P-WAVE VELOCITY STRUCTURE OF THE UPPER MANTLE IN NORTHERN CHINA
ZHAO ZHU(Seismological Bureau of Sichuan Province)
In this paper, the data of 140 earthquakes recorded at different azimuths from the Beijing network are used. By using both the smoothing method and dT/d△ method, the slowness curves of P-wave are obtained. By incorporating the travel-time and slowness data, continuous travel-time curves can be obtained. A simpler formcla than Gerver-Marku-shevish's formcla is derived and used to invert the intial veloctity models. Through computing theoretical travel-time curves of th initial velocity models, the final velocity models which agree better with the observational data are obtained. The average velocity model shows that: a shallow low velocity zone begins near 60 km in depth: two abroupt increases in velocity from 360 to 420 km and from 660 to 680 km are also obtained; a possible minor zone of high-velocity gradient near 500 km is detected. Differences in depth of the zone of 600 km between the south-east and south-west directions in Northern China can clearly be seen.