The objective of our paper is to develop a workflow that allows us to calculate more accurate hypocenter locations in seismic event clusters of aftershock sequences or artificial events. Due to the increased sensitivity of the seismological instruments and density of the network, we are able to record small natural and artificial events. The discrimination of these events is necessary to investigate the recent tectonic movements in the Pannonian Basin. As a first step, we performed a hierarchical cluster analysis on the events in the Hungarian National Seismological Bulletin using the spatial distances between the events to obtain event clusters. We selected 5 different test clusters from the list of clusters where two clusters consist of quarry blasts, another two consist of earthquakes, and the last one is a mixture of earthquakes and anthropogenic events. In the second step, to prepare for the double-difference multiple event location analysis, we manually revised the arrival time picks in the Hungarian National Seismological Bulletin in order to increase the consistency and accuracy of the arrival times. We obtained improved single-event locations with the iLoc algorithm using the RSTT 3D global velocity model to provide initial locations for the double-difference relocation. We applied waveform cross-correlation at every station to obtain the differential times and correlation matrices. In order to discriminate the events in the mixed event cluster, we repeated the hierarchical cluster analysis, but this time, we used the correlation matrix as a distance metric. Examining the shape of the resulting dendrogram, it is clear that certain subclusters are well separated. In these subclusters, the coordinates of the events are close to the mines, where explosive quarrying takes place. With this technique, we are able to identify explosions that were listed as earthquakes in the catalogue.
Between February 16 and April 5, 2019, a series of earthquakes took place around the village of Somogyszob, Somogy county, Hungary. The mainshock occurred on March 7 with a local magnitude $$M_L=4.0$$
M
L
=
4.0
and epicentral intensity of 5 on the EMS scale. The main event was preceded by four foreshocks and followed by four aftershocks. The largest foreshock ($$M_L=2.6$$
M
L
=
2.6
) was also felt with maximum intensity of 4 EMS. This earthquake sequence is the first remarkable one in the region that was recorded by a significant number of high-quality broadband digital seismographs. We have estimated the hypocenters of the 9 earthquakes using the hypoDD multiple-event location algorithm. The events occurred in a tight region around the mainshock at around 13–14 km depth. For the main event, we obtained an average moment magnitude of $$M_w=3.75$$
M
w
=
3.75
, source radii of $$r^P=509$$
r
P
=
509
m and $$r^S=400$$
r
S
=
400
m, and static stress drops of $$\varDelta \sigma ^P=1.19\times 10^6$$
Δ
σ
P
=
1.19
×
10
6
Pa and $$\varDelta \sigma ^S=4.00\times 10^6$$
Δ
σ
S
=
4.00
×
10
6
Pa from the analysis of displacement P- and S-wave spectra, respectively. The resulting spectral source parameters for the investigated events agree well with the results of earlier research. We have also shown that our recently developed probabilistic waveform inversion techniques applied in this study are suitable to retrieve the source mechanism for weak local earthquakes. We have successfully estimated the focal mechanism for the mainshock, a foreshock and an aftershock. Each earthquake was a thrust faulting event with a sub-horizontal P-axis pointing towards N-NE, coinciding with the general trend of the compressional stress field in the epicentral region.
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