DELINEATION OF SUBSURFACE STRUCTURE BENEATH 2018 LOMBOK EARTHQUAKE ZONE USING AMBIENT SEISMIC NOISE TOMOGRAPHY

• Main Author: Fajar Narotama Sarjan, Achmad
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/47951
• Institution: Institut Teknologi Bandung

The geological setting of Lombok Island captures the existence of the westward extension of Flores back arc thrust fault to the north of Lombok, Lombok Strait Strike-Slip Fault (LSF) in the West of Lombok, and Sumbawa Strait Strike-Slip Fault (SSF) in the East of Lombok. The implication of the active tectonic system is the occurrence of earthquake sequences on July 29th until August 19th. The earthquake series caused significant damage in several areas including in Mataram City. One important reason to study ambient noise seismic tomography (ANT) in Lombok Island is to improve knowledge of geohazard risk by delineating the upper crustal structure. In this thesis, we used seismic waveform data from 20 seismograph stations recorded for 36 days (August 3rd – September 9th, 2018) which installed around Lombok Island. 20 stations installed consist of 16 temporary station from collaboration between ITB, BMKG, and EOS-NTU, whereas 4 permanent stations are PVMBG networks. From frequency-time analysis (FTAN), we successfully pick 120 Rayleigh waves for obtaining group velocity maps. This study used fast marhincg surface tomography (FMST) to obtain group velocity maps. FMST implements forward and inverse problem using fast marching method (FMM) and subspace, respectively. Shear wave velocity (Vs) with depth profile obtained by inverting the group velocity dispersion curve using the neighborhood algorithm (NA).The tomogram shows low Vs anomaly in the northern part of Lombok assumed as an unconsolidated sediment layer or thermal activity of Rinjani Volcano that migrating along the Flores thrust nudging the stress field closer to brittle failure. A massive mass characterized by high Vs appears in the northeast of Rinjani Volcano. The existance of low velocity anomaly in the eastern part of Lombok indicates there are loose matriks and sediment layer that thicken eastward.