because of associated frictional sliding on faults and colder temperatures of earth's crust ( most brittle part of the earth) the large and great earthquakes can occur there which has many fault systems .The subducting slabs Earthquakes the parts of the tectonic plates that are being pushed down into the earth, the depths of These slabs are 700 km. the higher temperatures at these depths make rock softer It is not yet understood how frictional sliding can occur there?
I never saw an answer to thus question in science literature. It is future work.
Perhaps it is not necessarily about friction but about the detachment of a part of slab that descends because mineral transformations that can lead to the emergence of higher density compounds.
And also lower temperatures are preserved to greater depths in fast subduction where heating fails to keep up. Isotherms are depressed there, and the large slab being absorbs heat slowly. Hence brittle behavior is promoted at deeper depths.
Perhaps at a depth of 700 km the rocks of the crust not only soften, but also melt? This is logical. That is why the subducting crust is not traced deeper. And then it turns out that convection cells do not exist.
See the book from
Cliff Frohlich
Deep Earthquake,
Cambridge University Press
ISBN: 9781107297562
doi: 10.1017/CBO9781107297562
Book Deep Earthquakes
for an in-depth discussion on these events.
I'm afraid the theory of pleiotectonics raises too many questions, which can not be answered.
Theoretical drivers of Earth's tectonic plates and global seismic energy may require some revision:
Preprint Spectral, spatial-statistical, and graphical evidence that g...
Tectonics it is very interesting subject to be discussed.
Subduction - still an open question.