Greinar (Vísindafélag Íslendinga) - 01.01.1977, Síða 62

60
In the oceanic ridge areas and in the volcanically active
zones in Iceland, the focal depth is mostly less than 5 km,
usually closer to 3 km.
Below that depth the material must then have yielded to
stresses by flowage, i.e. without breaking, which corresponds
to the water in our above example, and only at the shallow
depth of around 3 km does the material yield by breaking,
i.e. by such a sudden yield which is necessary to transform
the strain energy into that of elastic waves. Only in this shallow
layer can the wind stress as a rule be summarized up to the
breaking point, while the strain energy is turned into heat at
greater depth.
We must emphasize very strongly, that when the plastic
layer under the brittle one is likened to the water in Fig. 8,
the duration of the wind stress is the essential point; in our
present case we must think in times like decades or even cen-
turies, while in Fig. 8 we were operating with the duration of
individual storms. In the present case of much longer dura-
tion, the inferred flowage suffices to so deform the thin brittle
crust, as to produce shear stresses that cause earthquakes.
It is not necessary to go here into such details as to derive
the global stress field whieh the atmospheric circulation creates
at a certain time, and thus withholds the strain energy against
the losses through seismicity. It suffices to point out that the
yearly seismic world energy is of the order of 1025ergs, while
the yearly energy of insolation, which drives the atmospheric
circulation is of the order of 1032ergs, or ten million times
greater. Earlier (39) I inferred global stresses in an upper
layer, for which provisorily I used the most neutral term
“tectonic layer”, I still thought in terms of internal forces. In-
ternal flow was to create stresses in the “tectonic layer“. But
it is now obvious that there are no such internal forces, or
sources of energy which are but a mere trifle in comparison
with the external energy source. We have but to point out a
mechanism which is capable of transforming a trifling part
of the external energy into an endogeneous one.
It is quite irrelevant that the material below the earthquake