Systematic Regional Variations Research Articles

Localized velocity anomalies in the lower mantle

Summary. Two localized regions of velocity heterogeneity in the lower mantle with scale lengths of 1000–2000 km and 2 per cent velocity contrasts are detected and isolated through comparison of S, ScS, P and PcP travel times and amplitudes from deep earthquakes in Peru, Bolivia, Argentina and the Sea of Okhotsk. Comparison of the relative patterns of ScS-S differential travel times and S travel-time residuals across North American WWSSN and CSN stations for the different source regions provides baselines for interpreting which phases have anomalous times. A region of low S and P velocities is located beneath Northern Brazil and Venezuela at depths of 1700–2700 km. This region produces S-wave delays of up to 4 s for signals from deep Argentine events recorded at eastern North American stations. The localized nature of the anomaly is indicated by the narrow bounds in azimuth (15°) and take-off angle (13°) of the arrivals affected by it. The long period S-waves encountering this anomaly generally show 30–100 per cent amplitude enhancement, while the short-period amplitudes show no obvious effect. The second anomaly is a high-velocity region beneath the Caribbean originally detected by Jordan and Lynn, who used travel times from deep Peruvian events. The data from Argentine and Bolivian events presented here constrain the location of the anomaly quite well, and indicate a possible short- and long-period S-wave amplitude diminution associated with it. When the travel-time data are corrected for the estimated effects of these two anomalies, a systematic regional variation in ScS-S station residuals is apparent between stations east of and west of the Rocky Mountains. One possible explanation of this is a long wavelength lateral variation in the shear velocity structure of the lower mantle at depths greater than 2000 km beneath North America.

Systematic Variations in Inshore Bathymetry

To specify and characterize systematic regional variations in inshore bathymetry, the eigenvectors of 504 profiles of the U.S. Atlantic and Gulf coasts were calculated. The first three eigenvectors calculated account for over 97% of the topographic variance. The first eigenvector represents nearly linear departures from the mean; the second and third eigenvectors represent curvilinear bathymetrie variation; i.e., bar/trough morphology. The orthogonality of the linear slope and curvilinear bar/trough forms does not support the concept of a functional relationship between slope and the frequency of bars. In addition no relationship was revealed between inshore bar/trough morphology and offshore slope within nine miles (14 km) of the shoreline.

Systematic Variations in Offshore Bathymetry

To specify and characterize systematic regional variations in inshore bathymetry, the eigenvectors of 504 profiles of the U.S. Atlantic and Gulf coasts were calculated. The first three eigenvectors calculated account for over 97% of the topographic variance. The first eigenvector represents nearly linear departures from the mean; the second and third eigenvectors represent curvilinear bathymetrie variation; i.e., bar/trough morphology. The orthogonality of the linear slope and curvilinear bar/trough forms does not support the concept of a functional relationship between slope and the frequency of bars. In addition no relationship was revealed between inshore bar/trough morphology and offshore slope within nine miles (14 km) of the shoreline.

P-wave travel times from deep earthquakes

A study of teleseismic P -wave travel times has been made using data from deep (450 to 650 km) earthquakes exclusively, for which the effect of near-source upper mantle heterogeneity is expected to be small and location parameters are considered reliable. About 3,300 arrival times selected from bulletins and winnowed to remove discrepant data were used. The shape of the travel-time curve and the average station anomalies found are in general agreement with the results of other works (e.g. Herrin et al. , 1968) but the scatter of the data is only about half as large (∼0.6 sec), verifying that deep earthquakes are indeed more suitable than shallow ones for travel-time studies. The shape of the travel-time curve is determined within 0.1 to 0.2 sec out to epicentral distances of 70°, but the absolute level of the curve remains uncertain within about ± 1 sec. This “base-line correction” has been estimated from Nevada Test Site explosion data, for which the upper mantle velocity structure near the source is known and could be corrected for. The results of this procedure indicate that the Jeffreys-Bullen (1940) times are about 1.5 sec late but the model of Herrin et al. (1968) has approximately the correct base line of P travel times at least for focal depth of 550 km. The station anomalies of this study, represented by constants, correlate very well with large-scale tectonic features and also give a better fit to the travel-time data than has been achieved in other studies which included azimuthal station terms. Systematic regional variations in travel time of 1 sec or more occur beyond 70°, and indicate that significant lateral variation occurs in the lower mantle (depth > 2000 km). A conservative lower bound of about 1 per cent can be placed on the magnitude of these variations in compressional velocity.

REGIONAL IMPACT OF WATER RESOURCE INVESTMENTS IN A DEVELOPING AREA1

ABSTRACT: Regional development and industrialization patterns are investigated and related via regression analysis to water resource investments for the island of Puerto Rico. Although results of this study indicate such investments have little immediate or short‐term impact, significant relationships and variations in regional responses appear over longer time periods. This is shown by applying a variation of Zellner's method of performing seemingly unrelated regressions jointly. By this method, subsets of parameter coefficients of specific economic variables were restricted across regional equations while unrestricted coefficients were interpreted as explaining systematic regional variations in response to public investment. Regional differences, obtained by using this method, are frequently neglected when simply examining the overall development process. Among the more interesting results in terms of policy implications is the apparent significant relationship, over the period considered, between changes in the distribution of income and the pattern of water resource development.

Two-stage least-squares model for the relationship between mappable geological variables

A method is developed for expressing a dependent variable that is subject to systematic regional variations (trend) in terms of a set of independent geological variables. Trend surfaces are fitted to the independent variables, and the dependent variable is regressed on both the fitted trend values and the original observations for the independent variables. The method is applied to evaluate the areal distribution of gold occurrences in the Greenbelt of western Quebec as a function of a set of lithological variables.

地震と火山との相補的分布

Active volcanoes of an island arc are situated in a specific belt, which is called a volcanic belt and parallels to an oceanic trench associated with the island arc. The trenchside border of a volcanic belt is named a volcanic front, as volcanoes are clustered rather near the border on the trench side than at the axis of the belt (Figs. 1 and 2).Mantle earthquakes of an island arc show a systematic distribution of the foci on a plane dipping away from an oceanic trench, the plane being called a seismic plane.Mantle earthquakes are closely related in space to volcanoes of the island arc. The depths of the seismic plane underneath the volcanic belt are limited to a range from 150 to 250km. On the other hand, basic differences in chemical composition among the basaltic primary magmas reflect differences in the depth at which the magmas were generated, instead of successive stages in the differentiation of magma. The depths of the seismic plane appear to be responsible for the magma-generation depths, as the systematic regional variation in chemical composition of the primary magmas shows a close correlation to the regional variation in depth of the seismic plane. Therefore, the generation of mantle earthquakes and the generation of primary magmas would have a common origin (Fig. 3).Complementary relationships in distribution between hypocenters of mantle earthquakes and the magma-generation zone are stressed on in this paper and are summarized as follows:1. The hypocenters of mantle earthquakes are abundant in the frontal area of the volcanic front, where no volcanoes are present. To the contrary, they are rare underneath the volcanic belt. The relationship is clearly given in a map of Japan (Figs. 4 and 5).2. Ratio of total numbers of mantle earthquakes at depths from 175 to 275km to total numbers of those from 75 to 125km is distinctly lower in island arcs than in non-islandarc regions. The relationship is given in a histogram and a table. Under the island arcs the hypocenters are relatively scarce at depths from 175 to 275km, where magmas would be generated (Fig. 7 and Table 1).3. Along the belt at the back of a volcanic front, either the bulk of the belt is occupied by many of volcanoes as is the case in Japan, or some sections of the belt are occupied by a series of volcanoes as is the case in the Pacific coast region of South America. In the latter case, nonvolcanic sections of the belt at the back of an extension of the volcanic front display higher seismicity in the mantle.Thus the greater part of epicenters of mantle earthquakes shows a complementary distribution to loci of volcanoes in island arcs. It would imply an exclusive relation between the generation of mantle earthquakes and the generation of primary magmas. Therefore, it seems very likely that the source of energy is common with the each other and the difference of generating whether shocks or magmas is attributed to a condition of circumstance, for instance, the temperature of the particular place.