IMPROVEMENT OF MOMENT TENSOR SOLUTION USING OPTIMUM INVERSION SETTINGS AND ITS APPLICATION FOR LIGHT-MODERATE EARTHQUAKES (M < 5.5) IN THE NORTHERN BANDA ARC REGION

• Main Author: Halauwet, Yehezkiel
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/62675
• Institution: Institut Teknologi Bandung

The complete catalog of Moment Tensor (MT) solutions is useful for various studies in solid earth science, however, the number of MT catalogs for small earthquakes is still very limited in several regions of the world, including Indonesia. In Indonesia, the northern region of the Banda Arc is one of the most difficult areas to obtain a quality MT solution, due to problems with observation coverage, strong seismic noise, and significant structural heterogeneity. To improve the quality of MT solutions with existing problems, it is necessary to analyze MT using the optimum inversion settings, which includes selecting the best station and determining the appropriate velocity model and frequency band for each sourcestation pair. This study proposes an analysis procedure with a process to identify the optimum inversion settings. The procedure consists of two inversion stages with the inversion process at each stage carried out using weights based on noise levels. The initial inversion is carried out to obtain an estimate of the source model which is then used to identify the optimum inversion settings at each station, and the final inversion is carried out using the obtained optimum inversion settings. The proposed approach was tested using synthetic data, then applied to M ? 5.5 earthquakes data in the northern region of Banda Arc. Using synthetic data, the sensitivity analysis of the source model deviation shows that the two-stage inversion approach can provide a precise optimum velocity model and frequency band even though the initial estimates of the source model deviate up to ~30° Kagan Angle. According to that, the comparison of solution quality against several other approaches shows that, in the case of noise and structural heterogeneity disturbance, the final solution of the proposed approach consistently improves waveform fitting and produces a source mechanism closer to the actual model. The application for earthquake data in the northern region of the Banda Arc was successful in analyzing more earthquakes than the Global Centroid Moment Tensor (GCMT) (i.e., 553 compared to 101). The resulting solutions were classified into 133 quality-A, 220 quality-B, 84 quality-C, and 116 quality-D with minimum Mw 3.3, 3, 3.6, and 2.7, respectively. Comparison with other data shows that the quality-A and -B solutions provide a good match of the source mechanism with the GCMT solution, as well as a good match of the P-axis orientation with the crustal stress data and tectonic conditions in the study area. Analysis of the distribution of quality-A and -B focal mechanisms confirmed the deformation in the north and east of Seram Island because of the Fold Thrust Belt formation process and not the subduction process. In addition, the distribution of the shallower focal mechanism (< 20 Km) found a normal fault focal mechanism in the KSZ which was previously identified as a strike-slip fault, confirmed the Amahai Fault with high activity, and found a Mw 4.7 earthquake with the same location and focal mechanism six years before the 2019 Ambon-Kairatu earthquake (Mw 6.5), which has not been previously identified.