Hetty Triastuty

3D seismic tomography Vp, Vs and Vp/Vs ratio beneath Gede Volcano, West Java, Indonesia

Gede Volcano (West Java, Indonesia), is located 60 km southern of Jakarta with altitude of 2,598 m above the sea level. This stratovolcano is categorized into type A volcano (magmatic eruptions after 17th century) and within one of the regions with the highest population density in the world. The characteristic eruptions occurred in Gede Volcano are commonly small and short in period. Almost every year, the seismicity of Gede Volcano significantly increased and was dominated with volcano-tectonic A-type event, volcano-tectonic B-type event, tectonic event, and tremor. The last swarm occurred in February-March 2012, was caused by the tectonic movement of the fault and intrusion. The objective of this study is to determine the low velocity zone due to the hot materials or weak zone. We picked manually the arrival times of P wave and S wave for the volcano tectonic events and local tectonic events of January 2016-January 2017 period. Most of the hypocenters are having a fix depth solution, due to the azimuthal GAP higher than 180°. We also found a cluster event between Gede Volcano and Pangrango Volcano at depth of 0 until 28.47 km under the mean sea level. Then the 1-D velocity model was updated and the hypocenters were relocated with VELEST33. The relocated hypocenters were getting closer to the volcano area, and the hypocenter with fix depth solution has been reduced. The updated velocity model, origin time, and hypocenter location were used as input to get the 3-D velocity model of P wave, S wave, and Vp/Vs ratio. We evaluated the resolution model by applying the checkerboard test and derivative weight sum, however, we acknowledge for the checkerboard tes did not show a fairly good recovery due to insufficient resolution in our study. The tomographic results show some prominent features as follows (i) high Vp anomaly and lowVp/Vs anomaly are observed at depths of about −2 km to -1 km which may be related to altered zone, (ii) high Vp anomaly and low Vp/Vs anomaly at depths of about −2.5 km to 2 km which may be associated with hot material, and (iii) low Vp anomali and high Vp/Vs at depths of 0 – 2 km which may be interpeted as weak zone (fractured zone).Gede Volcano (West Java, Indonesia), is located 60 km southern of Jakarta with altitude of 2,598 m above the sea level. This stratovolcano is categorized into type A volcano (magmatic eruptions after 17th century) and within one of the regions with the highest population density in the world. The characteristic eruptions occurred in Gede Volcano are commonly small and short in period. Almost every year, the seismicity of Gede Volcano significantly increased and was dominated with volcano-tectonic A-type event, volcano-tectonic B-type event, tectonic event, and tremor. The last swarm occurred in February-March 2012, was caused by the tectonic movement of the fault and intrusion. The objective of this study is to determine the low velocity zone due to the hot materials or weak zone. We picked manually the arrival times of P wave and S wave for the volcano tectonic events and local tectonic events of January 2016-January 2017 period. Most of the hypocenters are having a fix depth solution, due to the azimuth...