Akademik Veri Yönetim Sistemi
Journal of Earth System Science, cilt.134, sa.3, 2025 (SCI-Expanded, Scopus)
The success of the well-known and widely used focal mechanism determination methods (the first motion polarities of the P waves and S-to-P amplitude ratio methods) depends on good azimuthal coverage and waveforms with a high signal-to-noise ratio. Although the number of seismic stations is relatively high today, seismological networks are sparse in some regions, and it is not always possible to obtain waveforms with a high signal-to-noise ratio, especially for small earthquakes. These limitations hinder polarity–amplitude readings and the ability to achieve good azimuthal coverage. In these cases, the moment tensor inversion method can be used as an alternative to these methods. Thanks to the method that uses the entire waveform, focal mechanisms of earthquakes can be determined without polarity or amplitude readings. This case study aims to shed light on moment tensor solutions of small earthquakes by examining the effects of azimuthal coverage and velocity models on the moment tensor solutions of light earthquakes. For this purpose, five selected light earthquakes that occurred in the Marmara Region were used for analysis. The Marmara region and these earthquakes were chosen because the focal mechanism parameters of the earthquakes are well known, and there are many velocity models for the region, allowing us to compare their effects on the solutions. First, moment tensor inversion was performed by determining stations with good azimuthal coverage. Then, moment tensor inversions were continued, decreasing the azimuthal coverage from 360° to 90° (per 90°) and to lower than 90°. Reliable and accurate solution conditions evaluated the parameters obtained from the inversions. Strike, dip, and rake angles were compared to the focal mechanism parameters obtained from the catalog. Moreover, moment tensor inversions were performed with single-station 3 component records. As a result, moment tensor inversion (MTI) solutions generally remain stable even when azimuthal coverage is reduced. As long as the coverage remains above approximately 180°, solution reliability is not significantly affected – except for single-station solutions. Although some strike, dip, and rake angles obtained from single-station three-component records appear consistent with catalog values, key reliability parameters such as FMVAR frequently exceed the accepted threshold of 30°. FMVAR frequently exceeds the accepted threshold of 30°, even when the azimuthal angle of the station is above 180° and waveform fits look good. Since single-station solutions often fail to meet reliability thresholds such as FMVAR < 30°, they should only be used cautiously, as the ISOLA software also emphasizes. While the analysis was conducted on light earthquakes, the methodology and findings are highly relevant to smaller events, such as micro-earthquakes, which often face similar challenges, including low signal-to-noise ratios and sparse station distribution. This research highlights the potential for applying these techniques to small- and micro-earthquakes by proving the reliability of moment tensor solutions under constrained conditions.