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Prague is the capital of the Czech Republic with 1.3 million inhabitants. It is situated in an intraplate region with low seismicity. The seismic hazard is low, but not negligible. In its evaluation, we face the following challenging issues: (1) there have been no M > 6 events during the period covered by historical catalog (Giardini et al. 2013) although there is evidence from paleoseismic research of earthquakes with magnitude of at least 6.5. It is difficult to determine a reliable magnitude frequency distribution from sparse data. (2) There are no GMPEs determined from strong ground motion data for the region of interest. Therefore, we studied the attenuation from recent weak earthquakes in order to determine which of the published GMPEs are suitable for the region. (3) There are plenty of faults in the city and its neighborhood, all of them believed to be seismically inactive. We have to quantify the probability of an unexpected earthquake on such faults. We used the concept of diffused seismicity for this purpose. (4) The hazard is dominated by the 140 km distant West Bohemia Source Zone with earthquake swarm activity, which does not follow the classical mainshock-aftershock model. The results in the present study were obtained through the OpenQuake engine, the open-source software for seismic risk and hazard assessment developed within the global earthquake model (GEM) initiative. The seismic hazard for Prague (expressed in terms of peak ground acceleration) is below 0.1 g for the returning period of 10,000 years. [ABSTRACT FROM AUTHOR]

We analyze refraction measurements along a short profile in western Krušné hory crystalline unit. The profile passed close to the seismically active zone of Nový Kostel. The measurements were carried out to distances of about 15 km using quarry blasts near the village of Horní Rozmyšl, located at the eastern margin of the crystalline unit. Smoothed P-wave travel times were interpreted using the Wiechert-Herglotz method, which yielded a 1-D velocity model of the shallow crustal structure of the crystalline unit down to a depth of 1.7 km. The P-wave velocity of the model increases from about 4.0 km/s at the surface to 5.9 km/s at a depth of 1.7 km. The superficial velocities of our model are somewhat higher than the superficial velocities of the model that is routinely used for earthquake location in the region. [ABSTRACT FROM AUTHOR]

The Orlík water reservoir was built on the Vltava River in the places where the river crosses the geological unit called the Central Bohemian Pluton. Previous geological studies have disclosed no significant lateral inhomogeneity or fault zone in the close vicinity of the reservoir. However, repeated levellings, performed there in the 1950s and 1960s, revealed a systematic subsidence of the Vltava River valley. Using these measurements, the idea of the tectonic origin of the valley was even formulated. Recent seismic observations have also attracted attention to this region. Namely, a macroseismically felt earthquake occurred there on January 13, 2007, and many weaker earthquakes have been recorded by local seismic networks since 1992. To increase the accuracy of locating these seismic events, recently we have carried out refraction measurements along a short profile across the Orlík reservoir. These authors smoothed the observed travel times and derived a 1-D velocity model of the shallow crustal structure for the Orlík region. In the present paper, using the same refraction measurements, we study some anomalies in wave propagation beneath the Orlík reservoir. In particular, it is shown that the passage of seismic waves beneath the reservoir leads to their time delays and spectral changes. Moreover, we have also recognised similar time delays in earlier data from the nearby international profile CEL09 in the places where the profile crosses the Orlík reservoir. [ABSTRACT FROM AUTHOR]