摘要:

Densities of impact craters on tessera, which is complex ridged terrain of tectonic origin, and on the remainder of the planet, which is mostly volcanic plains, were studied using Magellan images for about 96% of the surface of Venus. The density of large (D>16 km) impact craters on tessera is higher by a factor of about 1.4 than on the remainder of the planet. This means that the tessera crater retention age is larger than the age of the plains. This is in agreement with the well known fact that tessera is embayed by the surrounding volcanic plains. The density of small (D<16 km) impact craters on tessera is lower than on the remainder of the planet, evidently an observational bias caused by a difficulty in recognizing the small craters on rough tessera terrain. The absence of recognizable tectonic deformation in most of the large on‐tessera craters means that during the period of crater emplacement most of the studied tesserae were tectonically stable and did not undergo noticeable degree of deformatio

ISSN:0094-8276    

DOI:10.1029/93GL02692

年代:1993

数据来源: WILEY

摘要:

The statistical properties of Earth, Venus, Mars, Moon, and a 3‐D mantle convection model are compared. The higher order properties are expressed by third and fourth moments: i.e., as mean products over equilateral triangles (defined as coskewance) and equilateral quadrangles (defined as coexance). For point values, all the fields of real planets have positive skewness, ranging from slightly above zero for Lunar gravity to 2.6σ³ for Martian gravity (σ is rms magnitude). Six of the eight excesses are greater than Gaussian (3σ4), ranging from 2.0σ4for Earth topography to 18.6σ4, for Martian topography. The coskewances and coexances drop off to zero within 20° arc in most cases. The mantle convective model has zero skewness and excess slightly less than Gaussian, probably arising from viscosity variations being onl

ISSN:0094-8276    

DOI:10.1029/93GL03086

年代:1993

数据来源: WILEY

摘要:

Thermal convection in a self‐gravitating rotating fluid shell is modeled using a hemispherical fluid shell that can be rotated about its axis of symmetry. In this apparatus, a tertiary convective state begins to exist at a Rayleigh number approximately equal to 2.1 times the critical value for the onset of convection (RC1). This state is characterized by the coexistence of three waves. In this tertiary state noise is always present. At a slightly higher Rayleigh number, a strong interaction was observed to develop. Frequency locking takes place at 2.4RC1. Later, the flow exhibits chaotic behavior as shown by the broad band Fourier spectra of the temperature records. Planetary implications of these findings are discusse

ISSN:0094-8276    

DOI:10.1029/93GL02817

年代:1993

数据来源: WILEY

摘要:

In an effort to confirm inner core anisotropy, we conducted a systematic search for PKP ray paths with various angles from the Earth's spin axis. In particular, we studied paths nearly parallel to the spin axis (polar paths) and those nearly parallel to the equatorial plane (equatorial paths). Data for earthquakes and explosions were collected from Worldwide Standardized Seismograph Network (WWSSN), Long Range Seismic Measurements (LRSM), and Global Digital Seismograph Network (GDSN). Absolute times (DF, BC) and differential times (BC‐DF, AB‐DF) as well as waveform data were examined. For all polar paths, differential times of BC‐DF consistently yield residuals of 1.5 to 3.5s larger than equatorial paths. Absolute DF time residuals exhibit anomalies of the same magnitude (1 to 4s) with DF being early along polar paths while BC residuals have no obvious correlation with the differential time anomalies. DF phases appear multi‐pathed for polar paths and are relatively simple for equatorial paths. These results coupled with previous studies suggest an axisymmetric anisotropy at the top of the inn

ISSN:0094-8276    

DOI:10.1029/93GL02812

年代:1993

数据来源: WILEY

摘要:

Convection studies of temperature‐dependent Newtonian and non‐Newtonian rheology in aspect‐ratio seven boxes have been carried out for volume averaged Rayleigh numbers between O(105) and O(107). Large lateral viscosity variations, O(104) are found for non‐Newtonian cases with total temperature‐dependent viscosity variations up to 250. Flow structures for both rheologies are distinguished by large‐scale circulations with relatively stable descending limbs. Much larger hot thermal anomalies are found near the surface for non‐Newtonian rheology. Lithospheric thinning is facilitated by non‐Newtonian rheology because of stress‐softening and the lubrication of the descending lim

ISSN:0094-8276    

DOI:10.1029/93GL02863

年代:1993

数据来源: WILEY

摘要:

We present a simple analytic model of the interaction of cold convective downwelling currents with an endothermic phase change. The model describes the ponding and lateral spreading of downflows along the phase transition interface. A simple comparison of the vertical forces on the ponding material provides a necessary condition for a downflow to penetrate the phase boundary. This condition is fundamentally dependent on the geometry of the downflow. For planar downwellings, the model predicts a minimum ponding time before the structure can penetrate the phase boundary. For columnar (axisymmetric) downflows, there is no minimum time of spreading required before penetration can proceed. The model thus provides an explanation for the observation that in numerical models of three‐dimensional convection with an endothermic phase change, cylindrical downflows penetrate the phase interface while planar ones do not. Since descending slabs in the Earth‧s mantle display a wide spectrum of geometries between planar and cylindrical (given various trench curvatures, as well as intersections of two or more subduction zones), this phenomenon may explain, in part, why some slabs appear to extend into the lower mantle while others are deflected at the 660 km discontinu

ISSN:0094-8276    

DOI:10.1029/93GL02691

年代:1993

数据来源: WILEY

摘要:

The effects of kinetics on the non‐equilibrium aspects of the olivine to spinel transition in a descending slab have been studied numerically. A one‐dimensional model consisting of the kinetic equations and the heat‐diffusion equation with latent‐heat release has been constructed. Numerical results show that the position and sharpness of the kinetic phase boundary is determined by the surface tension and the activation volume. For slow slab velocities, less than 6 cm/yr, near equilibrium conditions are found. Finger‐like structures emanating from the phase boundaries are obtained in this regime. These phase boundary prolusions may cause earthquakes. For slab velocities of around 10 cm/yr the metastable olivine region may be pushed down to a depth of about 600 km, with a sharp phase boundary. A direct transition from olivine to perovskite may be

ISSN:0094-8276    

DOI:10.1029/93GL02811

年代:1993

数据来源: WILEY

摘要:

A spatial slip distribution of the 1993 Kushiro‐Oki earthquake (Mw; 7.6) is undoubtedly constrained using near‐field data and the detailed aftershock distribution. This earthquake has peculiar features in that the slip on the fault plane is restricted within an extremely small area (about 40 km × 20 km) between the upper and lower planes of the double seismic zone in the subducting slab beneath Hokkaido. The total seismic moment, averaged slip, and stress‐drop in this area are 3.3 × 1020Nm, 5.5 m, and 4.2 × 107Pa, respectively. This earthquake indicates that the region of the slab between the planes of the double seismic zone beneath Hokkaido is strong enough to sustain a stress of about 4 × 107Pa. The slip motion of this event is consistent with retrograde slab migration along the Southern Kuril trench, and may represent a process by which retrograde motion of subducting slabs co

ISSN:0094-8276    

DOI:10.1029/93GL02864

年代:1993

数据来源: WILEY

摘要:

The 1992 Landers earthquake (Mw=7.3) occurred in the middle of the TERRAscope network. Long‐period Rayleigh waves recorded at the TERRAscope stations (Δ≤3°) after traveling around the Earth show large amplitude anomalies, one order of magnitude larger than spherical Earth predictions up to a period of about 600 s. The ground motions over the epicentral region at and after the arrival of R4–5 are in phase at all stations. These observations are inconsistent with the nearly vertical strike slip mechanism of the Landers earthquake. Synthetic seismograms for a rotating, elliptic and laterally heterogeneous Earth model calculated by the variational method agree well with the observed waveforms. Calculations for various 3D Earth models demonstrate that the amplitudes are very sensitive to the large scale aspherical structure in the crust and the mantle. The anomalies for modes shorter than 300 s period can be explained by lateral heterogeneity shallower than the upper mantle. Rotation of the Earth and lower mantle heterogeneity are required to explain mode amplitudes at longer periods. Current whole mantle seismic tomographic models can fully explain the observed amplitudes longer than 300 s. To assess the effect of the high order lateral heterogeneity in the mantle more precise estimate of the crustal correction is r

ISSN:0094-8276    

DOI:10.1029/93GL02910

年代:1993

数据来源: WILEY

摘要:

Approximately three minutes after the magnitude 7.4 Landers mainshock on 28 June 1992, a M5.7 aftershock occurred south of the mainshock epicenter, with a location of 34° 7.65′N, 116° 23.82′W. This aftershock was recorded on an array of portable digital seismic instruments deployed in Morongo Valley, 21 km southwest of the event. Although peak accelerations are found to differ by approximately 50% at stations with similar general site conditions within 500 m of each other, there is good coherence of arrivals across the array for frequencies ≤1 Hz. We use the recordings to determine the apparent phase velocity and azimuth of propagation across the array, and show that the event clearly ruptured to the south, with a rupture length of ∼11 km and a rupture velocity of approximately 3.0 km/s. Our results suggest that at least some of the mapped surface rupture south of the town of Yucca Valley (∼11 km in extent; maximum displacement of 20 cm) may have been associated with this aftershock. If this is the case, then the fault that produced the southern end of the Landers mainshock (the Johnson Valley fault) need not continue at depth across the active left‐lateral, east‐west trending Pinto

ISSN:0094-8276    

DOI:10.1029/93GL00666

年代:1993

数据来源: WILEY