摘要:

Remote sensing of meteorological parameters helps to provide the initial conditions for numerical weather prediction (NWP). Desired fields include those of temperature, moisture, winds, clouds, and surface properties. For high horizontal resolution and global coverage, satellite data are an unrivaled source of information. The basic form of this information is satellite sensor‐incident, wavelength‐dependent radiance (or equivalently, brightness temperature). The process of retrieving meteorological information from the observed radiances involves solving an inverse problem. Often the inverse problem is ill posed or poorly conditioned. A variety of methods have been used to make the inverse problem more tractable: in essence, some a priori information is required. The inverse problem is the opposite of the calculation of radiances, given the relevant meteorological profiles, surface conditions, and sensor geometry. This so‐called forward problem is an integral part of physical retrieval methods which are now generally favored over conventional statistical retrieval methods. Physical retrieval methods, in practice, actually rely to a certain degree on a priori statistics. Temperature retrievals, especially in the southern hemisphere, are of proven importance in NWP involving global scale models. Vertical temperature profiles are obtained from observed radiances in a set of emission bands with varying optical depths. In comparison to conventional radiosonde data, current temperature retrievals have coarse vertical resolution. There is also some evidence that retrieved synoptic features are somewhat smoothed horizontally in spite of the high horizontal resolution of the temperature retrievals. The usefulness of other geophysical parameters for NWP is not well established and is largely untested, although a great many retrieval methods have been proposed or developed for a variety of potentially interesting parameters. The reason for this is twofold: these data are expected to have the most impact on mesoscale modeling, but mesoscale analysis systems are still relatively primitive. Moisture variables—that is, specific humidity, clouds, and precipitation—are retrievable and are potentially very useful. It is theoretically possible to retrieve specific humidity profiles by using methods analogous to those used to retrieve temperatures. However, results to date with available sensors have not been wholly satisfactory. Other moisture parameters are somewhat easier to retrieve; for example, cloud cover may be obtained by a variety of techniques from imagery in visible and/or infrared spectral regions, and vertically integrated water vapor is well correlated with emissions at microwave frequencies, especially over the ocean. Unfortunately, these parameters are not readily assimilated by a global NWP model, and special methods are required. A variety of other meteorological parameters which may be used for NWP are retrievable. These include winds at the surface from microwave sensors and cloud drift winds aloft, as well as other surface properties, such as soil moisture, albedo, snow cover, temperature, and fluxes. In summary, remote sensing is important for NWP. The potential for a much greater impact exists. Greater accuracy and vertical resolution, better retrieval methods, and better and more flexible analysis methods are

ISSN:8755-1209    

DOI:10.1029/RG024i004p00701

年代:1986

数据来源: WILEY

摘要:

Recent results from climate models have led to the prediction that a global warming due to increasing atmospheric CO2is now imminent, if it has not already occurred. In an effort to develop more definitive information on this question, a detailed review has been conducted of prior efforts to unravel climatic change from the various types of recorded observational data available. Most of the more definitive of the prior analyses—along with evaluative comments by the various authors—have been assembled herein. There appears little doubt that the average surface air temperature of at least northern hemisphere has been increasing since the beginning of recorded data with most of the warming occurring in a brief period circa 1920. The fragmentary early data suggest significant cooling prior to 1883 such that 25–50% of the subsequent warming may represent a return to earlier levels. Whether the overall warming constitutes a climate change remains an unresolved problem, as does the cause of the wa

ISSN:8755-1209    

DOI:10.1029/RG024i004p00745

年代:1986

数据来源: WILEY

摘要:

A review of the techniques underlying the basic analysis of local and regional earthquakes is presented, describing modern methods for earthquake location, velocity inversion, and fault plane solution. These three topics require a method for ray tracing, determining the seismic ray path connecting the earthquake source to the observing station. A basic introduction to common ray tracing techniques is presented. Earthquake location based on Geiger's method remains the most common in use. Our understanding of this fundamental problem has been enhanced by a variety of studies, and by progress in geophysical inverse theory, but difficulties with the method remain. Alternatives to Geiger's method are reviewed to emphasize the continuing need for study of earthquake location techniques. The more complex problem of simultaneous inversion is receiving increasing attention. Algorithms have been developed to deal with a wide range of velocity structures, including layered, continuous one‐dimensional, and three‐dimensional models. Modern techniques for manipulating and transforming large matrix equations have been critical to the success of simultaneous inversion. Finally, methods for determining fault plane solutions are reviewed, such as first motion, amplitude ratio, and polarization, and techniques are discussed for inferring stress orientation from sets of focal mechani

ISSN:8755-1209    

DOI:10.1029/RG024i004p00793

年代:1986

数据来源: WILEY

摘要:

The depths of moderate‐sized shallow earthquakes can be routinely determined to an accuracy of several kilometers using teleseismic data alone, making it possible to determine the vertical positions of earthquakes within the lithosphere. This capability provides a valuable tool for plate tectonic studies since earthquake depths can be used to constrain lithospheric thermal and mechanical structure at plate boundaries and in intraplate regions. Two basic techniques are used for depth analyses. Surface wave spectra can be used to determine focal depths, as mode excitation reflects the behavior of the eigenfunctions with depth. The primary limitation on such studies stems from lateral heterogeneity of velocity structure. The waveforms of body waves are also diagnostic of focal depth, which controls the time separation between the direct arrival and near‐source surface reflections. These studies can be limited by a partial trade‐off between depth and source time function duration. The reliability of the body wave modeling approach, now in general use, is shown by excellent agreement between depths determined by different investigators using different algorithms. The depth determination is robust to typical uncertainties in focal mechanism and near‐source structure. The trade‐off with source time function duration generally does not produce major difficulties for small events (Ms<6.5). For larger events, the trade‐off can be resolved using a approach in which, for a given depth, the time function yielding the best fit to a suite of seismograms is found by deconvolution. Deconvolution at a series of depths yields a misfit function, whose minimum indicates the focal depth. Tests with synthetic data show that the deconvolution process yields reliable depth estimates even given uncertainties in focal mechanism and near‐source structure. It successfully handles multiple sources, moderate‐sized finite sources, and horizontally propagating ruptures. Good results are obtained for vertically propagating ruptures until the fault area becomes so large that waveforms differ significantly from the point source approximation. A number of important tectonic results have emerged from studies using these depth determination methods. The maximum depth of oceanic intraplate seismicity increases with lithospheric age and is approximately bounded by the 750°C isotherm. This depth roughly equals the flexural thickness of the lithosphere but is much less than the depth to the low‐velocity zone indicated by surface wave dispersion. The maximum depth of continental intraplate seismicity is also temperature limited, but by a lower temperature. These observations are consistent with the rapid weakening of the appropriate rocks with depth predicted by standard temperature dependent rheologies. The maximum depth of oceanic transform seismicity corresponds to an isotherm of about 400°C. This temperature is lower than would be predicted from the intraplate results, and may indicate that transforms are either hotter or weaker than expected. The depths of ridge crest earthquakes large enough for teleseismic analysis are extremely shallow (0–8 km). The depth distribution of seismicity in continental normal faulting regions provides constraints on the nature of the extension process. At subduction zones, precise depth determinations can resolve the upper and lower flexural regions in the bending downgoing plate, precisely locate the plate interface, and map the vertical extent and position of faulting

ISSN:8755-1209    

DOI:10.1029/RG024i004p00806

年代:1986

数据来源: WILEY

摘要:

In this paper we present a region‐by‐region review of the Wadati‐Benioff zone structure of most of the world's seismically active subduction zones, focusing primarily on the intermediate and deep seismicity. Lateral changes in Wadati‐Benioff zone structure are common in every major subduction zone. In this study we use these changes to define possible boundaries between portions or “segments” of lithosphere with differing subduction geometries. Although earthquake data seldom have the resolution to show conclusively whether these boundaries separate independent blocks of lithosphere, the available data indicate that the active process at most of these segment boundaries is ductile deformation of the subducting plate, rather than tearing. We found the strongest evidence for the existence of tears where tears are geometrically necessary, such as where a transform boundary terminates a trench, as at the New Hebrides, Tonga, and South Sandwich Trenches. Weak evidence suggesting other tears does exist in some regions, such as Taiwan, the Japan/Izu‐Bonin corner, and the Philippines. The causes of these changes in structure are, in most cases, unclear. Only about 34% of the possible segment boundaries coincide with subducting bathymetric features. Some boundaries occur where there is apparent lateral strain caused by anomalous trench geometry. We have designed a simple modeling procedure which incorporates published plate motion and the observed geometry of trenches and Wadati‐Benioff zones to estimate the lateral strain in subduction zones throughout the circum‐Pacific region. Although no observed subduction zone has a perfectly strain‐free geometry, there is a broad range of geometries for which the lateral strain is small. Indeed, the observed geometry of most subduction zones involve relatively little lateral strain. Comparison with centroid moment tensor focal mechanisms indicates that in zones where the modeling predicted little lateral strain, the mechanisms of intermediate and deep earthquakes show no effects of lateral stress, and downdip stresses are clearly dominant. In regions such as the Mariana Arc, where the model predicts very large lateral extension, lateral tension is very evident in the focal mechanisms. In regions such as the Hokkaido corner, where the modeling predicts large compressional strains, the plate appears to buckle, and bending stresses parallel to the trench are evident. In general this study finds that subducted lithosphere is remarkably cohesive and rigid, and only rarely deforms by bre

ISSN:8755-1209    

DOI:10.1029/RG024i004p00833

年代:1986

数据来源: WILEY

摘要:

The first part of this paper is a review on the main mineralogical, petrological and structural criteria which can be used to unravel the history of the various types of mantle peridotites met at the earth's surface. These criteria are applied in the second part to show that most peridotites share a common history which is that of an asthenospheric uprise followed by specific fates corresponding to their lithospheric history. The unifying concept is that of adiabatic ascent rate, related to rifting and spreading rates. Rates lower than 0.5 cm/yr in continental graben correspond to departure from adiabatic conditions at depths greater than 30 km. This inhibits further melt extraction with as consequences a limited melt extraction and comparatively fertile spinel lherzolites as residue. Higher rates, probably not exceeding 1 cm/yr, correspond to oceanic rifts with the main melt extraction completed around 15 km, generating a thin oceanic crust and plagioclase lherzolites as residue. Finally, rates greater than 1–2 cm/yr correspond to oceanic ridges with melt being extracted at Moho depth, thus generating a 6‐km‐thick crust and leaving depleted harzburgites as residue. Thus examination of the peridotite type and associated crustal formations give some clue to trace back the environment of origin. This conclusion must however be tempered by the fact that the spreading rate, envisioned here as the principal controlling parameter, can change with space and time in a given environment (mantle diapirism, rifting, seafloor spreading, crust genera

ISSN:8755-1209    

DOI:10.1029/RG024i004p00875

年代:1986

数据来源: WILEY