INVESTIGATION OF UPPER CRUSTAL STRUCTURE BENEATH EAST JAVA AND BALI, INDONESIA USING AMBIENT NOISE TOMOGRAPHY METHOD

• Main Author: ADI MARTHA (NIM : 32411301), AGUSTYA
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/20833
• Institution: Institut Teknologi Bandung

Physiographically, the research area is divided into six zones: (a) Quaternary volcanoes, (b) Alluvial Plains of Northern Java, (c) Rembang-Madura Anticlinorium, (d) Bogor, North Serayu and Kendeng Anticlinorium, (e) Central Depression zone of Java and Randublatung and (f) Southern Mountains. Based on structure and stratigraphy, the eastern part of Java is divided into four tectonostratigraphic zones from south to north: the Southern Mountains Zone, Present-day Volcanic Arc, Kendeng Zone, and Rembang Zone. The complexity of geological structures in East Java and Bali provide important opportunities for natural resource exploitation, but also harbor perils associated with natural disasters. <br /> <br /> Such a condition makes the East Java region an important area for exploration of the subsurface seismic wave velocity structure, especially in its upper crustal. This research aim to obtain information on the geometry of upper crustal structure beneath East Java and Bali using ambient noise tomography. We employed the Ambient Noise Tomography (ANT) method to image the upper crustal structure under this research area. We used seismic data recorded at 24 seismographs of BMKG spread in East Java and Bali. In addition, we installed 28 portable seismographs in East Java from April 2013 to January 2014 for 2-8 weeks, and we installed an additional 28 seismographs simultaneously throughout East Java from August 2015 to April 2016. <br /> <br /> We obtained more than 1,500 inter-station cross correlations from the 2,000 available station pairs (some were dropped because Rayleigh wave signals were not well represented in some of the cross-correlations). The results of cross-correlation between PCJI station to other stations, showing average Rayleigh wave group velocities of about 3 km/s. This relatively high velocity may be due to the area around PCJI being dominated by igneous rocks. <br /> <br /> Several checkerboard tests have been conducted to ascertain the resolution of the Rayleigh wave group velocity maps produced in the tomographic inversion for each period. These tests have been conducted using the configuration of ray paths from source to the receiver that resulted from the seismometer deployments described above. Based on the results test, we decide to use parameters grid size of 27.8 km x 27.8 km with maximum velocity perturbation of 1 km/s. We selected values for the <br /> <br /> damping and smoothing parameters from a range of 500 to 5000 to get the optimum values, and found optimum values of 1800 and 1800, respectively. <br /> <br /> In this research, we used the Neighborhood Algorithm (NA) developed by Sambridge (1999), to invert the ANT group velocity dispersion curves for Vs depth profiles at each point in a regular grid covering the research area. Using a grid spacing of 13.9 km, we took 512 sample points of group velocity results in the period range from 0.5 to 12.2 seconds in the research area. The dispersion curve at each of these points was inverted to obtain the profile of Vs as a function of depth. <br /> <br /> Low shear wave velocity is associated with basins and sedimentary layers, while high velocity anomaly is well associated with igneous rocks and carbonates. The Kendeng basin thickness reaches 8 to 10 Km, but Madura Sea has thicker sediment. The Kendeng and Southern Mountains Zones are bounded by active volcanoes. The existences of volcanoes in the eastern Java region are associated with moderately low velocity structures. The existence of mud volcano beneath the research area can be identified by the lower Shear wave velocity anomaly than the surrounding sediments.