The double-difference earthquake location algorithm was applied to the relocation of 10,057 earthquakes that occurred in central-western China (21 • N to 36 • N, 98 • E to 111 • E) during the period from 1992 to 1999. In total, 79,706 readings for P waves and 72,169 readings for S waves were used in the relocation. The relocated seismicity (6,496 earthquakes) images fault structures at seismogenic depths that are in close correlation with the tectonic structure of major fault systems expressed at the surface. The new focal depths confirm that most earthquakes (91%) in this region occur at depths less than 20 km.

Abstract. A one-dimensional Burridge-Knopoff spring-block model with slip-dependent friction was studied to explore the possibility of a solitary wave solution existing for the problem of earthquake faulting. The result may be used as an alternative case of the crack model (e.g., Madariaga and Cochard, 1996) and the spring-block model with velocity-dependent friction (e.g., Carlson and Langer, 1989) in the understanding of the mechanism of the self-healing slip pulse proposed by Heaton (1990). In general, the conditions for a solitary wave solution to exist are discussed by the trajectory in the phase space. By taking the first order approximation, it is demonstrated that a solitary wave solution exists in which the slip behaves as a propagating solitary wave with the propagation velocity less than that of an acoustic (seismic) wave, and the source time function at each position remains the same. As an alternative approach other than numerical calculations, the analytical solution, although simple, sheds light on some of the properties of the self-healing slip pulse. From the solution it is seen that the width of the pulse depends on its propagation velocity and the friction, consistent with experience in physics. It is pointed out that the self-healing slip pulse may exist for a broad class of frictional constitutive laws which, to some extent, explains the fact that the self-healing slip pulse may be observed for a variety of earthquakes occurring within different seismogenic environments.

The China Digital Seismograph Network (CDSN) program was initiated in May 1983. On October 1, 1986, the CDSN began to distribute the network-day tapes to the research community. On October 22, 1987, the CDSN began full operation. The CDSN are supported by the State Seismological Bureau, People's Republic of China and the United States Geological Survey. The operation and maintenance of the network are taken by the staffs at the 10th Division of the Institute of Geophysics, State Seismological Bureau, QGSSB). At present the CDSN includes ten field stations, Le Beijing (BR), Lanzhou (LZH), Enshi (ENH), Kunming (KMI), Qiongzhong (QIZ), Shanghai (SSE), Urumqi (WMQ), Hailar (HIA), Mudanjiang (MDJ), and Lhasa (LSA), two national centers, i.e. the Network Maintenance Center (NMC) and the Data Management Center (DMC), both at the Institute of Geophysics, State Seismological Bureau, Beijing.

For the realistic modeling of the seismic ground motion in lateral heterogeneous anelastic media, the database of 3-D geophysical structures for Beijing City has been built up to model the seismic ground motion in the City, caused by the 1976 Tangshan and the 1998 Zhangbei earthquakes. The hybrid method, which combines the modal summation and the finite-difference algorithms, is used in the simulation. The modeling of the seismic ground motion, for both the Tangshan and the Zhangbei earthquakes, shows that the thick Quaternary sedimentary cover amplifies the peak values and increases the duration of the seismic ground motion in the northwestern part of the City. Therefore the thickness of the Quaternary sediments in Beijing City is the key factor controling the local ground effects. Four zones are defined on the base of the different thickness of the Quaternary sediments. The response spectra for each zone are computed, indicating that peak spectral values as high as 0.1 g are compatible with past seismicity and can be well exceeded if an event similar to the 1697 Sanhe-Pinggu occurs

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