Abstract: On March 11, 2011, an M_S9 earthquake occurred in the trench near Japan, caused tremendous casualty and attracted extensive concern. Based on results of related researches, this paper analyzed observations, phenomena and understandings of the earthquake from varied aspects, and obtained four main conclusions. ① The earthquake located in the northwest boundary of the pacific plate, and has two zones of concentrated coseismic slip at different depths, with relatively small slip in the deep zone where it experienced many M7 historical earthquakes, and large slip in the shallow zone where it has few historical strong earthquakes. ② Studies of fault movement constrained by GPS data show that fault movement in the Japan trench has a background of widely distributed stability and locking (area of locking zone is equivalent to that of coseismic ruputure zone), perturbation occurred after the 2008 Hokkaido M8 earthquake, several ~M7 has afterslip lager than coseimic slip, two obvious slow slip events was recorded in 2008 and 2011, eventually, the March 9, 2011 M7 foreshock and the March 11, 2011 M9 mainshock occurred. The pre-earthquake changing of the fault movement in Japan Trench is quite clear. ③ Possibly due to monitoring reason, traditional precursory observations show no obvious anomaly. Anomaly before earthquake consists of high stress state in focal zone reflected by some seismic activity parameters, short period anomaly in regional ground motion, etc. ④ Analysis of physical property in focal zone aroused more scientific issues, for example, is there obvious difference between physical property in focal zone and its vicinity? Does frictional property of fault determine seismogenic ability and rupture process? Whether pre-earthquake fault movement include pre-slip? Could deep fluid affect fault movement in focal zone? Experience is the best teacher, and the authors hope this paper could be a modest spur to induce others in basic research in earthquake forecast and prediction.

Key words: The 2011 M_W9.0 Tohoku-oki Earthquake, Seismic Monitoring, Earthquake prediction, Earthquake Research