CAN THE DIFFERENTIAL SENSITIVITY OF BODY WAVE, MANTLE WAVE, AND NORMAL-MODE DATA RESOLVE THE TRADE-OFF BETWEEN TRANSITION ZONE STRUCTURE AND BOUNDARY TOPOGRAPHY
Em. Lavely et al., CAN THE DIFFERENTIAL SENSITIVITY OF BODY WAVE, MANTLE WAVE, AND NORMAL-MODE DATA RESOLVE THE TRADE-OFF BETWEEN TRANSITION ZONE STRUCTURE AND BOUNDARY TOPOGRAPHY, Physics of the earth and planetary interiors, 86(1-3), 1994, pp. 117-146
Large-scale seismic models of the three-dimensional (3D) variations in
elastic properties will be biased by topography on mantle boundaries
to the extent that volumetric and topographic structures produce simil
ar effects in the data. To date, seismic inversions for global-scale 3
D elastic models of the mantle have largely ignored the effect that to
pography on the major mantle discontinuities would have on estimating
these models. In this paper we address three questions: (1) to what ex
tent does unmodeled structure on the 410 and 660 km boundaries bias vo
lumetric structure in inversions based on normal mode-mantle wave stru
cture coefficients, absolute S-wave travel times, and differential SS-
S travel times? (2) Can the differences in the sensitivity of S waves,
SS-S phase pairs, and normal mode-mantle wave data be exploited to es
timate transition zone volumetric models that are relatively unbiased
by topographic structure? (3) Have current volumetric models resolved
the trade-off that exists between volumetric and topographic structure
s? To address question (1), synthetic experiments were performed which
show that volumetric models inferred from normal mode-mantle wave dat
a can be biased by an average value of 20-25% in r.m.s. amplitude in t
he transition zone relative to current aspherical Earth models, depend
ing on the model parametrization employed. The average transition zone
volumetric bias of models inverted from absolute travel times from bo
th transition zone and lower-mantle bottoming S rays that are imprinte
d with the same topographic signatures is reduced by at least by a fac
tor of five relative to models inverted from normal mode-mantle wave d
ata alone. A reduction in bias by a factor of about four is obtained u
sing SS-S phase pairs in which the SS legs bottom in both the transiti
on zone and lower mantle. The use of lower-mantle bottoming S rays or
SS-S phase pairs with the SS legs bottoming in the lower mantle reduce
s the bias only by an average factor of about two to three relative to
the normal mode-mantle wave inversion. These estimates of bias reduct
ion can vary with the type of damping or smoothness constraints that a
re applied in the inversion. With respect to question (2), these resul
ts suggest that, in principle, absolute S-wave travel time data can be
used to desensitize volumetric inversions to the bias caused by topog
raphy on the transition zone boundaries. In practice, however, near-so
urce structure that contaminates S-wave travel times would diminish th
is capability. The use of SS-S differential times for SS waves which b
ottom in the transition zone and mantle waves sensitive to transition
zone structure but insensitive to the 660 km boundary, should be the m
ost effective means of overcoming the trade-off. To address question (
3), we adopt the criterion that the combination of an unbiased volumet
ric model with an accurate topographic model should provide a better f
it to normal mode structure coefficients than the volumetric model alo
ne. The boundary model Topo660a when added to the volumetric model S12
_WM13 fits the normal mode structure coefficients significantly better
than the volumetric model alone. However, more recent models of 410 a
nd 660 km boundary topography degrade the fit of the volumetric models
S12_WM13 and SH.10c.17 to the normal mode structure coefficients, sug
gesting that there is not yet a conclusive answer to this question.