Showing papers on "Fault (geology) published in 1969"

The relation between fault plane solutions for earthquakes and the directions of the principal stresses

Abstract: The stresses involved in shallow earthquakes and their occurrence along fault planes suggest that they occur by failure on weak planes, rather than by brittle fracture of a homogeneous material. Possible orientations of the stress tensor are examined to determine what limits fault plane solutions can place on the orientation of the greatest principal stress. For the general case of a triaxial stress, the only restriction is that this stress direction must lie in the quadrant containing P , but may be at right angles to the P direction. Thus shallow earthquakes impose a few limitations on the orientation of the stress tensor. In contrast the fault plane solutions from deep earthquakes are best explained by fracture of a homogeneous material, with the greatest principal stress directed down the dip of the earthquake zone.

Heat flow, stress, and rate of slip along the San Andreas Fault, California

Abstract: The absence of a heat flow anomaly greater than ∼0.3 µcal/cm2/sec associated with the San Andreas fault is used to estimate the upper limit for the steady state or initial shear stress. Under the assumption that the long-term rate of motion along the fault is 5 cm/yr and occurs primarily in the form of creep, this upper limit is about 100 bars. If the motion is primarily accomplished by faulting during large earthquakes and if the frictional stress is equal to the final stress as suggested by E. Orowan (1960), the upper limit is estimated to be about 200 bars. Without Orowan's assumption, the estimation of the upper limit is about 250 bars, based on earthquake energy-magnitude-moment relations. If the long-term rate of motion along the San Andreas fault is only ∼2 cm/yr, these results are increased to 250, 350, and 400 bars, respectively.

Fault‐graded beds interpreted as seismites

Abstract: SUMMARY Fault grading is expressed by a regular structural sequence of (from top) liquified zone, rubble zone and step-faulted zone, with gradational contacts between these zones and the bottom, but with a sharp boundary at the top. It is interpreted as an effect of strong earthquakes on gradationally compacted muds in quiet water basins. Fault-graded beds are geopetal and paleoslope criteria. If interpreted as seismites they may also be used as paleo-seismograms.

Seismicity and tectonics of the western Pacific: Izu-Mariana-Caroline and Ryukyu-Taiwan regions

Abstract: The hypocenters of about 1000 earthquakes in the Izu-Bonin, Mariana, Palau, Caroline, and Ryukyu-Taiwan regions were relocated by computer, and earthquake mechanism solutions based on the first motions of P, pP, and S were determined for 26 earthquakes. The spatial distribution and the mechanisms were compared with major tectonic features such as volcanic zones, island arcs, and trenches. Although a nearly continuous zone of shallow earthquakes can be traced from Honshu to Palau, deep shocks were not detected south of 16°N; intermediate-depth events were not found south of the Marianas. A region of low seismicity for shallow earthquakes and a discontinuity for deeper events occurs in the Volcano Islands adjacent to a prominent gap in the trench where the Marcus-Necker ridge intersects the arc. In some profiles of this series of arcs, the deep seismic zone is nearly planar, but in other cases it is more contorted and the thickness of the zone appears to vary from place to place. Maxima and minima in seismic activity occur for deep and intermediate depths, and these features migrate in depth similar to that found in the Tonga arc. Activity as deep as about 680 km occurs in the northern Marianas where the deep focal zone is nearly vertical for depths greater than about 200 km. This nearly vertical distribution suggests that gravitational tectonics may play an important role as a motive force for global tectonics. Seismicity in the Ryukyu arc is concentrated in a thin planar zone that dips 35° to 45° northwest to a depth of about 280 km. Near the northwest coast of Taiwan all tectonic elements change strike abruptly, and the mechanisms change from thrusting of the Philippine Sea plate beneath the Ryukyu arc to strike slip near the east coast of Taiwan. South of this point, most of the events are shallower than 100 km, and the focal zone is not well defined in vertical sections. No events were detected from the Kyushu-Palau ridge, from the Central Basin fault, or from the rest of the interior of the Phillipine Sea. Thus, this region appears to be a single lithospheric plate. Normal faulting is found for some earthquakes either in the deepest parts of the trench or along the seaward wall. A zone of shallow earthquakes also can be identified on many of the profiles near the volcanic axis. Mechanism solutions for two events near this zone are also of the normal-fault type. Mechanism solutions for shallow earthquakes beneath the islandward margins of the Izu-Bonin and Mariana trenches indicate that the Pacific plate is underthrusting the Philippine Sea plate in a westerly direction. The results of the relocations and other pertinent data are given in a separate appendix, which is available on microfiche along with the entire article. Order from the American Geophysical Union, Suite 435, 2100 Pennsylvania Ave., N.W. Washington, D.C. 20037. Document J69-002; $1.00. Payment must accompany order.

The Mudurnu Valley, West Anatolia, Turkey, earthquake of 22 July 1967

Abstract: A severe earthquake in the west-central part of the Anatolian fault zone occurred on 22 July 1967, causing loss of life and widespread damage. The earthquake was associated with 80 kilometers of fresh faulting, part of which occurred in a zone ruptured ten years earlier. The sense of movement along the fault break is right lateral with the north side downthrown. Maximum relative displacements of 190 centimeters lateral and 120 centimeters vertical were measured. Damage caused by shaking in the immediate vicinity of the fault-break was equal to or smaller than that caused at some distance from the fault. Thus, proximity to the fault-break was found to be an unjustified criterion for higher intensities. The instrumental epicenter of the main shock had been located near the east end of the fault break. The bulk of the aftershocks is concentrated at the other end of the break, in the extreme west.

Active Faulting in Northern Turkey

Abstract: The North Anatolian fault zone of Turkey has become widely publicized in recent years because of the remarkable series of earthquakes that began along it in 1939 -- most of which have been associated with dextral surface displacements that have successively delineated the fault trace from east to west (Ketin and Roesli, 1953; Ambraseys and Zatopek, 1968). It is not so generally recognized that even prior to 1939 the fault zone could easily have been recognized on the basis of abundant and through-going features of Quaternary displacements, and that the North Anatolian fault is almost completely analogous to the better-known active transcurrent faults of the circum-Pacific region, such as the San Andreas fault of California and the Alpine fault of New Zealand.

Aftershocks of the Benham nuclear explosion

Abstract: The Benham nuclear explosion, a 1.1 megaton test 1.4 km beneath Pahute Mesa at the Nevada Test Site, initiated a sequence of earthquakes lasting several months. The epicenters of these shocks were located within 13 km of ground zero in several linear zones that parallel the regional fault trends. Focal depths range from near surface to 6 km. The earthquakes are not located in the zone of the major ground breakage. The earthquake distribution and fault plane solutions together indicate that both right-lateral strike-slip fault movement and dip-slip fault movement occurred. The explosion apparently caused the release of natural tectonic strain.

Wrench Faulting and Rocky Mountain Tectonics

Abstract: Wrench faults are near vertical fractures characterized by Important lateral slip components. They form under regional compression when the greatest and least principal pressures (axes) lie In the horizontal plane. With the aid of conceptual models (block diagrams) it is shown that since faults are finite and confined within an essentially incompressible crust, vertical separation and an important component of vertical slip must accompany lateral movement along wrench faults. Thus separation and slip along wrench faults may vary importantly, a geometric condition considered singularly characteristic. The reversal of fault dip along a fault line (“propeller faulting”), the development of belts of en echelon parafolds (“drag folds”) or pinnate tension fractures, or abrupt stratigraphic change across the fault zone, also are considered indicative of wrench faulting. In the central Rocky Mountain foreland area many of these wrench fault features can be recognized along major west-northwest (left wrench) and northeast (right wrench) trending fault zones. Considering the nearly north-south orientation of primary folds and of the low angle thrust faults of the disturbed belt, therefore, it appears that a single north-northeast–south-southwest direction of principal horizontal stress (PHS) can account for all of the Laramide structural features of the central Rocky Mountain foreland. An idealized tectonic diagram and a regional tectonic map of the central Rocky Mountain area are presented to show graphically the writer’s interpretation of the various faults and folds of this area according to the wrench fault concept. These diagrams portray the genetic relationships between first and second order wrench fault zones and other structural features. A check list of characteristic features is also presented as an aid in identifications of other wrench fault zones in the Rocky Mountain Area. Such identifications are considered economically important as the structures around and along wrench fault zones commonly are the habitat of mineral deposits.

Displacement and stress field along part of the Cobequid Fault, Nova Scotia

Abstract: The east-trending Cobequid Fault separates pre-Carboniferous rocks of the Cobequid Mountains to the north from Carboniferous clastic rocks along the southern flank of the mountains. A detailed study of the fault zone revealed tie predominance of right-lateral displacements. The orientation of the stress field that existed during deformation along the fault trace was determined by the study of systematic fractures in pebbles within Carboniferous conglomerate. Maximum compressive stress was aligned in a NW–SE direction, being compatible with the orientation of the displacement vectors in the fault zone. Transcurrent movement along the Cobequid Fault occurred in late Pennsylvanian time and involved both Carboniferous and pre-Carboniferous rocks; total displacement is unknown.

Fault displacements and motion related to nuclear explosions.

Abstract: Studies of some of the geologic effects of underground nuclear explosions that have been carried on during the last 3 years by the U. S. Geological Survey on behalf of the U. S. Atomic Energy Commission are summarized. Movement has been triggered on some faults within a few thousand feet of explosions in Nevada and in Amchitka Island, Alaska. The permanent relative vertical displacement on the faults always has been in the same direction as the last recognizable natural tectonic displacement and has been as much as 1 m (meter); horizontal displacement of as much as 15 cm (centimeters) has occurred on a few faults although no previous horizontal displacement had been apparent. The strike length of triggered faulting ranges from about 1 to 8 km (kilometers) and seems to correlate linearly with yield and magnitude of the explosions. High-speed motion picture photography of two faults from both the air and the ground provides the following data on the motion of the faults. 1 Fault motion started at the first arrival of seismic energy. 2 Rupture velocity along triggered faults, measured over a distance of 0.210 km, was 2.1 km/sec (kilometers per second) in alluvium of Hot Creek Valley, Nevada, which has a compressional velocity that ranges from about 2 km/sec to 3.3 km/sec and which has a density of 2.3 g/cc. 3 The velocity of displacement of alluvium at the Yucca Fault plane in Yucca Flat was a little more than 1 m/sec (meters per second). Velocity of displacement of alluvium at the fault plane in Hot Creek Valley was between 6 and 30 m/sec.

Tectonic Correlations, Klamath Mountains and Western Sierra Nevada, California

Abstract: The Klamath Mountains province of northwestern California has long been regarded by geologists as a probable northern continuation of the Sierra Nevada province. Attempts at stratigraphic correlations between the two provinces date back to the turn of the century, but attempts at correlation of tectonic elements have been hindered by a lack of data. Clark (I960) proposed a general correlation of north-striking faults in the Klamath Mountains and in the western Sierra Nevada. High-angle faults of the latter area constitute the Foothills fault system, which Clark believed to be of probable strike-slip origin. Recent studies in the southeastern Klamath Mountains demonstrate that the major faults of that region are thrust faults along which thrust plates have moved relatively westward. The contrast in geometry between subhorizontal to moderately dipping Klamath faults and steeply dipping faults of the Foothills system does not refute correlation of the two but can be explained by an eastward steepening of the Klamath thrusts into faults continuous with those of the western Sierra Nevada. Geologic relationships indicate that the Melones fault of the Foothills fault system extends northward into the eastern Klamath region as the Trinity thrust fault. Correlation of the two faults appears probable on the basis of (1) stratigraphic and structural similarities in their hanging wall blocks, (2) the presence of thick bodies of serpentinized peridotites below the Trinity thrust plate and below the hanging wall of the Melones fault in the northwestern Sierra Nevada, (3) permissive evidence for a Late Jurassic age of the two faults, and (4) a postulated curvilinear trace for the Trinity-Melones fault which is parallel to observed curvilinear structural trends farther west. It is believed that steep minor structures within fault zones of the Foothills system, cited by others as evidence for strike-slip faulting, are not incompatible with an origin for the system by thrust faulting. Three of the four fault-bounded Klamath subprovinces are interpreted as having stratigraphic and structural counterparts in the northwestern Sierra Nevada. The Klamath central metamorphic subprovince is regarded as a thrust slice of Devonian(?) metamorphic rocks, which thins northward and southward and is not present in the Sierran region. Eastward steepening of the proposed Klamath-Sierran thrust faults is related to their increasing proximity to the Sierran synclinorium and to their rooting into its western flank. Formation of the thrusts occurred in the Jurassic during a lengthy interval of profound crustal downbuckling and shortening. Rooting of the thrusts into a crustal zone undergoing contemporaneous subsidence and shortening rules out an origin for them by gravitational tectonics from an orogenic high or by extrusion tectonics from a dilating zone. The geometry of Mesozoic orogenesis in the Klamath-Sierran region, including thrust faulting, can best be explained by the convergence of subcrustal convection cells along the axial zone of the Sierran synclinorium.

Strain adjustments associated with earthquakes in southern California

Abstract: A technique for the calculation of strain changes in a two-dimensional elastic body with arbitrary internal dislocations is presented. This technique is applied to the southern California region by assigning a specific fault and fault slip function for each major earthquake that has occurred since 1812. Although the model used has serious shortcomings when applied to the real Earth, certain important features concerning strain energy changes associated with earthquakes are brought out. The occurrence of earthquakes over the past 150 years has resulted in net increases in stored strain energy in a number of regions including the northern end of the Gulf of California, the Cajon Pass area, and the northern part of the Carizzo Plain. Large regions of strain energy decrease can also be seen, the most important of which is in the vicinity of Fort Tejon.

Microearthquakes near The Geysers, Sonoma County, California

Abstract: In 120 hours of recording in October 1968, nineteen small earthquakes were found on the fault system associated with Geysers steam zone in Sonoma County, California. These earthquakes, as distinguished from steam-generated seismic noise, provide further evidence of the genetic relationship between the two phenomena and suggest that the mapping of microearthquakes could be used in the exploration for geothermal power.

Northeast-Trending Faults of Scotland and Ireland, and Chronology of Displacements: Chapter 53: Late Orogenic Stratigraphy and Structure

Abstract: The NE-trending faults of Scotland and Ireland form a structurally and chronologically coherent system of steep fractures on which there have been repeated movements, all of which exerted some control on sedimentation. At about late Early Devonian-early Middle Devonian time, important displacements variously combined dip slip of southeast throw with sinistral strike slip. Of the several periods of reactivation in which relatively minor strike slip commonly can be proved, the most important were during the Carboniferous. In some units of the system, pre-Devonian activity is suspected, and the Highland Boundary fault, in particular, marks the site of a long-established lineament which has acted as the locus of movement since pre-Arenigian time. The courses of several great aults are traced beyond Scotland, especially into Ireland, where, off the west coast, the fractures may converge into an important compound line. It should be sought on the shelf--especially in view of the possible correlation with the Cabot fault zone of eastern Canada.

The Fairbanks earthquakes of June 21, 1967; aftershock distribution, focal mechanisms, and crustal parameters

Abstract: A series of moderately severe earthquakes occurred in the vicinity of Fairbanks, Alaska, on the morning of June 21, 1967. During the following months, many thousands of aftershocks were recorded in order to outline the aftershock zone and to resolve the focal mechanism and its relation to the regional tectonic system. No fault is visible at the surface in this area. Foci were found to occupy a relatively small volume in the shape of an ablate cylinder tilted about 30° from the vertical. The center of the zone lay about 12 kilometers southeast of Fairbanks. Focal depths ranged from near-surface to 25 kilometers, although most were in the range 9-16 km. In the course of the investigation, it was found that the Jeffreys and Bullen velocity of 5.56 km/sec for the P wave in the upper crustal layer is very near the true value for this arec, and that the use of 1.69 for the V p / V s ratio gives good results in most cases. The proposed faulting mechanism involves nearly equal components of right-lateral strike slip, and normal faulting with northeast side downthrown on a system of sub-parallel faults striking N40°W. The fault surface appears to be curved—dipping from near vertical close to the surface to less steep northeast dips at greater depths. The relationship of this fault system with the grosser aspects of regional tectonism is not clear.

Geologic Structure of a Part of the Barinas Mountain Front, Venezuelan Andes

Abstract: The following lithologic units are exposed in the Barinitas-Santo Domingo region, southeastern Venezuelan Andes: (1) the Precambrian (?) Iglesias Group, forming the metamorphosed basement; (2) the Lower Paleozoic (?) Cerro Azul Formation, a phyllite sequence; (3) the Upper Paleozoic Palmarito Formation, mostly fossiliferous limestones and shales; (4) the Triassic-Jurassic La Quinta Formation, a continental sequence of red beds; (5) the rocks of the Cretaceous transgression and regression, consisting of the Rio Negro, Apon, Aguardiente, Maraca, La Luna, and Colon-Mito Juan Formations; and (6) the Upper Eocene Gobernador and Paguey Formations, disconformably overlying the Cretaceous sequence. Three granitic intrusions crop out, two of which, the La Soledad Quartz Monzonite and the hornblende granodiorite of Cerro Azul, intrude the Cerro Azul Formation. They were dated by the potassium-argon method as Middle Pennsylvanian and Early Devonian, respectively. The principal structural units present in the map area are enormous blocks of basement rocks (Iglesias Group) which are separated by steeply northwestward-dipping, or nearly vertical reverse faults (upthrusts), along which the blocks have been uplifted or depressed. Along the east flank of the mountain range toward the Barinas Basin the sedimentary cover has been folded into a monocline which is itself broken into blocks by steep faults. In the higher parts of the region, toward the central part of the mountain range, is a zone of extension which produced grabens. horsts, and stepped faults, represented by the Bocono fault zone. The present uplift of the Venezuelan Andes took place in post-Eocene time. Upper Eocene sediments of the Paguey Formation, poorly consolidated during the initial stages of the uplift, were deformed by slumping, into cascade folds (overturned to the southeast), and also slid down-slope as chaotically oriented blocks which cover the Upper Eocene sediments in situ. Vertical uplift, probably combined with arching or tilting of the region, produced the steep thrust faults along the margin of the range, and also the zone of extension represented by the Bocono fault zone. Strike-slip movement of great magnitude probably has not taken place along this fault.

Tectonics of the San Andreas Fault System strain studies

Abstract: The recording of surface displacement and tilts that appear to occur prior to large earthquakes offers one of the more promising approaches to the problem of earthquake prediction. Strain and in-situ stress studies are fundamental to solving the problem of the mechanics of faulting. The U. S. Geological Survey has installed a network of monuments for measurement of displacements and strains along the San Andreas fault system (Figure 1). The measurements taken are of three types: 1. Trilateration and triangulation with geodimeter and theodolite on quadrilateral figures spanning the fault and having side lengths of 1 to 10 km. Measurements of two of these figures, at San Andreas Lake and Black Mountain, have been repeated several times over the last 2½ years. Three additional figures have recently been installed, two along the San Andreas fault and one on the Calaveras fault. 2. Alignment on monument arrays 100–200 meters in length crossing the active trace of the fault. The measurement yields the horizontal component of motion parallel to the fault trace for 10 to 20 monuments in each array. 3. A precise level line at San Andreas Lake.

Late Cenozoic Movement on the Fairweather Fault in Southeastern Alaska

Abstract: A tentative history of late Cenozoic displacement on the Fairweather fault is presented. Geomorphic observations suggest predominantly dextral strike-slip movement at a rate of a few centimeters per year for at least the last millenium. The associated dip-slip component of motion is several times smaller than the strike-slip component. Little variation in the rate and the nature of displacement on the Fairweather fault from early Pliocene time to the present is consistent with interpretations of sea-floor spreading in the North Pacific Ocean.

Actividad ígnea y tectónica en la Cordillera Central durante el Meso-Cenozoico

Abstract: The history of the igneous and tectonic activity which took place in the Cordillera Central during the Mesozoic and Cenozoic Era may be divided into two definite periods. The first stage, of Jura-Triassic age, is closely associated with the regional uplift of the cordillera and is expressed largely at its Eastern and Western margins. This period is characterized by plutonic intrusions of granodiorite and quartzmonzonite followed by a great outpouring of felsic material of rhyodacitic composition. During this period of igneous and tectonic activity the Cordillera Central existed as un uplifted mountain are undergoing erosion; evidences of this fenomenon are indicated by studying the clasts of the Jura-Triassic formations. In the final stage of this period a complex fault system developed which, trend in NW direction. The second stage, far which we propase the term "Andean Geotectonic Cycle", is characterized by a volcanic-plutonic association of orogenic regions. The time interval of igneous and tectonic activity embraces from the Early Cretaceous until the present day. Four phases of igneous activity characterize the Adean Geotectonic Cycle. 1. Eruption of dominatly basic lavas, during the geosynclinals stage of the cycle. This initial vulcanism, of fissure type, is closely related with the birth of the fundamental Romeral fault. 2. Injection of ultrabasic and basic plutonic intrusions, in the internal zone of the cretaceous geosyncline, during the embryonic stage of folding. At the end of this phase the Cordillera Central evolved from an uplifted are to a geosinclynal ridge over which Cretaceous (Aptian) sediments are deposited far the first time in the Cordillera. Later, an important fault system, striking mainly NE and NW (Otu and:Palestina Faults), is developed. 3. Development of quartzdioritic batholiths in the central part of the cordillera. As a consequence, an intensive fracturing of the host rock is produced along with tectonic reorientation and reativation of the prexistent faults. 4. Surface eruption of basalts, andesite and rhyolite following the folding but concomitant with the final regional uplift of the cordillera. This second series of effusives, unlike the first, occurs in the axial zone of the cordillera and it is represented by volcanoes of central type which started forming during the Miocene. It is proposed the name of Andean Geotectonic Cycle to embrace all the phenomena of subsidence, vulcanism, plutomism, folding, faulting and uplifting, which have taken place since early Cretaceous time until the present day and are responsible of the formation of the Colombian Andes.

Preliminary Instrumental Measurements of Fault Creep Slippage on the San Andreas Fault, California

Abstract: Recent studies have delineated the occurrence of fault creep slippage on the San Andreas, Hayward, and Calaveras faults in central California. Preliminary instrumental measurements show that fault creep has different time character at different sites, being eventful at some places and continuous at others. Creep events measured at two sites indicate a creep propagation rate of of the order of 10 km/day.

Microearthquake seismicity of the Denali Fault

Abstract: The first detailed microearthquake survey of a major part of the Denali fault was carried out during July and August of 1967. The microseismicity of the fault was sampled by high-gain portable seismographs at 22 sites between Mt. McKinley National Park and Haines, Alaska. The rates of very local microearthquake activity (S-P times less than 2 sec) varied from 0 to about 40 events per day. The average microseismicity was found to be about the same as that recently found for the San Andreas fault. This result is surprising because in the last 60 years only one small-magnitude teleseism has been located near the Denali fault east of Mt. McKinley National Park. The significant microearthquake activity on this section of the fault could not have been predicted on the basis of recent major earthquake activity. A new fault is suggested in the Dezadeash Lake region of the Denali fault zone based on a concentration of microearthquake activity as well as on geological and topographic evidence. The new fault provides a mechanically more acceptable trend for large transcurrent displacements by straightening the Denali fault zone near the junction of the Shakwak Valley and the Haines fault segments.

The focal mechanisms in Iran and their relations to tectonics

Abstract: Data from focal mechanism solutions obtained by different authors and those of 8 fault-plane solutions found in this study have been used to search for the distribution of the main stress axes in Iran. For this purpose, the area has been divided into three regions as southern, central and northern Iran. The results indicate that the characteristics of the motion at the foci are different in each of the three regions. — By examining the B axes in south Iranian earthquakes, direction of tectonic motion has been obtained as N 66°E. Since the maximum and intermediate stress axes are nearly horizontal, it is concluded that focal movements in this region are of reverse fault type. Thus, there is a similarity between recent crustal movements and those occurring during Alpine orogeny which is in the form of an overthrusting to the southwest. — In central Iran earthquakes however, tension is predominant, and, therefore, in this region faultings are dip-slip normal or strike-slip, and the horizontal components of displacements are dextral. The mean direction of maximum tension axes is nearly perpendicular to the central Iranian complexes. — It is deduced from north Iranian shocks that, in this region, the earthquakes studied are of nearly almost pressural type, and horizontal components of the oblique displacements in foci are sinistral.

Dip angle of faults as calculated from surface deformation

Abstract: Theory shows that the ratio of the absolute displacements of the hanging wall and the foot wall at the fault line is determined by the dip angle for faults intersecting the surface. The ratio is independent of the distribution of stress released at the fault surface and independent of whether motion is strike-slip or dip-slip, except in some cases for which the ratio of length to depth is small. Dip angles calculated from surface displacements for Tango, Idu, Niigata, San Francisco, and Imperial Valley earthquakes were found to be consistent with values estimated from other data. For the Tango and the San Francisco earthquakes, decades elapsed between measurement periods, and the effect of tectonic strain accumulation had to be considered for a correct interpretation of data. A comparison of the rate of strain accumulation with the strain released indicates that about 1000 years was required to build up the strain released in the Tango earthquake, and several hundred years was required for the San Francisco shock.

A gravity survey of a region of the Highland Boundary Fault in Scotland

Abstract: The results of 1591 gravity measurements made in parts of the Midland Valley and the Grampian Highlands of Scotland, flanking the Highland Boundary Fault-zone between Callander and Cowal are described and density units in the Palaeozoic rocks of the area distinguished on the basis of some 800 laboratory measurements. Steep gravity gradients across the Highland Boundary Fault at Callander and Aberfoyle show that its small hade to the north-west, observed at surface, is maintained to a depth of 2400 ft below sea-level, indicating that the fault behaved as a high-angle reversed fault in mid-Devonian times. West of Loch Lomond, the fault continues in general alignment with its course to the east and is inferred to underlie the Carboniferous cover in the Greenock–Inverkip region. The existence of a deep sedimentary basin bounded to the north-west by three step faults and to the south-east by the Highland Boundary Fault is revealed by a gravity low west of Loch Lomond. The basin is probably partly filled by Lower Old Red Sandstone. The maximum thickness of the Lower Old Red Sandstone sediments in the surveyed area is between 5000 and 6000 ft and the formation thins south-eastwards in the region of the Ochil Fault. On the flanks of the Highland Boundary Fault, the survey has provided information of regional importance: the slate-belt probably occupies a downfolded synform within the Dalradian rocks; the Ochil Fault can have only a small magnitude in the surveyed area; the maximum thickness of the Lower Old Red Sandstone sediments in the area is between 5000 and 6000 ft and the formation thins south-eastwards in the region of the Ochil Fault.

Structural geology of the Malvern, Abberley and Ledbury hills

Abstract: The geological structure of the Malvern, Abberley and Ledbury Hills is described in detail. The apparently anomalous association of high- and low-angle reverse faults, vertical faults, high- and low-angle normal faults, wrench faults and isoclinal folding is interpreted as the result of upthrust tectonics related to the differential movement of basement blocks. The western boundary of the Pre-Cambrian outcrop appears to be the outcrop of an upthrust fault, with downthrow to the west, and with a vertical separation as large as 4000 ft. Upthrust faults are characterized by a strongly curved profile, and their geometry is a function of the tectonic level at which they are viewed. The curved fault-planes can be shown to plunge in such a manner that highly complex outcrop patterns emerge. The mechanics of plunging upthrust faults are discussed. All folding adjacent to the major Malvern upthrusts appears to be related to them and important variations in the type and intensity of folding can be discerned in vertical sequence. The Ledbury and Mathon folds appear to have originated as the result of major sinistral wrench movement along the Ledbury and Colwall faults respectively. The eastern boundary of the Pre-Cambrian and Palaeozoic rocks appears to be a fault in most areas, but it is unlikely that it anywhere has a vertical separation in excess of 1000 ft. The presence of the deep Worcestershire basin immediately to the east of the Pre-Cambrian outcrop suggests the existence of a major upthrust fault with downthrow to the east, now covered by Mesozoic sediments. This fault, together with the upthrust fault which forms the western boundary of the Pre-Cambrian outcrop, would have defined the Malvern horst during late Palaeozoic times.

Geology of the Fort Hill quadrangle, Lincoln County, Wyoming

Abstract: The Fort Hill quadrangle lies along the boundary between the Idaho-Wyoming thrust belt and the Green River Basin. A sequence of Paleozoic and Mesozoic sedimentary rocks about 24,000 ft thick has been intensely deformed along the front of the thrust-belt mountain system. Only Cambrian to Ordovician and Triassic to Cretaceous rock units are exposed now within the quadrangle. Middle and upper Paleozoic rocks are covered by partly deformed, but well-exposed, Tertiary strata that help date some of the tectonic events. Drilling within the quandrangle has provided data which help outline the third dimension of structures mapped. Westerly dipping lower Paleozoic to Mesozoic units overrode gently folded Cretaceous rocks eastward along the Hogsback fault. Above the Hogsback fault are 3 other thrust faults, interpreted as slices of the Hogsback fault that cut across the section moderately abruptly. Billions of cubic feet of natural gas and millions of barrels of oil have been produced from stratigraphic traps, in part controlled by structure in the NE. part of the quadrangle. Other natural resources of the area include helium, oil shale, coal, and phosphate. (140 refs.)

Some Characteristics of the Tectonic Stress Pattern in Alaska

Abstract: Summary Horizontal azimuths of tectonic ‘pressure’ are plotted for 38 small and intermediate earthquakes recorded in Central Alaska by a six station network during the period October 1967-September 1968. Two potential mechanisms were considered in evaluating P-wave data from the shocks. The first assumed lateral motion on vertical fault planes, while the second considered the possibility of normal faulting on vertical, or near vertical planes. Although many deeper-than-normal shocks occur in the region under investigation, they do not appear to be consistent with either of these simplified mechanisms. Of the 38 events considered, only one was at a depth significantly in excess of crustal thickness in the region. The results of this study suggest that maximum tectonic pressure is being exerted in a direction normal to the continental margin in the area of Cook Inlet, while block-faulting is occurring in the northernmost portions of the Alaska Range. Between these two areas, on the inside of the sharp bend formed by the Alaska Range, maximum compressive stress is being exerted in a direction parallel to the mountain front, implying that further ‘bending’ of the range may be occurring. This is the condition which would be expected if this portion of the Alaska Range is performing in the manner of a hinge as is stated in Carey's (1956) ‘Alaskan Orocline’ theory of continental drift.