jgg     2013 33 (6):  20-23    ISSN: 0412-1961  CN: 21-1139/TG GRAVITY BACKGROUND OF MEDIUM AND SMALL  EARTHQUAKES OCCURRED IN GUANZHONG REGION Earthquake Administration of Shanxi Province, Xi’an 710068 Received: 2013-05-01  Revised: null  Online: 2013-12-13 Supporting info