摘要:

Any seismologist trying even to carry, let alone read, the EOS abstract volumes for recent AGU Meetings knows full well of the substantial growth in seismological research during this reporting period, the four years of 1983 through 1986. Indeed, the number of Seismology Section abstracts has grown from 188 (Fall, 1982) to about 320 (Fall, 1986), to be more or less precise. At a time when research monies seem to be no better than stable (and declining in real terms) and when job opportunities for seismologists seem to have never been worse, at least in the professional lifetimes of most of us, something must be amiss, but certainly this is not the great vitality and diversity in seismological research during the past four years.The current reporting period saw the consortium approach brought to full flower in several fields of seismology, and these include CALCRUST, a consortium of California universities to investigate the crustal structure of the southwestern United States with seismic reflection data; DOSECC (Deep Observation and Sampling of the Earth's Continental Crust), a consortium to drill and make measurements within scientifically dedicated deep holes to sample active processes that make and remake the continents; EDGE, a consortium of university, government, and private industry scientists intent on exploring the oceanic/continental transitions along U.S. continental margins, using seismic and potential field methods; and IRIS (Incorporated Research Institutions for Seismology), whose prospectus includes a major upgrading of the global seismic network, an advanced portable array of 1000 seismic units for a host of active and passive experiments, and a data management center to store and utilize the vast quantities of data forthcoming from the first two activities. Each of these fledglings can trace their basic nature and motivation, if not their specific scientific agendas, to COCORP (Consortium for Continental Reflection Profiling), now a teenager, whose activities were summarized byPhinney and Odom[1983] for the last reporting period and by W. D. Mooney for the current one.

ISSN:8755-1209    

DOI:10.1029/RG025i006p01131

年代:1987

数据来源: WILEY

摘要:

While significant progress toward earthquake prediction has occurred in the past four years, the current guarded optimism for continued progress reflects in part revised goals and new definitions. First, recognition [e.g.,Evernden, 1982] of the failures of the search during the 1970s for a reliable readily‐observable earthquake precursor has reemphasized the need to understand the fundamental physics of the earthquake generation process [e.g.,Stuart, 1984/85] and brought acceptance of a probabilistic rather than a deterministic approach to earthquake prediction. That is, we now accept the notion that earthquake predictions must be couched in terms of probability gain and likelihood estimates. Second, a prediction terminology [Wallace et al., 1984] has been largely adopted that separates and expands the tasks into long‐term earthquake potential (no specific time window) and long‐term prediction (time windows of a few years to a few decades), where progress has been significant, and intermediate‐term prediction (time windows of a few weeks to a few years) and short‐term prediction (time windows up to a few weeks), where progress has lagged. In the 1970s, earthquake prediction usually was taken to mean a deterministic short‐term or intermediate‐term warning; now, probabilistic estimates of earthquake potential [e.g.,Lindh, 1983;Sykes and Nirhenko, 1984] are accepted as the most reasonable and valid form for expressing the likelihood of a future earthquake and its hazards, as well as the

ISSN:8755-1209    

DOI:10.1029/RG025i006p01135

年代:1987

数据来源: WILEY

摘要:

Research on seismicity and its relation to current tectonic processes by U.S. scientists and institutions advanced over the last four years on a wide front both topically and geographically. Common to much of this research is a growing recognition that preexisting structures and heterogeneous fault geometry strongly influence broad seismicity patterns [e.g.,Wesnousky et al., 1983;Wheeler and Bollinger, 1984] as well as the faulting style and rupture pattern of individual earthquakes [e.g.,Sibson, 1986;Jones et al., 1984]. Recognition of the latter, in particular, has enhanced the physical meaning of the widely‐used concepts “asperity,” “barrier,” “seismic gap,” and “characteristic earthquake” [Beck and Ruff, 1984;Mendoza and Dewey, 1984;Sanders and Kanamori, 1984;Sykes and Seeber, 1985; also seeSchwartz, this issue]. One topic that stands out in particular because of the increased attention given it over the last four years is volcanic seismicity. The eruptions of Mount St. Helens in 1980, El Chichon in 1982, Nevada del Ruiz in 1985, and the unrest in Long Valley caldera beginning in 1980 renewed interest in the long‐standing question regarding the nature and significance of volcanicb‐type (low‐frequency) earthquakes, harmonic (volcanic) tremor, and the relation of earthquakes to magmat

ISSN:8755-1209    

DOI:10.1029/RG025i006p01139

年代:1987

数据来源: WILEY

摘要:

Strong‐motion seismology is concerned with earthquake ground motion in the amplitude range that poses the threat of human injury or property damage. Because the occurrence of strong motion cannot be predicted in advance and because the time between significant events at any one place may range from tens to thousands of years, strong‐motion data is recorded on triggered instruments that typically must be maintained in readiness for years between periods of significant recording. As a consequence the strong‐motion data set is sparse. Because of the engineering need for estimates of future ground motion, the scope of the field of strong‐motion seismology encompasses seismic source theory and any other aspect of seismology that can provide insights helpful in making the best possible estimates given the limited data. This relationship with the rest of seismology is a reciprocal one in that strong‐motion data gives perhaps the best information available on earthquake source

ISSN:8755-1209    

DOI:10.1029/RG025i006p01149

年代:1987

数据来源: WILEY

摘要:

The deep interior structure of the earth has been extensively analyzed using a wide variety of seismic phases and techniques during the last four years. Most studies have emphasized quantitative three‐dimensional mapping of the lateral velocity heterogeneity of the mantle and core. These aspherical velocity variations are believed to be direct manifestations of thermal and compositional heterogeneity associated with convective processes. A first generation of global models for the lateral velocity variations in the deep earth has been produced, providing tantalizing images of large scale structures suggestive of a non‐steady state thermal convection system.The upper 400 km of the mantle has the strongest lateral velocity variations, of up to ±10% for shear velocity. Surface‐wave analyses that do not requirea prioriregionalizations have demonstrated that there is a strong association between surface tectonic provinces and the uppermost mantle velocity variations. Thus, the thermal and convective state of the upper 200 km of the mantle can be reliably interpreted in the context of plate tectonics. The upper mantle models support the contention that continents have deep roots, with differences in velocity structure from oceanic and active tectonic regions extending as deep as 400 km. Very long‐wavelength lateral velocity variations of a few percent have been detected in the transition zone at depths from 400 to 670 km, as well as throughout the lower mantle. These deep‐seated variations have little correspondence to surface tectonics, and efforts to interpret their nature are just beginning. The lowermost 200 km of the mantle (D″ region) has lateral velocity fluctuations comparable to those in the upper mantle, and evidence has been presented for the presence of a sizable velocity discontinuity at the top of the D″ layer. A combined thermal and compositional boundary layer, roughly mirroring the lithosphere, is a likely explanation for this anomalous zone. The core‐mantle boundary appears to have significant (10 km) long‐wavelength topography, presumably sustained by dynamic stresses from deep mantle convection. The inner core may have strong lateral heterogeneity or axially symmetric anisotropy, suggesting a complex thermal and compositional state.Significant progress has been made in characterizing the frequency dependence of anelastic attenuation in the mantle in the short‐period body‐wave band. Models for teleseismic P‐wave attenuation operators have converged, with t*values of 0.7–1.0 s appropriate at 1 Hz, and t*values of 0.4–0.6 s appropriate at 4 Hz. Regional variations of attenuation are slo

ISSN:8755-1209    

DOI:10.1029/RG025i006p01161

年代:1987

数据来源: WILEY

摘要:

More seismological studies of the continental crust and sub‐crustal lithosphere of the United States have been completed in the past four years than at any other similar period, and a continued growth in activity is likely to continue for years to come. Several trends account for this phenomenon. First, the interest in seismic reflection studies generated initially by COCORP results in this country [Brown and others, 1986], and later by the British BIRPS results [Matthews and Gheadie, 1986], has led to the development of several other seismic reflection programs. Among the most active of these research programs are those of the University of Wyoming [Smithson and others, 1986], Virginia Polytechnic Institute, CALCRUST [Henyey, 1986], and the U.S. Geological Survey (USGS) [Hamilton, 1986]. In Canada, the Lithoprobe program has achieved remarkable results in a variety of geographic locations [Green et al., 1986]. A second trend is the resurgence of interest in seismic‐refraction/wide‐angle reflection profiling. The year 1978 marked the beginning of this increased activity when several large projects were conducted in the western U.S., such as the Yellowstone‐Snake River Plain experiment organized by the University of Utah. Since 1979, a large amount of refraction/wide‐angle reflection data has been collected by the USGS and by university groups utilizing large numbers of state‐of‐the‐art industry seismographs in cooperative experiments. A third trend is the increased sophistication of other seismic methods such as teleseismic delay‐time methods, tomography, and receiver‐transfer functions. In these studies, permanent or temporary recording arrays have been used to determine local and regional crustal and upper‐mantle structure with

ISSN:8755-1209    

DOI:10.1029/RG025i006p01168

年代:1987

数据来源: WILEY

摘要:

The past four years have witnessed the introduction of a variety of new instruments and methods for the study of the seismic structure of the oceanic crust and lithosphere. The application of these and existing tools has led to the discovery of a number of new phenomena and to a fuller understanding of the genesis and evolution of the oceanic ithosphere. Borehole seismic instrumentation has become more widely employed; ocean bottom seismographs, while generally decreasing in number, have become significantly more reliable and useful; and multichannel seismic systems have been employed in innovative experiments ranging from studies of fracture zones to the regular detection of magma chambers beneath rise axes. The techniques available for the analysis of seismic data have become more sophisticated. Waveforms collected in seismic experiments can now be used directly in constructing and evaluating seismic velocity models, and travel time data are regularly inverted directly for structure. Trial and error modeling has become increasingly unimportant. Marine seismologists are becoming increasingly involved in understanding the coupling between the ocean and the underlying oceanic lithosphere. This has led to a more complete understanding of seafloor noise processes and the partitioning of energy between acoustic and elastic waves.

ISSN:8755-1209    

DOI:10.1029/RG025i006p01177

年代:1987

数据来源: WILEY

摘要:

Geologic studies of earthquakes involve mapping of coseismic surface faulting and secondary deformation from historical events, trenching and geomorphic analysis to define the timing and size of past earthquakes, and investigations of fault zone structure and geometry in both unconsolidated sediments and bedrock. This research is now being referred to as paleoseismology, seismic geology, and earthquake geology. Since the mid‐1970s, it has led to some of the most exciting and important contributions to the understanding of earthquake behavior in space and time [Hanks, 1985; Allen, 1986]. The present report is the first of what is hoped will become a regularly contributed summary of the geologic aspects of the study of earthquake

ISSN:8755-1209    

DOI:10.1029/RG025i006p01197

年代:1987

数据来源: WILEY

摘要:

IRIS is a university consortium organized to provide modernized seismographic networks and data distribution facilities for the university research community and funded by the National Science Foundation (NSF). There are currently 50 member institutions, each of which is represented on the Board of Directors. Overall policy and scientific guidance is provided by this Board acting through a 7‐member Executive Committee and three 9‐member Standing Committees representing each of the program elements. Technical and management support is provided by the President, and a Program Manager or Director for the three operational programs which are 1) Global Seismographic Network (GSN), 2) Portable Array Studies (PASSCAL), and 3) Data Management Center (DMC). This paper presents the historical background of development of IRIS and a review of current operational progr

ISSN:8755-1209    

DOI:10.1029/RG025i006p01203

年代:1987

数据来源: WILEY

摘要:

The period 1982 to 1986 has seen greatly increased activity in a number of problems related to the verification of nuclear test ban treaties. The greater activity results from a combination of scientific advances, major decisions by the United States Government about the negotiation of test ban treaties, the Soviet moratorium on testing, and much greater interest by the U.S. Congress in nuclear testing, test bans and other aspects of arms control. This review seeks to cover a range of activity that extends from scientific research to governmental decisions and public policy. A major aim has also been to provide a detailed bibliography on what is a wide range of topics. It is still not complete for what is a huge so‐called “gray literature” of reports to various government agencies. The discussion that follows only covers a selected number of these publications and re

ISSN:8755-1209    

DOI:10.1029/RG025i006p01209

年代:1987

数据来源: WILEY