In my Diplom thesis I am investigating the
Mantle structure beneath the Tonga-Fiji subduction zone.
Data :
The dataset consists of 306 deep earthquakes from the Tonga-Fiji region
from 1978 till 1984 which were registered at the Warramunga Array in North
Australia.
Most of the events are clearly deeper than 300 km up to a maximum depth
of about 650 km.
The time windows of the seismograms have a maximum length of 600 seconds.
Array:
The Warramunga Array has an aperture of about 20x20 km and consists of
two linear branches which are aligned in north-south and in east-west direction
respectively.
Five of the 20 stations are three-component stations, the other 15
stations are only vertical instruments. The characteristic distance from
the Tonga-Fiji events to Warramunga is between 42 and 45 degrees, the backazimuth
is about 100 degrees.
Aim:
1) Data processing - search for reflections from the top of D" (the lowest
200-400 km of the earth's mantle) using Vespagramms, DoubleBeam, ...
2) Determination of the thickness, velocity contrast and topography
- comparison with synthetics
3) Interpretation and comparison with regional and global models
Figure 1:
Great Circle Paths from the three clusters of events (the north, central
and the south cluster) to the Warramunga Array. Also shown are the theoretical
bounce points at the Core Mantle Boundary (CMB) for one event of each cluster.
Figure 2:
Raypaths of the phases P, PcP and PdP referred to an epicentral distance
of 42 degrees and a depth of 400 km, the used earth modell is PWDK. (the
estimated thickness of D" is 300 km, the contrast of vp is 3%)
Things to do:
At the moment (spring '97) I am searching for evidences of reflections
on the top of D", e.g. for the phases PdP and SdP which are precursors
of the CMB reflections PcP and ScP.
(Gunnar, 1.04.97)