SHEAR STRENGTH ANALYSIS OF PALU-KORO AND MATANO FAULTS BASED ON NEW FOCAL MECHANISM DATA AND ITS RELATION WITH THE SEISMICITY

• Main Author: Kusumawati, Dian
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/86535
• Institution: Institut Teknologi Bandung

The Palu-Koro and Matano faults are active faults that pass through the central part of Sulawesi. Both faults have been recorded producing earthquakes with significant magnitudes. The Palu-Koro Fault has a high slip rate of 35-40 mm/ year. However, the seismicity is relatively low. Meanwhile, the Matano Fault has a slip rate of 20 mm/ year, which is relatively lower than the slip rate of Palu-Koro Fault, with a low level of seismicity as well. Physically, the relatively low slip rate of the Matano Fault is still under discussion among scientists. It is concerned that Matano Fault reserves potential of significant earthquake, such as the Palu-Koro Fault. On September 28, 2018, an earthquake with a moment magnitude (Mw) of 7, 5 occurred in Palu, Central Sulawesi. The earthquake produced a very large rupture. This condition is relatively rare. The rupture reaches 177 km long and can pass through two large fault separations, 3,5 km at sea and 7 km on land. To understand the characteristic of the faults and the rupture phenomenon of the Palu earthquake, seismicity studies and shear strength analysis for both on the Matano Fault and the Palu-Koro Fault, need to be carried out. Given the importance of understanding in Palu-Koro and Matano faults characteristics, this study aims to: (1) analyze the seismicity pattern and stress regime around the Palu-Koro and Matano faults from the focal mechanism data, (2) conduct shear strength analysis in the Palu-Koro Fault to analyze the 2018 Palu earthquake rupture phenomenon, and (3) conduct shear strength analysis in the Matano Fault to analyze the earthquake potential in the Matano Fault. The data used in this study are catalog data and earthquake waveforms from the Meteorology, Climatology and Geophysics Agency (BMKG). The focal mechanism data from the Bulletin of the International Seismological Center (ISC) is also used in the preliminary study. The approach used for shear strength analysis are through estimating the fault friction coefficient and analytically calculating fault shear strength with information on fault geometry and stress orientation. This method has been applied by other researchers for similar studies in the Dead Sea transform fault system and the Japanese subduction zone. In the preliminary studi of shear strength analysis, fault friction (?s) estimation from stress inversion and mathematical calculations using ISC focal mechanism data was carried out. Bootstrap and Jackknife analysis are applied to the stressinversion to obtain a range of frictional uncertainty values. Friction was observed in three segments of the Palu-Koro Fault (Palu, Saluki and Moa segments) and two segments of the Matano Fault (Pamsoa and Ballawai segments), as well as the offshore rupture area of the 2018 Palu earthquake. We found an increasing friction pattern from the offshore rupture area to the Palu Segment, namely 0,26 < ?s < 0,58 in the offshore rupture area and 0,50 < ?s < 0,72 in the Palu segment. The friction pattern then decreased sequentially from the Saluki (0,68), Moa (0,45) to Pamsoa and Ballawai segments (~0,25). The high friction in Palu and Saluki segments explained the termination of the Palu earthquake rupture. Regarding the potential for earthquake in the Matano Fault, low friction in the Pamsoa and Ballawai segments does not rule out the possibility of a large earthquake, as long as a homogeneous shear strength area is met and there is a fairly high stress accumulation at the beginning. In the second stage study, focal mechanism data is added by utilizing BMKG data. The number of previous focal mechanism data in the Palu-Koro and Matano Faults is ?30 events. The focal mechanism data is increased to 140 events. Estimation of friction from stress inversion is carried out to analyze the fault characteristics. The b-value estimation was conducted to support the analysis of the stress level of the fault. The Saluki Segment has a higher stress level than the Palu Segment, indicated by a low b-value, as low as 0.86, and high friction. This high stress level can be the cause of the termination of the Palu Earthquake rupture in the Saluki Segment, consistent with the results of the initial study. At the Matano Fault, high stress level segment is found, namely at the middle to the end of the Matano Segment. This region is characterized by a low b-value, as low as 0.84, and high friction of 0.55. Based on earthquake records, earthquake gap is found at the Matano Segment. Considering the seismic gap, along with observations of fault characteristics, the Matano Segment is potential for seismic hazard. In the third stage of the study, synthetic data modeling is carried out with a simple model (rectangular prism) using the finite difference method, to understand the relationship between friction and stress fields, specifically maximum stress rotation. Modeling with simple blocks has been carried out in previous studies, but in this research several modifications will be applied, namely: (1) displacement per year as driving force for block deformation, (2) 3D modeling over a simulation time span of 200 years, and (3) application of the half-space fault model in an elastic medium. Numerical modeling results show a sharp change in maximum stress rotation at the tips of the fault when the fault slips. This stress rotation occurs as a result of contraction/traction zones at the ends of the fault. The average value of shear stress on a fault versus the average value of Coulomb shear stress or fault shear strength is observed to be correlated with the appearance of a sharp change in maximum stress rotation. Thus, it can be used as an indicator of stress rotation on fault deformation. In the fourth stage, the fault shear strength is estimated using analytical calculation methods, from information on fault geometry, friction and principal stress orientation obtained from the previous stage. Fault shear strength values on the Palu-Koro Fault segments show shear strength values that increase at the middleof the fault. The shear strength from the north to the south: 168,61 MPa for the Donggala Segment; 208,99 MPa for the Palu Segment; 276,47 MPa for the Saluki Segment; and 172,60 MPa for the Moa Segment. A significant increase was found at the Saluki Segment, which coincided with the termination of the 2018 Palu Earthquake rupture. High shear strength values likely blocked the rupture so that it did not continue. In the Matano Fault segments, the value of fault shear strength is relatively lower than in the Palu-Koro Fault. The shear strength from west to east of the fault: 78,69 MPa for the Kuleana Segment; 106,18 MPa for the Pewusai Segment; 75,68 MPa for the Matano Segment; 77,53 MPa for the Pamsoa Segment; and 79,66 MPa for the Ballawai Segment. However, low shear strength values do not eliminate the potential for significant earthquakes. Further studies of shear strength gradient calculations and numerical modeling of faults can be carried out to see the stress distribution in the area around the fault.