ESTIMATION OF MAGMA MIGRATION PATH IN SINABUNG VOLCANO BASED ON FEBRUARY 2017 HYPOCENTER DISTRIBUTION

• Main Author: Ramayanti, Suci
• Format: Theses
• Language: Indonesia
• Subjects: Fisika
• Online Access: https://digilib.itb.ac.id/gdl/view/47566
• Institution: Institut Teknologi Bandung

The subduction of the Indo-Australian plate along the island of Sumatra resulted in the formation of an active line of volcanoes at the boundary of the plate, one of which is the Sinabung volcano. After 1200 years of no history of eruption, the Sinabung volcano experienced the first phreatic eruption on August 27, 2010, which was followed by a series of eruptions up to September 7, 2010. Around 12,000 residents had to be evacuated due to the sudden eruption. Monitoring activities began to be carried out by the Center for Volcanology and Geological Hazard Mitigation (CVGHM) two days after the first eruption. One of the monitoring activities is monitoring seismic activity using several seismographs scattered around the Sinabung volcano. To understand volcanic activity in the Sinabung volcano, it is necessary to map the hypocenter distribution and determine the b-value around the Sinabung volcano. The hypocenter distribution is used to determine the estimation of the magma migration path at Sinabung volcano and to describe the seismicity under Sinabung volcano indicated by the b-value. This study uses waveform data recorded by 8 observation stations during February 2017. The arrival time of P waves and S waves at each observation station is used to determine the hypocenter distribution. Determination of the initial hypocenter using hypomh software that has been proven to provide optimal results in previous studies. Picking the arrival time of the P wave and S wave at each station is carried out at clearly recorded events of at least 4 stations. Earthquake parameters including origin time and hypocenter location of 454 events were successfully determined using hypomh software with horizontal error of 0.122 - 0.184 km, vertical error of 0.184 - 0.245 km, and RMS of 0.276 - 0.337 seconds. To get more accurate earthquake parameters, hypocenter relocation is needed. 258 events were successfully relocated using the double-difference method by hypoDD software with horizontal error of 0 - 0.0612 km, vertical error of 0 - 0.0612 km, and RMS of 0.0306 - 0.0918 seconds. Earthquake parameters from relocation give more accurate results than the initial earthquake parameters before being relocated. The hypocenter distribution is centered northeast of Sinabung volcano at a depth of 5 km - 15 km below sea level. The hypocenter distribution is not vertical below the peak of Sinabung volcano and is closer to the Sibayak volcano, but the calculation of the temporal variation of the hypocenter shows the distribution of the hypocenter faster towards the Sinabung volcano compared to the Sibayak volcano with a large speed of 3.5439 m/min towards the Sinabung volcano and 0.5233 m/min in the direction of the Sibayak volcano. The hypocenter distribution is divided into two clusters and there is an aseismic zone between the two clusters at a depth of about 12 km below sea level. The aseismic zone is estimated as the location of the magma chamber while the hypocenter distribution shows the magma migration path of the Sinabung volcano. Magma migration path estimation is determined by connecting hypocenter points that are in the same depth range. The direction of magma migration was determined by robust regression using the Theil-Sen estimator on the temporal hypocenter variation towards the depth, south-north direction, west-east direction. In the February 2017, magma is moving further northeast toward Sibayak volcano at a speed of 179.6679 m/day. Seismic activity around the magma migration path is illustrated by the b-value. Overall, the b-value for the February 2017 period shows a value of 1.7429 with an R of 90% which states that the source of the earthquake was not the tectonic activity but magmatic activity. Temporal variation of b-value can be used as an indicator of an eruption in Sinabung volcano and spatial variation in b-value can strengthen the existence of the magma chamber along with the magma migration path in Sinabung volcano.