Observation Conditions and Initial Data

[3]  The temporary small aperture RUKSA array (Institute of Physics of the Earth, Russian Academy of Sciences (IPE RAS)) was located in the Hautavaara settlement area (Karelia) 100 km west of the town of Petrozavodsk, between lakes Ladoga and Onega. The array observations (Figure 1) were conducted by a group of IPE RAS reserchears headed by M. V. Nevsky and S. G. Volosov within the framework of the large-scale SVEKALAPKO seismic tomography experiment for the study of the deep structure and evolution of the Baltic Shield. The array consisted of 3 three-component and 6 one-component ( Z ) instruments (SM-3KV seismometers) integrated with the digital recording system of the Ekspress station. The seismometers were positioned symmetrically on concentric circles of diameters of 640 m (outer ring C) and 300 m (inner ring B). The dynamic range of the recording instrumentation is 120 dB, and the recording sampling rate is 100 Hz. Time control was performed via GPS. The RUKSA recording channel after extending its frequency response toward lower frequencies [Bashilov et al., 1985] ensures reliable recording in the range 0.5-20.0 Hz.

[4]  The location of the array is characterized by a rather low level of microseismic noise. The noise spectrum includes stationary components of anthropogenic origin at frequencies of 12.5 Hz and 25 Hz and an intense episodic component with a maximum at 5 Hz. Upon examining the level and spatiotemporal variations in seismic noise at the RUKSA array, a frequency range of 0.5-3.0 Hz was chosen for the subsequent analysis.

[5]  Over 30 days of observations, the array recorded 100 seismic events of various origins including the largest earthquakes of 1999 in Turkey (Izmit), Taiwan (Chi-Chi), and Greece. This work uses records of the 12 strongest teleseismic events of magnitudes mb of 5.0 to 7.7 at epicentral distances of 20o to 70o (Table 1).


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