Documented historical records of large-magnitude earthquakes in China, in addition to providing data for preparing catalogs for seismostatistical investigations, have been used by seismologists for constructing isoseismal maps. Isoseismals of earthquakes tend to be influenced by the surficial sedimentary terrain, and in the case of the historical earthquakes, the ambient population density affected earthquake reporting used in preparing isoseismal maps. However, isoseismal geometry in the epicentral region also reflects on the pattern of faulting associated with the earthquake, whether the faults are visible on the surface or not. Isoseismals for a large number of earthquakes in China have been examined and interpreted. An interesting deduction is the definition of a boundary separating the East China mini-plate from the West China mini-plate which strikes almost north-south between longitudes 102° and 105°E and extends from about 25° to 36°N. Isoseismals on the west of this boundary are elongated from NW to NNW while on the east from NS to NNE. Beginning with 1800 A.D., available isoseismals of all earthquakes of magnitude greater than or equal to 6 have been examined for this area. These isoseismal geometries, combined with the inferred left-lateral, strike-slip faulting on satellite imagery, as well as on some recent earthquakes on the western side of the boundary, and the rotation measured by Chinese scientists near the city of Shan Tan during the February 11, 1954 earthquake (38°51′N, 101° 22′E, focal depth 12.5 km, M=714) are consistent with a clockwise rotation of the West China mini-plate.

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