- © Seismological Society of America
Surface rotational ground‐motion records that were collected by a network of deep mining exploration monitoring in the Polish region of the Upper Silesian Coal basin are reported. A set of 51 rotation records acquired during the first two months of the recording program is analyzed. The three strongest records are discussed in detail in this article. Thus far, the maximum measured rotation velocity has reached 0.03°/s, whereas the respective maximum rotational acceleration was equal to 2.53°/s2. Based on the maximum horizontal ground velocity of 2 cm/s, this corresponds to the modified Mercalli intensity (MMI = IV). The recorded “time history” records of ground rotations have similar durations to those of their respective translational records. However, their Fourier spectra are shifted to higher frequencies in a characteristic way, as predicted by earlier, theoretical considerations. The long‐term objective of the research program is to acquire robust surface rotation records that correspond to MMIs VI or even VII, since mine tremors with a magnitude of 4 or higher are expected to occur in this area in the near future.