IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS011 Oral Presentation 2104
Lowermost mantle structure of the western part of the Pacific low velocity
province constrained by the Vietnamese broadband seismograph array
Dr. Nozomu Takeuchi
Earthquake Research Institute Univ of Tokyo IASPEI
Takeuchi (2007, GJI) recently conducted broadband waveform inversion for the whole mantle SH
velocity structure. The inversion fully utilize later phase date (including major and multi-orbit body
waveforms) to improve the resolution. The resultant model reveals the different natures of the two
major upwelling systems: the strong low velocity anomalies beneath Africa extend for more than 1000
km from the core-mantle boundary (CMB), whereas those beneath the Pacific are restricted to 300-400
km from the CMB. The final goal of this study is to obtain independent evidence for the different
natures of the upwelling regions through analyses of seismograph array data. The western part of the
Pacific low velocity province is focused on in this study, because the array data which well resolve the
velocity profiles of this region were not previously available. The Vietnamese broadband seismograph
array, which was deployed by the Ocean Hemisphere Project, has ideal geometry for this purpose: the
array and epicenters of Fiji-Tonga and Vanuatu events are nearly on the same great circle, which
provides S and sS data with wide variety of bottoming depths (distributing between about 150-1300 km
from the CMB). The analysis of S and sS apparent velocity data indicates the existence of the low
velocity zone at the lowermost mantle. Its thickness is smaller than 400 km, and it seems to be smaller
than 150 km in some region. The horizontal scale length of thickness variations is small (about 500 km).
The analysis of ScS-S and sScS-sS travel times indicates the existence of sharp side boundary of the
Pacific low velocity province. The location of the boundary is precisely consistent to that indicated by
Sdif travel time analysis (Toh, Romanowicz & Capdeville 2003, PPV workshop). These results suggest
the existence of chemical heterogeneities which are confined in 400 km from the CMB.
Keywords: cmb, seismology