The analytic expressions for the displacements and stresses at any point of an infinite orthotropic elastic medium as a result of an inclined line load have been obtained. This plane strain problem has been solved by using eigenvalue approach and the use of matrix notation avoids unwieldy mathematical expressions. The technique developed in the present paper is simple, straightforward and convenient for numerical computation. The variations of the displacements and stresses with the horizontal distance have been shown graphically.

View full abstract

The vector fluxgate magnetometer of the Ørsted satellite is routinely calibrated by comparing its output with measurements of the absolute magnetic intensity from the Overhauser instrument, which is the second magnetometer of the satellite. We describe the method used for and the result obtained in that calibration. Using three years of data the agreement between the two magnetometers after calibration is 0.33 nT rms (corresponding to better than 1 nT for 98% of the data, and better than 2 nT for 99.94% of the data). We also report on the determination of the transformation between the magnetometer coordinate system and the reference system of the star imager. This is done by comparing the magnetic and attitude measurements with a model of Earth's magnetic field. The Euler angles describing this rotation are determined in this way with an accuracy of better than 4 arcsec.

View full abstract

Ore mineral and host lithologies have been sampled at 14 sites (89 oriented samples) in the Naica District, northern Mexico. The following properties were measured to characterise samples : saturation magnetization, high-temperature magnetic susceptibility, remanence intensity, Koenigsberger ratio, Curie temperature and hysteresis parameters. Rock magnetic properties seem to be controlled by variations in pyrrhotite, (titano) magnetite, (titano) hematite and (titano) maghemite content, and hydrothermal alteration. Post-mineralization hydrothermal alteration seems to be the major event that affected the minerals and magnetic properties. Continuous susceptibility measurements with temperature in most cases yield Curie points close to that of almost pure magnetite. Hysteresis curves and associated IRM (isothermal remanent magnetization) acquisition plots, however, in some cases points to the higher coercivity minerals. Hematite or a mixture of hematite-titanomagnetite-titanomaghemite are probably present in the Naica samples although their contribution in remanent magnetization is minor. Judging from the ratios of hysteresis parameters, it seems that all samples fall into the pseudo-single domain (PSD) grain size region. The Koenigsberger ratio (Q) was used as a measure of the relative contribution of remanent and induced magnetization into the magnetic anomalies. Q ranges from 0.05 to 34 and, generally, it is higher than 1, indicating the predominance of remanence over induced magnetism. The location and geometry of the magnetic source is shown as a single long subhorizontal body slightly inclined (-20°) to the SE. This modelled body suggests an andine-type nature and emplacement of granitic magmas.

View full abstract

A direct approach for determining the asymptotic MHD Mach cone is formulated and solved. An implicit analytical solution enables the calculation of the asymptotic downstream slope of MHD Mach cones at any clock angle for arbitrary M_s, M_a, and V_bv. The solution obtained includes all previously known symmetric cases. The elongation and shift of the asymptotic fast mode shock cross-section are studied for a wide range of upstream plasma parameters as well as its unusual ‘chopped’ shape under certain conditions. Our results may be useful for planetary shock modeling and MHD numerical codes verification.

View full abstract

The crater size-frequency distribution of lunar maria is characterized by the change in slope of the population between 0.3 and 4 km in crater diameter. The origin of the steep segment in the distribution is not well understood. Nonetheless, craters smaller than a few km in diameter are widely used to estimate the crater retention age for areas so small that the number of larger craters is statistically insufficient. Future missions to the moon, which will obtain high resolution images, will provide a new, large data set of small craters. Thus it is important to review current hypotheses for their distributions before future missions are launched. We examine previous and new arguments and data bearing on the admixture of endogenic and secondary craters, horizontal heterogeneity of the substratum, and the size-frequency distribution of the primary production function. The endogenic crater and heterogeneous substratum hypotheses are seen to have little evidence in their favor, and can be eliminated. The primary production hypothesis fails to explain a wide variation of the size-frequency distribution of Apollo panoramic photographs. The secondary craters are likely the major source of the steepening of the distribution. It is ambiguous, however, which primary craters can produce sufficiently numerous secondary craters. The regional variation of the sizefrequency distributions shows that few large impacts produce enough secondary craters to affect the distributions in the surrounding area. We emphasize that a crater size-frequency distribution of small craters on the moon should not be taken as an indication of the surface age. More data obtained from future lunar missions should be viewed in this context, and continued to be examined for further insight into the possible formation mechanism for secondary craters.

View full abstract

We have estimated a criterion for escape of silicate vapor from the disk formed by a giant impact. Escape from the disk affects the mass distribution and specific angular momentum of the disk. We applied the results to the disk formed by the Moon-forming impact. In the case of a hot (>6000 K) and highly vaporized (>63%) disk, the material outside of the Roche radius escapes. This indicates that the formation of the Moon is influenced by the thermal state of the disk just after the giant impact.

View full abstract

Clear images of the seismic velocity discontinuities in the crust and the uppermost mantle beneath southwest Japan have been obtained with Receiver Function (RF) analyses of teleseismic waveforms observed at extraordinary dense short-period and broadband stations. For imaging, we construct several 2-D profiles of depth-transformed RFs to delineate the fine crustal structure and the 3-D configuration of the Philippine Sea plate (PHS). The Moho is clearly seen and locally depressed just beneath the Chugoku Mountains. The Conrad, the velocity discontinuity within the crust, in the northern Kinki region is recognized and depressed just beneath the Chugoku Mountains as well as the Moho. RF analyses also clearly elucidate the largely contorted configuration of the PHS including aseismic portions. The PHS subducts aseismically under the western Chugoku region with a high dip angle of 35 degrees at least to a depth of 65 km. In the western Kii Peninsula, it descends steeply to a depth of 90 km with some curvature, while it subducts with a low dip of 20 degrees to a depth of 60 km from the eastern Kii Peninsula to the Lake Biwa.

View full abstract