摘要:

The one‐dimensional diffusion accelerated neutral‐particle transport (ONEDANT) code is augmented to explore the effects of gravity on neutron flux spectra near planetary surfaces. The lifetime of the neutron is also explicitly accounted for. The results show a qualitatively new feature in planetary neutron leakage spectra in the form of a component of returning neutrons having kinetic energies less than the gravitational binding energy (0.132 eV for Mars). The net effect is an enhancement in flux at the lowest energies that is largest at and above the outermost layer of planetary matter. This effect diminishes with increasing depth. Fluxes for kinetic energies larger than the gravitational binding potential are minimally changed by gravity. All energy spectra can be well characterized by a model consisting of the super‐position of relatively simple thermal and epithermal functions that are completely specified by four parameters; the thermal and epithermal amplitudes, α and β, respectively; the thermal temperature,Tα; and the epithermal power law exponent,p. Instrumental parameters for the initial version of the neutron mode of the Mars observer gamma ray spectrometer are used to demonstrate the ability to use measured count rates to determine thermal and epithermal am

ISSN:0148-0227    

DOI:10.1029/JB094iB01p00513

年代:1989

数据来源: WILEY

摘要:

Oriented archaeomagnetic samples were collected from 158 in situ features at 33 archaeological sites in the southwestern United States. Seventy‐three independently dated features were used for analysis of secular variation. A moving‐window technique with outlier rejection was developed to compute a smoothed secular variation curve. This technique incorporates weighted Fisher statistics to account for imprecision in both the age of remanence acquisition and the direction of magnetization for each feature. A mean direction and measure of dispersion is generated for each window of time. The secular variation record covers the period A.D. 750–1425, during which time the virtual geomagnetic pole (VGP) moves in a generally counterclockwise fashion from 86°N, 83°E at A.D. 750 to the lowest latitude position of 74°N, 192°E at A.D. 1075, and then to 85°N, 236°E at A.D. 1425. The median rate of VGP movement is 0.036° yr−1, similar to present‐day rates. The Southwestern record shows close correspondence to other North American archaeomagnetic, limnomagnetic, and speleomagnetic records. The archaeomagnetic record can serve as the master curve for the derivation of archa

ISSN:0148-0227    

DOI:10.1029/JB094iB01p00527

年代:1989

数据来源: WILEY

摘要:

Paleomagnetic secular variation (PSV) records have been recovered from three marine sediment cores from Santa Catalina basin, California continental borderland, in order to more accurately date these late Quaternary sediments. The PSV records are not high in resolution due to low sedimentation rates coupled with a 3‐cm sampling interval and some inherent smoothing of the PSV signal during remanence lock‐in. However, the PSV waveforms are sufficiently clear to permit their correlation among the three separate cores. These PSV records can also be correlated with four independent PSV calibration curves from western North America that have detailed radiocarbon age control. The four calibration curves are developed in this paper to improve the dating and regional comparison of PSV records from western North America. The PSV correlations establish time‐depth curves for the three cores which indicate that the sediments are all younger than about 11,000 years B.P. The relative accuracy of the time‐depth curves is approximately 200 years, which represents an order‐of‐magnitude improvement in the chronology of these sediments. Sedimentation rates derived from the three time‐depth curves indicate a constant rate of 20–25 cm/kyr for the last 6700 years throughout Santa Catalina basin, and more variable rates (but constant within each core) of 13–86 cm/kyr prior to 6700 years B.P. In all three cores, the change in sedimentation rate corresponds to a subtle but distinct change in lithology. These changes probably indicate a major shift in paleoceanographic processes within Santa Catalina bas

ISSN:0148-0227    

DOI:10.1029/JB094iB01p00547

年代:1989

数据来源: WILEY

摘要:

The principal finding of this paper is that the far‐field slope has a first‐order effect on model age determinations of scarplike landforms in weakly consolidated terrains. Observationally, this can be demonstrated in two ways using the Lake Bonneville and Lahontan shoreline scarps as separate and combined data sets. Use of the reduced scarp slope, tan θs‐b(where θsis the maximum scarp angle andbis the far‐field or fan slope), instead of tan θsalone as the measure of scarp slope measurably reduces separation between the two data sets induced by different average fan slopes for the two data sets and significantly reduces scatter in the slope‐offset plot for both the separate and combined data sets. Theoretically, the argument can be put even more strongly, at least within the range of linear and nonlinear diffusion models that we consider here together with a mathematical transformation of the empirical approach of R. C. Bucknam and R. E. Anderson: When one correctly takes into account the far‐field slope, one will basically get the same age determination no matter which of these models one uses; conversely, without accounting properly for the effect of far‐field slope, one is virtually guaranteed to get an erroneous age determination, no matter whi

ISSN:0148-0227    

DOI:10.1029/JB094iB01p00565

年代:1989

数据来源: WILEY

摘要:

We have designed a borehole tiltmeter using two horizontal pendulums which have periods of 1 s. We have installed the instruments at seven sites in Colorado and Wyoming to evaluate the secular tilt, the tides, and the coherence between nearby instruments. Using 28 days of data from Boulder, Colorado, the standard deviations of the tidal estimates are 5% for the semidiurnal component M2and 9% for the diurnal component O1. The estimates agree with models that include the body tide, the ocean load, and the topographic correction to better than the estimated uncertainty. Tidal measurements at Erie, Colorado, have larger, possibly nonrandom variability that may be caused by a coupling between the tides and long‐period tilts. The coherence between nearby instruments is ≳0.5 for frequencies ranging from 0.5 to 2 cycles per day, but drops to a much smaller value outside of this range. The secular tilt rate is quite variable and ranges from 0.1 to 1 μrad/yr. Measurements at Erie, Colorado, and in Yellowstone National Park, Wyoming, exhibit an annual or biannual periodicity with an amplitude of about 1 μrad, which is consistent with a 1/f2extrapolation of the diurnal noise power to longer pe

ISSN:0148-0227    

DOI:10.1029/JB094iB01p00574

年代:1989

数据来源: WILEY

摘要:

We have installed borehole tiltmeters at five sites in Yellowstone National Park, Wyoming, and have used these instruments to measure the spatial variation of the amplitude and phase of the principal semidiurnal tide. The measured tides vary both with position and azimuth and differ from the sum of the body tide and the ocean load by up to 50%. The difference predicted by a finite element model constructed from seismic, refraction, and gravity data has a maximum value of only 12%, although the discrepancy between our observations and the model is only marginally significant at some sites. The disagreement between the model and our observations is much larger than we observed using the same instruments at other sites and cannot be attributed to an instrumental effect. We have been unable to modify the model to explain our results while keeping it consistent with the previous observations.

ISSN:0148-0227    

DOI:10.1029/JB094iB01p00587

年代:1989

数据来源: WILEY

摘要:

Improved methods of radiocarbon analysis have enabled us to date more precisely the earthquake ruptures of the San Andreas fault that are recorded in the sediments at Pallett Creek. Previous dates of these events had 95% confidence errors of 50–100 calendar years. New error limits are less than 23 calendar years for all but two of the dated events. This greater precision is due to larger sample size, longer counting time, lower background noise levels, more precise conversion of radiocarbon ages to calendric dates, and better stratigraphic constraints and statistical techniques. The new date ranges, with one exception, fall within the broader ranges estimated previously, but our estimate of the average interval between the latest 10 episodes of faulting is now about 132 years. Variability about the mean interval is much greater than was suspected previously. Five of the nine intervals are shorter than a century; three of the remaining four intervals are about two to three centuries long. Despite the wide range of these intervals, a pattern in the occurrence of large earthquakes at Pallett Creek is apparent in the new data. The past 10 earthquakes occur in four clusters, each of which consists of two or three events. Earthquakes within the clusters are separated by periods of several decades, but the clusters are separated by dormant periods of two to three centuries. This pattern may reflect important mechanical aspects of the fault's behavior. If this pattern continues into the future, the current period of dormancy will probably be greater than two centuries. This would mean that the section of the fault represented by the Pallett Creek site is currently in the middle of one of its longer periods of repose between clusters, and sections of the fault farther to the southeast are much more likely to produce the next great earthquake in California. The greater precision of dates now available for large earthquakes recorded at the Pallett Creek site enables speculative correlation of events between paleoseismic sites along the southern half of the San Andreas fault. A history of great earthquakes with overlapping rupture zones along the Mojave section of the fault remains one of the more attractive possibilitie

ISSN:0148-0227    

DOI:10.1029/JB094iB01p00603

年代:1989

数据来源: WILEY

摘要:

During a seismic reflection survey conducted by the California Consortium for Crustal Studies in the Basin and Range Province west of the Whipple Mountains, SE California, a piggyback experiment was carried out to collect intermediate offset data (12–31 km). These data were obtained by recording the Vibroseis energy with a second, passive recording array, deployed twice at fixed positions at opposite ends of the reflection lines. The reflection midpoints fall into a 3‐km‐wide and 15‐km‐long region in Vidal Valley, roughly parallel to a segment of one of the near‐vertical reflection profiles. This data set makes three unique contributions to the geophysical study of this region. (1) From forward modeling of the observed travel times using ray‐tracing techniques, a shallow layer with velocities ranging from 6.0 to 6.5 km/s was found. This layer dips to the south from 2‐km depth near the Whipple Mountains to a depth of 5‐km in Rice Valley. These depths correspond closely to the westward projection of the Whipple detachment fault, which is exposed 1 km east of the near‐vertical profiles in the Whipple Mountains. (2) On the near‐vertical profile, the reflections from the mylonitically deformed lower plate at upper crustal and mid crustal depths are seen to cease underneath a sedimentary basin in Vidal Valley. However, the piggyback data, which undershoot this basin, show that these reflections are continuous beneath the basin. Thus near‐surface energy transmission problems were responsible for the apparent lateral termination of the reflections on the near‐vertical reflection profile. (3) The areal distribution of the midpoints allows us to construct a quasi‐three‐dimensional image on perpendicular profiles; at the cross points we determined the true strike and dip of reflecting horizons. This analysis shows that the reflections from the mylonitically deformed lower plate dip to the southwest westward of the Whipple Mountains and dip to the south southward of the Turtle Mountains. The results of this study support the interpretation of crustal reflectivity in the near‐vertical reflection profiles to be related to the mid‐Tertiary episode of extension which produced the Whipple metamorphic core complex. This association geometrically suggests a more regionally distributed mechanism for crustal thinning as compared wi

ISSN:0148-0227    

DOI:10.1029/JB094iB01p00625

年代:1989

数据来源: WILEY

摘要:

The procedure of volume averaging is applied to the problem of seismic propagation in a fluid‐filled porous medium. The interplay of temperature variation and the mechanical motion is taken into account to first order. A complete set of equations is obtained, which carry, in addition to permeability, two mechanical parameters and one thermal parameter. When thermomechanical coupling is ignored, a subset of the equations reduces to those of Biot (1956a), allowing a correspondence between the parameters to be establishe

ISSN:0148-0227    

DOI:10.1029/JB094iB01p00637

年代:1989

数据来源: WILEY

摘要:

The low‐frequency nature of the great 1960 Chilean earthquake, the Earth's largest recorded event, is characterized, and a source model composed of three events with a combined seismic moment of 5.5 × 1023N m and a duration of 1500 s is presented. The results show conclusively that large‐scale slip, which is only observed at low frequencies, preceded the main shock. Complex amplitude measurements of normal mode data in the band 1.0–5.0 mHz, recorded at eight vertical component stations, comprise the data set. In this band the earthquake source is assumed to be described by a line source propagating at a constant rupture velocity. The data set is the basis for a sequence of least squares inversions to determine the seismic moment, rupture velocity, spatial slip distribution, and the temporal slip function. The data are best explained by a source model that begins 1150 s before the main shock with an event of moment 1.9 × 1023N m and rise time of 300 s, a main shock with moment of 3.2 × 1023N m and for which most of the low‐frequency energy was released beginning about 50 s after the short‐period origin time, and an event 350 s later with moment of 0.4 × 1023N m. The uncertainty in the moment estimate of the great 1960 Chilean earthquake is about 50% and is due primarily to the probable range of fault plane dip, which is not well constrained

ISSN:0148-0227    

DOI:10.1029/JB094iB01p00643

年代:1989

数据来源: WILEY