Dear Saddek Samai

Your query is attractive, and to give you an answer it is necessary to take into account some considerations critical geological and geophysical features:

1- Due that you are talking about “seismic activity”. Seismic activity is described as the types, frequency and size of earthquakes that take place in a specific area in an interval of time. So, you are talking about earthquakes and isostasy equilibrium.

2- There are several categories of earthquakes, based on a few reasons such as:

· Types of plate boundaries: interplate, and intraplate.

· Focus´ depth: shallow, intermediate, deep

· Ranging from the amount of energy release (magnitude) or amplitude of the seismic waves generated and recorded by seismographs: Richter scale.

· Based on origin: tectonic associations, volcanic eruptions, post-glacial rebound, anthropogenic triggering.

3- Due that you are talking about “isostasy equilibrium”, it means: the process by which the Earth supports differences in topography o bathymetry in order to bring about a condition of hydrostatic equilibrium at depth, and include both, static and dynamic compensation mechanisms.

4- It means: there are a number of different types of isostatic compensation, the simplest is Airy – Heiskanen isostasy, or local compensation.

5- Isostasy implies that topography depends on lithospheric buoyancy, which depends both on the crust and mantle-lithosphere.

6- The isostatic residual gravity maps should not be casually interpreted in terms of “undercompensation” or “overcompensation” because large-amplitude anomalies can be produced by crustal bodies in complete local isostatic equilibrium. (Simpson et al., 1986): you need to associate based on tectonic environment.

7- Some key points to examples:

8- On the active tectonic environment: The fact that we have well-defined plates boundaries which correlate with isostatic anomalies highlights (See the pictures) and the fact that the plates boundaries criteria are mainly associated with earthquakes, enhances the likelihood that there is a correlation between seismic activity and isostasy equilibrium.

9- On another scenario: passive margins such as North Atlantic: if you are thinking about the loading of the lithosphere causes subsidence, then uplift can occur in the form glacial-isostatic rebound / denudation unloading such as Iceland, Scotland, Norway, where the seismic activity is checked closely related to deglaciation and rebound. The paleoseismic and microseismic events, on the basis of fault displacement and sediment deformation effects, enhances the likelihood that there is a correlation between seismic activity and isostasy equilibrium.

10- Thus, both analysis considering extremity geodynamic context (active and passive margins) provides the correlation concerning seismicity and isostatic disturbances, such as isostatic gravity anomalies.

Best wishes, Mario E. Sigismondi

Very good question!

According to Artemjev et al. (1972), the isostatic gravity anomaly and the zones of seismicity are in good correlation. The regions which are not in isostatic equilibrium are the vulnerable zones for seismic activity due to the development of large shear stress with in the crust. The high positive horizontal gradient of isostatic anomalies are the potential places of isostatic disturbance and tending regions for seismic activity.

Interesting topic for discussion. Should be correlated.