Abstract
The occurrence of double seismic planes beneath some island arcs can be explained in terms of an unbending of the descending lithosphere. Stress distributions in a representative cross section of the unbending lithosphere are calculated under reasonable physical conditions and physical properties of the lithosphere. The results show that various types of possible seismic zones appear in the descending lithosphere depending on the conditions mentioned above, and that the appearance of double seismic zones with a plane structure is quite sensitive to small changes in the physical conditions. The temperature at the double seismic zones in the descending lithosphere is estimated from the permitted temperature limit under which the double-plane structure of the seismic zone appears. The temperature of the dual seismic zone at a depth of about 180km ranges from 850° to 1, 000°C.
In addition to the pure unbending of the lithosphere, uniaxial extension along the descending direction of -1% is favored during the unbending for the appearance of a dual seismic zone. That is, double seismic zones having nearly the same thickness do not appear in the case of the unbending lithosphere with an additional compressional strain of more than 1% or with an additional extensional strain of more than 2% during unbending.
