This document explores the concept of isostatic compensation and its relation to gravitational anomalies in the context of tectonic episodes. It delves into the mechanics of faulting, isostatic balance, and the creation of relief. The document provides mathematical expressions and calculations to understand the phenomenon.
Extract
[...] External Geophysics Isostasy and Gravitational Anomalies 1.a. Isostatic equilibrium was achieved before this tectonic episode. This is written by the fact that the weight of the crust and the Archimedes' push of it on the mantle were compensated: . This gives the depth of the isostatic compensation surface at the initial state:" And the initial uplift of the crustal surface. After the tectonic episode and the addition of the new nappe, the new isostatic balance is written as: . What gives the new depth of the isostatic compensation surface: And the new rise in the surface of the crust The continental lithosphere thus sinks let us observe a creation of relief of . [...]
[...] The expression of simplifies to . It is an affine expression in (of the type ) with a slope that is worth . To find the value of the speed of waves P in the mantle, it is therefore sufficient to read the slope of the line corresponding to the wave Pn on the hodograph. It is worth: Thus: 2.c. À , on a so III- Mechanism at the hearth 3.a. The mechanism at the hearth associated with this earthquake is the following: On observes two almost straight lines crossing perpendicularly, which is characteristic of an earthquake caused by a detaching fault. [...]
[...] The actual plan of the mechanism above is therefore the plan in red on the figure. We can also say that it is a left shear failure. The second plan is the auxiliary plan. In the case of a shear failure, the actual plan of the mechanism is almost vertical, which corresponds to a angle of inclination close to 90°. The actual plan being almost horizontal on the figure (east-west direction) its azimuth is either close to 90°, or close to 270°. [...]
