3. Historical earthquakes
1556 Shaanxi, ~8 Ms

This earthquake in central eastern China is approximated to be of magnitude 8 on the surface-wave magnitude scale Ms. This scale is an accepted alternative to the well known Richter scale and is related to the amplitude of earthquake waves travelling in the upper layers of the Earth (the difficulty with the original and famous scale of Charles Richter is that it is distance-limited, applicable up to local epicentral distances to the earthquake of about 600 km maximum). The Shaanxi earthquake is considered to have led to the largest ever loss of life from an earthquake, killing an estimated 830,000 people. Countless thousands died from earthquake induced landslides which destroyed houses and cave dwellings constructed of silty soil and clay.

1920 Haiyuan, 8.4 Ms

One of the most destructive earthquakes of the 20th century occurred near Haiyuan in central China in 1920, with an epicentral intensity of XII (figure 4). More than 220,000 people were killed with many thousands injured and much infrastructure destroyed. A 230 km long left-lateral surface rupture zone was formed along the Haiyuan Fault during the earthquake with a maximum horizontal displacement of 10 m (Ran, 2004). Weiqi et al. (1987) show from fault investigation that the recurrence interval for an event similar to 1920 is between 800 to 1,600 years.

 
Figure 4. Distribution of intensity for the Haiyuan earthquake (after Zhang and Wang, 1995)

1927 Gulang, 8.0 Ms and 1932 Changma, 7.5 Ms

Also in northwestern China and situated in the Hexi corridor, an important communication route during the silk trade, a magnitude 8 earthquake in 1927 resulted in the loss of 200,000 lives. This remote area was struck again in 1932 by the magnitude 7.5 Changma earthquake, leading to a further 70,000 deaths. These two earthquakes and the 1920 Haiyuan event seemed to indicate that seismic energy had been released progressively from east to west along the length of the Haiyuan fault.

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