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 2205
Anelasticity in the mantle surrounding Izu-Bonin subduction zone inferred
from BBOBS waveform data
Dr. Azusa Shito
Ocean Hemisphere Research Center University of Tokyo
Hajime Shiobara, Hiroko Sugioka, Aki Ito, Claudia Adam, Hitoshi Kawakatsu,
Toshihiko Kanazawa
Anelasticity in the upper mantle surrounding Izu-Bonin subduction zone is estimated by using the
amplitude spectra of P wave recorded by Broad Band Ocean Bottom Seismometer (=BBOBS). As a part
of Stagnant Slab Project we deployed 12 BBOBS from October 2005 to October 2006 in and around the
Philippine Sea . The sites were equipped with three components Guralp CMG-3T sensors recorded at
200 Hz. This is the first challenge of regional scale and long term BBOBS array. The BBOBS waveform
data will provide us much information on physical property of subduction zone. We analyze regional (<
15 degree) earthquake that sample the Pacific slab, mantle above the Pacific slab (=mantle wedge),
and the mantle beneath the Pacific slab. The waveform data from events in Izu-Bonin slab were
recorded by the BBOBS array with adequate signal to noise ratio. We estimated t* of P wave using high
frequency spectral decay and calculated the path averaged quality factor; Qp by using the observed
travel time. Time domain signals are windowed with the window length of 6s; before 1s and after 5s
onset of manually picked P wave. This window includes main pulse of the waveform and excludes later
phases. Then amplitude spectra and the slope are calculated over the frequency range 0.75 to 4.0 Hz.
The results clearly show low-Q mantle and high-Q slab. For shallow event (< 150km), the estimated Qp
are 100-200 above the Pacific slab, and are 500-1000 in the mantle beneath the Pacific slab. For deep
event (> 400km), the estimated Qp are 200-300 both above and beneath the Pacific plate. However,
the waveforms especially from shallow events are complex may be due to effects of multi-path.
Therefore, we should trace the raypath more carefully for further discussion. In the presentation, we
will show the results of estimation of t* and discuss anelastic structure and the tectonic implications.
Keywords: anelasticity, mantle, bbobs