Riedel megashears R' and the trend to gravitational equilibrium as main factors of tsunamigenic earthquakes

Abstract

An earthquake source is characterized by two nodal planes oriented parallel to two planes of maximum shear stresses ( Fig. 1, left). A rapid displacement of the shear type (in mechanical, rather than in the geological meaning) occurs along one of the planes and causes an earthquake.The concept of plate tectonics with one of its main components, subduction zones, provides, at first sight, the unique opportunity to select one of the two nodal planes -a gently dipping plane which is parallel to the roof of the subducting oceanic plate (Fig. 1, bottom right). The other nodal plane that is steeply dipping in the opposite direction ( Fig. 1, top right) seems 'unpromising', considering the aspect of seismicity, for two reasons. First, displacement along this plate is contrary to the general direction of oceanic plate subduction. Secondly, such displacement is directed against the direction of gravity, which is energetically disadvantageous.However, it should be taken into account that in the stress field of the subduction zone, as in any stress field, the two above-mentioned maximum shear stresses have equal values. At the same time, it is the sub-vertical displacement that excites rapid uplifting of the seabed which causes a tsunami. Researchers who support the traditional choice of a gently dipping nodal plane have to reckon with it and therefore create complex models, such as the 'splay fault' model that seem most successful, though being quite complicated and controversial (Figs. 56 and 57).In our opinion, the geological reality is more adequately refelected by the geological and geophysical model shown in Fig. 1 (right). It is based on the wide range of information and assumes that both nodal planes are equivalent and interchange in generation of strong earthquakes.The aim of this article is to consider this model in terms of tectonophysics. For this purpose, earthquake sources indicated on ( Fig. 1, right) are classified as Riedel megashears, R (bottom right) and R' (top right top), which occur in the geodynamic setting of sub-horizontal shearing (in this case, subduction of the oceanic plate) along the sub-horizontal plane (Fig. 3). This situation is one of five elementary geodynamic settings (see Fig. 2). It is similar in everything, except the position of the shearing plane, with the geodynamic setting of horizontal shearing along the vertical plane ( Fig. 4). Riedel shears formed in the latter situation were subject to the most detailed studies using purpose-made devices (Fig. 5, and 6). This study gave grounds to conclude that Riedel shears, R are developed much better than shears R'.Our experiments (Fig. 7) confirm the above conclsuion. Moreover, it is revealed that shears R', that develop poorly in samples made of wet clay (Figs. 8,9, 12, and 13), cannot develop in a granulated medium such as a mixture of sand and solid oil (Fig. 10, 11, and 14) and do not develop in other granulated media (Fig. 17), which are similar to the block structure of the uppermost crust ( Fig. 18-20). In such mediums, shears R res...

Structural Control of a Dissolution Network in a Limestone Reservoir Forced by Radial Injection of CO2 Saturated Solution: Experimental Results Coupled with X-ray Computed Tomography

“…In most of the studied cases, shear failure within the strike-slip zone starts with the primary development of a set of R 1 fractures. Continuous, but narrow, zones of R 1 structures are composed of individual shear fractures, R 1 , linked by transfer zones containing en-echelon arranged dilatational jogs-tensile joints and normal faults, T [48,49]. Closely spaced, but relatively short, R 2 fractures develop later due to secondary restraining in the spatially overlapping bridges between the already developed R 1 fractures [47].…”