  The analysis of long term seismological data collected underground in the
M\'atra Mountains, Hungary, using the facilities of the M\'atra Gravitational
and Geophysical Laboratory (MGGL) is reported. The laboratory is situated
inside the Gy\"ongy\"osoroszi mine, Hungary, 88m below the surface. This study
focuses on the requirements of the Einstein Telescope (ET), one of the planned
third generation gravitational wave observatories, which is designed for
underground operation. After a short introduction of the geophysical
environment the evaluation of the collected long term data follows including
the comparison of a two-week measurement campaign deeper in the mine. Based on
our analysis and considering the specialities of long term data collection,
refinements of the performance and evaluation criteria are suggested as well as
performance estimation of a possible M\'atra site.
