Surface Ground Motion From Rockbursts

 Recent Research Activity

Between March and April 2001 a research study of mine induced ground motion was carried out at Technical University of Opole, Poland (Chmielewski et al., 2001).  The research was motivated by a need to properly estimate actual intensity of rockburst effect on the surface of the ground.

Surface ground motion may be an effect of either an earthquake or man made effects like traffic, industrial blasts or rockbursts. In some areas of intensive mine exploitation the rockbursts may cause quite substantial ground motion inducing even some damage to buildings and causing concerns among residents. Rockbursts occur when due to accumulation of stresses breaking of intact rock, usually ahead of an advancing mine face, takes place. The event from March 13, 1989 in Germany near Merker (M_L=5.4) which caused injuries to three people and substantial damage to buildings proved that these problems should be treated seriously by civil engineers. Some rockbursts may directly be linked with the underground activity, whereas other occur randomly, usually with 3-6 months return period.

The research reported in this note was prompted by unusually high Peak Ground Accelerations (PGA) recorded by a network of surface accelerometers of “Rudna” Copper Mine located in south-western Poland. The mine is carrying out exploitation of copper ore at a depth of about 600m almost directly below a small town Polkowice. The registered PGA reached level of about 0.15g. Geophysical services of the Mine calculate for each quake the location of its epicenter and energy released whereas civil engineers classify the quakes according to Mercalli intensity scale. On the one hand the rockbursts cause usually only minor damage, mostly to non-structural elements of buildings which would suggest intensities of MSK not more than V-VI. On the other hand high PGA values required further studies and clarifications. These studies are needed not only to ensure safety of buildings but also to reliably asses eventual claims for repairs of damaged parts of buildings by the Mine.

The research was carried out on a sample of 31 records registered between January 2000 and May 2001 by 7 surface recording stations located in Polkowice and its vicinity. The main feature of the records of ground motion was typical short 1-2 seconds duration. When analysing deeper their spectral properties another characteristic features were noted. These were very small PGV (Peak Ground Velocity) and PGD (Peak Ground Displacement) values. Typical value of PGA/PGV ratio for far field Californian earthquakes equals about 10 (e.g. El Centro, 1941 record) whereas for the rockbursts these values reached level of 80, far more than it is usually attributed to near field earthquakes for which PGA/PGV may be close to about 30-40. Another characteristic feature of the analysed rockbursts was their almost 1:1 proportion of vertical to horizontal motion also a characteristic for near field earthquakes. When comparing the Fourier spectra of rockburts and low intensity earthquakes of PGA 0.1-0.15g one could see a result of these unusual PGA/PGV ratios – a shift of dominating components of the spectra from about 4-6Hz to about 15-20Hz. The spectral bandwidth of 0-5Hz is almost not present in the rockburst records, whereas it usually dominates in the earthquake accelerograms. This explains why the rocbursts with rather large PGA are not as destructive as typical earthquakes with the same PGA values. As a conclusion one may say that for the description of intensity of rockbursts PGV values are much better than the PGA.  Comparing the energy released by the quakes with the surface PGA and PGV values has shown that rockbursts with greatest energies usually did not resulted in the strongest surface effects. Detailed results of the reported research will be subject of further publications.

Zbigniew ZEMBATY

Technical University of Opole