NORMAL-MODE CONSTRAINTS ON THE STRUCTURE OF THE EARTH

We have analyzed the splitting characteristics of 75 spheroidal free o scillations excited by the Great 1994 Bolivia and Kuril Islands earthq uakes. These spheroidal modes may be roughly subdivided in terms of 8 radial modes, 40 mantle modes, and 27 core-sensitive modes. The splitt ing of each mode is corrected for the effects of rotation and hydrosta tic ellipticity. The remaining signal is due to lateral variations in the mantle and core and may be expressed in terms of so-called splitti ng functions, which represent a local radial average of the Earth's ev en three-dimensional heterogeneity. In the surface-wave limit, splitti ng functions are the equivalent of an even-degree phase velocity map. Each mode is uniquely sensitive to the Earth's structure. Some modes a re predominantly sensitive to compressional velocities in the upper ma ntle, others to shear velocity variations in the lowermost mantle, and some modes ''see'' the inner core. As part of our analysis, we determ ine the center frequency and quality factor of each individual mode; t hese observations constrain the terrestrial monopole. Collectively, th e normal-mode splitting observations presented in this paper put const raints on the large-scale, even structure of the entire Earth. We comp are the observed splitting functions with predictions from three recen t Harvard models: SH12WM13, SKS12WM12, and PS12WM13. These models are constrained by traveltime and waveform data but contain no normal-mode information. We demonstrate that large-scale, even structure is quite accurately represented in current Earth models, but that the splittin g of some predominantly compressional modes is not satisfactorily expl ained. Although a distinct mantle signal is observed in the splitting functions of core-sensitive modes, a characteristic zonal degree 2 pat tern is missing. This missing signal is believed to be the result of i nner core anisotropy.