ANALISIS ASPEK KINEMATIK PERAMBATAN RUPTURE GEMPA SUMATERA-ANDAMAN 26 DESEMBER 2004

Gempa Sumatera-Andaman 2004 memicu tsunami global yang menyapu habis pesisir barat Provinsi Aceh dan wilayah terdampak lain di berbagai belahan dunia. Banyak hal penting, misalnya proses kinematik rupture yang bertanggung jawab terhadap distribusi spasial energi gempa belum terjawab tuntas. Pertanyaan tentang parameter kinematik rupture (rupture duration, rupture extent, rupture speed dan rupture directivity) dan mengapa Pulau Phuket, Thailand yang terpisah oleh daratan Pulau Sumatera bagian utara ikut terdampak tsunami dijawab melalui penelitian ini. Oleh karena itu, penelitian ini bertujuan menganalisis dan mendiskripsikan proses dan karakteristik rupture gempa Mw 9,1 Sumatera-Andaman 2004 dengan metode 0,25-1,0 Hz multiple signal classification back-projection imaging (MUSIC BPI) berbasis sinyal seismik gelombang P koheren berfrekuensi tinggi yang diperoleh dari jaringan stasiun seismik Eropa (EU array). Hasil analisis citra proses rupture beresolusi tinggi mendiskripsikan rambatan unilateral energi gempa megathrust menuju ke arah utara sepanjang jalur zona subduksi pertemuan Lempeng India, Lempeng Australia dan Lempeng Burma melewati Kepulaun Nicobar dan menuju Kepulauan Andaman sejauh hampir 1400 km. Analisis lebih lanjut memanfaatkan radiator seismik berfrekuensi tinggi menemukan radiasi gempa berlangsung selama 480 s (durasi rupture) yang berkorelasi dengan panjang ekstensi rupture 1400 km dan memindahkan energi gempa dengan laju sebesar 2,9 km/s (kecepatan rupture). Semua temuan sama dengan prediksi terdahulu (Ishii et al., 2005; Ishii et al., 2007). Selain itu, mekanisme pelepasan energi gempa berlangsung dalam beberapa tahap, dimana pada detik ke 160 yang berkorelasi dengan jarak tempuh sekitar 400 km, energi gempa mencapai wilayah Kepulauan Nicobar dan mengalami amplifikasi akibat efek lokal (topografi wilayah) sehingga memicu pembentukan episenter baru sesudah episenter utama yang berada di off westcoast Pulau Sumatera bagian utara. Episenter baru inilah yang memicu gelombang tinggi tsunami yang menerjang wilayah Phuket, Thailand.

Kata kunci: kinematika rupture, gempa Sumatera-Andaman 2004, MUSIC BPI

The 2004 Sumatra-Andaman earthquake triggered a global tsunami striking the west coast of Aceh Province and other affected areas around the world. Many things, such as rupture kinematics have not been resolved. Questions about kinematic parameters (rupture duration, rupture extent, rupture speed, rupture directivity) and why Phuket, Thailand, separated by northern Sumatra mainland, was affected by the wave are explained in this study. Thus, this study aims to analyze rupture processes of the 2004, Mw 9,1 Sumatra-Andaman event using 0.25-1.0 Hz multiple signal classification back-projection imaging (MUSIC BPI) on coherent P-wave signals. The high-frequency signals were obtained from a network of European stations (EU array). The results of high-resolution rupture image analysis showed unilateral propagation of the energy, heading north along the subduction zone where Indian, Indo-Australian and Burmese plates meet and passing through Nicobar Islands towards Andaman Islands. Further analysis of high-frequency radiators found that the radiation lasted for 480 s (rupture duration) which correlated to a rupture length of 1400 km and transported the energy at a rate of 2.9 km/s (rupture velocity). All findings agree well with previous work. In addition, mechanisms for energy release took place in several stages, where at 160 seconds equivalent to a distance of 400 km, the energy reached Nicobar Islands and experienced amplification owing to local effects, causing a new epicenter after the main one, located off the westcoast of northern Sumatra Island. This new epicenter triggered a tsunami wave that hit Phuket, Thailand.

Keywords: rupture kinematics, Sumatra-Andaman 2004 earthquake, MUSIC BPI