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Differential and source terms locations of the 2015 Teresva (East Carpathians) series and their tectonic implications

Gnyp Andriy, Malytskyy Dmytro

Acta Geophysica

2021

Vol. 69, no. 6

2099--2112

Earthquakes of the 2015 Teresva series have been relocated using differential arrivals only of their P-waves at the same set of seismic stations and source-specific station terms. At least six distinct groups had been identified in the series as a result of single linkage clustering analysis of cross-correlations between their waveforms. Differential arrivals were estimated separately in each group, and not directly relative to the master event, but through the chains of events with the largest cross correlations. Time drift at some Ukrainian stations had been detected by comparing intervals between the first P-waves from the same earthquakes at pairs of stations and taken into account, assuming a linear drift rate. The relocated epicenter of the main MSH3.5 earthquake was only~2.3 km to the east of the macroseismic one and almost exactly at the intersection of the two major local faults, perpendicular and parallel to the Carpathians arc. The almost linear alignment of the other earthquakes in azimuth~320° almost coincided with the parallel fault and with the nodal plane of the almost purely strike-slip focal mechanism estimated for the strongest earthquake from its first polarities at 26 stations, and by moment tensor inversion of the ground displacement amplitudes and duration of the first P-wave pulses at 18 stations. A very interesting oscillatory (cyclic) pattern of the epicenter migration along the SE–NW axis, obtained as a result of relocation, was also confirmed by variations in S-relative to P-wave delays: During the cycle, the epicenters gradually shifted to NW and at the beginning of the new cycle returned.