摘要:

Daily estimates of the Earth's pole position have been obtained with measurements from a worldwide network of Global Positioning System (GPS) receivers, obtained during the three week GIG'91 experiment in January–February, 1991. For this short‐term study, the GPS based polar motion series agrees with the other space based geodetic techniques (Very Long Baseline Interferometry and Satellite Laser Ranging) to ∼0.4 mas rms, after the removal of mean biases of order 1–3 mas. The small error in day‐to‐day variability is not sensitive to the fiducial strategy used, nor are fiducial sites even necessary for monitoring high frequency pole position variability. The small biases indicate that the applied reference frames of the three geodetic techniques are nearly aligned, that the GPS fiducial errors are small, and that systematic errors in GPS are also small (of order 5 ppb). A well determined reference frame is necessary for monitoring the long‐term stability of polar motion and for separating it from other long‐term signals such as tectonic motion and internal sy

ISSN:0094-8276    

DOI:10.1029/92GL00576

年代:1992

数据来源: WILEY

摘要:

Daily determinations of the Earth's pole position from Global Positioning System (GPS) observations acquired during the first GPS IERS and Geodynamics (GIG) experiment held during January 22, 1991 to February 13, 1991 are interpreted in terms of atmospheric wind and pressure fluctuations as parameterized by the atmospheric angular momentum (AAM) χ‐functions. All available AAM χ‐functions spanning the GIG '91 period have been collected and compared to the observations in the time and polar motion domains. The greatest level of agreement is obtained using the total (wind plus pressure) χ‐functions from the Japanese Meteorological Agency (JMA) where the pressure term is computed by assuming the inverted barometer approximation. In this case, the correlation coefficient between the observed and JMA‐induced polar motion series is 0.91 for the x‐component and 0.72 for the y‐component, the rms of the difference between the observed and JMA‐induced series is 0.71 mas for thex‐component and 0.69 mas for the y‐component, and the percentage of the variance of the observed series explained by the JMA‐induced series is 83% for the x‐component and 48% for the y‐component. Thus, it is demonstrated that atmospheric wind and pressure fluctuations are largely responsible for exciting the rapid polar motio

ISSN:0094-8276    

DOI:10.1029/92GL00935

年代:1992

数据来源: WILEY

摘要:

Using 21 days of Global Positioning System (GPS) data from 21 globally distributed receivers operating during early 1991, we solve for a 7‐parameter transformation between a GPS free‐network solution and coordinates of 12 stations listed in the International Terrestrial Reference Frame (ITRF). Standard errors of GPS coordinates are derived by applying an orthogonal projection operator to the free‐network covariance. The weighted RMS difference between 33 transformed GPS and ITRF coordinates is 12 mm in the northern hemisphere. Best results are obtained by mapping ITRF coordinates to the epoch of this experiment assuming no vertical site motions. Fixing selected sites in the GPS solution to ITRF'90 does not improve the agreement. We conclude the use of fiducial constraints is unnecessary for global net

ISSN:0094-8276    

DOI:10.1029/92GL00775

年代:1992

数据来源: WILEY

摘要:

Late Holocene sea‐level observations at sites along the east coast of Hokkaido in Japan indicate a gradual decrease of the altitude of relative sea‐level eastward toward the tip of Nemuro Peninsula. These observations in seismically active areas can be explained by glaciohydroisostatic adjustment due to the last déglaciation for an Earth model with a thin lithosphere of 30–40 km thickness and with no low viscosity layer, or with a 25 km lithosphère overlying a low viscosity layer less than 50 km, although more data as a function of time are needed to distinguish these models. Thus the vertical crustal displacement associated with the subduction of the Pacific plate seems to has not been cumulated on a time scale of 103

ISSN:0094-8276    

DOI:10.1029/92GL00936

年代:1992

数据来源: WILEY

摘要:

Current models of the Laurentide deglaciation predict only 15% to 30% of the observed Hudson Bay Free‐air gravity anomaly low. Unless these models are able to be modified, the source of the remainder of this Free‐air gravity anomaly must be sought elsewhere, such as in mantle convect

ISSN:0094-8276    

DOI:10.1029/92GL00687

年代:1992

数据来源: WILEY

摘要:

A variable pressure wave propagation velocity is developed and analyzed in ground materials which may be characterized by an exponential density ‐ pressure relationship. The peak overpressure dependence is minor in the low pressure range and the propagation velocity of the pressure wave is essentially a constant, in fact, it is the seismic P‐wave propagation velocity. Pressure dependence becomes increasingly more significant at peak overpressures exceeding 1 kbar (108Pa). At very high pressure levels encountered only in shockwaves at and above 10 kbar (109Pa), the initial mechanical characteristics of the ground materials become less significant, and the propagation velocity depends primarily upon the peak overpress

ISSN:0094-8276    

DOI:10.1029/92GL00754

年代:1992

数据来源: WILEY

摘要:

Permeability of granite specimens heated up to 650°C were measured in the laboratory with respect to both confining and pore pressure. An unexpected behavior (permeability decrease) was found in the low temperature range (20–125°C). Permeability variations are interpreted with the help of a theoretical model of porosity. Complementary measurements of porosity and absolute surface area have also been obtained. A discussion in terms of microstructural controlling parameters concludes to a decrease of crack aperture at low temperatures (20–125°C), followed by an increase at higher temperatures (T>

ISSN:0094-8276    

DOI:10.1029/92GL00579

年代:1992

数据来源: WILEY

摘要:

A series of large earthquakes occurred off east coast of Honshu, Japan, during the period of October to November 1989. Mechanisms were studied for the foreshock (M=6.5), the mainshock (M=7.1) and the two aftershocks (M=5.2 and 5.3) using the STS‐I broadband records at station INU about 700km away from the epicenters. The lowpass filtered seismograms of these earthquakes show mutually very similar waveforms, in good agreement with those calculated for a known mechanism by an extended reflectivity method. The mainshock is 10 times as large as the foreshock and 2000 times as large as the aftershocks in terms of seismic moment. Assuming that the mainshock (or foreshock) is a collection of some elementary sources at a focal point and that aftershock seismograms represent the responses to such elementary sources, the broadband seismogram of the mainshock (or foreshock) for the first 60s was deconvolved with that of an aftershock to obtain the moment rate function. The moment rate functions of the mainshock and foreshock are both triangle‐like with durations of 14 and 9s respectively. Their rising slopes are mutually very different, a greater slope is associated with the greater shock. This and the similar results for other pairs of earthquakes suggest that the rising slope of moment rate function is causaly related to the eventual size of seismic rupt

ISSN:0094-8276    

DOI:10.1029/92GL00367

年代:1992

数据来源: WILEY

摘要:

In August 1989 a sequence of mb5–6 normal faulting earthquakes occurred in the area between the Ghoubbet Asal‐Manda Inakir and Manda Haroro‐Abhe Bad rifts in central Afar. Relocation of the events suggest they occurred on major faults in the Dobi graben, located midway between the principal rift zones. The earthquakes are evidence that extensional strain is still occurring in the area. The bookshelf tectonic model for central and southern Afar proposed byTapponnier et al. [1990] is modified to account for ongoing extension in the area between the principal rifts as well as distributed

ISSN:0094-8276    

DOI:10.1029/92GL00686

年代:1992

数据来源: WILEY

摘要:

We have obtained maps of lateral variations of structure in the crust and lithosphere in Tibet and surrounding regions by inversion of single station measurements of fundamental mode Rayleigh wave phase velocities in the period range 25 ‐ 100 sec, for earthquakes located in central Asia and observed at teleseismic digital stations in Eurasia. Phase velocity maps are obtained using a tomographic method without a priori constraints on region boundaries. The lateral variations of S velocity reveal a symmetric pattern on each side of crustal low velocity zone centered on the Chang Tang region (north central Tibet). This zone is bounded by rather steep velocity gradients to the south and east, about 250 and 400 km north and west of the frontal thrusts of the Himalayas and Lungmen Shan, respectively. The northern boundary of the zone crosses the northern limit of the Tibet plateau (Altyn Tagh and Kunlun) extending into the southwest Tarim and Qaidam basins. The low crustal velocities are consistent with a thick, hot crust in the central part of Tibet, consistent with Quaternary volcanism in Chang Tang. The steep gradients towards higher velocities in the south and in the north are compatible with the documented continental subduction in the south, and suggest that a similar situation might exist in the nort

ISSN:0094-8276    

DOI:10.1029/92GL00261

年代:1992

数据来源: WILEY