SHEAR WAVE VELOCITY MODEL BENEATH AMBON AND SURROUNDING ISLANDS BASED ON AMBIENT NOISE TOMOGRAPHY

• Main Author: Fachrul Rozi K., Muhammad
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/62102
• Institution: Institut Teknologi Bandung

Ambon and its surroundings Islands are located at the convergence zone between the Eurasian, Australia, and Pacific Plates, that caused a major potential geological hazard, especially earthquakes. Based on the historical seismicity, Ambon Island has been hit by several strong earthquakes in which one of them caused a large tsunami that has run up 100 m high in 1674 (one of the largest tsunamis ever in Indonesia). On September 26, 2019, Ambon was shaken by a 6.5 MW earthquake, which according to BMKG report has a depth of 10 km. The 6.5 MW earthquake has been suspected to trigger fault movements in eastern part Ambon Island causing 5.2 MW aftershock. To further understand the aftershock and the geometry of the fault, ITB, BNPB and BMKG collaborated for monitoring the aftershocks in Ambon and surroundings Islands. The monitoring activity was carried out using 11 temporal seismometers deployed by ITB–BNPB and 4 permanent seismometers owned by BMKG that spread across Ambon and surroundings Islands for approximately 2 months recording time. The author tried to utilize the vertical component of seismic monitoring data to image the subsurface model using ambient noise tomography (ANT) in Ambon and surroundings Islands firstly. First stage of ANT is preparing data that enhance the ambient noise signal. Then cross-correlation of vertical component to obtain empirical Green’s function (EGF). Fifty-two 52 EGFs with good quality were obtained from 91 potentially EGFs that can observed in periods of 0,1-4,0 s. The EGFs are analysed using time-frequency analysis to obtain dispersion curve of Rayleigh wave. The dispersion curve can extract travel time value of Rayleigh wave for each periods. Tomographic inversion carried out for group velocity of Rayleigh wave for 1-4 s periods using fast marching surface tomography (FMST). The group velocity of Rayleigh wave maps show that the Eastern Ambon Island and Haruku Island has low Rayleigh wave group velocity of which consistent to up to 4 s. The group velocity maps parted grid each other points 0,1o to build apparent dispersion curves. Dinver modul used to inverse apparent dispersion curve to obtain 1D Vs. The result of 1D Vs made interpolation to create distribution maps of shear wave velocity (Vs). The shear wave velocity (Vs) maps show the consistency results with the geological study. Piru Bay has a high Vs ranging in depth of ~2100 m to 3500 m indicating that the bedrock of Piru Bay is composed of a combination of metamorphic rocks and cordierite granites extending to eastward to Seram Island. Low Vs to a depth of ~1400 m may indicate the presence of volcanic deposit on top the metamorphic rocks characterized as a basin. The eastern islands of Ambon Island and Haruku Island have a consistent low Vs to a depth of ~4000 m. The lineation between Ambon Island and Haruku Island, which in previous study, was indicated as a fault has high Vs in the north region and low Vs in the south region. The low Vs area indicates that the south region is a relatively weaker zone than the northern region.