Showing papers on "Fault (geology) published in 1974"

Petroleum Traps Associated with Wrench Faults

Abstract: The interaction of three factors results in several different predictable patterns of wrench-fault-related petroleum traps: (1) the evolutionary stage or magnitude of the wrench faulting; (2) the configurations of the laterally moving plates and their orientations to regional movement vectors; and (3) the structural response of the deformed terrane. In the more common fold and fault responses small strike-slip displacements develop narrow trends of en echelon anticlinal culminations which straddle an incipient or underlying wrench fault. Traps commonly are structurally complex. Wrenches with intermediate strike-slip displacement have offset-truncated half-fold culminations and structural bowings closed upplunge by the wrench fault, and less complex intact en echelon antic inal-culmination traps away from the fault. Folds adjacent to many large-displacement wrench faults often are adversely disrupted structurally, or deeply eroded. Potential hydrocarbon traps form downstructure where basinward-plunging en echelon folds cross sedimentary wedges associated with basin margins, and where large anticlinal culminations are preserved. Fault responses to wrenching deformation result in various patterns of en echelon fault traps. Degrees of regional-plate convergence or divergence enhance compressive or extensional structuring, respectively, and further modify prospective structures.

Seismicity and Structure of the Zagros (Iran): The Main Recent Fault between 33 and 35 degrees N

Abstract: The most recent tectonic deformation of the Zagros, and in particular the late Quaternary right-lateral wrench movement along the Main Recent Fault, is summarized in the context of the general tectonic history of the range. The seismicity along the Main Recent Fault between latitudes 33 and 35 degrees N is examined, and details are given for several destructive earthquakes, including the 1909 Selakhor earthquake which was associated with over 40 km of surface faulting along a segment of the Main Recent Fault and which is described here for the first time. The relation between the seismicity and the individual fault segments forming the Main Recent Fault is studied and interpreted in terms of a continuing right-lateral strike slip deformation. The implications of this contemporary deformation for the seismotectonics of the Zagros are considered, and in particular its bearing on the problem of the relative motion of the Arabian Plate with respect to Central Iran.

Regional Geophysics of the Basin and Range Province

Abstract: Nearly one hundred years ago, Gilbert (23, 24) and other geologic pioneers introduced the idea that much of the seelIling jumble of mountains and valleys in western North America was the result of far different processes than fold mountain systems such as the Appalachians or Alps. After a century of geologic and geophysical investigations in the region, it is now generally accepted that the physiography of the Basin and Range province (Figure 1) is one of sculptured and partially buried fault-bounded blocks that have been produced by the extension of the region during late Cenozoic time. Crustal blocks composed of complexly deformed, diverse pre-Cenozoic rocks and relatively undeformed, predominantly nonmarine volcanic rocks of early and middle Cenozoic age have been variously uplifted, tilted, and dropped along numerous normal faults throughout a broad region from Mexico to Canada-from as far west as California and Oregon to as far east as western Texas (e.g. Cook, 13; Gilluly, 27 ; Thompson, 76). The distribution of late Cenozoic normal faults in the western United States is shown on Figure 2 (note that the regional extent of faulting is somewhat larger than the Basin and Range physiographic province of Figure 1). The recent seismicity (Figure 3) shows that small earthquakes are widespread in what Atwater (5) called a wide soft zone accommodating oblique divergence between the Pacific and North American plates. The net effect of fault movements within this region is a crustal extension oriented roughly WNW-ESE. The actual motion on individual faults is quite variable, however, and appears to be controlled by the orientation of faults with respect to this principal extension (Thompson & Burke, 78). In the northern portion of the region -across Nevada and western

Why and where great thrust earthquakes occur along island arcs

Abstract: For many subducting margins of the Pacific there appears to be a characteristic maximum for the source dimensions of large shallow earthquakes The major conclusion of this investigation is that this characteristic maximum for a particular island arc or segment of an arc is strongly influenced by the geometry of the interface zone, particularly the width of the interface between underthrust and overthrust slabs of lithosphere By width of interface we refer to the downdip dimension of the contact zone between the two abutting slabs of lithosphere Maps displaying inferred variations in interface geometry along five major island arcs agree generally with the known locations and extent of rupture during large shallow earthquakes of this century Ruptures of extraordinary length (>400 km) occur near gently dipping slabs of lithosphere that abut over a broad contact zone against the overthrust slab Moderately large earthquakes (maximum rupture length, <150 km) occur along slabs that dip more abruptly and have a thin zone of contact with the overthrust lithosphere If the major conclusion of this study is valid, then detailed knowledge of interface geometry may provide a technique for estimating the extent of the greatest shallow earthquakes likely to occur in an existing seismic gap That is, a narrow width of interface apparently places severe restrictions on the maximum extent of rupture that can occur during an earthquake The extent and the location of ruptures along large strike-slip faults may also be influenced principally by the width of interface (vertical extent of hypocenters for strike-slip faults) Along the two great transform fault systems near western North America (that starting near spreading centers in the Gulf of California and extending northward into the San Andreas system and that starting near the Juan de Fuca spreading center and extending northward into the Queen Charlotte Islands-Fairweather fault system) both the width of interface and the typical dimensions of large earthquakes tend to increase in size with distance from the spreading center

An analysis of strain accumulation on a strike slip fault

Abstract: An analysis of strain accumulation on a strike slip fault is given. The fault between two lithospheric plates is assumed to be locked to a finite depth; owing to plastic flow the fault is free to slide at greater depths. The base of each plate is also a free boundary. The periodic stress accumulation and stress release associated with the elastic rebound hypothesis are modeled. A solution for the two-dimensional stress and strain field near the fault is obtained. It is shown that significant strain accumulation extends to distances of the order of the length of the fault break. The rates of accumulation of stress and strain energy for the San Andreas fault are obtained. Predicted distributions of shear strain are compared with observations. Because of the scatter in the data and possible complexities where observations have been obtained, poor agreement with theory is found.

Parkfield, California, earthquake of June 27, 1966: A three-dimensional moving dislocation

Abstract: Recordings from five strong-motion accelerograph stations have been used to derive a three-dimensional dislocation model for the Parkfield Earthquake. The model consists of a buried fault which extends from a depth of 3 km to a depth of 9 km below the ground surface. It appears from the analysis, which considers various fault lengths, that the zone of significant faulting was the 20-km-long northwestern section of the fault. The rupture velocity has been found to be between 2.4 and 2.5 km/sec and the dislocation amplitudes have been found to be about 120 cm. There have been comparisons made of the model results with geodetic data on static deformations and creep measurements following the event. In contrast with several other source mechanism studies of the Parkfield event, this model yields a picture which appears to be very consistent with both the dynamic strong-motion measurements as well as the available geodetic and creep data.

Near-field observations and source parameters of central California earthquakes

Abstract: This is a study of source characteristics of 13 earthquakes with magnitudes between 2.4 and 5.1 located near the San Andreas fault in central California. On the basis of hypocentral locations and fault-plane solutions the earthquakes separate into two source groups, one group clearly related to the throughgoing northwest-trending San Andreas fault zone and the other apparently associated with generally north-trending bifurcations such as the Calaveras fault. The basic data consist of broad-band recordings (0.03 to 10 Hz) of these earthquakes at two sites of the San Andreas Geophysical Observatory (SAGO). Epicentral distances range between 2 and 40 km, and maximum ground displacements from 4 to 4000 microns were recorded. The whole-record spectra computed from the seismograms lend themselves to source parameter studies in that they can be interpreted in terms of low-frequency level, corner frequency, and high-frequency slope. Synthetic seismograms have also been used to estimate source parameters in both the time domain and frequency domain, and the results compare favorably with those estimated directly from the spectra. The influences of tilts and nonlinear response of the seismometer were considered in the interpretation of the low frequencies. Seismic source moments estimated from the low-frequency levels of the spectra show a linear dependence on magnitude with a slope slightly greater than 1. The geology at the recording site can contribute an uncertainty factor of at least 3 to the estimated moments. Observed corner frequencies are only weakly dependent on magnitude. Interpreted in terms of source dimension, these corner frequencies imply values of 1 to 2 km for the earthquakes of this study. The corner frequencies may also be interpreted in terms of the rise time source function, yielding values in the range 0.5 to 1.0 sec. The data indicate that the earthquakes of this study are all surprisingly similar in their fundamental source parameters, with only the seismic moment showing a strong dependence on magnitude.

Seismo-tectonics of the 1923 kanto earthquake

Abstract: Kanto earthquake of 1923 was associated with various sorts of seismic and tectonic effects such as surface displacements, seismic damages, tsunamis and volcanism. The writer previously proposed a fault-origin model of the earthquake from a geodetic viewpoint. The present paper is to develop the geodetic model on the idea that the various sorts of the associated effects in 1923 can be interpreted with a simple fault-origin model, equally well. The geodetic model is slightly revised to fit the effects. Thus it is found out that the fault model can be much simplified except for the area immediately adjacent to the fault. The fault is tectonically interpreted as one of the transform faults along the northeastern boundary of the Philippine-Sea plate with the Asian plate.

Potentially active faults in dam foundations

Abstract: The Paper contains information on existing dams founded on active faults, a summary of pertinent lessons learned from study of historic fault breaks and fault mechanisms, recommended practice for evaluation of active faults, and opinions concerning design of dams on active faults. While a dam site with an active fault should be avoided if possible, if a reservoir is vitally needed and a better site is not available, it is reasonable practice to construct a conservatively designed embankment dam. Concrete dams on active faults, or near some major active faults, are not advisable. For evaluation of fault activity, geological studies usually must be carried a considerable distance from the dam site, a departure from recent past practice. Experience of the last few years with many fault studies indicates that thorough geological investigations with modern techniques will usually provide sufficient evidence to allow a judgement on the activity or inactivity of a fault.

Generation of tsunami by a fault model

Abstract: Characteristics of tsunami waves caused by a fault model are investigated theoretically in detail. Effect of focal parameters on tsunami amplitudes, relation with the static deformation at the source area, sense of initial motion and directivity are discussed.Main results are summarized as follows: The dip-angle, fault length and focal depth play important roles in the generation of tsunami. As an estimate of the directivity, we define an index α, which is the ratio of the maximum amplitude in the wave train at φ=90°, where φ is measured from a direction along the strike of the fault plane, to that at φ=0°. The index α is strongly affected by the change of the dip-angle and the ratio (fault width)/(fault length). The moving directions of initial motions of tsunami waves at φ=90° and 270° reflect the sense (upheaval or subsidence) of the static deformation but not at φ=0°.

Offset Plutons and History of Movement along the McKinley Segment of the Denali Fault System, Alaska

Abstract: The Foraker and McGonagall plutons, bodies of granodiorite with nearly identical mineralogy and chemistry, are considered to be parts of a single igneous mass that has undergone right-lateral displacement of about 38 km along the McKinley segment of the Denali fault system since the igneous mass crystallized about 38 m.y. ago. These offset plutons place severe constraints on the amount and rate of movement along the McKinley segment since the beginning of the Oligocene Epoch. The 38-km displacement indicates an average rate of 0.1 cm/yr if movement began immediately after crystallization in early Oligocene time or an average rate of 0.4 cm/yr if movement began 10 m.y. ago in late Miocene time. These rates, however, are considerably less than Holocene movement rates measured along the fault further to the east, which suggests that the Holocene displacement rate is greater than the pre-Holocene rate or that right-lateral movement along the fault may diminish to the west.

The karstic groundwater basins of the Kaibab Plateau, Arizona

Abstract: The Kaibab plateau north of the Grand Canyon is characterized by the absence of live surface streams because most of the surplus water drains through the groundwater system. Over 95% of the primary porosity that can be attributed to the 4000-ft sequence of Paleozoic sediments under the plateau occurs in the upper 900 ft of the section. However, almost all the water that discharges from the region exits through perennial karst springs developed in carbonates 3000 ft below the rims of the Grand Canyon and the Marble Canyon. The locations of these springs are fault- and joint-controlled, and they drain between 65 and 97% of the area underlying the Kaibab plateau. The relatively permeable upper Paleozoic rocks in the section collect and transmit water to fault zones where the water enters regional drains dissolved along fractures in the lower carbonates. Flow in the fault zones is largely unconfined and perched above an impermeable shale unit that underlies the lower carbonates. Consequently, the areal extent of the groundwater basins that drain the Kaibab plateau as well as potential saturated zones along the faults can be delineated by using gradients taken from a structural contour map of the top of the shale.

A New Interpretation of the 1968 Fernandina Caldera Collapse and its Implications for the Mid-Oceanic Ridges

Abstract: Summary The earthquake swarm which accompanied the Fernandina Caldera collapse is shown to consist of two different types of earthquake: lowstress events occurring on the main caldera fault and higher stress events on minor faults. The difference in scale of these faults allows the two types of earthquake to be distinguished by magnitude. Events occurring on the main caldera fault have a high ‘b’ value, those on minor faults a normal ‘b’ value. The fall in magma chamber pressure which led to the collapse is shown to have been quite small and hence a model of the volcano is developed which can account for the magnitudes of the earthquakes observed, the stress on the main caldera fault and the seismic efficiency. The seismicity of the rift zone of the Mid-Atlantic Ridge has many similarities with that of the Fernandina collapse and it is proposed that the median valley which is characteristic of slow-spreading ridges is a zone of caldera collapse. No such collapse occurs along the axes of fastspreading ridges, so these are relatively aseismic.

Seismic displacement and ground motion near a fault: The Saitama Earthquake of September 21, 1931

Abstract: The dislocation parameters of the Saitama earthquake (M = 7.0, 36.15°N, 139.24°E) of September 21, 1931, are determined on the basis of first-motion data, aftershock area, and close-in seismograms obtained by a low-magnification long-period seismograph. The earthquake represents a left-lateral strike slip faulting on a plane dipping 80° toward N 196°E with dimensions of 20 km (length) × 10 km (width). The strike of the fault plane is found to be almost parallel to that of the eastern extension of the median tectonic line. A synthetic study suggests that the rupture grows bilaterally at a velocity of 2.3 km/s. The rise time, the final dislocation of the linear ramp dislocation time function, and the particle velocity of the fault dislocation are estimated to be 2 s, 100 cm, and 50 cm/s, respectively. The near-source ground displacements and ground motions derived from the above seismic fault model are consistent with high-precision leveling data and with the field survey that determined the directions of collapse of structures.

Geology and petrochemistry of weakly metamorphosed rocks in the upper Wakatipu district, southern New Zealand

Abstract: The area near the head of Lake Wakatipu lies between the Livingstone Fault to the west, and the Otago Schist terrane to the east. The area is divided into three major tectonic slices or subareas by north-striking faults. A eastward-younging sequence of Caples Group strata in the western-subarea is subdivided into four informal units consisting predominantly of volcanogenic sediments, feldspathic sandstone, tuff and tuffaceous sandstone, and grey sandstone and slate respectively. The Greenstone Ultramafite Belt, which separates the western subarea from the middle subarea, is interpreted as a tectonic melange of serpentinite, metagabbro, mafic volcanic and meta-sedimentary rocks. The synclinal Bold Peak unit, predominantly rhythmically alternating green sandstone and slate, occupies the middle subarea and is separated by an important inferred fault, the West Wakatipu Fault, from the eastern subarea which represents the western limit of the Haast Schist of Central Otago. Five metamorphic zones are r...

Tectonic Elements of the Central Part of the Gulf of California

Abstract: Geophysical data from closely spaced ship tracks in the central Gulf of California delineate tectonic features associated with the Pacific–North America plate boundary. Three en echelon fracture zones extend southeast from Delfin Basin through Salsipuedes Channel to the northern part of San Pedro Martir Basin, southwest from the southern part of San Pedro Martir Basin to the northeastern flank of Guaymas Basin, and southeast from Guaymas Basin. Large magnetic anomalies and abundant volcanism are associated with segments of these faults. The faults are interpreted to represent a transform fault zone here designated as the “Guaymas fault zone.” Active spreading is taking place within the Guaymas and possibly the San Pedro Martir Basins, although patterns of sediment distribution within these basins preclude a model of simple stationary positions of active spreading centers for more than a few tens of thousands of years at a time.

Seismicity and structure of the Kopet Dagh (Iran, U.S.S.R.)

Abstract: The region considered under the general term 'Kopet Dagh' is located east of the Caspian Sea, and includes Northeast Iran and southern Soviet Turkmenia. The regional tectonics are reviewed with special emphasis on the post-Alpine 'Diagonal Fault System'. The seismicity over the last 100 years is studied, and the four strongest earthquakes in Northeast Iran, i.e. 1871/2, 1893 and 1895 Quchan and 1929 Baghan-Germab, are described in detail for the first time on the basis of new bibliographical and field data. These four earthquakes were located on the NNW-SSE 'Bakharden-Quchan Zone', which forms part of the Diagonal Fault System. The 1929 earthquake in particular was accompanied by a surface fracture over 50 km long caused by reactivation of one of the faults of this Zone. Russian work on the seismotectonic aspects of the 1948 Ashkhabad earthquake which occurred in an adjoining zone, and migration of seismic activity in the Kopet Dagh since about 1870, are examined. The overall seismotectonics of the Kopet Dagh are interpreted in terms of an eastern 'NNW trend' which is separated by a longitudinal zone of relative quiescence near 56-57 degrees E from a western 'NNE trend'. Active surface structures throughout the region are on average consistent with a tectonic model based on a NNE motion of Iran with respect to the Turan Plate. Many tectonic features are characteristic of the margins of converging continental plates.

Seismicity, earthquake mechanisms, and tectonics along the western coast of North America, from 42°N to 61°N

Abstract: Seismicity maps of the region are presented for the periods January 1900 to June 1973 and January 1961 to June 1973. Focal mechanism solutions for 24 earthquakes ( m b ≧ 5.5) occurring between January 1963 to July 1973 have been determined. Four earthquakes which occurred in the vicinity of the Gorda ridge have a component of normal faulting, thus identifying it as a spreading ridge. However, departure of the trend of the axis of tension, determined in the focal mechanism solutions for these earthquakes, from being perpendicular to the trend of the ridge axis indicates that the tectonics of the Gorda basin is complex. The existence of the Blanco fracture zone and the Queen Charlotte Islands fault as dextral transform faults, east block moving south, is confirmed. Evidences derived from the focal mechanism solutions indicate that the Revere-Dellwood and Sovanco fracture zones are also right-lateral transform faults. The northern extent of the Queen Charlotte Islands fault probably terminates near 54.5°N, 135.2°W or 57.5°N, 136°W. Focal mechanism solutions for an earthquake occurring near Vancouver Islands and another near the Puget Sound region in Washington indicate normal faulting and suggest the presence of a downgoing slab of lithosphere in this region. The interpretation is that the former results from tension caused by the bending of the plate as it dips under the continent and the latter results from tension within the sinking slab. Two recent earthquakes which occurred on July 1 and July 3, 1973 at a latitude of about 58°N, have thrust fault solutions and together with the seismicity data suggest a zone of underthrusting of the lithospheric plate between latitudes 58°N to 59.5°N. The possibility of a third zone of subduction north of latitude 59.5°N between longitudes 139°W to 142°W is indicated.

Fault parameters determined by near- and far-field data: The Wakasa Bay earthquake of March 26, 1963

Abstract: The source process of the Wakasa Bay earthquake ( M = 6.9, 35.80°N, 135.76°E, depth 4 km) which occurred near the west coast of Honshu Island, Japan, on March 26, 1963, is studied on the basis of the seismological data. Dynamic and static parameters of the faulting are determined by directly comparing synthetic seismograms with observed seismograms recorded at seismic near and far distances. The De Hoop-Haskell method is used for the synthesis. The average dislocation is determined to be 60 cm. The overall dislocation velocity is estimated to be 30 cm/sec, the rise time of the slip dislocation being determined as 2 sec. The other fault parameters determined, with supplementary data on the P -wave first motion, the S -wave polarization angle, and the aftershocks, are: source geometry, dip direction N 144°E, dip angle 68°, slip angle 22° (right-lateral strike-slip motion with some dip-slip component); fault dimension, 20 km length by 8 km width; rupture velocity, 2.3 km/sec (bilateral); seismic moment, 3.3 × 1025 dyne-cm; stress drop, 32 bars. The effective stress available to accelerate the fault motion is estimated to be about 40 bars. The approximate agreement between the effective stress and the stress drop suggests that most of the effective stress was released at the time of the earthquake.

Structural Analysis of the Snake Range ‘Décollement,’ East-Central Nevada

Abstract: Minor structures associated with the Snake Range “decollement” at six sites in east-central Nevada are used to infer direction of movement of rocks above the “decollement” with respect to rocks below. Analysis indicates latest movement varied at each site and was down the present dip of the “decollement.” The directions inferred range from westward through southward to eastward. When combined with evidence of Tertiary age for latest movement on the “decollement,” the data support a denudation fault model rather than regional Mesozoic decollement models. This suggests that the tectonic pattern may have more to do with post-Eocene evolution of the Great Basin ignimbrite-block fault province than with the Mesozoic Sevier orogenic belt.

Two-dimensional kinematic fault modeling of the Pacoima Dam strong-motion recordings of the February 9, 1971, San Fernando earthquake

Abstract: A simple, uncontrived model of the rupture during the San Feruando earthquake can explain the main features of the particle velocity traces derived from the accelerograms recorded at Pacoima Dam. This result, combined with the probable small effect of surface topography on the velocity traces, strengthens the case for acceptance of the peak particle velocity at Pacoima Dam (115 em/sec) as a valid ground-motion parameter for design purposes ill earthquake engineering. Most of the conspicuous motion on the velocity traces during the first 4 sec after triggering seems to result from thrust faulting, starting at a focal depth within several kilometers of 14 km, on a fault surface dipping at least 50 ° and extending only part way to the surface at a velocity near 2.5 km/sec. The data also indicate that this faulting continued to the surface at a slower rupture velocity (less than 2 km/sec) along a less steeply dipping surface. The amount of relative offset across the fault surface is difficult to determine, both because of inherent limitations in the two-dimensional model and because of nouuniqueness in the fitting of the data. The estimates of this dislocation, however, are consistent with the wide range of values reported by other authors in studies using various types of data. The data are also consistent with a model suggested by Alewine and Jordan (1973) and Trifunac (1974) in which the total dislocation has a minimum near the center of the fault surface, with approximately equal amounts of total offset on the fault near the hypocenter and near the Earth's surface.

Relation between P- and S-wave corner frequencies in the seismic spectrum

Abstract: A comprehensive set of body-wave spectra has been calculated for the Haskell fault model generalized to a circular fault surface. These spectra are used to show that in practice the P -wave corner frequency ( ƒ p ) may exceed the S -wave corner frequency ( ƒ s ) when near-sonic or transonic rupture propagation obtains. The explanation appears to be that in such cases ƒ s is so large that it is not identified within the recorded band, but rather a secondary corner is mistaken for ƒ s . As a consequence of failing to detect the true asymptotic trend, the high-frequency falloff of the spectrum with frequency is substantially less for S waves than for P waves. This explanation appears to be consistent with the demonstration by Molnar, Tucker, and Brune (1973) that ƒ p may exceed ƒ s .

Vertical Crustal Movement in Iceland

Abstract: The rift zones of Iceland are subsiding at relatively constant rate. The absolute subsidence has not been determined but the rate of tilt of the flanks of the rift zones is 0.2 to 0.7 microradians per year and the width of the zone that is being tilted appears to be more than 40 kilometers. It is estimated that the central part of the rift zone is subsiding 0.5 to 1.0 centimeter per year relative to surrounding areas. The ground deformation associated with this subsidence is characterized by gentle bending of the earth’s crust with no measureable fault displacement except during earthquakes and possibly during a period of one or two years after earthquakes.

Magnetic Anomalies in the Northern and Central Gulf of California

Abstract: Magnetic anomalies found in the northern and central Gulf of California do not resemble typical sea-floor spreading anomalies, but are consistent with the existence of spreading centers in this region. The magnetic anomalies appear to delineate the extension of the San Jacinto fault into the Gulf of California and some of the transform faults that connect the inferred spreading centers within the gulf.

Surface wave study of earthquakes near northwestern Hudson Bay, Canada

Abstract: Source parameters of an earthquake in northwestern Hudson Bay that occurred on October 2, 1971, with magnitude mb = 5.0 were determined primarily by using surface waves. An estimate of focal depth of 21 km, which in this particular case was insensitive to minor variation in azimuth and focal mechanism, was determined mainly by the Rayleigh wave amplitude spectrums at periods between 7 and 20 s. The focal mechanism solution determined from Love and Rayleigh wave amplitude and phase spectrums in conjunction with P initial motions suggests a thrust fault type with maximum compressional direction nearly NE-SE. The maximum possible fault length was estimated to be 7 km by examining the Rayleigh amplitude spectral variation with azimuth at periods less than 20 s. Another earthquake in the same region, mb = 4.5, on May 14, 1964, was estimated to have a similar focal mechanism with a focal depth of possibly about 17 km. An implied tectonic map of eastern North America is given in Figure 15. High Q values for periods between 7 and 20 s for Rayleigh waves were found also from Hudson Bay earthquakes, in agreement with a similar result for Love waves from earthquakes on Baffin Island.

Evidence for postearthquake slip in the Fairview Peak, Dixie Valley, and Rainbow Mountain fault areas of Nevada

Abstract: The U.S. Coast and Geodetic Survey ran first-order level surveys in 1934, 1955, and 1967 across the fault zone associated with the 1954 sequence of earthquakes at Rainbow Mountain (M = 6.6 and 6.8), Dixie Valley (M = 6.8), and Fairview Peak (M = 7.1) in Nevada. The difference between the 1955 and 1967 surveys clearly shows distinct anomalies over distances of several kilometers at all but one of the fault scarps mapped after the 1954 earthquakes. The anomalies resemble deformation produced by normal faulting extending to a depth of at least several kilometers and, consequently, are interpreted as implying continued slip on the earthquake faults in a period beginning at least 6 months after the earthquake. At the Fairview Peak Fault, the inferred postearthquake slip is about 5 per cent of the displacement observed at the time of the earthquake. The difference between the 1955 and 1967 surveys suggests an overall tilt of 2 mm/km down to the west extending over a 90-km distance crossing the fault zone. The difference between the 1934 and 1955 surveys suggests an overall tilt of 0.8 mm/km down to the east extending over a 200-km section. However, these regional tilts might be due to unusually large systematic errors in the level surveys. Gravity and seismic-refraction surveys indicate that the region as a whole is isostatically compensated, although the mountain ranges and intervening basins are not individually compensated. Thus, the regional elevation changes, if they exist, cannot be accounted for by isostasy.