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Earthquake in Żerków |
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Monday, 09 January 2012 15:04 |
 On Friday January the 6th, 2012, at 4:37 pm, seismic shocks of short duration occurred in Żerków, 50 km north-west from Kalisz. Witnesses felt the ground uplifting and then rolling from side to side. Phenomenon, which lasted for several seconds, was accompanied by dull sounds coming from the inside of the Earth.
Quakes were also felt by residents of the neighbouring town of Jarocin. Nobody was injured; there was no damage to buildings and infrastructure. Only in few houses in Żerków scratches were noted on buildings' walls and in one house plaster fell of the ceiling. Services were set in a state of emergency and a district emergency panel was called. Reports from Żerków were broadcasted by many TV stations and the event was described in press.
According to the European-Mediterranean Seismological Centre the shock was of a magnitude of 3.8 and its epicentre was at a depth of 10 km. The phenomenon was recorded by some 100 European seismic stations.
Natural quakes of a magnitude between 3 and 4 occur relatively often, some 130 000 shocks per year, according to the USGS. They occur also in non-seismic areas, such as Poland. They are very distinct to people but do not cause damage. Similar earthquakes are generated by mining activities - a day after the incident in Żerków, on the 7th of January 2012, in early morning hours shocks of a magnitude of 3.2 were recorded in the copper basin near Polkowice. It shall be noted, that over the past 100 years there were no major seismic shocks recorded in a radius of 100 km of Jarocin. There are also no such information in historical listings (Guterch, 2009), which is not an evidence though that such quakes did not occur in historical times at all.
All information so far suggests that shocks in Żerków were of natural tectonic genesis, i.e. were caused by the movement of rock masses along a fault line, resulting from a tectonic stress that excided the critical level, needed for reactivation of the fault. Comprehensive analysis of seismographic records from seismographs that recorded the quake will allow defining geometry of the translocation.
The general tectonic context of the localisation of the quake suggests, that the reactivated fault belongs to a fault zone stretching from Cracow, via Kalisz and Poznań towards Szczecin (zone Kraków-Szczecin), which makes the south western fringe of the Teisseyre'a-Tornquist tectonic zone (TTZ). The TTZ is over 100 km wide and is one of the most important tectonic seams in Europe. It divides the stable East European craton of the old Precambrian consolidation, from the West European platform of younger Palaeozoic consolidation. In vicinity of the seam, underneath few kilometres of sedimentary rocks of Perm and Mesozoic age, there are older rock complexes that are locally folded and cut by faults. The concentration of tectonic damages in vicinity of the TTZ makes this part of the earth crust to be mechanically weakened.
Location of the epicentre of the earthquake on a background of the Geological Map of Poland devoid Cainozoic formations, in a scale of 1 to 1000000, Polish Geological Institute, Warsaw 2002
So far, seismic shocks have been usually registered in the northern - Baltic section of the TTZ and in the southern section of Świętokrzyskie. Rarely, they occur in the central, so called Kujavian section, where the latest earthquake occurred.
Stress analyses within the limits of the TTZ zone suggest the occurrence of a stress regime conducive to horizontal movements of tectonic blocks. Numerical models (Jarosiński, 2006) suggest that the Kraków-Szczecin fault zone has a potential to reactivate in a current field of tectonic tensions as a right-handed advance fault. Pace of the earth crust deformation in this region is so slow that it does not threaten systematic recurring shocks.
Since there has been a broad discussion about potential for seismic shocks occurring in response to hydraulic fracturing used for exploring and exploitation of shale gas, it shall be discussed, whether the earthquake near Jarocin could have been potentially excited by this type activities. In this case the answer is explicitly negative. This possibility is excluded by: the depth of the quake’s epicentre – which exceeds depths of exploration boreholes by few times; as well as by the fact, that the closest hydraulic fracturing in shale had been undertaken at a distance of more than 200 km away from Jarocin and that this was undertaken few weeks before the quake happened. Hydraulic fracturing in connection with tight gas exploration near Poznań was undertaken over 50 km away from the earthquake’s epicentre and was performed many months before the quake. It is not possible to create a seismic shock by a procedure that happened in different place and in different time. The range of increased pore pressure in response to hydraulic fracturing does not usually exceed 1 km, and it takes usually few hours to release this pressure. It shall be added, that for exploitation of conventional gas fields that occur near Jarocin, hydraulic fracturing is not used and therefore no increase in pore pressure, which might cause destabilisation of faults, occurs in the area. In addition, hydrocarbon production is so small that is does not cause evident subsidence of the earth surface.
Marek Jarosiński, Mirosław Rutkowski
Translation Anna Kuczyńska
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