PEMODELAN STRUKTUR BAWAH PERMUKAAN ZONA SUBDUKSI DAN BUSUR GUNUNGAPI JAWA TIMUR BERDASARKAN ANALISIS DATA GRAVITASI

MUHAMAD RAGIL SETIAWAN, Dr.Ing. Ari Setiawan, M.Si.

2015 | Tesis | S2 Ilmu Fisika

Zona subduksi di selatan Jawa Timur merupakan bagian dari subduksi panjang yang terbentuk akibat tumbukan lempeng Indo-Australia dan Eurasia. Keberadaan subduksi tersebut menjadi menarik untuk dikaji karena turut mempengaruhi aktivitas gunungapi dan potensi bencana alam di daerah tersebut. Pengkajian zona subduksi dan gunungapi dilakukan dengan melakukan pemodelan struktur bawah permukaan menggunakan anomali gravitasi dengan bantuan program Grablox dan Bloxer. Pemodelan dilakukan terhadap data sekunder anomali gravitasi udara bebas, anomali regional, dan anomali lokal. Data anomali gravitasi diperoleh dari Bureau Gravimetrique Internationale (BGI). Pembuatan model dilakukan dengan teknik inversi menggunakan metode Singular Value Decomposition (SVD) dan optimasi Occam. Hasil penelitian menunjukkan adanya penipisan kerak samudera rata-rata dari 20 km menjadi 15 km ke arah utara. Penipisan tersebut disebabkan adanya lapisan mantel atas yang menonjol dan mendesak kerak yang disebut flexural bulge. Ketebalan batuan sedimen pada cekungan busur belakang sebesar ~4,50 km, dan semakin ke timur nilainya semakin berkurang. Keberadaan dapur magma pada busur gunungapi di Jawa Timur pada umumnya diperkirakan pada kedalaman ~6 km.

The subduction zone to the south of East Java is a part of a long subduction formed by collision of Indo-Australian and Eurasian plate. The existence of the subduction is interesting to be studied since it also affects the volcanic activity and potential of geohazards in the area. The study of subduction zone and volcano was conducted by modelling the subsurface structure using gravity anomaly with the help of program Grablox and Bloxer. The modelling was conducted on secondary data of free air gravity anomaly, regional anomaly, and local anomaly. The data of secondary gravity anomaly was obtained from Bureau Gravimetrique Internationale (BGI). The modelling construction was performed by inversion technique applying Singular Value Decomposition (SVD) method and Occam optimization. Result of the study indicated there was oceanic crust thinning in average that was from 20 km to 15 km to the north. The thinning was caused by the protuding upper mantle pressuring crush called flexural bulge. The thickness of sedimentary rocks in the back-arc basin was ~4.50 km, and the more it was to the east, the more the value decreased. The existence of magma chamber generally was estimated at depth of 6 km.

Kata Kunci : Anomali gravitasi, Zona subduksi dan gunungapi, Grablox, Bloxer, Pemodelan anomali gravitasi