#TITLE_ALTERNATIVE#

• Main Author: SUANTIKA (NIM 32404005), GEDE
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/12146
• Institution: Institut Teknologi Bandung

Seismic tomography has been successfully applied to constraint subducted slabs into the Earth's mantle using global tectonic earthquakes. This method is applied to study a smaller area like in an active volcano. The internal structure of volcano has been generally studied by using petrology, geochemistry, geophysics, chemical gas and water analysis, volcano deformation, hypocenter of volcanic earthquake distribution, and focal mechanism of volcanic earthquake. Application of seismic tomography to an active volcano can be expected to image precisely the internal structure or magma chamber of the volcano, hence the seismic tomography model could complement other models.<p> <br /> <br /> <br /> <br /> The object of this study is Guntur volcano located at 35 km southeast of Bandung city and close to Garut city. Guntur volcano is chosen as the study area because it has a high risk of volcanic hazard due to the long dormant activity, high seismicity, and close to a tourist area i.e. at the southeastern flank of the volcano.<p> <br /> <br /> <br /> <br /> The study of seismic tomography at the Guntur volcano complex has used seismic waves of volcanic earthquakes recorded at seismic stations around Guntur volcano. The seismic ray travels from the source to the receiver through a medium which has physical characteristics. Some of the physical characteristics of the medium are seismic velocity and seismic attenuation. Existence of subsurface structure can be represented by the seismic anomaly in the medium.<p> <br /> <br /> <br /> <br /> Amplitude decay of wave form, arrival time of P wave (tp) and S wave (ts) have been used in seismic tomography to image the internal structure of the volcano through solving linear equations for velocity deviation (dV) and seismic attenuation (Q-1). The study area in Guntur volcano covers a volume of 20x20x20 km3. The first step in seismic tomography is to parameterize the study volume into smaller volume elements of 2x2x2 km3. The block size has been determined based on the hypocenter and seismic station distributions in Guntur volcano.<p> <br /> <br /> <br /> <br /> The data processing steps of seismic tomography consist of hypocenter determination using both spherical and grid search methods, travel time calculation using pseudo bending method, and tomographic inversion. The input data of seismic tomographic inversion are delay time (&#948;t) i.e. the difference between observed and calculated travel times, weighted travel time of P wave (tp*) and S wave (ts*) obtained from spectral fitting analysis, and differential attenuation of S and P waves (&#916;tsp*=ts*- tp*) obtained from spectral ratio analysis.<p> <br /> <br /> <br /> <br /> The seismic tomographic study at Guntur volcano provides good results in which the internal structure of the volcano can be imaged precisely. All tomograms (velocity deviations, seismic attenuation from spectral fitting and spectral ratio, Vp/Vs ratio, Poisson's ratio, and bulk sound velocity) show a negative anomaly located at the same location beneath the Guntur summit (4-6 km beneath reference elevation), Gandapura caldera (3-6 km beneath reference elevation) and Kamojang caldera (6-8 km beneath reference elevation). The reference elevation is elevation of z=0 km located 4 km above mean sea level. Physical characteristics of the anomalous area are unconsolidated, weak, hot material, and heterogeneous (based on velocity deviations), compressible (negative bulk sound velocity anomaly is stronger than negative shear wave anomaly), weak fluid characteristics (Vp/Vs < 1.82 and Poisson's ratio < 0.28), and partially fluid saturated (Qs/Qp >1).<p> <br /> <br /> <br /> <br /> The seismic velocity and attenuation tomograms depict a consistent low velocity zone and a high attenuation zone beneath the Guntur summit, and the Gandapura and Kamojang calderas. This zone is interpreted to be associated with hot materials that may indicate the magma chamber or rupture area caused by previous volcanic and tectonic activities.