Josef Havíř

Stress Analyses in the Epicentral Area of Nový Kostel (Western Bohemia)

The focal mechanisms of events from three micro-earthquake swarms (swarm in the years 1985/86, swarm of December 1994, and swarm of January 1997) in the epicentral area of Nový Kostel (West Bohemia region) were used as input data for stress analyses. The simple graphical method of Angelier and Mechler (1977) and inversion by the program BRUTE3 (Hardcastle and Hills, 1993) were applied to the data collections. The results of the stress analyses for the 1985/86 swarm and for the swarm of December 1994 are similar. For the January 1997 swarm, the results of the analyses differ from those for older swarms. The axis of maximum extension is oriented in the NE-SW direction and is subhorizontal, the axis of maximum compression is oriented in the NW-SE direction and perspicuously dips towards SE. These results are similar to older results of stress analyses carried out for the 1985/86 swarm (Antonini, 1988; Sonnleitner,1993). They are also consistent with most of other published results of stress analyses conducted at different sites in the western part of the Bohemian Massif. The orientation of the principal stresses confirms the sinistral strike-slip movement along the Nový Kostel-Počátky-Zwota line (the trend is about 355°) defined by the epicentres of the micro-earthquakes. The eastern tectonic limit of the Cheb Basin (and other respective parallel faults ) could be characterised by normal rather than strike-slip faulting.

Zvýšená seismická aktivita v epicentrální oblasti Lašťany(střední Morava)

Weak natural seismic activity is known from the NE part of the Bohemian Massif. More than 2 200 tectonic events were recorded in this region on the basis of records of seismic stations operated by Institute of Physics of the Earth (IPE) during period 1996–2011. Since 2008 a relatively signifi cant seismic activity has been observed in epicentral area Lasťany situated SE of Sternberk. In this area, 221 tectonic events were observed during period September 2008 – December 2011. Local magnitude of strongest event reached value 2.

ZÁZNAMY SEVEROKOREJSKÝCH JADERNÝCH EXPLOZÍ NA SEISMICKÉ STANICI VRAC

Seismic station VRAC operated by IPE (Institute of Physics of the Earth) is part of the International Monitoring System CTBTO (Comprehensive Nuclear-Test-Ban Treaty Organization). This monitoring system is built for verifi cation of the compliance with the Comprehensive Nuclear-Test-Ban Treaty. Station VRAC provided important registration of the nuclear explosions. Last explosions were carried out on the territory Democratic People’s Republic of Korea (DPRK), latest North Korean nuclear test was conducted this year (12. 2. 2013). Station VRAC recorded signal of all North Korean nuclear explosions. Registrations by stations of International Monitoring System, including station VRAC, significantly contributed to improving of the discrimination techniques, mainly to improving the discrimination between natural earthquakes and explosions based on the analyses of Ms and mb magnitudes.

VÝJIMEČNÉ ZEMĚTŘESENÍ JV. OD POZNANĚ (POLSKO) ZAZNAMENANÉ 6. 1. 2012

On January 6, 2012, a moderate earthquake was observed in a region SE of Poznan (local magnitude ML = 3.6 according to Institute of Physics of the Earth, IPE). In this region, there haven‘t been known any historical earthquakes so far, and no natural seismic activity has been observed up to present. Similar rare occurrences of weak and moderate earthquakes were observed in a region near Kaliningrad in 2004 (sequence of events, local magnitude of strongest event being 5.0) and in south Moravia region near Znojmo in 2000 (local magnitude ML = 2.5). These facts show that even in seismically quiet regions occurence of weak to moderate seismic events (with value of magnitude ranging from 3 to 5) could be expected.