INVESTIGATION OF RUPTURE OF 2018 LOMBOK EARTHQUAKE SERIES BASED ON SEISMICITY STUDY AND SHEAR WAVE SPLITTING

• Main Author: Trisnia Sasmi, Annisa
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/77492
• Institution: Institut Teknologi Bandung

Lombok is one of the islands in the transitional zone of Sunda Arc and Banda Arc Tectonic System, Indonesia. In the mid-2018, Lombok area was hit by a series of strong earthquakes, started with a magnitude 6,4 earthquake on July 29 2018, followed by earthquakes on August 5 (M 7,0), August 9 (M 5,9), and August 19 (M 6,3 and 6,9). Some researchers suspected that this phenomenon occurred due to a segmentation rupture in the northern part of Lombok Island. This study aims to obtain information on how is the distribution of the Lombok earthquake rupture zone 2018, through the Shear Wave Splitting (SWS) study. Splitting, or S-wave separation, occurs when the S-wave passes through an anisotropic medium. The S wave is split into fast S wave and slow S wave with almost orthogonal polarizations and has parameters such as delay time and polarization direction. To determine the SWS parameter, we used a Lombok earthquake aftershock data set which was recorded during 4 August – 9 September 2018, using 16 ITB and EOS/NTU temporary seismic stations. The steps taken to obtain the SWS parameters were: event selection, windowing using the Short Time Fourier Transform, and the Rotation-Correlation process. The tomographic delay time SWS algorithm was also applied to obtain the distribution of the subsurface seismic anisotropy percentage. The results of the determination of SWS parameters indicate that the fast polarization direction probably have a linkage with the local fault system and the fault related to Lombok earthquake rupture zone, so that the seismic anisotropy conditions in the Lombok region are controlled by a structureinduced anisotropy mechanism. The trend of SWS delay time over period can be an indication of an increase in the formation of fractures which was quite intensive before the significant earthquake occurred on 19 August 2018. While the results of the SWS delay time tomography show a pattern of high seismic anisotropy degrees which is thought to be related to the Lombok earthquake crack system 2018, and the volcanic activity of Mount Rinjani. The low seismicity zone in the Rinjani Volcano region is thought to be caused by the presence of fractures filled with hydrothermal fluids. The pattern of aftershock cluster positions relative to the seismic anisotropy degree distribution indicates the potential for new asperity zones in the future.