Physics
Scientific paper
Nov 1985
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985e%26psl..75..403h&link_type=abstract
Earth and Planetary Science Letters, Volume 75, Issue 4, p. 403-409.
Physics
10
Scientific paper
With the same principle as being used in medical X-ray imaging, the absorption of S-waves is utilized to map attenuation variations in the upper mantle beneath the Kanto district, Japan where the existence of two sinking slabs, the Pacific and the Philippine Sea slabs, is inferred. Nearly 4500 seismic intensity reports on local shallow and intermediate earthquakes are used as measures of the maximum ground acceleration of S-waves which are assumed to be radiated isotropically from earthquake foci. An ordinary damped least-squares inversion is employed to obtain the attenuation structure. The inversion reveals a remarkable contrast of the attenuation coefficient and three prominent features. The first is a low-attenuation zone corresponding to the descending Philippine Sea slab. The attenuation mapping suggests that an aseismic portion of the Philippine Sea slab continues to a depth of 120 km toward the north. The second feature is a hard ``root'' (low-attenuation zone) of the plate bearing the Japanese islands, attaining at least a depth of 60 km beneath the northeastern part of the Kanto region. This ``root'' appears to be responsible for the anomalously deep interplate seismicity in this area, where a strong seismic coupling exists at a depth of 40-70 km between the surficial plate and the underlying Philippine Sea slab. The third feature is a zone of extremely high attenuation in the upper mantle, lying on the continental side of the volcanic front, where many Quaternary volcanoes are located. Especially, a spot of very high attenuation is found at a depth of 60-90 km beneath an active volcano, Mt. Fuji.
Hashida Toshihiko
Shimazaki Kunihiko
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