Showing papers on "Fault (geology) published in 1973"
TL;DR: In this article, the authors simulate the formation of en echelon folds and faults caused by wrenching in sedimentary basins and show that these folds form early in deformation and are accompanied or followed by conjugate strike-slip, reverse, or normal faulting.
Abstract: En echelon structures which may trap oil and gas develop in a systematic pattern along wrench zones in sedimentary basins. Laboratory clay models simulate the formation of en echelon folds and faults caused by wrenching. Folds form early in the deformation and are accompanied or followed by conjugate strike-slip, reverse, or normal faulting. Deformation may cease at any stage or may continue until strike slip along the wrench zone produces a wrench fault and separation of the severed parts of early structures. Oblique movements of fault blocks on opposite sides of a wrench fault cause divergence or convergence and enhancement, respectively, of extensional or compressional structures. Basins form in areas of extension and are filled with sediment, whereas upthrust blocks e erge in areas of compression and become sediment sources. The combined effects of wrenching in a petroliferous basin are to increase its prospectiveness for major hydrocarbon reserves.
958 citations
TL;DR: In this paper, the average relative right lateral motion is estimated to be 32 ± 5 mm/yr for the period 1907-1971 and it appears that most, if not all, of the plate motion is accommodated by fault creep.
Abstract: Geodetic data along the San Andreas fault between Parkfield and San Francisco, California (latitudes 36°N and 38°N, respectively), have been re-examined to estimate the current relative movement between the American and Pacific plates across the San Andreas fault system. The average relative right lateral motion is estimated to be 32 ± 5 mm/yr for the period 1907–1971. Between 36°N and 37°N it appears that most, if not all, of the plate motion is accommodated by fault creep. Although strain is presumably accumulating north of 37°N (San Francisco Bay area), the geodetic evidence for accumulation is not conclusive.
710 citations
TL;DR: In this article, it has been suggested that pseudotachylyte material on fault planes puts an upper limit on the magnitude of shear stresses associated with earthquake faulting, which has important implications with regard to the seismic source mechanism and the nature of cataclastic rocks produced by rapid faulting.
Abstract: At depths in the crust greater than perhaps 1 km, local melting with production of pseudotachylyte should take place on fault planes during seismic faulting1,2. But, although many ancient fault zones are now exposed at erosion levels which correspond to depths of several kilometres when the faults were active, very few of them contain pseudotachylyte; it has been suggested3,4 that the general absence of this material on fault planes puts an upper limit on the magnitude of shear stresses associated with earthquake faulting. In refs 1–4 earthquake fault models with heat generated by dry frictional sliding on a single plane across which there is a constant normal stress are used. Two other factors are probably dominant in controlling the temperature rise on a fault, and the second has important implications with regard to the seismic source mechanism, and the nature of cataclastic rocks produced by rapid faulting.
515 citations
TL;DR: In this paper, the authors present several possible criteria for forecasting the locations of large shallow earthquakes of the near future along major plate boundaries and assign a crudely determined rating to those forecasts.
Abstract: This study presents several possible criteria for forecasting the locations of large shallow earthquakes of the near future along major plate boundaries and for assigning a crudely determined rating to those forecasts. These criteria, some of which were proposed by other investigators, are based on the past space-time pattern of large earthquakes, the lateral extent of their rupture zones, and the direction of rupture propagation. The criteria are applied in two stages. Application of the first set of these criteria to major plate boundaries along the eastern, northern, and northwestern margins of the Pacific from Chile to Japan and also to the Caribbean loop east of about 74°W results in delineation of several areas of special seismic potential along each of the boundaries. The phrase ‘special seismic potential’ is used in this paper only to indicate those segments of plate margins that fulfill certain specific criteria. However, if the criteria are valid, at least some and perhaps most large shallow earthquakes of the near future within the zones examined will occur near these locations. At present the validity of the criteria is not firmly established, and profound social changes based on these predictions are uncalled for, but the forecast presented here can, at the very least, serve as a guide in selecting areas for intensive study and instrumentation prior to the occurrence of a major earthquake. The criteria have greater usefulness in those regions where the rupture zones of large earthquakes have nearly covered the seismic zone in recent years and only a few gaps remain. In certain areas where additional information is available the subsequent application of a second set of supplementary criteria focuses special attention on certain of the areas delimited by the first set of criteria. Rupture zones of large shallow earthquakes as determined from aftershock locations, intensities, tsunami descriptions, and coastal uplift tend to cover plate boundaries without significant overlap. Because of this tendency to fill in the plate boundaries and because large earthquakes account for such a high percentage of the total seismic moment released in a seismic zone, segments of the zone that have not experienced a large earthquake recently are likely locations for future shocks. Along the Kurile-Kamchatka area, the Alaska-Aleutian arc, and much of western South America, the ‘large’ earthquakes tend to be great earthquakes with rupture zones sometimes extending many hundreds of kilometers. Along western Middle America, however, the large earthquakes have rupture zones no larger than 100–200 km. Although great earthquakes with ruptures hundreds of kilometers in length have occurred along some segments of the Caribbean loop, certain other segments have no known history of great earthquakes. Thus there is no recognizable class of large earthquakes around the Caribbean loop. Nearly all the extensive parts of the plate boundaries examined in this study tended to be covered by rupture zones of large shallow earthquakes. Therefore tectonic strain along segments of plate boundaries considered here appears to be relieved in most cases by periodic large earthquakes, regardless of the possible presence of fault creep and small-magnitude earthquakes. Thus fault segments characterized by aseismic creep should still be considered potential sites for large earthquakes unless persuasive evidence to the contrary is forthcoming. For all island arcs examined, the epicenters of the main shocks tended to be located near the inner or landward side of the aftershock zone. Since most large earthquakes near island arcs occur on dipping fault planes, this phenomenon suggests that, during large thrust earthquakes, rupture initiates at depth and propagates upward and outward along the plate interface. This phenomenon occurs not only for shocks of moderate.
396 citations
TL;DR: In this paper, the underthrust lithosphere is broken along tear faults into 100 to 300 km long segments and the separate segments descend into the mantle with different strikes and dips and give the geology its segmented character.
Abstract: Central America is divided into seven segments based on differences in the strikes and positions of volcanic lineaments in the historically active volcanic chain. This block-like pattern is also seen in the contrasting volcano morphology, recent fault patterns and the distribution of shallow earthquakes. Discontinuities in the deep seismic zone can be identified at some segment margins. The boundary areas between the segments are characterized by faulting transverse to the volcanic lineaments, frequently accompanied by basalt cinder cone fields, by concentrations of shallow earthquakes and by catastrophic historic eruptions. This volcanological, structural and seismological data can be explained by a segmented converging plate margin. At shallow depths the underthrust lithosphere is broken along tear faults into 100 to 300 km long segments. The separate segments descend into the mantle with different strikes and dips and give the geology its segmented character. Evidence for similar segmented converging plate margins is found in Mexico and in other parts of the circumpacific and alpine belts.
206 citations
TL;DR: In this paper, the authors interpret the Garlock fault as an intracontinental transform structure which separates a northern crustal block distended by late Cenozoic basin and range faulting from a southern, Mojave block much less affected by dilational tectonics.
Abstract: The northeast- to east-striking Garlock fault of southern California is a major strike-slip fault with a left-lateral displacement of at least 48 to 64 km. It is also an important physiographic boundary since it separates along its length the Tehachapi–Sierra Nevada and Basin and Range provinces of pronounced topography to the north from the Mojave Desert block of more subdued topography to the south. Previous authors have considered the 260-km-long fault to be terminated at its western and eastern ends by the northwest-striking San Andreas and Death Valley fault zones, respectively. We interpret the Garlock fault as an intracontinental transform structure which separates a northern crustal block distended by late Cenozoic basin and range faulting from a southern, Mojave block much less affected by dilational tectonics. Earlier ideas that the Garlock fault terminates eastward at the Death Valley fault zone appear to us to be in error, although right-lateral offsetting of the Garlock along that zone by about 8 km is necessary. Displacement along the Garlock fault must increase westward from its eastern terminus, a point of zero offset now buried beneath alluvial deposits in Kingston Wash to the east of the Death Valley fault zone. Much of the displacement on the Garlock fault due to east-west components of basin and range faulting appears to have been derived from block faulting in the area between Death Valley and the Nopah Range. Westward displacement of the crustal block north of the Garlock by extensional tectonics within it totals 48 to 60 km in the Spangler Hills–Slate Range area and probably continues to increase westward at least as far as the eastern frontal fault of the Sierra Nevada. Westward shifting of the northern block of the Garlock has probably contributed to the westward bending or deflection of the San Andreas fault where the two faults meet. Many earlier workers have considered that the left-lateral Garlock fault is conjugate to the right-lateral San Andreas fault in a regional strain pattern of north-south shortening and east-west extension, the latter expressed in part as an eastward displacement of the Mojave block away from the junction of the San Andreas and Garlock faults. In contrast, we regard the origin of the Garlock fault as being directly related to the extensional origin of the Basin and Range province in areas north of the Garlock. Recent models for development of that province related to intracontinental spreading east of an east-dipping subduction zone along the Cenozoic margin of western North America may best account for the differential east-west extension which has occurred in the crustal blocks to the north and south of the Garlock fault. Other possible examples of intracontinental transform faults in the southwestern Cordillera with geometries similar to that of the Garlock fault include the left-lateral Santa Cruz–Sierra Madre fault zone along the southern margin of the western Transverse Ranges, and the right-lateral Las Vegas shear zone and Agua Blanca fault of Baja California.
190 citations
TL;DR: In this paper, the authors show that regional contemporaneous faults of the Texas coastal area are formed on the seaward flanks of deeply buried linear shale masses characterized by low bulk density and high fluid pressure.
Abstract: Regional contemporaneous faults of the Texas coastal area are formed on the seaward flanks of deeply buried linear shale masses characterized by low bulk density and high fluid pressure. From seismic data, these masses, commonly tens of miles in length, have been observed to range in size up to 25 mi in width and 10,000 ft vertically. These features, aligned subparallel with the coast, represent residual masses of undercompacted sediment between sandstone-shale depoaxes in which greater compaction has occurred. Most regional contemporaneous fault systems in the Texas coastal area consist of comparatively simple down-to-basin faults that formed during times of shoreline regression, when periods of fault development were relatively short. In cross-sectional view, faults in hese systems flatten and converge at depth to planes related to fluid pressure and form the seaward flanks of underlying shale masses. Data indicate that faults formed during regressive phases of deposition were developed primarily as the result of differential compaction of adjacent sedimentary masses. These faults die out at depth near the depoaxes of the sandstone-shale sections. Where subsidence exceeded the rate of deposition, gravitational faults developed where basinward sea-floor inclination was established in the area of deposition. Some of these faults became bedding-plane type when the inclination of basinward-dipping beds equaled the critical slope angle for gravitational slide. Fault patterns developed in this manner are comparatively complex and consist of one or more gravitational faults with numerous antithetic faults and related rotational blocks. Postdepositional faults are common on the landward flanks of deeply buried linear shale masses. Many of these faults dip seaward and intersect the underlying low-density shale at relatively steep angles. Conclusions derived from these observations support the concept of regional contemporaneous fault development through sedimentary processes where thick masses of shale are present and where deep-seated tectonic effects are minimal.
159 citations
Abstract: Anomalous high fluid potentials exist within the miogeosynclinal Great Valley and eugeosynclinal Franciscan sequences of Jurassic-Cretaceous age within the Coast Ranges and at depth on the west side of the Central Valley, California. These rocks are dominantly mudstones with low fluid transmissibilities. Certain problems exist as to the probable regional distribution of these high fluid potentials. Low fluid potential areas such as The Geysers geothermal district are present in the Franciscan of northern California within a region generally characterized by high fluid potentials. The low potential areas are attributed to fracture zones with channel-type flow whose transmissive characteristics exceed those of intergranular flow. It is concluded that the Franciscan of northern California probably is characterized regionally by near-lithostatic fluid pressures at depth, but fracture zones with both low (i.e., near-hydrostatic) and high (i.e., near-lithostatic) fluid potentials probably exist at various depths from the surface. The Geysers dry-steam occurrence is envisioned as a fracture one with low fluid potentials by virtue of a decrease in transmissive characteristics of a fracture system with depth, in a local region of high heat flow, possibly caused by the existence at depth of a magma chamber. An abundance of direct fluid-pressure measurements within the Great Valley section of the Sacramento Valley demonstrates the existence of high fluid potentials. The only direct fluid-pressure measurement that has been made within the Great Valley section in the central or southern San Joaquin Valley indicates high fluid potentials. The regional chemistry of the lower Tertiary waters of the San Joaquin Valley (membrane effluent type) suggests that these waters have been extruded from a widely distributed series of mudstones and other rocks that are undergoing compaction. The presumed source for this widespread compacting sequence is the underlying Great Valley sediments with their postulated high fluid potentials. It is concluded that the anomalous high fluid potentials of Tertiary rocks within folds on the west side of the San Joaquin Valley reflect indirectly the presence at depth of high fluid potentials in the underlying Great Valley section. The origin of the folds is attributed to dynamic tectonic compression caused by current deep-seated linear diapirism of Great Valley mudstones and related rocks that possess near-perfect plastic properties by virtue of their near-lithostatic fluid pressures. The closed gravity minimum over the south end of South Dome-Lost Hills anticline is postulated as being the result of a diapir of serpentine or similar material. It is postulated that a fault zone, named herein the "West Side" fault, probably exists at depth along the west side of the Central Valley. This buried fault is envisioned as having an intermittent near-surface expression in the form of faults such as the Midland fault, or long linear folds such as the Kettleman folds. Diapirism along this fault is presumed to be responsible for these folds. Subsidence along the West Side fault is postulated as having occurred contemporaneously with deposition of the Great Valley sequence and thus provided a local trough in which the thick (maximum 60,000 ft) Great Valley section was deposited. The depositional barrier between the Franciscan and Great Valley sequences is postulated as a zone of serpentinite-ultrabasic rocks that intruded intermittently to form a sediment trap on the continental slope throughout Jurassic-Cretaceous geosynclinal deposition. The final conclusion reached is that an extensive geographic zone is present in which the pore-fluid pressures of the thick Franciscan and Great Valley geosynclinal sediments reach near-lithostatic values. This zone is 400-500 mi long and 25-80 mi wide; it is bounded on the west by the San Andreas fault and the granitic Salinas block, on the east by the buried West Side fault and the granitic Sierran-Klamath block, on the south by the granitic San Emigdio-Sierran block; the northern boundary is interpreted as being the northern termination of the San Andreas fault in the Cape Mendocino region. Structural deformation of this zone by diapirism and thrusting is facilitated by the lithic plasticity caused by high fluid pressures. Known diapirism and thrusting and possible diapiric folding suggest a late Cenozoic age for the development of the high fluid potentials. The origin of the anomalous fluid pressures adjacent to the San Andreas fault is attributed to compression between the granitic Sierran-Klamath and Salinas blocks resulting from late Cenozoic extension of the central Great Basin in Nevada and Utah. The San Andreas is a transform fault which separates the independent stress field of the Pacific plate (Salinas block) that is moving northwestward relative to the North American plate (Sierran-Klamath block and the Great Valley-Franciscan sediments). The Sierran-Klamath block also is moving westward or southwestward by continued late Cenozoic central Great Basin extension; this westerly motion is terminated by compression of End_Page 1219------------------------------ the rocks on both sides of the San Andreas. This compression has the greatest effect within the Franciscan and Great Valley shale mass just east of the fault; the effect is greatly reduced within the granitic basement and overlying sediments of the Salinas block west of the fault but has been responsible for folding of the sedimentary veneer. The high fluid potentials are caused by the squeezing of this belt of highly compressible shales east of the San Andreas in a vise whose jaws are formed of relatively incompressible granite; these anomalous fluid potentials are envisioned as being late Cenozoic phenomena dynamically active today. Diapirism and diapiric folding instead of thrusting have been the preferred modes of late Cenozoic structural deformation within this high fluid potential belt. The dominance of diapirism is attributed to the limited crustal shortening related to the development of this compressive field, as opposed to the dominance of the shearing stresses related to plate movements on both sides of the San Andreas fault. Diapirsm and more limited thrust faulting related to the current generation of high fluid potentials may develop in the future. Among the possible consequences of the existence of this postulated extensive zone of near-lithostatic fluid pressures are the shallow-focus earthquakes and extensive aftershocks along the San Andreas fault. The near-continuous fault creep along the San Andreas and related Calaveras and Hayward faults also may be a result of these postulated high pore-fluid pressures adjacent to these faults. An important implication of this paper is the demonstration that fluid pressures within rocks can serve as extremely sensitive and unique strain gauges for the detection of local or regional structural movements.
132 citations
TL;DR: The largest events in the San Fernando earthquake series, initiated by the main shock at 14h 00m 41.8s UT on February 9, 1971, were chosen for analysis from the first three months of activity, 87 events in all.
Abstract: The largest events in the San Fernando earthquake series, initiated by the main shock at 14h 00m 41.8s UT on February 9, 1971, were chosen for analysis from the first three months of activity, 87 events in all. C. R. Allen and his co-workers assigned the main shock parameters: 34°24.7′N, 118°24.0′W, focal depth h = 8.4 km, and local magnitude M_L = 6.4. The initial rupture location coincides with the lower, northernmost edge of the main north-dipping thrust fault and the aftershock distribution. The best focal mechanism fit to the main shock P wave first motions constrains the fault plane parameters to: strike, N67°(±6°)W; dip, 52°(±3°)NE; rake, 72° (67°−95°) left lateral. Focal mechanisms of the aftershocks clearly outline a down step of the western edge of the main thrust fault surface along a northeast-trending flexure. Faulting on this down step is left lateral strike slip and dominates the strain release of the aftershock series, which indicates that the down step limited the main event rupture on the west. The main thrust fault surface dips at about 35° to the northeast at shallow depths and probably steepens to 50° below a depth of 8 km. This steep dip at depth is a characteristic of other thrust faults in the Transverse ranges and indicates the presence at depth of laterally varying vertical forces that are probably due to buckling or overriding that causes some upward redirection of a dominant north-south horizontal compression. Two sets of events exhibit normal dip slip motion with shallow hypocenters and correlate with areas of ground subsidence deduced from gravity data. One set in the northeastern aftershock area is related to shallow extensional stresses caused by the steepening of the main fault plane. The other set is probably caused by a deviation of displacements along the down step of the main fault surface that resulted in localized ground subsidence near the western end of the main fault break. Several lines of evidence indicate that a horizontal compressional stress in a north or north-northwest direction was added to the stresses in the aftershock area 12 days after the main shock. After this change, events were contained in bursts along the down step, and sequencing within the bursts provides evidence for an earthquake-triggering phenomenon that propagates with speeds of 5–15 km/day. Seismicity before the San Fernando series and the mapped structure of the area suggest that the down step of the main fault surface is not a localized discontinuity but is part of a zone of weakness extending from Point Dume, near Malibu, to Palmdale on the San Andreas fault. This zone is interpreted as a decoupling boundary between crustal blocks that permits them to deform separately in the prevalent crustal shortening mode of the Transverse ranges region.
120 citations
TL;DR: In this article, the influence of oxidation induced stacking faults on the electrical characteristics of p-n junctions in silicon has been studied by employing scanning and transmission electron microscopy in conjunction with electrical measurements.
Abstract: The influence of oxidation induced stacking faults on the electrical characteristics of p‐n junctions in silicon has been studied by employing scanning and transmission electron microscopy in conjunction with electrical measurements. Diodes were fabricated with a junction depth and an integrated boron concentration of ohm‐cm n‐type epitaxial silicon.The presence of decorated stacking faults in the field region of the diodes examined is found to introduce excess reverse leakage currents in the junction. However not all faults introduce the same degree of leakage and the electrical activity of the faults is found to vary. The concept of the "threshold voltage" of a stacking fault is introduced which is a measure of the specific electrical activity of the fault. Electrically active faults introduce excess reverse currents in the diodes which are characterized by a relationship between the reverse log ratio measured at low voltages and the characteristic "threshold voltage" of the faults. The electrical activity is related to both the size and the structure of the faults. The smaller faults which are also more prone to impurity decoration are electrically more active than larger faults which are decorated to a lesser degree. Models are presented to account for the electrical activity of the faults. These models are based on the strain effects of the decorated faults and on the distortions produced in the p‐n junctions by the presence of faults.
85 citations
TL;DR: A study of the Alpine fault zone and the Fiordland region of the South Island of New Zealand from February through April 1972 indicates high but diffuse microearthquake activity as mentioned in this paper.
Abstract: A study of the Alpine fault zone and the Fiordland region of the South Island of New Zealand from February through April 1972 indicates high but diffuse microearthquake activity. Composite focal mechanism solutions show that a regional northwest-southeast compression dominates the tectonic pattern. This direction is nearly normal to the Alpine fault, indicating that the Alpine fault is now undergoing a large component of thrust faulting. This agrees with geologic data for uplift of the Southern Alps along the Alpine fault beginning in mid-Miocene time and accelerating in the Pliocene, the time of the Kaikoura orogeny. Before the Kaikoura orogeny, the Alpine fault apparently was a transcurrent fault. This major change in the New Zealand tectonic pattern could have been produced by a relatively minor migration of the nearby Indian-Pacific pole of rotation. Incipient underthrusting of the Tasman Sea appears to be occurring off the Fiordland coast, terminating at the point where the Lord Howe Rise intersects the coast. To the north is a zone of oblique continental convergence, with the Southern Alps being rapidly uplifted along the Alpine fault. North of the Alps, much of the motion is transferred to several faults that have more easterly strike; these formed in the Kaikoura orogeny and constitute a new transform fault system.
TL;DR: In this paper, a machine program is developed by which contour map of vertical components and vector map of horizontal components around the focal area can be drawn, designating focal para-meters as the inputs.
Abstract: Surface deformations due to the fault spreading over several layers in a multi-layered medium are investigated. Machine program is developed by which contour map of vertical components and vector map of horizontal components around the focal area can be drawn, designating focal para-meters as the inputs.Some numerical examples of surface deformations are presented for the purpose of comparison with those obtained for a homogeneous semi-infinite model.For a vertical dip-slip fault, for example, it is known that the vertical displacement at the surface is upheaval on one side of the strike of the fault and subsidence on the other side. For a multi-layered model, however, subsidence area appears even in the upheaval area mentioned above. We call this reverse area. Thus the displacement field shows different pattern from that for a semi-infinite model.
TL;DR: In this article, the authors analyzed the kinematics of the fault-creep process and found that the maximum slip velocity ranges from 0.1 to 10 p.m/s.
Abstract: Fault slip and strain during fault-creep episodes are continuously recorded by a dense network of creepmeters deployed along several major traces of the San Andreas fault system in central California. These data are analysed on the basis of theoretical faulting models to delineate the kinematics of the fault-creep process. The results indicate that fault creep is a failure propagation phenomenon, kinematically similar to seismic faulting, but with very low characteristic rates. The speed of creep propagation is not constant and is of the order of 10 km/day or less. The maximum slip velocity usually ranges from 0.1 to 10 p.m/s. Both of these are five or more orders of magnitude smaller than the corresponding rates of seismic faulting. The slowness in particle motion can account for the ineffectiveness of the creep process in exciting observable seismic waves. However, the tectonic strain released by a creep event may be sizable. The largest event recorded so far has a rupture length of 6 km and a maximum offset of 9 mm, comparable to similar parameters of a shallow earthquake of magnitude 4.7.
TL;DR: In this paper, a continuous seismic survey along closely spaced ship tracks in the northern Gulf of California is presented in terms of the tectonics of this region, indicating they are the loci of active tectonism.
Abstract: Results from a continuous seismic survey along closely spaced ship tracks in the northern Gulf of California are presented in terms of the tectonics of this region. Apparent vertical offsets of the most recent sediments, ranging in height from several to a few hundred meters, are associated with the central basins (Delfin and Wagner basins), indicating they are the loci of active tectonism. Structural relations inferred from mapping these features are consistent with plate tectonic concepts of the Gulf. Delfin basin represents a single, complex, northeast-southwest–trending, spreading center. Two parallel transform faults, which flank Angel de la Guarda Island and strike northward into Delfin basin from the south, and a complementary transform fault to the north represented by the Wagner basins, end at this spreading center. With the possible exception of the San Jacinto fault, no correlation of active faults was found between the northern Gulf and contiguous land areas. Interpretations of other geophysical and geological data are complicated by the high sedimentation rate in the northern Gulf, yet are generally consistent with our conclusions. Spatial and temporal characteristics of plate boundaries in the northern Gulf are probably influenced by the proximity of continental structures.
TL;DR: In this article, the authors examined regional seismicity data to determine the nature of seismicity in the years preceding these moderate earthquakes in California: 1952 Kern County (M = 7.7), 1963 Watsonville (m = 5.4), 1964 and 1967 Corralitos (m= 5.3), 1966 Parkfield-Cholame (m.1, 5.5), 1968 Borrego Mountain (m., 6.4).
Abstract: Regional seismicity data have been examined to determine the nature of seismicity in the years preceding these moderate earthquakes in California: 1952 Kern County (M = 7.7), 1963 Watsonville (M = 5.4), 1964 and 1967 Corralitos (M = 5.0, 5.3), 1966 Parkfield-Cholame (M= 5.1, 5.5), 1968 Borrego Mountain (M = 6.4), 1969 Santa Rosa (M = 5.6, 5.7), and 1971 San Fernando (M = 6.4). In each instance the moderate earthquakes occurred in areas characterized by a relatively high level of small earthquake activity. In most cases the preceding activity in the area immediately surrounding the epicenter was high relative to other segments of the same fault zone or other nearby fault zones that, on the basis of geologic criteria, would be considered equally likely locations for moderate earthquakes. Detailed observations for a moderate earthquake that occurred in 1972 near Bear Valley (M = 5.0), where seismograph station coverage is adequate to obtain reliable focal depths for small earthquakes, indicate that concentrations of small earthquakes were present in the immediate hypocentral region in the months before the earthquake. The regional observations have potential applications for the analysis of seismic risk in that fault zones or segments of fault zones characterized by a relatively large number of small earthquakes seem more likely to sustain moderate earthquakes than do adjacent fault zones or fault segments with equivalent geologic evidence for recency of movement. The detailed observations provide a potential tool for mapping stress concentrations or areas of anomalously low strength along the parts of fault zones capable of seismic slip.
TL;DR: In this article, the authors investigated the faulting process of the San Fernando earthquake of February 9, 1971 using the following seismic and geodetic data: vertical and horizontal displacements, strain and tilt changes, dynamic ground motions in the near field, focal mechanism, spatial distribution of aftershocks and features of surface fault breaks.
Abstract: The faulting process of the San Fernando earthquake of February 9, 1971 has been investigated using the following seismic and geodetic data: vertical and horizontal displacements, strain and tilt changes, dynamic ground motions in the near-field, focal mechanism, spatial distribution of aftershocks and features of surface fault breaks. A synthetic study suggests that the earthquake was caused by thrust faulting with a slip of 233° to 244° over a fault plane with dimensions 19 by 14 km, dip 50° to 52° and strike N64° to 70°W, which ruptures the ground surface over a distance of about 12 km. The fracture initiating at the hypocenter of the main shock seems to have propagated radially over the fault plane with a velocity about 2.5 km/sec. A small dislocation less than 30 cm at initiation probably increased rapidly during propagation and reached 3.5 to 4 m at the ground surface. A pronounced uplift and small subsidence of the ground north and south of the fault traces, and the overall pattern of the observed vertical and horizontal displacements can be explained well by the above model, but the recorded strain and tilt offsets are not always consistent with theoretical predictions. The wave forms and amplitudes for some of the integrated ground displacements from accelerograms at the Pacoima Dam and Pasadena are in fairly close agreement with those of the computed displacements. The seismic moment and stress drop of this earthquake were found to be 1.1 × 10 26 dyne·cm and 40 to 65 bars, respectively.
TL;DR: The horst was probably a southern source of detritus for the Upper Dalradian turbidites now on both sides of the Leek fault, and it was concluded that the case for a pre- Nitidus Zone age is unsatisfactory and that a mid-Ordovician age is possible as discussed by the authors.
Abstract: The Highland Boundary Fault is considered to extend from Stonehaven in Scotland to the major Leek fault of Clare Island in western Ireland. This conspicuous fault of the island has however been erroneously correlated with the Leannan fault of Donegal, a probable branch of the Great Glen Fault. South of the Leek fault Silurian sediments rest unconformably upon a metamorphic basement. This consists of amphi-bolite facies metasediments intruded by basic and ultrabasic rocks that have also undergone amphibolite facies metamorphism. These high grade rocks of uncertain age are considered to be part of a horst which was uplifted within the Moine Dalradian basin of western Ireland during Cambrian times. The horst was probably a southern source of detritus for the Upper Dalradian turbidites now on both sides of the Leek fault. The large uplift to the south which formed this horst conforms with and expands previous ideas on the early history of the Highland Boundary Fault. The age of the Dalradian metamorphism in western Ireland is reconsidered, and it is concluded that the case for a pre- Nitidus Zone age is unsatisfactory and that a mid-Ordovician age is possible.
TL;DR: For simple one-slip events on a long fault, the peak particle velocities near the center of the fault average about half the value Δσβ/μ, with Δσ β/μ apparently being a good upper bound as mentioned in this paper.
Abstract: Stick-slip along precut surfaces in stressed foam rubber is similar to earthquake faulting, stick-slip in rock specimens, and theoretical predictions. An additional feature is the common occurrence of multiple events. A significant amount of slip occurs as fault creep.
For simple one-slip events on a long fault, the peak particle velocities near the center of the fault average about half the value Δσβ/μ, with Δσβ/μ apparently being a good upper bound. The variation is probably due to focusing by rupture propagation. On a circular fault, the peak values average 3 to 4 times less, partly a result of a greater amount of fault creep, and probably, partly a result of focusing by rupture propagation. Total stress drops are typically about 10 to 20 per cent of the absolute stress, and during individual events about 80 to 90 per cent of the released strain energy is dissipated as friction.
For multiple events the cumulative source time function is usually much longer than the source dimension divided by the shear-wave velocity. Thus, the far-field spectrum would have the shape for fractional stress drop, but the low-frequency spectral corner would not correspond to the fault dimension; thus, the inferred stress drops would be too low. Multiple events may explain some anomalously low inferred stress drops for small earthquakes and may partly explain the success of surface-wave excitation as a method of distinguishing underground nuclear explosions from earthquakes. Foam rubber models may be used to study strong motion around various types of faults and, thus, aid in the problem of microzonation.
TL;DR: In this article, the authors inferred that the southern Iran earthquake of April 10, 1972 (Ms = 6.9) killed more than 5,000 people in the region of Ghir, Fars Province, in the folded belt of the Zagros Mountains.
Abstract: The southern Iran earthquake of April 10, 1972 ( Ms = 6.9) killed more than 5,000 people in the region of Ghir, Fars Province, in the folded belt of the Zagros Mountains. Aftershocks of the earthquake, relocated with respect to the hypocenter of the main shock by the method of joint hypocenter determination, and the radiation pattern of P and S waves suggest that the earthquake occurred on a west-northwest-striking reverse fault. This fault had a moderate dip and a strike that was parallel to the local trend of the folds and thrust faults of the Zagros folded belt, which are of latest Cenozoic (Pliocene and Quaternary) age. The inferred fault rupture of the main shock propagated approximately 35 km east-southeast from its epicenter, terminating in the epicentral region of a Ms = 5.6 earthquake that occurred on September 14, 1968. The zone of heaviest damage to adobe structures follows closely the trend indicated by the relocated epicenters of the main shock and principal aftershocks. Beyond 25 km from this trend, damage to adobe structures was negligible.
TL;DR: In the Los Angeles basin, the Mesozoic geologic boundary between Peninsular Ranges basement and Catalina Schist (Franciscan) basement is now covered by middle and late Cenozoic strata as discussed by the authors.
Abstract: In the Los Angeles basin, the Mesozoic geologic boundary between Peninsular Ranges basement on the east and Catalina Schist (Franciscan) basement on the west is now covered by middle and late Cenozoic strata. Most earlier workers have assumed that the basement boundary is a fault, and that its expression in the younger strata is the Newport-Inglewood zone of faults and folds. However, the basement rocks of the Newport-Inglewood zone, the Alondra oil field west of it, and the Brea-Olinda oil field east of it contain actinolite-bearing greenschist and serpentine, as assemblage unlike either the Catalina Schist or Peninsular Ranges basement. The closest Catalina blueschists are 5 km southwest of the zone. Retrograde plutonic rocks found beneath the Long Beach, Inglewood, and Las Cienegas oil fields are petrographically unlike Peninsular Ranges basement, but are similar to amphibolite-facies tectonic blocks associated with the Franciscan in northern California and are so interpreted here. Retrograde metamorphism and cataclastic textures in Los Angeles basin basement rocks may be related to thrusting accompanying Mesozoic subduction. The distribution of these rocks suggests that the Peninsular Ranges-Franciscan basement boundary does not follow the Newport-Inglewood zone in the Los Angeles basin, but instead departs from it north of Sunset Beach oil field, trending northerly between the Anaheim nose and Las Cienegas oil field. Cenozoic structural patterns in the Newport-Inglewood zone are quite diverse. Sunset Beach, Huntington Beach, and West Newport oil fields within the zone and Wilmington oil field west of it are characterized by north-trending normal faults with no movement younger than early Pliocene. Dominguez, Rosecrans, and Howard Townsite oil fields are characterized by west-trending reverse faults of the same age as the normal faults. Northwest-trending faults, with no more than 3 km of total right-lateral slip, cut across these diversely oriented structures and affect beds as young as Pleistocene. The western Los Angeles basin is visualized as a sedimentary blanket which, except for bedding, is structurally isotropic, overlying a basement with diversely oriented structural anisotropies. As the area was subjected to simple right-lateral shear in late Miocene and early Pliocene time, these basement anisotropies propagated upward into the sedimentary blanket as fault systems on which the displacements were controlled by their orientation. West-trending faults were reverse, north-trending faults normal, and northwest-trending faults right-lateral. As distortion continued, the high-ductility-contrast Peninsular Ranges-Catalina Schist basement boundary southeast of the Los Angeles basin propagated itself as the right-lateral slip Newport-Inglewood fault zone northwestward across the ower ductility-contrast Franciscan and greenschist basement of the Los Angeles basin. Localization of shear within this zone caused the older, diversely oriented normal and reverse faults to become inactive and produced the shear system of today. The older, diversely oriented fault systems are in part parallel with adjacent outcropping fault systems of middle Miocene age in the San Joaquin Hills. Both systems may have been controlled originally by the breakup of the northern end of the Peninsular Ranges as the East Pacific Rise reached the edge of the continent. This caused the Channel Islands-San Nicolas Island block and the Santa Monica Mountains block to move west from the Peninsular Ranges, leaving behind a rift which became the Los Angeles basin.
TL;DR: In this article, a 152m-deep well was used to examine the possibility of any variations in fluid pore pressure during fault creep movement, and the results indicated the possibility that deeper wells could be used to monitor stress changes along active fault zones.
Abstract: A 152-meter-deep well was drilled into the San Andreas fault zone near Hollister, California, to examine the possibility of any variations in fluid pore pressure during fault creep movement. Anomalous water level changes were recorded that coincide, within hours, with the only creep episodes recorded at a nearby site. The ratio of maximum water level change to total creep offset observed is 14 mm/mm. The calculated change in pore pressure for creep events is about 20 mb. Assuming 1.2 cm of total creep displacement per year along this section of the San Andreas fault, we obtain a total stress drop of 60 mb/yr associated with creep at shallow depths. The results are most encouraging, indicating the possibility that deeper wells could be used to monitor stress changes along active fault zones.
TL;DR: In this paper, the McKinley strand of the Denali fault near the Delta River in the east-central Alaska Range was shown to have experienced right-lateral displacement during the last 10,000 yrs.
Abstract: Offset Holocene alluvial fans and drainages along the McKinley strand of the Denali fault near the Delta River in the east-central Alaska Range indicate as much as 50 to 60 m of right-lateral displacement during the last 10,000 yrs. Vertical movement of 6 to 10 m during the same time interval is reflected by south-facing scarps along the trace of the fault. All but possibly 1 m of the lateral movement is thought to predate the 1830 neoglacial ice advance. Older drainages have been offset in a right-lateral sense since early Wisconsin or Illinoian time by as much as 6.5 km or, alternatively, by as little as 1 km.
TL;DR: The theory of wrench-fault tectonics as discussed by the authors postulates that lateral compression formed wrench faults in the earth's crust early in geologic history, and the shear pattern thus formed has controlled subsequent deformation in a continuing lateral compressive stress field, which has produced second-, third-, and higher order shears.
Abstract: Many, if not most, of the faults, fractures, and lineaments in the worldwide regmatic shear pattern appear to be wrench faults, along which the dominant motion is horizontal and the fault planes are essentially vertical. The theory of wrench-fault tectonics postulates that lateral compression formed wrench faults in the earth's crust early in geologic history. The shear pattern thus formed has controlled subsequent deformation in a continuing lateral-compression stress field, which has produced second-, third-, and higher order shears. In the third order, however, shear directions become repetitive, hence the ultimate shear pattern in any region contains only eight preferred directions of wrench faulting and four preferred directions of drag folding. A meridional shear direction consisting of two primary systems has been superimposed on a somewhat older equatorial shear system, which is similarly structured. With four primary shear directions, there are six types of shear intersections: intersections of complementary shear sets form angles that approximate 60°; shears of the same sense intersect at 90°; meridional and equatorial shears of opposing sense intersect approximately at 30°. The 30° and 60° intersections have lateral compressive components that favor vertical movements, whereas the 90° intersections show a tendency for block rotation. Wrench-fault tectonics have direct application to petroleum exploration, particularly in delineating sedimentary basins whose history of deformation and sedimentation is critical in the accumulation and preservation of hydrocarbons. Wrench faulting according to the postulated pattern also forms traps of four or more types. Brecciation associated with wrench faulting also may develop fracture-type porosity such as is found in the Lima-Indiana, Scipio-Albion, Panuco (Mexico), and some Ellenburger (Texas) fields. The scale on which wrench faulting operates ranges from such single oil-producing trends as the drag folds of Elk Hills and Kettleman Hills in California, to such regional oil provinces as Venezuela-Colombia, Alaska, and Gulf of Guinea. Wrench-fault tectonics appear to be a prim factor in respect to petroleum accumulation.
TL;DR: A metamorphic zonal map of the Central and Northern Highlands of Scotland has been drawn based on the mineralogy of Moinian calc-silicate gneisses.
Abstract: A metamorphic zonal map of the Central and Northern Highlands of Scotland has been drawn based on the mineralogy of Moinian calc-silicate gneisses. The regional distribution of metamorphic maxima and minima indicates a post-metamorphic sinistral shift of 160 km along the Great Glen Fault.
TL;DR: The U.S. Geological Survey's telemetered seismic network in central California was supplemented over a 7-week period with 13 portable seismograph stations in the area of the Calaveras Fault southeast of San Jose as mentioned in this paper.
Abstract: The U.S. Geological Survey's telemetered seismic network in central California was supplemented over a 7-week period with 13 portable seismograph stations in the area of the Calaveras Fault southeast of San Jose. Travel-time data from small quarry explosions indicate a vertical low-velocity zone apparently associated with the fault and extending to a depth of at least 6 km.
When this zone is taken into account, the hypocenters of 55 earthquakes recorded to study this effect show an arrangement of a near-vertical dipping plane that coincides remarkably well with the mapped trace of the fault. The previously determined distribution of foci was off the fault by several kilometers to the east.
TL;DR: In this article, the Siqueiros Fracture Zone was mapped and it was shown that it extends at least 850 km east of the crest of the East Pacific Ridge with an overall azimuth of 082°.
TL;DR: In this paper, an inactive fault is proposed to extend between the Bering Strait and the Arctic Ocean continental shelf east of the Northwind escarpment, separating northern Alaska from northeast Siberia.
TL;DR: In this paper, the authors show that the distribution of mineral parageneses in the northeastern Diablo Range, revealed by analyses of more than 300 thin sections of metaclastic rocks, shows no spatial relation between highest grade rocks and either exposed segments of the Coast Range thrust fault or the margins of ultramafic masses undergoing present-day ser-pentinization.
Abstract: Fault contacts in the northeastern Diablo Range, California, between the partially melanged late Mesozoic Franciscan Complex and the broadly coeval, less deformed sedimentary rocks of the Great Valley sequence have been called, by definition, the Tesla-Ortigalita fault. “Coast Range thrust” is the name applied by Bailey and others (1970a) to the fault of regional extent that originally separated subducted oceanic crust and sedimentary rock of the Franciscan Complex from structurally overlying ophiolite plus shelf-slope facies sedimentary rock of the Great Valley sequence. The two faults are not equivalent. High-angle Neogene segments of the Tesla-Ortigalita fault truncate older fault surfaces of the Coast Range thrust-fault system. Franciscan metamorphic rocks contain low-to high-pressure, low-temperature mineral parageneses characterized by the phases pumpellyite, prehnite, aragonite, lawsonite, glaucophane, and jadeitic pyroxene. Metamorphism has been variously ascribed to metastable re-crystallization, metasomatism by fluids generated in and around serpentinite, structural burial produced by subduction of an oceanic lithospheric plate, and burial plus tectonic overpressures generated beneath the Coast Range thrust fault. Suppe (1970) and Ernst (1971a) argued against metastable recrystallization and metasomatism on the basis of available field and laboratory evidence. If metamorphism is related to serpentinization, a metamorphic aureole should surround ultramafic bodies undergoing present-day ser-pentinization. Metamorphism resulting from tectonic overpressures generated beneath the Coast Range thrust fault will be revealed by an increase in metamorphic grade toward the thrust, whereas structural burial would result in an increase in metamorphic grade with structural depth. Distribution of mineral parageneses in the northeastern Diablo Range, revealed by analyses of more than 300 thin sections of metaclastic rocks, shows no spatial relation between highest grade rocks and either exposed segments of the Coast Range thrust fault or the margins of ultramafic masses undergoing present-day serpentinization. Thus the available evidence fails to support the metastable recrystallization, metasomatic, and tectonic overpressure concepts. Only the hypothesis of structural burial is not negated by the observed relations.
TL;DR: In this paper, geomorphic and seismic data relative to one of the major faults of Iran, the Doruneh Fault, are presented and it is shown that along its eastern section, the fault is presently active and connected with two destructive earthquakes during the twentieth century Contrary to previous assumptions, recent and probably contemporary movements along it are essentially in the vertical direction
TL;DR: The Sur fault zone is a complex series of high-angle and low-angle faults that juxtapose a northeast block of competent crystalline rocks against a southwest block of generally sheared Franciscan rocks.
Abstract: The Sur fault zone is a complex series of high-angle and low-angle faults that juxtapose a northeast block of competent crystalline rocks against a southwest block of generally sheared Franciscan rocks. The Franciscan rocks are an assemblage of sandstone, siltstone, greenstone, and chert. This assemblage can be divided into a northwestern terrane characterized by unmetamorphosed sheared and unsheared units containing detrital K-feldspar and a southeastern terrane with a pervasive dip-slip tectonite fabric throughout, metamorphic mineral assemblages indicative of high-pressure, low-temperature metamorphism, and no K-feldspar. A lawsonite isograd in Franciscan sandstone crosses the southeastern terrane. K-Ar dates indicate a minimum age of Late Cretaceous for high-pressure metamorphism of the Franciscan.
During mid- to Late Cretaceous time, accumulation of Franciscan rocks was closely followed by subduction and high-pressure, low-temperature metamorphism. Low-pressure, high-temperature metamorphism and plutonism occurred concurrently in the Salinian block. In Late Cenozoic time, the San Andreas fault probably offset the Sur-Nacimiento fault from the Great Valley subduction zone.