BAYESIAN MOMENT TENSOR ANALYSIS OF THE 2012 MW 6.3 PALU EARTHQUAKE SEQUENCE FOR UPDATING THE PALUKORO FAULT MODEL AND COULOMB STRESS DISTRIBUTION

• Main Author: Yusril Iqbal Habibana, Mohammad
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/81605
• Institution: Institut Teknologi Bandung

The Palu-Koro fault in Sulawesi, Indonesia, has recorded at least two earthquakes above Mw 6 in the last decade, i.e. 18 August 2012 (Mw 6.3) and 28 September 2018 (Mw 7.5). Previous research stated that the 2012 earthquake had no significant influence on the distribution of Coulomb stress in the 2018 earthquake. The magnitude of Coulomb stress is influenced by fault parameters. We assume that the possible influence of Coulomb stress towards the fault that caused the 2018 earthquake can be observed if the parameters of the fault segments around Palu-Koro are updated. To prove this, we conducted several analyses to obtain an updated parameter value. The analysis process is divided into: calculation of tensor moments from earthquake sequences; finding the updated parameter values based on the tensor moment results; and applying the updated model to the Coulomb stress calculation. We have performed moment tensor inversion of 16 earthquake events in the 18 August 2012 earthquake sequence. The sources of this information are catalogues and waveform data recorded by the regional stations of the Meteorology, Climatology and Geophysics Agency (BMKG). The range used was magnitudes ranging from Mw 3.8 to 6.3, and the period between 18 August and 31 December 2012. The inversion process was performed by applying Bayesian moment tensor inversion in the ISOLAObspy code. This approach allows us to assess the uncertainty of the centroid moment tensor (CMT) parameters including their positions. In this study, the maximum values of CMT uncertainties in horizontal (latitude & longitude), vertical (depth), time shift, magnitude, and DC were 7.74 km, 4.94 km, 2.2 sec, 0.48, and 22%, respectively. The moment tensor inversion performed on the 2012 main shock shows that the results of this study have close values to the BMKG moment tensor catalogue in terms of strike. The dip value is around 82° with the direction of rake movement still within the range of a left-slip fault earthquake. These results are aligned with three agencies that released CMT catalogues, i.e. BMKG, Geo Forschungs Zentrum (GFZ), and Global Centroid Moment Tensor (GCMT). Compared to these three agencies, the DC value of this study is higher (83%) with a relatively good quality with qualified VR and CN values (VR>60%; CN<10). Our results show that many aftershocks are distributed near the northern end of the Saluki segment of the Palu-Koro Fault. Most solutions show a left strike-slip mechanism or rake values of -20 to 20°, with the main earthquake located west of Lake Lindu. The moment tensor solutions generally have a North-Northwest to South-Southeast strike. Previous research states of the three fault parameters (strike, dip, rake), the Coulomb stress is affected more significantly when the dip value is varied. Therefore, this study will focus on analysing the calculations for the dip fault based on all the moment tensor results. The histogram of dip values shows a skewed shape, so the median value of the dip data distribution is selected as a solution that represents the data. Using this method, a dip value of about 82.5° was found. This value updates information from previous records by the National Earthquake Centre (PusGeN). The update with our findings will be applied to the Palu-Koro Fault at the Saluki and Palu segments according to the moment tensor distribution at these locations. In the PusGeN records, these two segments have 90° (vertical) dipping. Furthermore, simulations of Coulomb stress distribution will be performed using two models: the PusGeN model without updating (vertical dipping model) and the PusGeN model with updating based on the moment tensor analysis performed in this study (non-vertical dipping model). Through this process, we found that the non-vertical dipping model exhibits greater Coulomb stress intensity, up to 5 bar, with a wider distribution area than the vertical dipping model. Observations on both models show very little Coulomb stress observed in the Makassar Strait Segment with values below 0.5 bar. This suggests that an earthquake with parameters like the 2012 earthquake could not significantly affect the Makassar Strait Segment, which is assumed to be the main segment of the 2018 earthquake. However, this finding should be concerned because we found a significant increase of Coulomb stress at the northern end of the Saluki segment with our updated model.