Once the ancient capital of China (Chang'an) and currently known worldwide as the home of the Terracotta Army, Xi'an is an important, modern, industrial centre of northwest China with steel, textile, chemical and machinery factories and a population of over 2.5 million. It lies on the River Weihe, approximately 1100 km southwest of Beijing in the Province of Shaanxi, of which it is the capital (Fig. 1). The surrounding valley is intensively farmed and is one of the main growing areas of northwest China. Climatically the area is part of humid and sub-humid, warm, temperate, north China.

Several major crustal faults occur in the Xi'an area and form a fault-bounded, depositional basin of Early Tertiary age on the southwest part of the Huabei plate. The latest fill comprises up to 700 m of fluvial, lacustrine and loessic deposits of Late Tertiary to Recent age. There are three distinct river terraces on the broad flood plain of the River Weihe and additional terraces on the foot slopes of the Qinling Dabie Mountains which form the southern margin of the valley. On the hills surrounding the valley, to the north and south, there are fine grained, clayey, loess deposits of Late Pleistocene age lying on Middle Pleistocene loess. The main loess deposits are found to the north on the loess plain whose southern margin forms the northern boundary of the valley.

The area is tectonically active and extension of the Xi'an region has resulted in the formation of a number of active

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