Triggered Earthquakes on the Sambia Peninsula

On September 21, 2004, the series of perceptible earthquakes occurred near the city of Kaliningrad. This series of seismic events affected not only the Sambia Peninsula, but also the entire Baltic region up to southern Sweden and eastern Denmark. The following events were recorded instrumentally: the foreshock with a magnitude of 5.0, the main shock with a magnitude of 5.3, and several aftershocks. The above magnitude estimates correspond to the highest values provided by various agencies. The magnitude estimates are conservative, but in this case it is even more difficult to explain the abnormally high intensities in some locations. Abnormally high seismic effects can be explained in only two ways: either the influence of the geological medium structure, or the occurrence of new earthquakes, which were triggered by the fore- and mainshock. The first assumption is not supported by the results of seismic microzoning, because the maximum amplification of seismic intensity was only 0.6 at two sites. To explain the observed intensity, such amplification is too weak. We attempted to find factual material confirming the possibility of earthquakes triggered by the fore- and mainshock. The article analyzes the distribution of seismic effects from the September 21, 2004 Kaliningrad earthquakes within Kaliningrad oblast. The observed intensities are compared with the calculated seismic effects in terms of the seismic intensity scale. As a result, it was found that the observed level of seismic effects significantly exceeds the calculated one and cannot be explained by the effects from the fore- and mainshock because of ground conditions. The macroseismic surveys in the territory of Kaliningrad oblast revealed excessively large cracks in the ground with significant horizontal movements along them and an anomalous lowering of the water level in a pond near the village of Veselovka, despite the torrential rains the day before. According to the macroseismic data, the average value of the attenuation coefficient for the region under consideration was b ≈ 2.7, which is significantly lower than the world average. The acquired data gave us grounds to hypothesize about certain earthquakes that accounted for the anomalously high seismic effects in Kaliningrad oblast: these seismic events were triggered by the fore- and mainshock of the 2004 Kaliningrad earthquakes, but were not recorded by seismic stations. A conclusion is drawn about the stress state of the crust beneath the Sambia Peninsula. Therefore, trigger earthquakes are quite possible here.