Imaging small-scale mantle heterogeneities using seismic arrays

In recent years seismology has been used extensively to detect and locate the small
scale (~10 km) structure of the Earth. In the mantle these structures likely represent
chemical heterogeneity and are essential in our understanding of mechanical mixing
processes within mantle convection. As subducted crust is chemically distinct from the
background mantle, imaging the remains of the crust provides a tracer for convectional
flow. In this study global and regional seismic heterogeneities in the mantle are found by
processing teleseismic earthquake data through array seismology methods. Scattered
energy from shallow earthquakes that arrives as PP precursors is studied in a 100 s
quiet window before the main PP.
Global average stacks of the PP wavefield are formed using data recorded at a global
distribution of seismic arrays, for distances 70-120°. The resultant global stacks of PP
have revealed that precursors to PP exist for all distances with the amplitudes increasing
with distance and time. Regional stacks for the Pacific and Atlantic Oceans are found
to contain very similar patterns of PP precursors suggesting scattering observed here
does not vary with tectonic region. Global averages of PP precursors are modelled using
a Monte Carlo phonon method that generates statistical scattering models for random
media. Modelling results show that heterogeneities in the crust and mantle contribute
to scattered PP wavefield. The best models found have 1% scattering strength in the
crust, with reduced scattering strength of 0.8% in the uppermost mantle and an increase
in scattering strength to 1% at ~700 km. Correlation length also varies from 2 km in
the crust to 6 km in the mantle. The extent of the deeper mantle layer of heterogeneity
is not well resolved and may be determined using larger epicentral distances.
Regional patterns of heterogeneity are found from PP precursors that are scattered
from small-scale heterogeneities in subduction zones. Array methods are applied to
data in the epicentral distance range of 90°-110° from Eielson Array in Alaska, to
calculate directivity and to enhance weak arrivals. Coherent precursors are selected
automatically based on a semblance weighted beampower spectrum. Assuming single
P-to-P scattering and using the directivity information from array processing, the origin
of scattering is found by ray-tracing through a 1D velocity model. Most scatterers
are imaged in western Pacific subduction zones with evidence for ~300 small-scale heterogeneities
in the region around the present day Japan, Izu-Bonin, Mariana and West
Philippine subduction zones. Most of the detected scatterers are located in the crust
and upper mantle, but 6% are located deeper than 600 km. Scatterers in the transition
zone correlate well with edges of fast features in tomographic images and subducted
slab contours derived from slab seismicity. Deeper scatterers are located beneath the
Izu-Bonin/Mariana subduction zones, which outline a steeply dipping pseudo-planar
feature to 1480 km depth, and beneath the ancient (84-144 Ma) Indonesian subduction
trench down to 1880 km depth. The cause of scattering is likely the underside reflection
of the subducted Moho of subducted crustal material and are related to past and
present subduction providing evidence that the subducted crust does descend into the
lower mantle at least for these steeply dipping subduction zones.
Combining the findings from both global and regional studies, it is likely that the
heterogeneities detected in these studies are related to different stages of the mantle
mixing cycle. As such a simple model for mid-mantle heterogeneity applicable to
subduction zones has been suggested, with a well mixed mid-mantle of 6 km scale heterogeneities
that have been thinned through mechanical stirring and a steeply dipping
slab with attached crust penetrating to the lower mantle.