عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Japan is a region of active plate tectonics, well known for its Japanese Trench a geological feature evolving as a result of North Pacific (NP) oceanic plate pushing and sub-ducting underneath the North American (NA) plate. Multiple plate tectonics have been continuously recorded in the region since 1979 by the network of GPS stations called GEONET. The network operating and monitoring more than 1200 GPS stations into which the movements in three dimensions in the GPS Cartesian coordinate system are continuously recorded. For the geodetic applications, the movements are transformed to the local North, East, and Upward directions at each station. In this research, the plate tectonics resulted in the disastrous March 11, 2011, M9.0 earthquake have been reviewed prior and after the earthquake and explained in the form of strain analysis using the accurately estimated type F3 temporal (daily) 3-D geodetic coordinates given in ITRF05 coordinates system of the GPS stations around the epicenter. Three periods of displacements comprehensively different in the pattern of motions of the plate were realized within 128 days before the disaster. The characteristics common to all periods is that the displacements at the stations are steady in time and almost in the same direction but may be slightly different in magnitude from place to place on the plate. Period 1 shows displacements all almost continued westward at the stations with the exception of a few stations close by volcanic activities. Period 2 show nonaligned displacements and negligible in magnitude pointing towards randomly distributed directions. It seems that the time of period 2 (30 days) is spent by the plate as the time needed to change its course of motion, so that when turning to period 3 (48 days), the course of motions are completely diverted from the motions in period 1. An strain analysis of the displacements is performed in a local 2-D horizontal Cartesian coordinates system defined at the center of the region, the unique system into which all GPS stations attain new horizontal positions while their displacements, already computed, remain unchanged. For the computation of strain elements (positional derivatives of displacements) at each station, the displacements in the neighboring stations are taken into account in a system of linear forms (Taylor expansion of displacements). Then, the least-squares optimization is applied to solve for the elements. Analysis of the diagonal strain elements separately in each period, show that the contraction phenomena happens during both periods 1 and 3 but the way (direction) contractions grow are completely different from period 1 to period 3. Also the phenomena of expansion show up and grow up along the east coast line of Japan explaining somehow overriding of the plate towards the PC ocean by the amount of 0.01 ppm before the disaster. The analysis of the displacements on the day of disaster show faster overriding of around 5 m in magnitude. Period 2 as a time interval could be assumed as the opportunity for the plate to change its course of action. It then may be considered as a precursor to the disaster of March 11, 2011.