SOCIAL SECURITY REFORM: HOW TO MAKE IT SECURE

Public Economics,

INTERSTATE TAX DIFFERENTIALS RELATED TO ECONOMIC GROWTH

Public Economics,

SOCIAL SECURITY REFORM: HOW TO MAKE IT SECURE

Public Economics,

THE CHANGING DISTRIBUTION OF THE BURDEN OF FEDERAL INCOME TAXES

Public Economics,

ECONOMIC GROWTH IN MINNESOTA COMPARED TO NEIGHBORING STATES AND THE NATION AS A WHOLE AND PROJECTIONS TO 1980 AND TO 1990

Community/Rural/Urban Development,

ECONOMIC GROWTH IN MINNESOTA COMPARED TO NEIGHBORING STATES AND THE U.S.

Community/Rural/Urban Development,

Seismological support for the metastable superplume model, sharp features, and phase changes within the lower mantle

Recently, a metastable thermal-chemical convection model was proposed to explain the African Superplume. Its bulk tabular shape remains relatively stable while its interior undergoes significant stirring with low-velocity conduits along its edges and down-welling near the middle. Here, we perform a mapping of chemistry and temperature into P and S velocity variations and replace a seismically derived structure with this hybrid model. Synthetic seismogram sections generated for this 2D model are then compared directly with corresponding seismic observations of P (P, PCP, and PKP) and S (S, SCS, and SKS) phases. These results explain the anticorrelation between the bulk velocity and shear velocity and the sharpness and level of SKS travel time delays. In addition, we present evidence for the existence of a D" triplication (a putative phase change) beneath the down-welling structure

Deep mantle structure and the postperovskite phase transition

Seismologists have known for many years that the lowermost mantle of the Earth is complex. Models based on observed seismic phases sampling this region include relatively sharp horizontal discontinuities with strong zones of anisotropy, nearly vertical contrasts in structure, and small pockets of ultralow velocity zones (ULVZs). This diversity of structures is beginning to be understood in terms of geodynamics and mineral physics, with dense partial melts causing the ULVZs and a postperovskite solid–solid phase transition producing regional layering, with the possibility of large-scale variations in chemistry. This strong heterogeneity has significant implications on heat transport out of core, the evolution of the magnetic field, and magnetic field polarity reversals

Evidence for a chemical-thermal structure at base of mantle from sharp lateral P-wave variations beneath Central America

Compressional waves that sample the lowermost mantle west of Central America show a rapid change in travel times of up to 4 s over a sampling distance of 300 km and a change in waveforms. The differential travel times of the PKP waves (which traverse Earth's core) correlate remarkably well with predictions for S-wave tomography. Our modeling suggests a sharp transition in the lowermost mantle from a broad slow region to a broad fast region with a narrow zone of slowest anomaly next to the boundary beneath the Cocos Plate and the Caribbean Plate. The structure may be the result of ponding of ancient subducted Farallon slabs situated near the edge of a thermal and chemical upwelling