South-central Pacific Research Articles

Measurements of mantle wave velocities and inversion for lateral heterogeneities and anisotropy: 3. Inversion

Lateral heterogeneity in the earth's upper mantle is investigated by inverting dispersion curves of long‐period surface waves (100–330 s). Models for seven different tectonic regions are derived by inversion of regionalized great circle phase velocity measurements from our previous studies. We also obtain a representation of upper mantle heterogeneities with no a priori regionalization from the inversion of the degree 6 spherical harmonic expansion of phase and group velocities. The data are from the observation of about 200 paths for Love waves and 250 paths for Rayleigh waves. For both the regionalized and the spherical harmonic inversions, corrections are applied to take into account lateral variations in crustal thickness and other shallow parameters. These corrections are found to be important, especially at low spherical harmonic order the “trench region” and fast velocities down to 250 km under shields. Below 200 km under the oceans, both S velocity and S anisotropy support a model of small‐scale convection in which cold blobs detach from the bottom of the lithosphere when its age is large enough. The spherical harmonic models clearly demonstrate (a posteriori) the relation between surface tectonics and S velocity heterogeneities in the first 250 km: all shields are fast; most ridges are slow; below 300 km, a belt of fast mantle follows the Pacific subduction zones. However, at greater depths, large‐scale heterogeneities that seem to bear no relationship to surface tectonics are observed. The most prominent feature at 450 km is a fast‐velocity region under the South Atlantic Ocean. Smaller‐scale heterogeneities that are not related to surface tectonics are also mapped at shallower depths: an anomalously slow region centered in the south central Pacific is possibly linked to intense hot spot activity; a very fast region southeast of South America may be related to subduction of old Pacific plate. Between 200 and 400 km, a belt of SV>SH anisotropy follows part of the ridge and subduction systems, indicating vertical mantle flow in these regions. The spherical harmonic results open new horizons for the understanding of convection in the mantle. Perspectives for the improvement of the models presented are discussed.

Regional Differences in Divorce in the United States

The authors examine regional variations in the US rate with the concept of social integration as an explanation for these variations. Social integration includes residential mobility community siex religious preference frequency of attendance of religious services and age at marriatge. The most striking aspect of the findings is the large regional variation in all of the indicators of marital dissolution; the divorce belt (the West South Central Mountain and Pacific census divisions) has rates about 50% higher than those of the rest of the country. Adjusting for differences in racial marital religious and socioeconomic composition reduced regional variations only a little. A negative indicator of social integration called the Residential Mobility Index is related to a measure of marital dissolution from which the effects of years of exposure to the risk of religious background socioeconomic background and several community and family background variables have been removed. The relationship among the 9 regions is very strong (r=.978) and is also very strong when the measure of marital dissolution is computed only for persons living in the same state they lived in at age 16 (r=.942). The extremely high level of marital dissolution in the American divorce belt can be accounted for by a high level of residential movement there; intervening variables are probably a symptom of variables associated with level of social integration. (authors modified)

Variation of amplitude of PKP from underground explosions in the southcentral Pacific

Taking advantage of the French nuclear test site in Southcentral Pacific for the analysis of PKP seismic waves, a systematic study around amplitude and arrival times was untertaken. A peak in the amplitude versus distance curves at 145°5 was observed with amplitudes up to 20 times larger than observed amplitudes in the 130‐140° distance range. This peak, together with six secondary maxima in the 142‐147° distance range is in good agreement with theoretical amplitude versus distance curves computed from interference phenomena and Airy diffraction theory. Some variation of blast site epicenter and energy provides the best check of the amplitude oscillations versus frequency and distance.

Teleseismic location techniques and their application to earthquake clusters in the South-Central Pacific

abstract Improved methods for single-and multiple-event hypocenter determinations are developed and applied to the problem of locating earthquake clusters in the South-Central Pacific Ocean. Bayesian statistical methods are used to incorporate a priori information about arrival-time variance into the derivation of hypocenter confidence ellipsoids, permitting a more realistic calculation of critical parameters in the case where the number of stations is small. The diagonal elements of certain projection operators, called “data importances” by Minster et al. (1974), are used to evaluate network balance. The hypocentroid of an event cluster is defined to be the average location of events within the cluster, and the deviations of individual hypocenters from the hypocentroid are called cluster vectors . The problem of estimating the cluster vectors can be decoupled from the problem of estimating the hypocentroid by a simple but fundamental mathematical result, here termed the hypocentroidal decomposition theorem. The algorithm based on this analysis appears to have many advantages over other published methods for multiple-event location, both in its efficient use of available information and its computational speed. The application of this method to three clusters of shallow intraplate seismicity in the South-Central Pacific, designated Regions A, B, and C, demonstrates that the seismicity within each cluster is very localized; the rms lengths of the cluster vectors for each group of epicenters are estimated to be only 9, 6, and 12 km, respectively. Estimates of the epicentroids are

Strontium isotopes in basalts from the Pacific Ocean basin

Basalts being erupted in the Pacific Ocean Basin vary in Sr isotopic composition in a simple geographic pattern. 87Sr/ 86Sr increases away from the East Pacific Rise to very high values for islands in the south-central Pacific. The 87Sr/ 86Sr variations are almost certainly related to past segregation of Rb, K, and other large cations. The segregation process was probably incipient partial melting which resulted in various mantle zones being enriched or depleted.

The vascular flora and terrestrial vertebrates of Vostok Island, South-Central Pacific

The vascular flora and terrestrial vertebrates of Vostok Island, South-Central Pacific

Geology of the Cook Islands

Abstract The geology of the 15 Cook Islands in the south-central Pacific is briefly described and their geological history outlined. All are the summit portions of extinct Tertiary volcanoes; six of the seven Northern Group islands are atolls, four of the Southern Group are makatea-type islands, and the others include a high mountainous volcanic island, a hilly near-atoll, an atoll, and a sand cay. Radiometric ages from Rarotonga are Late Pliocene, foraminifera from the makatea (raised coral reef) of Mangaia are Oligocene or Miocene, and much of the limestone underlying the atolls is probably middle Tertiary.