Two-step interface and velocity inversion |
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Authors: | Xiao-Ling Lai Xian-Kang Zhang and Xu-Yao Zheng |
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Institution: | (1) Research Center of Exploration Geophysics, China Seismological Bureau, 450003 Zhengzhou, China |
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Abstract: | This paper studies the computation method of two-step inversion of interface and velocity in a region. The 3-D interface is
described by a segmented incomplete polynomial; while the reconstruction of 3-D velocity is accomplished by the principle
of least squares in functional space. The computation is carried out in two steps. The first step is to inverse the shape
of 3-D interface; while the second step is to do 3-D velocity inversion by distributing the remaining residual errors of travel
time in accordance with their weights. The data of seismic sounding in the Tangshan-Luanxian seismic region are processed,
from which the 3-D structural form in depth of the Tangshan seismic region and the 3-D velocity distribution in the crust
below the Tangshan-Luanxian seismic region are obtained. The result shows that the deep 3-D structure in the Tangshan seismic
region trends NE on the whole and the structure sandwiched between the NE-trending Fengtai-Yejituo fault and the NE-trending
Tangshan fault is an uplifted zone of the Moho. In the 3-D velocity structure of middle-lower crust below that region, there
is an obvious belt of low-velocity anomaly to exist along the NE-trending Tangshan fault, the position of which tallies with
that of the Tangshan seismicity belt. The larger block of low-velocity anomaly near Shaheyi corresponds to a denser earthquake
distribution. In that region, there is an NW-trending belt of high-velocity anomaly, probably a buried fault zone. The lower
crust below the epicentral region of the Tangshan M
S=7.8 earthquake is a place where the NE-trending belt of low-velocity anomaly meets the NW-trending belt of high-velocity
anomaly. The two sets of structures had played an important role in controlling the preparation and occurrence of the M
S=7.8 Tangshan earthquake.
Contribution RCEG97006, Research Center of Exploration Geophysics, China Seismological Bureau, China.
This project is supported by the Chinese Joint Seismological Science Foundation. |
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Keywords: | two-step interface and velocity inversion 3-D velocity pattern of crust below Tangshan seismic region seismicity zone |
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