摘要:

An experiment was performed to test Newton's inverse square law of gravitation. The test compared accurately measured gravity values along a 600‐m tower with upward continued gravity estimates calculated from ground measurements using algorithms based on Newtonian potential theory. Gravity measurements with an internal consistency of better than 20 μGal (1 μGal = 10−8m/s2) were made on the tower at intervals of about 90 m using a single LaCoste‐Romberg model G gravimeter. Field measurements of gravity (using the same instrument) and corresponding geodetic positions were performed at 77 points within a radius of 5 km from the tower. These and other gravimetric data within a radius of about 220 km were used to perform the upward continuation using two independent computational techniques. One is least squares collocation, which is based on a statistical interpretation of the local gravity field, and the other determines a local harmonic expansion of the field from the given data. The upward continuation from both methods gave almost identical results and is estimated to have an uncertainty comparable to the in situ measurements. The results of the experiment show a substantial departure from the inverse square law, asymptotically approaching about 500 μGal at the top of the tower, with a standard deviation of better than 40 μGal. This indicates that there is a non‐Newtonian attractive force which falls off rapidly wit

ISSN:0148-0227    

DOI:10.1029/JB094iB02p01563

年代:1989

数据来源: WILEY

摘要:

Color images of bright red dust deposits at the Mutch Memorial Station were acquired at variable incidence angles during sol 611 (subsolar longitude ∼70°, northern spring season). After removing effects due to atmospheric scattering and absorption, the data were used to estimate the independent variables in the Hapke (1986) photometric function. In blue, green, and red coordinates the vector representing the space radiance factor of the landing site extracted from Viking orbiter images acquired on sol 609 is separated by a Euclidean distance of only 0.022 units and an angle of only 1.5° from the vector estimated from the station data for the orbiter lighting and viewing geometries. This result implies that light reflected from dust exposures dominates the orbiter signal; multiplicative and additive atmospheric terms cancel one another and surface roughness is a second‐order effect in the orbiter data. Dust radiance factors computed from station data are most like laboratory spectra for fine‐grained Hawaiian palagonite and are indistinguishable from Earth‐based spectra of classical bright areas. Color composites of orbiter images show that the dust is found immediately south of Acidalia Planitia and in association with topographic barriers such as craters and cliffs. Examination of Viking infrared thermal mapper data shows that the dust deposits typically do not have distinctive thermal inertia signatures, implying that the deposits are optically thick (hundreds of micrometers) but thinner than the diurnal thermal skin depth (centimeters). Dark gray material with thermal inertia values (8–12 × 10−3cal cm−2s−1/2K−1) indicative of sand grain sizes (0.5–1.5 mm) dominates the Acidalia Planitia lowlands and parts of Kasei Vallis. This material also occurs as dark streaks extending from craters in Xanthe Terra and Oxia Palus. Space radiance factors of dark gray material are similar to spectra of mafic rock mixed with a minor amount of palagonitelike material. Material that is darker than but just as red as the dust deposits occurs in Lunae Planum, Xanthe Terra, and Oxia Palus, areas of intermediate elevation. Multiple phase angle orbiter images suggest that the dark red exposures are a mixture of bright red and dark gray materials, with the dark red exposures being relatively rough at a subpixel scale as compared to exposures of the other two materials. Thus only two types of materials can be detected in the data covering the study area: palagonitelike dust and mafic rock fragments. Thermal inertia values (4–8 × 10−3cal cm−2s−1/2K−1) for the dark red material are consistent with the presence of fine to medium sand size particles (0.10–0.5 mm); such material should be easily moved by winds. The lack of aeolian features implies that the dark red deposits are not composed of loose material. Rather, they are probably more eroded versions of blocky soil or duricrust (cemented dust and rock fragments) exposed at the station. Both bright red dust (aeolian suspension load) and dark gray materials (saltation, traction loads) migrate over the dark red substrate. The overall distribution of the surficial units is controlled by topography at a variety of length scales. Lower wind threshold friction velocities associated with higher atmospheric densities in lowlands keep dust from accumulating and duricrust from forming, while higher threshold velocities in highlands lead to net accumulation of dust. Local topographic obstacles (craters, walls, ridges) perturb wind flow and lead to local accumulation or erosion at a variety of elevations. In addition, the bright red dust ubiquitously found between exposures of dark gray and dark red materials may accumulate as the wind velocity gradient decreases at the transition from smoother dark gray exposures to rougher dark red exposures. The distribution of the materials must also be modulated by climatic variations induced by quasi‐periodic oscillations in obliquity, eccentricity and spin axis direction, constraining

ISSN:0148-0227    

DOI:10.1029/JB094iB02p01573

年代:1989

数据来源: WILEY

摘要:

The Intermountain and Idaho seismic belts within Idaho, Wyoming, and Montana form an unusual parabolic pattern about the axis of the aseismic eastern Snake River Plain (SRP). This pattern is also reflected in the distribution of latest Quaternary normal faults. Several late Cenozoic normal faults that trend perpendicular to the axis of the eastern SRP extend from the aseismic region to the region of latest Quaternary faulting and seismicity. A study of the late Miocene to Holocene displacement history of one of these, the Grand Valley fault system in southeastern Idaho and western Wyoming, indicates that a locus of high displacement rates has migrated away from the eastern SRP to its present location in southern Star Valley in western Wyoming. In Swan Valley the studied area closest to the eastern SRP, isotopic ages, and paleomagnetic data for over 300 samples from 47 sites on well‐exposed late Cenozoic volcanic rocks (the tuff of Spring Creek, the tuff of Heise, the Huckleberry Ridge tuff, the Pine Creek Basalt, and an older tuff thought to be the tuff of Cosgrove Road) are used to demonstrate differences in the displacement rate on the Grand Valley fault over the last ∼10 m.y. Tectonic tilts for these volcanic rocks are estimated by comparing the results of paleomagnetic analyses in Swan Valley to similar analyses of samples from undeformed volcanic rocks outside of Swan Valley. Basin geometry and tilt axes are established using seismic reflection profiles and field mapping. Combining these data with the tilt data makes it possible to calculate displacement rates during discrete temporal intervals. An average displacement rate of ∼1.8 mm/yr is calculated for the Grand Valley fault in Swan Valley between 4.4 and 2.0 Ma. In the subsequent 2.0‐m.y. interval the rate dropped 2 orders of magnitude to ∼0.014 mm/yr; during the preceding 5.5‐m.y. interval the displacement rate is ∼0.15 mm/yr, or about 1 order of magnitude less than the rate between 4.4 and 2.0 Ma. Mapping of fault scarps and unfaulted deposits along the Grand Valley fault system shows that latest Quaternary fault scarps are restricted to the portion farthest from the eastern SRP, the southern part of the Star Valley fault. Surface displacements estimated from scarp profiles and deposit ages estimated from soil development suggest a latest Quaternary displacement rate of 0.6–1.2 mm/yr for the southern portion of the Star Valley fault. Morphologic evidence suggests that this displacement rate persisted on the Star Valley fault throughout most of the Quaternary. The latest Quaternary displacement rate calculated for the southern portion of the Star Valley fault is similar to the rate calculated for Swan Valley during the interval from 2.0 to 4.4 Ma. This similarity, together with evidence for a low Quaternary displacement rate on the fault system in Swan Valley, suggests that the location of the highest displacement rate has migrated away from the eastern SRP. Other normal faults in southeastern Idaho, northwestern Wyoming, and southwestern Montana, while less well described than the Grand Valley fault system, exhibit a similar outward migrating pattern of increased fault activity followed by quiescence. Furthermore, a temporal and spatial relationship between fault activity and the 3.5 cm/yr northeastward track of the Yellowstone hotspot is observable on the Grand Valley fault system and on other north‐northwest trending late Cenozoic faults that border the eastern SRP. The temporal and spatial relationship of Miocene to present high displacement rates for other circumeastern SRP faults and the observable outwardly migrating pattern of fault activity suggest that a similar parabolic distribution of seismicity and high displacement rates was symmetrically positioned about the former position of the hotspot. Moreover, the tandem migration of the hotspot and the parabolic distribution of increased fault activity and seismicity are closely followed by a parabolic‐shaped “collapse shadow,” or region of fault inactivity and aseismicity. We suggest that the outwardly migrating pattern of increased fault activity (active region) results from reduced integrated lithospheric strength caused by thermal effects of the hotspot. Conversely, the outwardly propagating quiescent region is the result of a reduction or “collapse” of crustal extension rates caused by increased integrated lithospheric strength. Lithospheric strength in this region is increased by addition of mafic materials at the base of the crust and at midcrustal levels. Although the strength of the mantle portion of the lithosphere is reduced, the increased strength of the crust results in a total integrated increase in lithospheric strength. Paradoxically, the surface heat flow data suggest that the region within the interior parabola has a higher heat flow (after accounting for the cooling effects of the eastern SRP aquifer) than the adjacent regions, yet the interior region exhibits significantly lower extension rates. It appears that in this region the surface heat flow is not a good predictor of rate

ISSN:0148-0227    

DOI:10.1029/JB094iB02p01589

年代:1989

数据来源: WILEY

摘要:

The accuracy of the thermoluminescence (TL) method for dating Holocene paleoearthquakes was tested on fine‐grained sediments from three trenches excavated across the American Fork segment of the Wasatch fault zone, Utah. Sediments exposed in the trenches targeted for TL dating include A horizons of buried soils, loessial silts, and sag pond muds. This study indicates that these sediments were fully light‐bleached prior to deposition and are suitable for TL dating. Equivalent dose estimates by the regeneration, total‐ and partial‐bleach methods are in general agreement, and mean TL age estimates are concordant with radiocarbon ages. Combined radiocarbon and TL age estimates indicate three major episodes of faulting at 5.3±0.3, 2.6±0.3 and 0.5±0

ISSN:0148-0227    

DOI:10.1029/JB094iB02p01622

年代:1989

数据来源: WILEY

摘要:

A detailed analysis of near‐source strong motion and leveling data, together with the results of teleseismic waveform modeling by Westaway and Jackson (1987) and aftershock studies by Deschamps and King (1984), allows a satisfactory kinematic description of the complex normal faulting associated with the magnitudeMs= 6.9, November 23,1980, Irpinia earthquake (southern Italy). The three main rupture episodes, starting at about 0, 20 and 40 s, are here associated with better constrained source parameters than in previous studies. We first evaluated the triggering time for the 11 closest accelerometers by using the well‐recorded 40‐s strong motion phases. This gave the absolute time and location of the dominant episodes of faulting. The first rupture propagated mainly toward the northwest on a NE dipping normal fault, at a mean velocity close to 3 km/s for about 20 km, and continued 15 km further on smaller subfaults. It was associated with surface breakage in the southeastern part. The second rupture started from the southeastern end of the first rupture, about 18 s after it, and propagated about 20 km toward the southeast on a low‐angle normal fault dipping 20°NE. It was associated with secondary faulting on steeper planes reaching the surface. The third and last episode at 39 s nucleated near the first hypocenter, at shallower depth, and the rupture possibly propagated on a 10‐ to 15‐km‐long normal fault, striking SE, antithetic to the first activated fault. A clear correlation appears between the strength of the geological formations and the existence of surface breakage and shallow after

ISSN:0148-0227    

DOI:10.1029/JB094iB02p01631

年代:1989

数据来源: WILEY

摘要:

On the basis of Akaike's Bayesian information criterion (ABIC), a new method is proposed for detecting a temporal change in a seismic velocity in a source region. The method of joint hypocenter determination was modified in order to determine a seismic velocity in a source layer as a function of time together with hypocenters and station corrections. Arrival times of initial waves of shallow earthquakes in a small area are analyzed in this method. The smoothness of the estimated temporal variation in the velocity is guaranteed by the introduction of a prior distribution of the parameter. The hyperparameter of the prior distribution of the velocity, the reading error of arrival times, and the initial velocity in the source layer are chosen to minimize ABIC. This procedure was applied to the 1983 eastern YamanashiM= 6.0 earthquake in central Japan. We analyzedParrival times of 374 earthquakes observed at 12 stations in the network of the National Research Center for Disaster Prevention by dividing the whole period (from October 1981 to May 1987) into 12 six‐month subperiods. Calculating ABICs for different combinations of the three parameters above, we searched for the minimum value of ABIC and found two minima. The first one corresponds to a model of a constant velocity in time, and the other corresponds to a model of a variable velocity with 5% velocity change at maximum. However, since ABIC in the former is 10 smaller than that in the latter, the former constant velocity model is statistically more suitable than the latter. Furthermore, generating artificial data with the same reading errors as the actual data, we used computer simulation to examine the lower limit of the velocity change detectable for this data set. In conclusion, the velocity in the source region is 6.24±0.18 km/s, and the velocity change exceeding 6–7% at maximum did not exist during the 6 years before and after theM= 6.0 earthq

ISSN:0148-0227    

DOI:10.1029/JB094iB02p01649

年代:1989

数据来源: WILEY

摘要:

Several measurements of vertical ground motion at Piñon Flat Observatory, California, indicate the overall weakness and instability of the Earth's weathered surface with respect to the underlying rock. Cumulative long‐period motions of order 0.5 mm per year dominate these records, though smaller elastic deformations caused by precipitation loading, atmospheric loading, and tidal strains are evident at higher frequencies; all of these help to characterize the near‐surface material. The long‐period records suggest that near‐surface weathering is the dominant influence on monument motion, at least at this site on crystalline rock in a semi‐arid environment. Rainfall loading gives an average vertical modulus of 2.6 GPa for the material in the uppermost 26 m of the ground, compared with 88 GPa for granite under moderate confining stress; atmospheric loading gives similar results but indicates the ground is permeable to airflow at periods longer than a few hours. Earth tide records show Poisson's ratio to be 0.09 in contrast to the normative range of 0.2–0.25, establishing that horizontal strains couple only weakly into vertical ones, so that vertical strains near the surface are a poor measure of areal strain. The form of the ground‐surface displacement power spectrum indicates that analyses of geodetic surveys would be improved with the inclusion of a monument‐positioning error budget that increases with time. Because of ground instability, and the generally small rate of crustal tectonic motions, deeply emplaced monuments will be needed for observational programs designed to detect short‐term changes in crustal deformation over baselengths of or

ISSN:0148-0227    

DOI:10.1029/JB094iB02p01655

年代:1989

数据来源: WILEY

摘要:

The present study is an attempt to model abyssal hill topography, i.e., the randomly varying topography that is superimposed on the subsidence of the ocean floor due to thermal contraction. We assume that abyssal hill topography is created by faulting, and that it can be represented as the convolution of a series of stochastic fault offsets with a fault impulse response. As a constraint, the power spectrum of the model‐generated topography will have to resemble the power spectrum of a profile of abyssal hill topography across a typical slow spreading ridge such as found in the South Atlantic. Using a series of fault offsets positively correlated in space and fault impulse responses limited to a finite widthw, profiles that statistically resemble abyssal hill topography have been generated. If the fault offsets are constrained to be positive, i.e., to cause uplift, the synthetic profiles contain a steady state depression at the axis. The fit of the synthetically derived topography to the observed topography is very sensitive to the widthwof the fault impulse response. Our results suggest that the effects of a faulting event must be limited to distances of about 3.5 km and that tectonic activity must take place in the immediate vicinity of the inner wall of the axial valley. If faulting is indeed the process that creates abyssal hill topography, the best fit flexural impulse response to faulting implies a flexural rigidity of 5 × 1017N m. If instead constructional volcanism contributes significantly to the topography, our study suggests that the emplacement of volcanic constructions must also take place within a narrow width w about the ax

ISSN:0148-0227    

DOI:10.1029/JB094iB02p01665

年代:1989

数据来源: WILEY

摘要:

Mantle xenoliths from western Saudi Arabia, some 220 km east of the Red Sea margin, indicate temperatures as high as 900°C in the mantle at about 40 km depth beneath an area of the Arabian Shield where surface heat flow is lower than the world average. This suggests that there has been a large increase in the mantle temperature that has not had time to equilibrate at the surface. The amount of uplift and the difference between the observed xenolith temperature and that obtained by projecting the surface heat flow to depth suggests that the lithosphere has been thinned by 30 to 100 km to its present calculated thickness of about 80 km. Calculations of the time necessary for thermal reequilibration indicate that the lithospheric thinning probably occurred less than 20 m.y. ago, so it postdates rifting which started about 30 Ma. If there was a prolonged upwelling of deep mantle material, resulting in a thinner lithosphere prior to rifting, there should have been enough time for some reequilibration at the surface, and the observed heat flow should be higher over the broad area of uplift adjacent to the Red Sea. Our calculations are consistent with a postrift age of uplift adjacent to the Red Sea as determined from regional geologic evidence and fission track ages on apatites from shield rocks. The Red Sea appears to have begun with alkaline volcanism on the Arabian Shield, followed by rifting. Uplift and lithospheric thinning adjacent to the Red Sea rift began 5–10 m. y. after rifting. This suggests that the Red Sea is a passive rift that formed because of two‐dimensional plate motions, rather than as a consequence of mantle upwelling. A possible cause for the stress in the plates might have been the change of plate motions in the Indian Ocean that resulted from the collision of India with Eur

ISSN:0148-0227    

DOI:10.1029/JB094iB02p01677

年代:1989

数据来源: WILEY

摘要:

Prior to the formation of the Red Sea the northeastern Afro/Arabian continent had low relief and was largely below sea level from the Late Cretaceous to the early Oligocene. The events leading to the formation of the Red Sea followed the sequence (1) alkaline volcanism and rifting beginning about 30–32 Ma affecting a narrow linear zone in the continent, (2) rotational block faulting and detachment faulting, well underway by 25 Ma, (3) gabbro and diorite magmatism, andesite to rhyolite volcanism, and fine‐grained nonmarine sedimentation in the rift between 20 and 25 Ma, (4) fine‐grained marine sedimentation in the rift as the early shelves started to subside in the middle Miocene, and (5) uplift of the adjacent continents (about 3 km) and subsidence of the shelves (about 4 km) between 13.8 and 5 Ma. The youth of the uplift is suggested by 44 fission track dates on apatites from rocks of the Proterozoic Arabian Shield that range in age from 13.8 to 568 Ma. The youngest of these ages, coupled with the present high relief along the Arabian escarpment and published heat flow measurements, indicate that 2.5–4 km uplift has occurred in the last 13.8 m.y. The sequence volcanism/rifting followed by uplift leads to our adoption of a passive mantle model for rift origin. Models that require uplift to create the rift are rejected, because of the late uplift. We advocate a model of lithospheric extension caused by two‐dimensional plate stress over those requiring tractional drag at the base of the lithosphere caused by vigorous flow in the asthenosphere. It is acknowledged that traction models could explain the observed data, but they imply a rigid, static lithosphere and seem to require a link between the direction of flow in the asthenosphere and plate motions. Neither requirement is necessary in the extension model. The rift starts with mechanical extension in a narrow zone of lithosphere between 25–32 Ma in our model. The thinned lithosphere is replaced by upwelling asthenosphere and by rocks from the adjacent deep continental lithosphere which flow into the rift. Ductile flow of the deep continental lithosphere is accelerated by partial melting as rocks flow upward toward the rift axis. Once partially melted, rocks formerly part of the continental lithosphere join the upwelling asthenosphere, resulting in a rapid erosion of the lithospheric mantle beneath the continent near the rift edge. The resulting density decrease explains the uplift. We think that the Red Sea began as a consequence of changing plate geometries resulting from the collision of India and Eurasia. After the collision, the segment of the Owens fracture zone north of the Carlsberg Ridge became locked, forcing the northeast corner of Afro/Arabia to rotate with the Indian plate away from the res

ISSN:0148-0227    

DOI:10.1029/JB094iB02p01683

年代:1989

数据来源: WILEY