115 / 2021-09-30 19:22:06

Relationship of variations in gravitational load of the rock mass with induced seismicity in the Upper Silesian Coal Basin, Poland.

USCB seismicity,Earth tide,mining tremor,tidal gravity meter

In seismology dealing with natural lithospheric quakes, one of the elements assumed as the cause of their occurrence at a certain time is influence of periodic changes of the gravity field on the Earth’s lithosphere. This influence being the effect of the mutual gravitational interaction of the system of celestial bodies, i.e. the Sun, Earth and the Moon is manifested in the so-called lithospheric tides and has a wave character. The search for a connection between tidal waves and the occurrence of earthquakes is a widespread issue in the scientific world.



Underground exploitation of hard coal deposits in Upper Silesia Coal Basin in Poland, carried out for several hundred years, has led to disturbance of the natural state of rock mass equilibrium. As a result of changes in the stress system, mass distribution (causing the changes in gravity) and occurring deformations in the strata, seismic tremors are generated. The energy of recorded tremors reaches 10E+09J which corresponds to the magnitude M_L= 4.2. Regardless a good recognition of the geological structure and tectonics of the mining area, mechanical properties of rocks and modelling of stress distributions, which makes it possible to determine the area at risk of a tremor, it is still very difficult to determine when such a tremor may occur.



            The presentation shows the results of the research undertaken to find the relationship between the occurrence of seismic tremors with variations of the gravity value caused by periodic, slowly varying loading and unloading of the rock mass with irregular characteristic. The tremors are considered to be caused by the underground mining of hard coal deposits in the area of Upper Silesian Coal Basin. The gravity changes taken to analysis were theoretical and measured by tidal gravimeter gPhoneX. The study included three datasets. Two sets contain data of the tremors that occurred in the period from 2016 to 2019 in the area of Upper Silesian Coal Basin. Third set contains data of seismic tremors that took place during the operations of the longwall in the hard coal deposits recorded by the seismological network of the ROW Mine - Mining Plant Rydułtowy. The time of occurrence of shocks in these sets were superimposed on the course of curves determined theoretically and observed by measurements of gravity changes .



As a result, the data was obtained which shows that for the collection compared with theoretical changes in gravity, almost 30% of induced shocks in the period 2016-2019 occurred at the moment of maximum loading or unloading of the Earth’s crust by gravity. A comparison of the 2019 collection with the actual changes in the acceleration of gravity measured by the tidal gravimeter showed that for 38% of the induced tremors, the time of occurrence can be associated with the moment of maximum loading or unloading of the Earth’s crust by gravity. For the data set coming from the direct environment of the exploited longwall, it is observed that this relation reaches the value of 27%. Additionally, in view of the general result, indicating that the time of occurrence of about 30% of the mining tremors coincides with the extreme values of changes in the intensity of gravity field resulting from the total effect of tidal waves, it was investigated with the Schuster spectrum whether there is a periodicity in the catalogue, that could be associated with the main lithospheric tidal waves. Results shows that there is no dependence in the time of occurrence of tremors in relation to the periods of the most important lithospheric tidal waves. In conclusion, therefore, it should be noted that in the analysis of the relationship between the occurrence of seismic shocks not only caused by mining and lithospheric tidal action, the total effect of gravity variation should be considered rather than the effect of individual tidal waves.