implicitly assume that the energy is radiated from a point (the focus), can seriously underestimate the real risk, especially for high intensity motions. As an illustration of the model, the seismic risk analysis of a site in downtown San Francisco is presented.

The Reykjanes Peninsula, Iceland, earthquake swarm of September 1972 and its tectonic significance

Abstract: A temporary network of up to 23 short-period seismic stations operated on the Reykjanes Peninsula during the summer of 1972 and recorded an earthquake swarm consisting of more than 17,000 events. The area studied is the immediate landward extension of the Mid-Atlantic Ridge, and seismic processes are probably similar to those on submarine parts of the ridge crest. The Reykjanes Peninsula forms the transition between the Reykjanes Ridge and the south Iceland transform fault. The peninsula is essentially a leaky transform fault, and focal mechanism solutions are of both normal and strike-slip types. The 12-km-long seismic zone active during the 8-day swarm is defined by 2514 hypocenters, which are mostly between 2 and 5 km deep. Individual structures within the seismic zone include several seismic lineations or faults trending obliquely to the zone. Also visible are two aseismic zones, which may be either rigid blocks or low-rigidity regions, such as magma chambers. Thus the observed seismic zone is not a single fault; instead it appears to be the shallow expression of a deeper-seated and aseismic deformation zone. In time, the seismic events cluster together into subswarms lasting a few hours and migrate laterally at speeds of 1–2 km/d. The energy released is equivalent to that of a single event of Icelandic local magnitude 4.9, and the total fault area is close to that expected for a single earthquake of this size. Although the high microearthquake activity and swarm seismicity can be generally related to the presence of an adjacent geothermal area, the observed earthquakes do not locate in or outline the hydrothermal upwelling zone visible at the surface. This contribution is the first study to reveal the details of faulting during an ocean ridge earthquake swarm, both in space and in time.

LISPB–III. Upper crustal structure of northern Britain

Abstract: Interpretation of upper crustal data obtained during the LISPB seismic experiment reveals the velocity structure of the pre-Caledonian basement in northern Britain. Lewisian-like basement with a relatively high seismic velocity (> 6.4 km/s) extends from the Caledonian foreland into the Midland Valley and probably terminates at the Southern Uplands fault. To the south, beneath northern England, the basement has a lower velocity (

Earthquakes related to hydraulic mining and natural seismic activity in western New York State

Abstract: We have monitored the seismic activity of western New York since 1970 using a combination of up to eight permanent seismograph stations and several portable stations. Our investigation centered on the Attica-Dale area, the site of several damaging earthquakes in this century. Although the background level of seismicity was found to be extremely low, less than one event per month in the first 12 months of our study, this near quiescence was broken in 1971 by a sharp increase in seismicity at our site near Dale following the initiation of fluid injection under high pressure (120 bars tophole) at a hydraulic mining operation nearby. This facility, which mines salt from the Vernon Formation of Silurian age, is centered near the Clarendon-Linden Fault, a major north-south trending system of high-angle thrust faults that extends for over 100 km from Lake Ontario to Allegheny County, New York. Although the seismic events were small (none were recorded at our station 30 km to the northwest), as many as 80 occurred per day, and many were felt locally. The marked increase in seismic activity after attaining high pressures, the closeness of these events to the bottom of the injection well, and the near cessation of activity within 48 hours of the shutdown of injection strongly suggest that this activity was caused by the triggering of tectonic strain on or near the Clarendon-Linden Fault by the high fluid pressures of the mining operation. The minimum pressure (41–48 bars) at which seismic activity occurred is consistent with predictions made by applying the Hubbert and Rubey theory of effective stress to hydrofracturing stress measurements from Alma, New York. Since 1972, five other wells at Dale have been hydrofractured at pressures of about 110 bars, but none had abrupt changes in seismic activity associated with them during the high-pressure phases of pumping. The well used in 1971 (0.43 km deep) is the closest (about 50 m) of the six to the Clarendon-Linden Fault. It was hydrofractured near the base of the salt layer, whereas the others were hydrofractured well within the salt layer. Thus fluids under high pressure appear to have reached the Clarendon-Linden Fault in 1971 but not in the other five cases. A focal mechanism involving thrust faulting on a plane nearly parallel to the Clarendon-Linden Fault was obtained from events that continue to occur but at a much reduced level in the brine field in 1974 and 1975. An earthquake of magnitude 2.7, which occurred about 7 km to the west of the brine field in 1973, is apparently unrelated to the injection operation and is of natural origin. This and other nearby natural events appear to be associated with the western and southwestern branches of the Clarendon-Linden Fault. Hydraulic diffusivity values calculated from the space-time relationship of earthquakes triggered by fluid injection at Denver and Rangely, Colorado, Matsushiro, Japan, and Dale are about 104 and 105 cm2/s and are similar to those obtained from precursory anomalies of earthquakes. The similarity of the diffusivity values suggests that the precursory changes do involve the movement of water. In each of these four places the wells used for fluid injection bottomed into or very close to major fault zones.

Photographic Atlas of the Mid-Atlantic Ridge Rift Valley

Abstract: Geologic Setting.- Photographic Techniques.- Submarine Volcanic Products.- Volcanic Vents.- Major Flow Units.- Individual Flow Units.- Bulbous Pillows.- Flattened Pillows.- Elongate Pillows.- Hollow Pillows.- Knobby Pillows.- Trapdoor Pillows.- Sheet Flows.- Flow Foot Rubble and Talus.- Features of Pillow Crust.- Faults and Related Tectonic Features.- Tensional Fissures.- Gjar.- Isolated Fault Block.- Small-scale Grabens.- Fault Scarps of Inner Floor.- Simple Fault Scarps.- Rotated and Tilted Blocks.- Slump Notches.- Patterns of Faulting.- Faulted Sedimentary Rock.- Rift Valley Walls.- References Cited.

The balance of energy in earthquake faulting

Abstract: Summary An earthquake is modelled kinematically by specifying the tangential slip history on a fault surface which expands within a uniformly rotating, self-gravitating, slightly anelastic earth model. The total amount of energy released by such an idealized earthquake is the sum of three distinct quantities: kinetic energy of rotation, gravitational potential energy and thermodynamic elastic internal energy. The first two of these quantities may also be interpreted as the work done throughout the earth model against the action of the apparent centrifugal and real gravitational body forces respectively. The total energy released by an earthquake fault is in general considerably smaller than any of its three individual constituents, since the work performed against body forces is very nearly balanced by the work performed against the initial hydrostatic pressure in the earth model. The smallest individual constituent is the change in the kinetic energy of rotation of the earth model, which may be as much as two orders of magnitude larger than the total energy released, even though the corresponding change in the angular velocity of rotation due to the redistribution of mass is extremely small. The total energy released by an earthquake fault may also be expressed in terms only of the final static displacement and the initial and final static traction on the fault surface itself. This alternative representation of the energy change is explicitly independent of both the rotation and the self-gravitation of the earth model. All of the energy released by an earthquake fault must be dissipated somewhere within the earth model. Energy may be dissipated during faulting either in heating on the walls of the fault surface, where work must generally be done against the action of the frictional traction acting to resist slip, or at the instantaneously expanding boundary of the fault surface, where some energy may be required to overcome cohesion and where there may be additional heating. The remainder of the energy released, which is generally referred to as the seismic energy, is dissipated both during and subsequent to faulting by the slight bodily friction which must be assumed to exist throughout the entire volume of any physically realizable earth model. The seismic energy may also be expressed in terms only of the displacement and incremental traction histories on the instantaneous fault surface during the course of faulting. This alternative representation of the seismic energy is explicitly independent of both the rotation and the self-gravitation of the earth model, and so therefore is the seismic efficiency, which is defined to be the ratio of the seismic energy to the total energy released. Classical formulae for the total energy released by an earthquake fault, the seismic energy and the seismic efficiency are based not only upon the neglect of rotation and self-gravitation, but also upon the assumption that the initial hydrostatic pressure and deviatoric stress are infinitesimal quantities; those classical formulae, upon which many seismological applications depend, are justified if the initial deviatoric stress at the hypocentre is small compared to the hypocentral rigidity.

Are spreading centers perpendicular to their transform faults

Abstract: A COMMON assumption in seafloor spreading is that mid-ocean ridge crests are aligned perpendicular to their transform faults and, hence, to their spreading directions. There are some well known exceptions to this rule, for example, the Reykjanes Ridge. Vogt et al.1 suggested that spreading systems may take one of two configurations: either a transform faultless, oblique configuration or a perpendicular one. He then assigned the Reykjanes and certain older anomaly sets to the first category and the rest, the segmented, faulted ridges to the latter. We agree with this bimodal separation of ridge types, and here we discuss only the latter, the transform-faulted ‘perpendicular’ group. We examined all available detailed data from this group, and wherever we could find a fine scale map which included both transform fault and spreading centre we measured their trends. Of eight segmented, slow-spreading centres (half-rate less than 3 cm yr−1) we did not find a case which was, in fact, perpendicular. All were 6–38° oblique, and all were oblique in the sense which shortens the connecting transform faults, that is, the configurations in Fig. 1a as opposed to those shown in Fig. 1b . Fast- and some intermediate-rate spreading centres, on the other hand, seem to be perpendicular within the errors of measurements. These results are particularly interesting for the constraints that they place upon models of spreading centres in which the ridge crest-transform fault angle is used as a measure of the relative amounts of energy dissipated by these two features as motion occurs across them.

Physiography and structure of the inner floor of the FAMOUS rift valley: Observations with a deep-towed instrument package

Abstract: A deep-towed instrument survey was made of the floor of the rift valley in the Mid-Atlantic Ridge near lat 37 °N (FAMOUS project). Near-bottom bathymetry, side-looking sonar (SLS), and wide-angle photography are among the data brought to bear on the definition of the American-African plate boundary and the intrusion-extrusion zone between these plates. The valley floor is 1 to 4 km wide and contains eight elongate shield volcanoes that occupy 40% of the axis. Where the volcanoes — called central highs — are absent, there are sometimes shallow depressions — called central lows. Photographic data show three components to the near-bottom environment: massive pillow lavas, well-sorted rock fragments, and sediment cover. Sediment is ubiquitous in all the photos but is scarcest along the axis of the valley floor. Pillows appear the freshest on the central highs except for one locality found at the extreme east side of the valley floor. The rock fragments are evidently pillow joint blocks and are associated with spalling off steep flow fronts and fracturing from faulting. Less well sorted fragments are associated with the large-throw step faults at the edges of the valley floor. Pillow elongation approaches 10:1 and possibly indicates flow directions both across and along contour. Numerous fissures and small-throw step faults were also seen. The SLS records show that the smoothest areas are the well-sedimented regions off the central highs that are yet to be broken by faulting at the floor edges. More than 700 faults were mapped in the valley-floor region. They trend parallel to valley strike (N17 °E) and are generally absent within 500 m of the axis. Fault density is highest at the east inner floor edge, reaching 35/km 2 . Fissures are present in photos but are not readily recognized with SLS data. Flow edges or ridges also align near N17 °E, but some trend across strike. SLS point targets 25 m high and about 50 by 50 m represent volcanic conelets (haystacks). These align in groups and often are associated with faults. They are not detected away from the axis. The floor is intensely fractured with fissures and small-throw vertical faults as are parts of the inner walls; this is evidence that the region of tensional faulting is at least 5 km wide. The absence of faults near the axis due to volcanic burial suggests that the extrusion zone is as much as 1 km wide. The construction process in the floor is virtually entirely volcanic, but much of this relief is obscured by the intensive shear (normal) faulting found beyond the inner walls. Many geomorphic features of the floor have direct analogy with those in the Icelandic rift valley and the East Pacific Rise, although there are differences in scale.

Plate boundary within Tjörnes Fracture Zone on northern Iceland's insular margin

Abstract: The current North American-Eurasian plate boundary on Iceland's northern insular margin is defined. Overlapping rift zones and en echelon post-glacial volcanic eruptions oblique to the apparent transform direction of the Tjornes Fracture Zone suggest that the fault is in a transient deformational stage.

Transcurrent faults and associated cataclasis in Shetland

Abstract: A series of transcurrent faults with northerly trend is well exposed in the cliffs of Shetland. The Walls Boundary Fault which cuts through the middle of Shetland has a continuously changing trend so that it forms a very flattened S. Dextral movement along the fault seems to have created further dextral transcurrent faults, including the Nesting Fault, across the eastern concavity in the trend. These main faults lie within broad zones of cataclasis, subsidiary faulting and local folding. The offsets on the subsidiary faults are very much less than on the main faults, and the crushed rocks have isotropic fabrics. The zones probably arose during faulting from varying local stresses caused by the interaction of the unevennesses on the two sides of the non-planar main faults. Gouge in the Walls Boundary Fault contains analcite. Gouge-like laumontite in subsidiary shears was probably formed by mechanochemical reactions. The occurrence of these minerals and of blastomylonitic scapolite veins near the Walls Boundary Fault may indicate up to three phases of movement taking place between Cretaceous and Devonian times under different depths of overburden. Slices of secondarily cataclastic mylonite occurring along the Walls Boundary Fault are probably relics of the Great Glen Fault now partially cut out by the nearly coincident Walls Boundary Fault.

State-of-the-Art for Assessing Earthquake Hazards in the United States. Report 6. Faults and Earthquake Magnitude

Abstract: : The short seismologic and historic records, in combination with the long recurrence intervals between potentially damaging earthquakes, create a need for developing geologic methods as alternatives or supplements to seismologic methods of establishing design earthquakes. The main goal of this report is to review geologic methods of determining the maximum probable earthquakes for active faults based on empirical relationships between magnitude, length of surface faulting, maximum fault displacement, and combinations of fault length and maximum displacement.

Stafford fault system: Structures documenting Cretaceous and Tertiary deformation along the Fall Line in northeastern Virginia

Abstract: Four en echelon northeast-trending structures, including southeast-dipping monoclines and northwest-dipping, high-angle reverse faults, have been mapped along the inner edge of the Coastal Plain in northeastern Virginia–an area generally considered to be undeformed. Although displacements are small (15 to 60 m), the structures markedly affect the present distribution and thickness of Coastal Plain sediments. Structure-contour maps on Cretaceous and Paleocene lithostratigraphic units show that the amount of displacement on the structures increases downward, indicating recurrent movement. The major deformation took place in the Cretaceous and the middle(?) Tertiary, but some latest Tertiary or Quaternary movement is possible. The structures, herein named the Stafford fault system, extend for at least 56 km parallel to the Fall Line and the northeast-trending reach of the Potomac estuary. This relationship supports the hypothesis that the Fall Line and major river deflections along it have been tectonically influenced.

Tectonic evolution of the FAMOUS area of the Mid-Atlantic Ridge, lat 35°50′ to 37°20′N

Abstract: The FAMOUS area, which straddles the Mid-Atlantic Ridge on the southwest extension of the Azores Plateau in the Atlantic Ocean, has been studied by means of morphotectonic and magnetic anomaly analysis. Major morphologic elements are (1) the rift valleys and associated fault blocks, (2) the transform faults, and (3) diagonal linear trends intersecting the azimuths of the rift valleys and transform faults. Within the past 6 m.y. the area has been characterized by a reorientation of the spreading axis, from an oblique trend toward an orthogonal pattern relative to the transform faults. The long and nearly continuous early spreading axis, with a strike of N50 °E, broke up into smaller rift segments, which progressively rotated to their present strike (about N23 °E) and now appear to be recombining through asymmetric spreading to form the continuous axis it now has. The process involved complex migrations of the larger rift segments through asymmetric spreading, as well as jumping of shorter rift segments and migration of transform faults, the latter mechanism leading to the diagonal troughs. The presence of parallel spreading axes and the propagation of rift axes may be due to coupling across transform plate boundaries. These boundaries are complex shear zones with tension cracks and en echelon fault scarps deviating from the overall trend of the zone; this implies that the fracture zones might be leaky. The reorientation of the spreading axis in the FAMOUS area seems not to be a unique event but a phenomenon that has been repeated periodically. It has tentatively been correlated with recurrent plume activity below the Azores Plateau.

Recent vertical crustal movements from precise leveling data in southwestern Montana, western Yellowstone National Park, and the Snake River Plain

Abstract: Repeated levelings in southwestern Montana, the western portion of Yellowstone National Park, and the Snake River Plain provide information on the pattern of relative vertical crustal movement throughout this region. Except for the coseismic deformation associated with the 1959 Hebgen Lake earthquake the most outstanding and best defined feature of the data is contemporary doming at a rate of 3–5 mm/yr involving approximately 8000 km2 including the epicentral area and aftershock zone of the 1959 Hebgen Lake earthquake. On the basis of observations over different time intervals, doming appears to have continued throughout the time the movements were monitored, beginning at least 25 years prior to the 1959 earthquake and continuing at least 1 year after the earthquake. The character of the coseismic deformation associated with the 1959 earthquake and the high regional elevation are consistent with the observed doming. It is suggested that doming preceded the earthquake for a considerable time (of the order of hundreds to thousands of years, perhaps longer), giving rise to tensional stresses in the upper crust. When these stresses, combined with the regional tectonic stresses, exceeded some critical value, faulting and collapse in response to gravity occurred, resulting in the 1959 earthquake. The voluminous Tertiary and younger volcanics in the vicinity of the doming region suggest that magma intrusion into the crust is the most likely cause of the observed uplift. The proximity of the doming region to the thermally active Yellowstone area supports this suggestion. Secondary features of the data include (1) a spatial correlation between tilting and historic seismic activity; (2) uplift within the Norris-Mammoth corridor in Yellowstone National Park relative to nearby bench marks to the north and south; (3) regional subsidence of the eastern Snake River Plain relative to points north and west of this physiographic province, including subsidence of the Pleistocene Island Park caldera floor relative to its rim fractures; and (4) rapid tilting in the vicinity of the Continental fault east of Butte, the intersection of the Gardiner, Mammoth, and Reese faults just north of Yellowstone National Park, and the Madison Range fault in eastern Idaho.

Estimation of future destructive earthquakes from active faults on land in japan

Abstract: Most of active faults (=Quaternary faults) known on land in Japan are ranked to class A(1

Kinetic Shear Resistance, Fluid Pressures and Radiation Efficiency During Seismic Faulting

Abstract: During earthquake faulting, radiation efficiency and the degree of stress relief are critically dependent on the kinetic shear resistance. This is often assumed to stay constant during slip, but geological evidence suggests that for moderate or large shallow earthquakes it may decrease dramatically to near-zero values once slip is initiated, either by melt formation or by transient increases in fluid pressure on the fault plane. The latter, probably more common process may arise partly through an interaction between temperature and water pressure, and partly through dilatancy recovery as shear stress is relieved. If the fault remains undrained, stress relief should be absolute with seismic efficiency reaching high values, so that stress drops give a measure of the level of tectonic shear stress in fault zones. Supporting evidence comes from the observation that apparent stress is generally about half the stress drop.

Velocity contrast across the San Andreas fault in central California: Small-scale variations from P-wave nodal plane distortion

Abstract: Systematic variations in P-wave radiation patterns, evident in a data set of 400 central California earthquakes, have been analyzed for variations in velocity contrast across the San Andreas fault zone. Vertical strike-slip faulting characterizes the region, with radiation patterns well constrained by the dense local seismographic station network. A discontinuity in crustal velocity occurs across the San Andreas fault. The distribution of systematically inconsistent first motions indicates that first arrivals observed along the fault plane within the northeastern block have followed refracted paths through the higher velocity crustal rocks to the southwest, retaining P-wave polarities characteristic of the quadrant of origin, and thus appearing reversed. A simple geometrical interpretation, with P waves refracted at the fault plane near the focus, yields the velocity contrast across the fault zone; the distribution of hypocenters allows its mapping in time and space. The velocity contrast so determined ranges up to 15 per cent, for a depth range of 1 to 10 km. The observed pattern of contrast values is coherent, with the greatest contrast related apparently in space, and possibly in time, to the larger earthquakes occurring on the fault. We suggest the phenomenon reflects changes in stress state at the fault and, by virtue of its ease of measurement, offers a new and valuable technique in earthquake studies.

The continental crust northeast of Newfoundland and its ancestral relationship to the Charlie Fracture Zone

Abstract: CONTINENTAL edges that are matched in pre-continental drift reconstructions have generally been defined by an isobath. When the isobath chosen is too shallow, isolated bathymetric highs within the ‘oceanic’ area are interpreted as continental fragments that may consequently be moved at will to fill any gaps in the reconstruction1. Orphan Knoll (which had a history of uplift, erosion and subsidence2) and Flemish Cap3 are two such fragments for which lateral movements across the northeast Newfoundland shelf have been invoked4. Detailed bathymetric, magnetic and gravity surveys5,6 have confirmed that the entire intermediate depth area of the northeast Newfoundland Shelf inshore of Orphan Knoll and Flemish Cap is continental in nature thereby invalidating reconstructions which assume otherwise. I show here that the offshore extension of the Hermitage–Dover fault in Newfoundland provided the line of weakness along which the Charlie transform fault developed.

Studies of Ventura Field, California, I: Facies Geometry and Genesis of Lower Pliocene Turbidites

Abstract: The Pliocene sediments of Ventura field, more than 10,000 ft (3,048 m) thick, were deposited in a deep-sea basin, and subsequently were folded into an anticline. Three approaches were used to investigate sand trends within the basin: (1) facies relations using isopach maps, fence diagrams, and textural analyses of core samples; (2) grain orientation by visual inspection, thin-section analysis, and dielectric-anisotropy measurements; (3) sedimentary structures, mainly cross-bedding. Facies relations suggest that the sands were deposited as elongate lenticular bodies by laterally restricted westerly flowing currents in deep parts of a deep-sea basin. The sand trends are oriented predominantly east-northeast, approximately parallel with the axis of the present Ventura anticline. Studies of grain orientation and cross-bedding support evidence for approximately east-northeast lower Pliocene sand trends in the Ventura field. Maximum sand development in the central parts of ancient deep-sea basins in tectonically active areas is in direct contrast to the sand-development patterns in shallower water paralic and platform types of depositional basins. This study suggests that the search for sand reservoirs in tectonically active regions might be directed toward the central parts of ancient depositional basins if paleoecologic and sedimentologic data suggest conditions of deep-sea sedimentation. Parallelism of structural axis and trend of sand bodies in folded deep-sea basins suggests an influence on structural trends by sediment-distribution pattern; anticlinal traps might be expected to coincide with maximum sand development, and stratigraphic traps might be expected on the flanks of such folds. From detailed isopach maps of sand trends offset by faults it is possible to determine fault displacements accurately. Such evidence indicates that the thrust faults in Ventura field have had right-lateral movements. The Padre Juan fault northwest of the Ventura field is perhaps also a right-lateral fault, offsetting an originally continuous structure into the en-echelon Rincon and Ventura anticlines.