摘要:

In providing a method for solving non‐linear optimization problems Monte Carlo techniques avoid the need for linearization but, in practice, are often prohibitive because of the large number of models that must be considered. A new class of methods known as Genetic Algorithms have recently been devised in the field of Artificial Intelligence. We outline the basic concept of genetic algorithms and discuss three examples. We show that, in locating an optimal model, the new technique is far superior in performance to Monte Carlo techniques in all cases considered. However, Monte Carlo integration is still regarded as an effective method for the subsequent model appraisa

ISSN:0094-8276    

DOI:10.1029/91GL02368

年代:1991

数据来源: WILEY

摘要:

We present an application of a genetics‐based optimization algorithm in an attempt to compute residual statics in seismic data processing. Genetic algorithms have a long history, but have only recently been applied to complex optimization problems. In this paper, optimization results are shown for a synthetic data set to test whether the genetic algorithm can obtain the correct qualitative and/or quantitative feature

ISSN:0094-8276    

DOI:10.1029/91GL02537

年代:1991

数据来源: WILEY

摘要:

The experimental values of the ratio of the radial seismic velocity perturbation with respect to the radial density perturbation, (R)T, for MgO at pressures of up to about 20 GPa show a conflicting pressure dependence. However, (R)T, calculated using free energy minimisation at 1000 K and pressures of up to 160 GPa for the lower mantle minerals MgO and MgSiO3‐perovskite, is predicted to decrease with increasing pressure. Previous calculations of the elastic constants of MgO using the Potential Induced Breathing model also predict that (R)Tdecreases as the pressure increases.The ratio (dlnνs/dlnνp)Tfor both MgO and MgSiO3−perovskite is also predicted to decrease with increasing pre

ISSN:0094-8276    

DOI:10.1029/91GL02795

年代:1991

数据来源: WILEY

摘要:

The differential thermal analysis (DTA) technique has been applied in a multi‐anvil high‐pressure apparatus to study the dehydration reaction of brucite (Mg(OH)2) to periclase (MgO) plus H2O at 4 to 6 GPa. At 4 GPa and 1030°C, endothermic and exothermic peaks due to dehydration and rehydration were observed during heating and cooling cycles, respectively. These peaks were shifted to higher temperature with increasing pressure (1080°C at 5 GPa; 1120°C at 6 GPa), suggesting that the reaction has a positive dT/dP slope within this pressure range. The stability of brucite under deep mantle condition is dis

ISSN:0094-8276    

DOI:10.1029/91GL02468

年代:1991

数据来源: WILEY

摘要:

Understanding of AMS in pristine basic igneous rocks in terms of flow‐orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains (1) typically are irregularly equant or skeletal rather than distinctly ellipsoidal, and (2) usually crystallize interstitially and late, after magma flow has ceased. However, the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly‐oriented (by flow) silicate “template”. This constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth, but in addition, we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of AMS. Experiments involving the casting of magnetite‐epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected AMS symmetry, i.e. K3perpendicular to glass plates or K1parallel to glass rods. Slicing of near isotropic synthetic blank‐ and natural rock‐cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from ∼0.4% to 2%, and the natural rock from ∼0.9% to ∼2.9%. In both the K3axis migrates to be perpendicular

ISSN:0094-8276    

DOI:10.1029/91GL01777

年代:1991

数据来源: WILEY

摘要:

We have studied the influences of depth‐varying rheologies on the structure of mantle convection and the near surface flow fields. Upwelling flows crossing the upper‐lower mantle boundary become considerably thinner and faster. Streamlines are concentrated near the surface and other plate‐like characteristics are enhanced by this type of rheological stratification in which the averaged effective viscosity of the upper mantle is about two orders of magnitude lower than that of the lower mantle. The presence of a lithosphere with a higher non‐linear dependence in the flow law further promotes plate‐like behavior. Plate‐like character of the top boundary‐layer is encouraged by a decreasing depth‐dependent viscosity in the non‐Newtonian upper‐mantle and by great

ISSN:0094-8276    

DOI:10.1029/91GL02573

年代:1991

数据来源: WILEY

摘要:

We constructed velocity models of subducting slabs with a kinetically‐depressed olivine → β‐ and γ‐spinel transition, and examined the effect that such structures would have on teleseismic P waveforms using a full‐wave finite‐difference method. These two‐dimensional calculations yielded waveforms at a range of distances in the downdip direction. The slab models included a wedge‐shaped, low‐velocity metastable olivine tongue (MOTO) to a depth of 670 km, as well as a plausible thermal anomaly; one model further included a 10‐km‐thick fast layer on the surface of the slab. The principal effect of MOTO is to produce grazing reflections at wide angles off the phase boundary, generating a secondary arrival 0 to 4 seconds after the initial arrival depending on the take‐off angle. The amplitude and timing of this feature vary with the lateral location of the seismic source within the slab cross‐section. Careful analysis of waveforms from earthquakes with depths near 400 km, simple sources, and adequate station coverage in appropriate geometries will be required to reso

ISSN:0094-8276    

DOI:10.1029/91GL02588

年代:1991

数据来源: WILEY

摘要:

ISC locations for earthquakes beneath Tibet indicate a random distribution of events down to a depth of about 50 km. This distribution would be expected from a relatively cool crust which would allow the seismo‐genic zone to extend to such depths. A detailed investigation of the Tibetan earthquakes, with magnitude greater than 5.5 from 1964 to 1986, yields a distinctly different picture. Waveform modeling of depth phases indicates that only a few events from this population is actually deeper than 25 km. These few events occurred near the edges of the Plateau where active subduction is occurring as suggested by the thrust‐like nature of their mechanisms. The events, averaging the entire population, occurred earlier than indicated by the ISC by about 3 seconds which leads to about a 1.5% and 0.5% over estimation of Pnand Snvelocities respectively applying ISC tables and standard flat‐layered models. A more serious error occurs if the Pnand Snvelocities are determined by correcting for source depth but assuming the ISC origin

ISSN:0094-8276    

DOI:10.1029/91GL02865

年代:1991

数据来源: WILEY

摘要:

The origin and evolution of Ishtar Terra is modeled using numerical simulations of crust/mantle interaction on Venus. Based on our modeling, we favor a two phase evolutionary sequence for Ishtar. The first phase is driven by bulk mantle flow due to thermal convection and results in the formation of a thickened crustal block over a mantle downwelling. The second phase, a result of the thermal perturbation exerted on the mantle by the region of thickened crust, is driven by the sinking of a lithospheric slab. This slab‐driven tectonic pulse leads to low angle subduction and orogenesis at the peripheries of the crustal block. Throughout its evolution Ishtar remains coupled to flow within the deep mantle, due to the lack of an asthenosphere, and ultimately bulk mantle flow comes to dominate as the effects of the slab‐driven pulse w

ISSN:0094-8276    

DOI:10.1029/91GL02734

年代:1991

数据来源: WILEY

摘要:

Paleomagnetic measurements of samples from the Mio‐Pliocene Purisima Formation demonstrate that the Pomponio tectonic block of central coastal California has rotated clockwise by approximately 35° to 55° within the last 2.5 million years. The most likely interpretation of this data is that the Pomponio block is broken into several small blocks which have rotated by various amounts. The data suggest that rotations contribute to vertical deformation and secondary faulting within the central San Andreas Fault System, and that they play an important role in the accommodation of shear along the fault sys

ISSN:0094-8276    

DOI:10.1029/91GL02760

年代:1991

数据来源: WILEY