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
JSS014 Poster presentation 2274
Fine crustal structure in active tectonic area at Northwestern Edge of
Tarism Basin Block
Dr. Zhaofan Xu
Geophysical Exploration Center,CEA 104 Wenhua Road, Zhengzhou 450002,China IASPEI
The northwestern edge of Tarism Basin and its adjacent area lies at the place where three tectonic
units, i.e. Tianshan fold zone, Pamir tectonic arc and Tarim block, meet there and is one of the most
active earthquake provinces at present in Chinese mainland. In the last century, about 3/4 strong
earthquakes in Chinese mainland took place there. Especially from the twenty-first of January to
sixteenth of April in 1997, at a very small area of 9x18km2 near Jiashi city, where there was no any
main fault found before, was hit by 7 strong earthquakes with the magnitude ranging from 6.0 to 6.9,
and also no any seismic fault found after these shocks besides the macroscopic appearances of the
destroyed villagers houses. It had never taken place before in Chinese mainland that a series of strong
earthquakes shook a very small area in such a shot period time of 85 days. In order to understand the
crustal structure features beneath the strong earthquake swarm area, active source seismic
investigation experiments with wide angle reflection/refraction, high resolution seismic refraction, near
vertical seismic reflection profiles and a three-dimensional temporary seismic transmission array were
carried out, respectively. The crustal structure features with different scales were determined by these
seismic data and the hidden crustal faults were inferred. The high resolution seismic refraction results
show that the upper crust is relatively well-distributed in laterally and the buried depth of crystalline
basement is deeper in SW and shallower in NE, which forms a slope toward Tianshan Mt with a little
uplift. The basement velocity is about 6.2km/s, which is comparatively higher than the other area in
Chinese mainland. Though the crust show obviously layered structure features, the geometries of
interior crust interfaces and Moho suggest that the crust had been suffered very strong deformations
under the extrusion from Kunlun in south and Tianshan in north, and also suggest that the crustal
deformations in the southern and deeper depth should stronger than in the northern and upper crust.
The different deformation images between upper and lower crust, which are obtained from wide angle
reflection/ refraction and high resolving seismic refraction surveys, respectively, imply that the
geodynamics process should come from a rather large depth. Though the seismogenic tectonic
background of Jiashi strong earthquake swarm maybe has something to do with the inferred crustal
faults, no structure features show that these faults stretch to the near surface. Acknowledgments: This
research was supported by the Joint Seismological Foundation of China Earthquake Administration,
Grant 106076
Keywords: active tectonic area, active source seismic experime, crustal structure