1) Possible Upper and Lower Inner Core Structure

Donald V. Helmberger, Xiaoming Ding, and Xiaodong Song


Recent papers have introduced an asymmetry in the anisotropic structure of the Inner Core, Su and Dziewonski (1995), and hemispheric variations, Tanaka and Hamaguchi (1997), McSweeney et al. (1995), with large scatter in individual picks. We propose an interpretation of this data in terms of a transitional structure mostly isotropic (upper) surrounding the anisotropic (lower) inner core with possible variable thickness. We suppose that the interface between these structures is complex and scatters short-period signals similar to Upper-Mantle structure and is most easily studied at longer periods. In this paper, we model broadband PKP triplication data at ranges 146 to 154 from South Sandwich Island to northwestern North America and show that these waveforms are consistent with an isotropic layer 300 km thick overlaying in anisotropic (5%) lower inner core.

2) Extremes in CMB Structure

Donald V. Helmberger, Xiaoming Ding, and Lianxing Wen


Recent broadband (BB) lower mantle studies has emphasized the variability of D", especially in regions of down-welling (Caribbean) and up-welling (Africa). In particular, the early evidence for a shear-wave triplication caused by a 3% jump in velocity a few hundred kms above the CMB beneath the Caribbean has been well established although with considerable variation in depthness. An equivalent P-wave triplication sampling the same region appears undetectable as commonly observed in other areas. Complicated seismic structure predicted from dynamic modeling of down-welled slab debris show considerable promise in explaining this paradox by suggesting an upper and a lower thermal boundary layer. A gentle positive gradient is initiated 350 km above the CMB with a sharper increase near 200 km reaching velocities of 13.5 km/sec in P and 7.5 km/sec in S. A strong negative gradient begins about 100 km above the CMB, similar to previous studies. This model explains the BB waveforms of P and S waves observed at TERRAscope quite well and is also compatible with the recent tomographic models, Grand (1995) and van Hilst et al. (1996).

Shear wave record sections across Africa and Europe containing the cross-over from S to SKS and extended core-phases (75 to 120) are presented from deep South American events. These are compared against corresponding 2D synthetics computed from various tomography models. Some of the most recent models, Grand's (1996), work quite well in explaining the observations from Africa. However, considerable fine-tuning is required in D" to explain abrupt changes in S and ScS waveforms and the extreme chances in SKS-S travel times.

The most spectacular examples of SKPdS distortions occur beneath mid-Africa and beneath Iceland. The latter is most easily explained by a ULVZ zone of dome shape, horizontal dimension of 500 km and height (30-60 km) depending on severity of S-wave velocity drop and density increase.

Don Helmberger ( cui@gps.caltech.edu)

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