jgg     2014 34 (2):  27-30    ISSN: 0412-1961  CN: 21-1139/TG STUDY ON NON-LINEAR VARIATION OF VERTICAL  GRADIENT OF GRAVITY 1）Key Laboratory of Earthquake Geodesy，Institute of Seismology，CEA，Wuhan 430071 2）Wuhan Base of Institute of Crustal Dynamics，CEA Wuhan 430071 Received: 2013-08-26  Revised: null  Online: 2014-04-21 Supporting info