Abstract
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.
Highlights
The seismicity of the Pannonian Basin can be described as moderate
The primary purpose of the hierarchical cluster analysis was to divide these events into smaller groups that would be suitable for the multiple event location method
We selected five test clusters to demonstrate the feasibility of our methodology to relocate event clusters and possibly discriminate between earthquakes and explosions
Summary
The seismicity of the Pannonian Basin can be described as moderate. The recent seismic activity is caused by the Adriatic microplate’s movement, which rotates counterclockwise relative to Europe. Additional geological structural studies require the most accurate earthquake catalogue. 40% of the Hungarian earthquakes are anthropogenic: quarry explosions and mine blasts. The earthquake catalogue may contain unidentified explosions that makes geological interpretation more difficult. In order to make the Hungarian National Seismological Bulletin (HNSB, Gráczer et al 2016) more accurate, it is necessary to identify these anthropogenic events. In the routine observatory practice, a single-event location algorithm is used to calculate the hypocenter parameters, which often suffers from high uncertainty and location bias. The location errors can be significantly reduced by multiple event relocation methods combined with high-quality data. We use one of the most commonly used algorithms, the doubledifference algorithm (Waldhauser and Ellsworth 2000)
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