Seismic structure above and below the core-mantle boundary (CMB) has been studiedthrough use of travel time and waveform analyses of several different seismicwave groups. Anomalous systematic trends in observables document mantle heterogeneityon both large and small scales. Analog and digital data has been utilized,and in many cases the analog data has been optically scanned and digitized prior toanalysis.
Differential travel times of S - SKS are shown to be an excellent diagnostic ofanomalous lower mantle shear velocity (V s) structure. Wavepath geometries beneaththe central Pacific exhibit large S- SKS travel time residuals (up to 10 sec), andare consistent with a large scale 0(1000 km) slower than average V_s region (≥3%).S - SKS times for paths traversing this region exhibit smaller scale patterns andtrends 0(100 km) indicating V_s perturbations on many scale lengths. These times arecompared to predictions of three tomographically derived aspherical models: MDLSHof Tanimoto [1990], model SH12_WM13 of Suet al. [1992], and model SH.10c.17of Masters et al. [1992]. Qualitative agreement between the tomographic modelpredictions and observations is encouraging, varying from fair to good. However,inconsistencies are present and suggest anomalies in the lower mantle of scale lengthsmaller than the present 2000+ km scale resolution of tomographic models. 2-Dwave propagation experiments show the importance of inhomogeneous raypaths whenconsidering lateral heterogeneities in the lowermost mantle.
A dataset of waveforms and differential travel times of S, ScS, and the arrivalfrom the D" layer, Scd, provides evidence for a laterally varying V_s velocity discontinuityat the base of the mantle. Two different localized D" regions beneaththe central Pacific have been investigated. Predictions from a model having a V_sdiscontinuity 180 km above the CMB agree well with observations for an easternmid-Pacific CMB region. This thickness differs from V_s discontinuity thicknessesfound in other regions, such as a localized region beneath the western Pacific, whichaverage near 280 km. The "sharpness" of the V_s jump at the top of D", i.e., thedepth range over which the V_s increase occurs, is not resolved by our data, and ourdata can in fact may be modeled equally well by a lower mantle with the increase inV_s at the top of D" occurring over a 100 krn depth range. It is difficult at present tocorrelate D" thicknesses from this study to overall lower mantle heterogeneity, due touncertainties in the 3-D models, as well as poor coverage in maps of D" discontinuitythicknesses.
P-wave velocity structure (V_p) at the base of the mantle is explored using theseismic phases SKS and SPdKS. SPdKS is formed when SKS waves at distancesaround 107° are incident upon the CMB with a slowness that allows for coupling withdiffracted P-waves at the base of the mantle. The P-wave diffraction occurs at boththe SKS entrance and exit locations of the outer core. SP_dKS arrives slightly later intime than SKS, having a wave path through the mantle and core very close to SKS.The difference time between SKS and SP_dKS strongly depends on V_p at the baseof the mantle near SK Score entrance and exit points. Observations from deep focusFiji-Tonga events recorded by North American stations, and South American eventsrecorded by European and Eurasian stations exhibit anomalously large SP_dKS -SKS difference times. SKS and the later arriving SP_dKS phase are separated byseveral seconds more than predictions made by 1-D reference models, such as theglobal average PREM [Dziewonski and Anderson, 1981] model. Models having apronounced low-velocity zone (5%) in V_p in the bottom 50-100 km of the mantlepredict the size of the observed SP_dK S-SKS anomalies. Raypath perturbationsfrom lower mantle V_s structure may also be contributing to the observed anomalies.
Outer core structure is investigated using the family of SmKS (m=2,3,4) seismicwaves. SmKS are waves that travel as S-waves in the mantle, P-waves in thecore, and reflect (m-1) times on the underside of the CMB, and are well-suited forconstraining outermost core V_p structure. This is due to closeness of the mantlepaths and also the shallow depth range these waves travel in the outermost core.S3KS - S2KS and S4KS - S3KS differential travel times were measured usingthe cross-correlation method and compared to those from reflectivity synthetics createdfrom core models of past studies. High quality recordings from a deep focusJava Sea event which sample the outer core beneath the northern Pacific, the Arctic,and northwestern North America (spanning 1/8th of the core's surface area), haveSmKS wavepaths that traverse regions where lower mantle heterogeneity is pre-dieted small, and are well-modeled by the PREM core model, with possibly a smallV_p decrease (1.5%) in the outermost 50 km of the core. Such a reduction implieschemical stratification in this 50 km zone, though this model feature is not uniquelyresolved. Data having wave paths through areas of known D" heterogeneity (±2%and greater), such as the source-side of SmKS lower mantle paths from Fiji-Tongato Eurasia and Africa, exhibit systematic SmKS differential time anomalies of upto several seconds. 2-D wave propagation experiments demonstrate how large scalelower mantle velocity perturbations can explain long wavelength behavior of suchanomalous SmKS times. When improperly accounted for, lower mantle heterogeneitymaps directly into core structure. Raypaths departing from homogeneity playan important role in producing SmKS anomalies. The existence of outermost coreheterogeneity is difficult to resolve at present due to uncertainties in global lowermantle structure. Resolving a one-dimensional chemically stratified outermost corealso remains difficult due to the same uncertainties. Restricting study to highermultiples of SmKS (m=2,3,4) can help reduce the affect of mantle heterogeneitydue to the closeness of the mantle legs of the wavepaths. SmKS waves are ideal inproviding additional information on the details of lower mantle heterogeneity.
