摘要:

Unusual parabolic features associated with impact craters have been observed by Magellan on Venus. A strong correlation exists between the orientation of the features and the zonal winds on Venus. We propose a quantitative model in which the parabolic features are produced by the interaction of the zonal winds with material ejected ballistically from the impact crater. As the ejecta particles fall through the atmosphere, the winds transport them downwind from their entry point, smaller particles being transported a greater distance. Since the ejecta distribution is initially axially symmetric and smaller particles are thrown farther from the crater, the winds blow the particles on the upwind side back upon one another, leading to a pile‐up of material. On the downwind side, the winds disperse the ejecta particles and no pile‐up occurs. The resulting thickness distribution on the Venusian surface matches the observed parabolic features closely. The dual parabolic feature associated with the crater Carson is also explained by this mo

ISSN:0094-8276    

DOI:10.1029/91GL02812

年代:1992

数据来源: WILEY

摘要:

In most of the recent determinations of the geocentric gravitational coefficient (GM) of the Earth, the laser ranging data to the Lageos satellite have had the greatest influence on the solution. These data, however, have generally been processed with a small but significant error in one of the range corrections. In a new determination of GM using the corrected center‐of‐mass offset, a value of 398600.4415 km3/sec2(including the mass of the atmosphere) has been obtained, with an estimated uncertainty (1 σ) of 0.0008 km3/

ISSN:0094-8276    

DOI:10.1029/92GL00259

年代:1992

数据来源: WILEY

摘要:

Comparison of Cartesian coordinates determined at collocated sites using two independent space techniques, very long baseline interferometry (VLBI) and Global Positioning System (GPS), shows remarkable agreement even when the points in question span transcontinental distances. The results corroborate the capabilities of commercial dual‐frequency GPS receivers to perform geodetic work at the highest available accuracy. Adjusted geocentric coordinates of a configuration of GPS stations well distributed along the eastern half of the United States were accurately determined (better than 10−8) in the rigorously defined International Earth Rotation Service (IERS) terrestrial reference frame ITRF

ISSN:0094-8276    

DOI:10.1029/92GL00262

年代:1992

数据来源: WILEY

摘要:

Data from a worldwide Global Positioning System (GPS) tracking experiment have been used to determine variations in Earth rotation (UT1‐UTC) over a time period of three weeks. Kalman filtering and smoothing enabled changes in UT1‐UTC over intervals of 2 to 24 hrs to be detected with the GPS data. Internal consistency checks and comparisons with other solutions from very long baseline interferometry (VLBI) and satellite laser ranging (SLR) indicate that the GPS UT1‐UTC estimates are accurate to about 2 cm. Comparison of GPS‐estimated variations in UT1‐UTC with 2‐hr time resolution over 4 days with predicted variations computed from diurnal and semi‐diurnal oceanic tidal contributions strongly suggests that the observed periodic sub‐daily variations ∼0.1 msec (5 cm) are largel

ISSN:0094-8276    

DOI:10.1029/92GL00563

年代:1992

数据来源: WILEY

摘要:

To measure the changes in fluid volume flow rates through an artificial fracture undergoing slip, silicone oil was pumped through a sawcut in welded tuff in the radial flow geometry during rotary shear. The flow rates at constant normal stress and constant fluid pressure gradient were measured at different amounts of fracture slip. The fluid volume flow rate for a smooth fracture increases at first then decreases with ongoing slip. For a rougher fracture the flow rate decreases over the whole range of slip. The results have implications for fluid withdrawal from reservoirs and underground waste disposal.

ISSN:0094-8276    

DOI:10.1029/92GL00197

年代:1992

数据来源: WILEY

摘要:

The Luzon, Philippines earthquake of July 16, 1990, with Ms=7.8 was generated by left‐lateral faulting in the central Luzon Island. Ground breakage was observed along a linear trace extending 110 km. The maximum horizontal offset as measured on the fault was as much as 6 m. A fault area of 120 × 20 km2was inferred from a field study and aftershock data. This event is one of the largest strike‐slip earthquakes ever recorded in the world. For long‐period surface waveform analysis, we used very broadband records from 14 stations of the GDSN, GEOSCOPE, IRIS and ERIOS networks. The Centroid Moment Tensor (CMT) solution from surface waves represents almost pure left‐lateral strike‐slip faulting on a steep fault with strike = 155°, dip = 88° and rake = −7°. The seismic moment is 3.9 × 1020Nm (× 1027dyn.cm), which gives Mw=7.7. The average dislocation on the fault is 5.4 m and the stress drop is 5.2 MPa (52 bars). Teleseismic P and SH waves from 14 stations are used to model the spatiotemporal distribution of subevents. Five subevents are detected in an interval of 44 sec. Their sequence indicates a bilateral rupture. The mean velocity of rupture toward the north is 2.7 km/sec. The total seismic moment of the subevents is 3.6 × 1020Nm. The largest aftershock (Ms=6.6) occurred on July 17 near the recognized northern end of the main fault. The CMT solution for the aftershock represents a reverse faulting with moment of 6.4 × 1019Nm (Mw=6.5). This aftershock was probably caused by compression produced by block motion of the eastern bloc

ISSN:0094-8276    

DOI:10.1029/91GL02467

年代:1992

数据来源: WILEY

摘要:

We study the November 13, 1990 off‐Primorskij earthquake, which occurred near the northern end of the Japan Sea off the coast of the Primorskij region, by using data from short‐period and broad‐band seismograph networks in Hokkaido, Japan. The main shock and four aftershocks are located from arrival times of P‐ and S‐waves recorded by these networks. The main shock epicenter is located about 50 km off the coast at a depth of about 10 km, and the largest aftershock is located about 20 km southwest of the main shock. It is difficult to read the polarities of P‐waves recorded by the short‐period network; the source mechanism of the main shock is determined by moment tensor inversion of long‐period wave‐forms recorded by broad‐band instruments. The source mechanism is characterized by a reverse faulting with a vertical T‐axis and a horizontal P‐axis trending in the WNW‐ESE direction. We have examined earthquake data reported by the ISC and by the Academy of Sciences of the USSR to study previous seismicity in the vicinity of the November 13 earthquake; no significant well‐located earthquake is reported in this region. The stress orientation in this region may have significant implications for the tectonics of the northern end of the Japan Sea, where a plate boundary between the Eurasian and North Americ

ISSN:0094-8276    

DOI:10.1029/92GL00426

年代:1992

数据来源: WILEY

摘要:

Tilt changes related to a summit dike emplacement at Piton de la Fournaise volcano have been recorded by a 4 continuously recording electronic tiltmeters network during the April 18, 1990 eruption. The real‐time processing of these data at the Observatory allows to calculate during the emplacement the relative position of the inflation center from the tilt vectors supplied by the nearby stations. Two main steps have been identified: an initial subvertical intrusion step, followed by a lateral dike migration from the summit cone. This monitoring allows to calculate a high average intrusion velocity of about 0.2 m.sec−1which can related to the highly fracturated pattern of the summit a

ISSN:0094-8276    

DOI:10.1029/91GL00438

年代:1992

数据来源: WILEY

摘要:

We have used data from the 80‐km‐diameter NORSAR array and the 3‐km‐diameter NORESS array to determine the Transverse Coherence Function (TCF) of arrival time for spatial separations ranging from a few hundred meters to 80 km. To accomplish our objective, we have devised a method to combine data from arrays of different aperture. The procedure is based on information about rms fluctuations and rms tilts on each array. Synthetic fluctuation fields with different statistical properties were generated to test the combination method. Our results for the TCF from NORSAR and NORESS are in reasonable agreement with the predictions of the Flatté‐Wu model of heterogeneities un

ISSN:0094-8276    

DOI:10.1029/92GL00267

年代:1992

数据来源: WILEY

摘要:

Heat production measurements from the West‐African shield have been compiled using various samples of crustal rocks. There is no clear indication for a systematic variation of heat production with age which could explain the regional heat flow density (HFD) variations by corresponding variations of the crustal component. HFD values are positively correlated with the P‐wave travel time residuals deduced from teleseismic observations and negatively correlated with gravity isostatic anomalies. We propose that regional HFD variations are significative of change in the upper mantle struct

ISSN:0094-8276    

DOI:10.1029/92GL00263

年代:1992

数据来源: WILEY