A GRAVITY MODEL OF THE NORTH EURASIA CRUST AND UPPER MANTLE. 3. STRESS STATE OF THE LITHOSPHERE INDUCED BY DENSITY INHOMOGENEITIES

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.