Electronic Theses and Dissertation

ABSTRAK

Karakteristik Sesar dan Struktur Pidie Jaya masih menjadi bahan diskusi antara peneliti. Pusat Gempa Nasional tahun 2017 mengusulkan bidang mengarah tenggara-barat laut sementara penelitian lain menunjukkan arah timur laut-barat daya. Sementara struktur tiga dimensi Pidie Jaya belum dipublikasikan sampai saat ini. Ketersediaan data tahun 2017 ditambah dengan data tahun 2020 memungkinkan untuk memperkaya pemahaman tentang karakteristik sesar serta struktur tiga dimensi Pidie Jaya. Tujuan penelitian ini adalah untuk mengkaji pola Sesar Pidie Jaya berdasarkan gabungan data rekaman seismik tahun 2017 dan 2020 serta struktur tiga dimensi yang diturunkan dari model kecepatan gelombang P dan rasio gelombang P dan S. Sebanyak 28 stasiun dipasang tahun 2016-2017 dan 157 stasiun dipasang pada tahun 2019-2020. Lokasi awal gempa bumi dihasilkan dari penentuan waktu tiba gelombang P dan S mengunakan Hypo71. Hasil lokalisasi awal diperoleh lebih dari 1000 gempa dan sebanyak 348 gempa dengan kriteria azimuthal gap angle dari coverage stasiun ≤ 180°. Hypocenter gempa bumi direlokasi menggunakan inversi 1 dimensi menggunakan Metode Simultanous earthquake relocation and velocity determination. Hasil model kecepatan 1-D kemudian digunakan sebagai model awal dalam inversi dan relokasi 3-D secara simultan. Sesar Pidie Jaya terdiri dari tiga jalur yang paralel dengan strike mengarah timur laut-barat daya dengan dipping mengarah ke barat laut. Tiga jalur gempa sistem sesar Pidie Jaya juga terkonfirmasi oleh model elevasi permukaan. Sampai kedalam 40 km, kerak bumi Pidie Jaya terdiri dari 4 lapisan. Model lapisan Vp juga menunjukkan dipping sesar ke arah barat laut. Batas Conrad discontinuity terdapat pada kedalaman sekitar 4 km sedang moho 20 hingga 27 km dan mantel bumi tergambar di bawahnya. Secara cross-section model kecepatan Vp membentuk pola V-shape dimana low-Vp dan high-Vp/Vs yang memiliki kesamaan dengan hasil penelitian sebelumnya menggunakan seismik refleksi. Berdasarkan nilai Vp/Vs yang berkisar 1,6 hingga 1,85 km/s, maka diperkirakan litologi kerak bumi Pidie Jaya didominasi oleh batuan sedimen dengan kandungan air yang tinggi pada area sekitar patahan.


Kata kunci: Pidie Jaya, tomografi seismik, kerak bumi, patahan aktif, struktur Vp dan Vp/Vs

ABSTRACT The characteristics of the Pidie Jaya fault and the three dimensional structure are still interesting discussion topics among researchers. The 2017 National Earthquake Center suggest that the fault strikes in southeast-northwest direction while other studies suggest a northeast-southwest direction. The three dimensional structure of Pidie Jaya has not been published until now. The availability of the 2017 seismic data complemented by the 2020 data make it possible to enrich the understanding of the fault characteristics and three-dimensional structure of Pidie Jaya. The purpose of this study was to examine the pattern of the Pidie Jaya fault based on the combined seismic data recorded in 2017 and 2020. That data can also be used image the three-dimensional structure derived from the P wave velocity model and the ratio of P and S velocity. Passive seismic experiments in Pidie Jaya were carried out using 2016-2017 to record the Mw 6.4 2016 aftershocks and the followed by the 2019-2020 seismic experiment. A total of 28 seismic stations were installed in 2016-2017 and 157 stations in 2019-2020. The initial location of the earthquake resulted from the determination of the arrival time of the P and S waves using Hypo71. More than 1000 earthquakes were recorded by and 348 events recorded with the coverage criteria of azimuthal gap angle ≤ 180°. The earthquakes were than relocated by using simultaneous earthquake relocation and 1D velocity determination. The results of the 1-D velocity model are then used as the initial model in the simultaneous 3-D inversion and relocation. The 3D inversion results three parallel lines with strike direction of the northeast-southwest and dipping direction to the northwest. The three lines parallel fault system of Pidie Jaya is also mimicked by the digital elevation model. Within 40 km depth, the structure of Pidie Jaya consists of 4 layers and shows northwest dipping structure beneath the fault. The Conrad discontinuity boundary is found at a depth of about 4 km while moho is 20 to 27 km. In cross-section, the Vp velocity model forms a V-shape pattern where low-Vp and high-Vp/Vs are similar to the results of previous seismic reflection studies. The Vp/Vs values ranging from 1.6 to 1.85 km/s and it is estimated that the lithology of the Pidie Jaya crust is dominated by sedimentary rocks with high water content in the area around the fault. Keywords: Pidie Jaya, seismic tomography, earth's crust, active fault, Vp and Vp/Vs structures.