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
JSS012 Poster presentation 2211
Body wave travel time anomalies caused by the stagnant slab
Dr. Hiroko Sugioka
IFREE JAMSTEC IASPEI
Daisuke Suetsugu, Masayuki Obayashi, Yoshio Fukao, Yuan Gao
Previous tomographic studies using short-period P times have commonly shown that the fast Pacific slab
is stagnant in the mantle transition zone beneath the Philippine Sea and eastern Asia (e.g., Fukao et al.,
2001). However, the fast S-velocity anomalies of the stagnant slab have been not commonly obtained
from short-period S-wave time, which is based mostly on the ISC bulletin. The purpose of the present
study is to determine whether the stagnant slab has fast S-velocity anomalies by analyzing arrival times
picked from broadband waveforms and, if any, to determine the velocity perturbation ratio δlnVs/δlnVp
(= R) of the stagnant slab. The parameter R is useful to constrain whether a lateral velocity variation is
caused by thermal and/or chemical origins. We analyzed seismograms of Izu-Bonin deep events
observed by three broadband seismic networks of F-net, IRIS and CDSN (China Digital Seismographic
Network). The CDSN data are important since body waves observed at the CDSN stations should travel
a long distance in the stagnant slab. The observed first arrivals are significantly early relative to the
Iasp91 model beneath northeastern China: Travel time residuals are -3~-4 s for P-wave and -6~-8 s for
S-wave, which implies that the stagnant slab has fast S-velocity anomalies as well as fast P-velocity
anomalies. We computed theoretical travel times of the first P arrivals with a three-dimensional P-
velocity model (Obayashi et al., 2006) using the wave-front propagation method (Sakai, 1992) in which
the stagnant slab beneath the studied region has fast P-velocity anomalies of 1-1.5 %. We calculated S-
wave travel times with the S-velocity model constructed from the P-velocity model using a constant R.
The theoretical travel times are in good agreement with the observed one for both P- and S-wave with
the value of R of ~1.7.
Keywords: stagnant slab, tomography