Finite-Frequency Tomography of D’’ Shear Velocity
Heterogeneity beneath the Caribbean and Central America
Shu-Huei Hung1, Ling-Yun Chiao2, Edward J. Carnero3, Ban-Yuan Kuo4, and Thorne Lay5
1
: Department of Geosciences, National Taiwan University, Taiwan
2
: Institute of Oceanography, National Taiwan University, Taiwan
3
: Department of Geological Sciences, Arizona State University, USA
4
: Institute of Earth Sciences, Academia Sinica, Taiwan
5
: Institute of Tectonics, University of California, Santa Cruz, USA
ABSTRACT
Differential shear-wave travel-time residuals measured by high-quality waveform
data from South America earthquakes recorded by seismic stations in North America
are utilized to explore shear velocity variations in the D’’ region beneath the
Caribbean and Central America (Garnero and Lay, 2003). The emergency of
triplication arrivals, Sab and Scd, refracted above and below the top of the D” region,
respectively, suggests the existence of a seismic discontinuity at the top of the D”
layer and the increase in S-wave velocity across this D” discontinuity (e.g., Kendall
and Nangini, 1996; Garnero and Lay, 2003). Classical ray-theoretical travel-time
analysis indicates that relatively high velocities with respect to one-dimensional (1-D)
PREM model are persistent throughout the study region, with a locally
short-wavelength variability related to lateral variations in the thickness and velocity
gradients of the D” layer. However, a typical teleseismic S or ScS arrival with a
dominant period of ~8 s has a Fresnel zone of ~500 km wide across the geometrical
raypath at the bottoming depth near the core-mantle boundary (CMB). Any
anomalous velocity structures in the D” region on the length scale smaller than 500
km can be substantially obscured in accrued travel-time shifts due to diffractive
wavefront healing and hence underestimated in the inferred heterogeneity strengths.
Recent development in the banana-doughnut kernel theory has integrated off-path
wave scattering effects to amend the infinite-frequency deficiency in ray theory.
The Born paraxial approximation in conjunction with body-wave propagation theory
yields a concise formulation that can be implemented efficiently for the inversion of
massive seismic dataset. This advanced theory takes into account intrinsic
wavefront healing and other finite-frequency effects and possesses the capability of
projecting the volumetric sensitivity for an actually finite-frequency travel-time shift
into “target” wavespeed perturbation (e.g., Dahlen et al., 2000; Hung et al., 2000,
2001). This finite-frequency approach has been launched in both global and
regional seismic tomographic studies; the resolved models have significantly
improved the resolution in both shape and strength of seismic velocity heterogeneity
(Montelli et al., 2004a, b; Hung et al., 2003).
To better understand the thermal, chemical and geodynamical causes for the
complicated environment in the D” region, we employ finite-frequency tomography
to image fine-scale shear velocity heterogeneity in the lowermost 500-km mantle
beneath the Central America using differential ScS-S, S(Sdiff)-SKS, Scd-S, and
ScS-Scd travel-time residuals as constraints. Prior to the tomographic inversion all
the differential times are corrected for contributions from aspherical mantle structure
based on high-resolution global models (Ritsema and Van Heist, 2000; Megnin and
Romanowicz, 2000; Grand, 2002). The 3-D banana-doughnut-shaped kernel for
individual pair of differential travel-time measurement is constructed in a
radially-symmetric model SKNA1 which allows a seismic discontinuity at the depth
of 2636 km and a positive velocity gradient on the top of the D” layer. Different
from the intuitive 1-D ray interpretation in which a ScS phase essentially samples the
velocity structure at the reflection point right on the CMB and a Scd wave is most
sensitive to the top D” layer at the bottoming depth of the raypath, alternatively the
regions besieged by finite-frequency ScS and Scd arrivals are interchanged because
of their widespread off-path sensitivity. We will present robust D” features beneath
the Caribbean and Central America derived from the kernel-based tomographic
model and discuss its implication on the nature of the D” layer in the transition from
the liquid iron-rich core to the solid silicate mantle.
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