On the Origin of Complexity in Core Phases Travel Times

by Hrvoje Tkalcic
Institute of Geophysics and Planetary Physics
University of California, San Diego

3-4pm
Friday Feb 7, 2003
Refreshments served at 2:45pm
Munk Conference Room
Cecil and Ida M. Green Institute of Geophysics and Planetary Physics
Scripps Institution of Oceanography
University of California, San Diego
http://mahi.ucsd.edu/seminar/

Abstract

I present a study of the deep mantle and the inner core structure,
using broadband differential and absolute travel time measurements
of mostly compressional waves that travel through the Earth's core,
and those that reflect from the core-mantle boundary. We collected
three such global datasets of hand-picked high quality measurements:
PKP(AB-DF), PKP(BC-DF) and PcP-P. By forward modeling and inversion,
we confirm that the lowermost mantle is a very heterogeneous region
and that in order to make conclusions about the physical and chemical
properties of the Earth's core, these heterogeneities have to be taken
into account. The obtained models of D'' are characterized by prominent
fast features under mid America and east Asia, a fast belt across Pacific,
a slow region under the southwestern Pacific and southern Africa, as
well as sharp transitions from fast to slow, for instance under Alaska
and south Atlantic. Heterogeneity itself cannot completely explain
the trends observed in differential time residuals, when plotted as a
function of the angle between P wave paths in the inner core and the
Earth's spin axis, but it can account for most of the trend. Inner core
anisotropy, with the fast P axis oriented parallel to the Earth's spin
axis, which was hypothesized to exist as the main cause of the trends in
travel time residuals and inner core sensitive normal modes splitting,
thus can not be ruled out. However, anisotropy has to be very complex,
which is difficult to reconcile with its plausible physical causes.