摘要:

The theoretical background to the application of linear viscoelasticity in describing the anelastic behavior of rocks is developed in this review. The constraints imposed on the behavior of the impulse response and system function of a linear system by the requirements that the system be causal and passive are examined. These include the existence of dispersion relations for the system function for causal systems and the role of the concepts of positive reality and minimum phase in passive systems. The uses of these linear system concepts in linear viscoelasticity and in the propagation of plane waves in an isotropic viscoelastic medium are considered. Finally, applications to seismology are presented, in particular the significance of the dispersion associated with a quality factorQthat is constant or varies as a power of frequency in comparisons of seismic data for different frequency ranges and in the evolution of the shape of propagating pulses.

ISSN:8755-1209    

DOI:10.1029/RG019i002p00233

年代:1981

数据来源: WILEY

摘要:

To determine whether reports of unusual animal behavior before earthquakes are plausible, we have compared these reports with recent laboratory studies of animal sensory thresholds. Our major conclusion is that some animals are much more capable than humans of perceiving certain kinds of geophysical stimuli which may precede earthquakes. These geophysical stimuli are seismic or acoustic waves at low frequency (below 50 Hz), electric field changes, and olfactory stimuli. For example, recent studies suggest that some birds and fish are more sensitive than humans to sounds with frequencies below 40 Hz, and many animals are exceptionally good at perceiving low‐frequency vibrations through their skin. Certain fish are sensitive to electric field changes as small as 10−5V/m, and some laboratory mammals also respond to significantly weaker fields than humans. For these electric and acoustic stimuli the reported levels of geophysical precursors are within the reported perceptible range of some animals which show unusual behavior prior to earthquakes. In addition, stimuli caused by the release of gases from small cracks may well be perceived by some animals before earthquakes. Recent research has confirmed the remarkable olfactory sensitivity of some animal species, but no quantitative comparisons with geochemical precursors can be made yet. We find no evidence that magnetic field precursors or precursory high‐frequency (above 10 kHz) sounds are the cause of unusual animal behavior before earthquakes. Although some animals are highly sensitive to these stimuli, observed magnetic precursors tend to be of low amplitude amid relatively high noise levels, and high‐frequency sound is severely attenuated within a short distance from the earthquake hypocenter. Knowledge of animal sensory capabilities may suggest an instrumental strategy for detecting earthquake precursors. We recommend further geophysical research in seismic areas to measure (1) seismic waves in the frequency range 10–50 Hz, (2) earthquake‐related electric field changes and air ionization, and (3) coseismic or preseismic release of trapped gases other than radon. Further biological research can refine our understanding of (1) responses to sounds and vibrations with frequencies below 50 Hz, (2) the effects of electrical and electrostatic changes on animal behavior, (3) behavioral responses to odors, and (4) sensory capabilities of common domestic animals like horses, dogs, and chickens, which are seldom studied but commonly mentioned in the preearthqu

ISSN:8755-1209    

DOI:10.1029/RG019i002p00247

年代:1981

数据来源: WILEY

摘要:

Recent investigations of the lower continental crust have progressed along three relatively independent fronts: geophysics, field mapping, and study of xenoliths brought to the earth’s surface in volcanic and kimberlitic eruptions. The general results of these studies have increased our appreciation of the scale of lower crustal structural and compositional complexity, but not to the point where we can construct an accurate crustal cross section for even one locality. The first part of this review summarizes the results of crustal geophysical and geological mapping studies, which have provided both the broad framework and, more recently, some local detail for the continental crust. In the second part the role of crustal xenolith studies is outlined, and the occurrence of xenoliths is summarized by tectonic setting. In the course of this review, several generalizations about the lower crust are emphasized. First, compositional inferences based on matching of mean seismic velocities and rock types are not unique. For example, the occurrence of hydrous minerals at depth is indicated by some xenolith populations and perhaps corroborated by electrical conductivity studies. The lower velocities and densities of hydrous rocks allow consideration of less silicic mean compositions than were indicated by several recent estimates. Second, the occurrence of lower crustal metasedimentary rocks in several xenolith suites and in exhumed lower crustal sections indicates that mechanisms exist for burying surface rocks to great depths, presumably in thrust zones. Given the existence of metasedimentary rocks, however, one must still recognize the preponderance of igneous and metaigneous rocks, commonly of broadly basaltic composition, in lower crustal xenolith suites. Finally, the regional variability of the lower crust most probably correlates with tectonic setting and should temper extrapolations of lower crustal characteristics found in exposed lower crustal sections to other region

ISSN:8755-1209    

DOI:10.1029/RG019i002p00271

年代:1981

数据来源: WILEY

摘要:

Most problems in applications are inverse source problems or identification problems. This review deals with a systematic treatment of the nonnegative sources (measures) relative to the Laplace equation and the heat conduction equation producing the same potential outside a domain Ω ⊂R³. This set is convex and weakly compact. Therefore the basic theorems on convex sets can be applied to such problems. Sets of extremal measures are characterized by geometric conditions. Almost nothing is known about the structure of the sources which produce the earth’s gravitational field. Therefore these investigations may be of interest for the inverse problem in gravimetry. The following question remains unanswered in this review: to find additional conditions to select uniquely a measure producing the earth’s gravitational field. This question is unanswered in most inverse source p

ISSN:8755-1209    

DOI:10.1029/RG019i002p00299

年代:1981

数据来源: WILEY

摘要:

A review is presented of the status of our knowledge of electromagnetic cyclotron instabilities in multi‐component magnetosphericlike plasmas consisting of a hot anisotropic population and of additional cold plasma components, with special emphasis on the nonlinear phenomena. To achieve this, the results of a large number of computer simulation experiments are considered. While the paper deals primarily with naturally occurring magnetospheric phenomena, implications for the active space (seeding) experiments aimed at producing artificial auroras and consequently the verification of magnetospheric theories are also discusse

ISSN:8755-1209    

DOI:10.1029/RG019i002p00307

年代:1981

数据来源: WILEY

ISSN:8755-1209    

DOI:10.1029/RG019i002p00344

年代:1981

数据来源: WILEY