Historical research and the revision of basic parameters for the earthquake in 849 A.D. in Zhenwu and Tiande area

Historical research of the earthquake in 849 A.D. in Zhenwu and Tiande area shows: ① The name " Hexi” refers to the " Hexi county” of Tang dynasty, in the area of the Yellow River in Heyang county of Shaanxi Province. ② " Yujia” refers to the Tang dynasty " Yujia militarised gate”, probably near the northwest Shimen of Baotou city. ③ Thousands of people were killed in this earthquake. The basic parameters of earthquake are revised based on research of historical materials. This earthquake on October 24, 849 A.D. in the Baotou city Donghe district（north latitude 40.6° and 110.1° east longitude）is determined to be a magnitude 7½ earthquake.