Mapping the lowermost mantle anomaly beneath northern Siberia using PcP precursors from near grazing incident P-waves
Abstract
A detailed study was conducted concerning
a new area of the lowermost mantle anomaly beneath northern Siberia.
We identified precursors to PcP phases in records
from the Network of Autonomous Registrating Seismographs (NARS)
located within, or nearby, the Netherlands.
With the aid of differential slowness versus differential time plots,
18 events were analysed.
The precursors
appeared clearly on some of the stacked records obtained from registrations
of this dense seismic network, for epicentral distances of 70 to 82 degrees.
The precursors arrived 2 to 8 s before
the PcP phase with a differential slowness, with respect to the direct
P arrival, of -0.0043 to -0.014 s/km.
Bounce points showing no precursors as well as
reflections from D''
were found to be lying closely together, within the Fresnel zone.
One-dimensional forward modeling explained the
precursors as reflections from the top of the D'' layer at 219 to 316 km
above the core-mantle boundary. The reflector was shown to be
characterized
by a 2.8 to 3.1% increase in P velocity compared to the overlying
lower mantle.
Synthetic seismograms
generated by a one-dimensional model could only partly be fitted to
the stacked data,
confirming the existence of lateral variations of the D'' layer.
Focusing and defocusing effects occurred within the Fresnel zone
due to topography on top of the D'' layer.
These features are also explained by the large variations found in the
thickness of the reflector.
Frequency dependent focusing
was used, in a qualitative way, to explain a large PdP versus
P amplitude ratio observed
in one of the stacked datasets.