Kacper R. Rybicki

Analysis of aftershocks on the basis of dislocation theory

Abstract In this paper aftershocks are analysed in the framework of dislocation theory. It is postulated that stresses which are created in the medium after a main rupturing are of great and direct importance in the aftershock process. These stresses, acting on inhomogeneities of the medium, result in aftershock generation. Four models are presented to illustrate the stress pattern associated with a main shock fault. On the basis of these models some conclusions regarding aftershocks are drawn. There is a good agreement between theoretical and observational data.

Faulting in vertically inhomogeneous media and its geophysical implications

Faulting of strike-slip type in vertically inhomogeneous media is analyzed using the classical fracture theory. The analysis makes it possible to get a better insight into various aspects of faulting and earthquakes in real media. The major conclusions are that both low rigidity media and those with regions in which a rapid transition from high to low pore pressure occurs may form barriers for a propagating rupture. The permanent deficit of the driving stress (small or negative stress drop) in sediments that represent the most distinguished class of such media may be one of the main reasons of the generally observed lack of seismicity and rupture suppression there.

Static deformation of a multilayered half-space by a very long strike-slip fault

SummaryThe formulas for the elastic residual field of a very long strike-slip fault in a multilayered elastic half-space are given. It is assumed that the medium is made up ofn parallel, isotropic and homogeneous layers lying over an isotropic, homogeneous half-space and being in welded contact. The formulas which allow the calculation of the displacement and stress fields due to an arbitrary very long strike-slip fault are derived. The explicit expressions of coefficients which appear in these formulas are listed for the casesn=1 andn=2.

Mechanical interaction between neighboring active faults—an application to the Atera fault, central Japan

Abstract Static stress changes across the Atera fault, central Japan, due to the neighboring large earthquakes are examined. Several large earthquakes in the past 100 yrs which might have dominantly affected the fault are selected in order to calculate coseismic changes in the fracture stress. The result suggests that the stress increased at both the northern and southern ends of the fault, which amounted to as much as 6 bar in total. Among these earthquakes, the Nagano-seibu earthquake ( M = 6.8) of 14 September, 1984, seems to have most seriously affected the Atera fault in such a sense as to increase the fracture stress.

FOCAL MECHANISM IN CONNECTION WITH ENERGY STORAGE BEFORE CRACK FORMATION

Abstract The pattern of internal energy storage depends on distribution of the internal failures, fractures and local stress sources. A simple model of energy storage along a tectonic plane is represented by an array of dislocations. Stress concentration leads to crack formation of tensile, shear or transverse type depending on the confining pressure and stress components prevailing at the tectonic plane. The conditions of energy storage and conditions of crack formation vary with depth. Hence it is expected that the focal mechanism is also subject to transformation with depth.

On faulting in a medium containing an inhomogeneity inplane and antiplane strain models

Faulting in a medium with an inhomogeneity is analysed applying two-dimensional models consisting of a shear crack in a presence of a circular inclusion. The stress drop and the stress intensity factor are calculated for mode II and III cracks of various positions in relation to the inclusion. The results demonstrate that the effect of an inhomogeneity on a shear zone strongly depends on the location of a zone for either mode II or mode III shear zone. This effect is mostly due to the spatial distribution of external effective shear stress around an inhomogeneity. Depending on the position, an inhomogeneity may have either a destabilizing effect (the stress intensity factor becomes greater) or a stabilizing influence (the stress intensity factor is decreased or faulting is prohibited so the inhomogeneity acts as an asperity or a barrier). There is a substantial difference, however, between mode II and mode III shear zones approaching an inhomogeneity centrally. Namely, the effect of inhomogeneity on the mode III shear zone located in the immediate vicinity of the inhomogeneity is in this case considerably more pronounced than that for mode II shear zone and depends to a far greater extent on the rigidity contrast between the inhomogeneity and the surrounding medium. Another important conclusion is that the quantitative effect of an inhomogeneity on faulting depends essentially on the initial value of the stress drop of a shear zone approaching an inhomogeneity, being decidedly higher for a shear zone of small stress drop. It means that in specified areas in the proximity of medium inhomogeneities one should expect substantially greater faulting activity in which weak events prevail than in other regions surrounding inhomogeneities where such activity should be distinctly reduced. Such conclusions apply to both high rigidity inhomogeneities, which, in particular, may be associated with intrusions from the upper mantle, and to low rigidity inhomogeneities such as volcanos. The present model sets forth the plausible explanation regarding why earthquakes from the same region are occasionally characterized by various values of the stress drop. The model also presents the quantitative insight concerning how heterogeneity of the medium, in the sense of spatial variation of elastic constants, affects faulting.