Russian Journal of Earth Sciences
Vol. 6, No. 2, July 2004
A gravity model of the North Eurasia crust and upper mantle.
3. Stress state of the lithosphere induced by density inhomogeneities
M. K. Kaban
Abstract
This work continues a series of papers investigating the density structure of
the North Eurasian lithosphere. Based on the 3-D density model of the
lithosphere, which was constructed at the preceding stage of the investigations,
local stresses in the upper crust due to the density inhomogeneities are
estimated. Numerical analysis showed that the stress estimates are comparable,
on the order of magnitude, with maximum stress amplitudes determined by
instrumental methods. In active orogenic regions, local stresses accommodating
density inhomogeneities amount, on average, to 40-50 MPa and reach a maximum
of
140 MPa. Such local stresses can produce the triggering effect against the
background of weakly varying regional stresses and therefore indicate the most
probable zones of earthquake occurrence. This effect is most noticeable at a
certain distance from active boundaries between lithosphere blocks. The
distribution of calculated stresses is compared with seismicity parameters in
the Baikal rift zone. The distribution of calculated maximums is shown to
correlate well with the distribution of earthquakes in this region. The
difference between maximum and minimum principal components (the twofold maximum
tangential stress) of the calculated stress tensor has two critical values. The
first, amounting to ~7 MPa, indicates the occurrence of
M>3 earthquakes. If
the twofold maximum tangential stress reaches a value of 33-35 MPa, seismicity
dramatically increases, largely due to strong ( M>5 ) earthquakes. It is
important to note that, even in tectonically passive platform regions, local
stresses can attain 20 MPa. Thus, these stresses can account for, at least in
part, the phenomenon of intraplate seismicity.