SEISMIC SIGNAL ANALYSIS OF NON-VOLCANIC TREMORS AROUND MOUNT TAMBORA, WEST NUSA TENGGARA

• Main Author: Mujur Mandela Simamora, Buha
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/81621
• Institution: Institut Teknologi Bandung

West Nusa Tenggara (NTB) is located between two subduction zones, the Flores Back Arc Thrust in the north and the Java Trench in the south. As a result, seismicity in the region is very active and dominated by small earthquakes with a magnitude of M2. Seismic activity around Mount Tambora is dominated by shallow earthquakes with small magnitudes. These seismic events can be caused by magma activity, pressure from the subduction zone and triggered by earthquakes with greater strength. In connection with this, it is necessary to have a better understanding of the causes of seismic activity. Non-volcanic tremor (NVT) is different from tremor, in that tremor is closely related to magmatic activity while non-volcanic tremor is a seismic event that usually occurs in subduction zones and can be triggered by large teleseismic earthquakes. There are several fundamental questions that remain unclear regarding the physical mechanisms that trigger nonvolcanic tremor and what factors can trigger non-volcanic tremor events. In addition, the location of non-volcanic tremors also has considerable uncertainty. Therefore, this study focuses on analyzing the seismic signal characteristics of nonvolcanic tremor triggers and the physical mechanism and location determination method of non-volcanic tremor. Seismic signal characteristics such as frequency, incidence angle, dynamic stress, and signal filtering can be used to identify nonvolcanic tremor events. Determination of the location of non-volcanic tremor in this study uses the envelope waveform cross correlation method. This study aims to identify non-volcanic tremor events in West Nusa Tenggara and determine the characteristics of the seismic signals that trigger these events. A systematic search was conducted on non-volcanic tremor (NVT) triggered by teleseismic earthquakes around Mount Tambora in West Nusa Tenggara by 35 teleseismic earthquakes with Mw > 7 and epicenter distance more than 1000 km during the period 2020 to 2022. Waveform data were taken from the Indonesian Meteorology, Climatology and Geophysics Agency (BMKG). The identification of triggered tremors is based on visual detection of non-impulsive and prolonged spikes of high-frequency (1-10 and 2-8 Hz) seismic energy, showing coherence across a number of seismic stations and modulation by teleseismic surface waves. From the 35 earthquakes observed, four teleseismic events triggered the NVT in West Nusa Tenggara. The 2021 Kermadec Mw 7.4 and 8.1 earthquakes, the 2021 Vanuatu Mw 7.3 earthquake, and the 2022 PNG Mw 7.7 earthquake triggered NVTs in this region. This study reveals that surface wave amplitude plays an important role in determining the triggering potential of NVTs, with an observed PGV threshold of about 0.1 cm/s, or a dynamic stress of 7-8 KPa. The triggered NVTs were initiated by Rayleigh waves. The results show that NVTs are controlled by PGV, frequency, and dynamic stress.