Electronic Theses and Dissertation

Wilayah Toba memiliki struktur tektonik yang kompleks dan zona seismik aktif Great Sumatra Fault (GSF) di bagian barat yang menyebabkan aktivitas seismik terus terjadi. Pada tahun 2021, gempa swarm terjadi di Wilayah Toba yang belum diketahui penyebabnya. Penelitian ini bertujuan untuk (1) melokalisasi gempa swarm yang terjadi di Wilayah Toba-Samosir berdasarkan data seismik 2021, (2) merelokasi gempa swarm Toba tahun 2021 menggunakan dua metode yaitu coupled velocity hypocenter dan double difference, (3) menentukan mekanisme fokal gempa berdasarkan polaritas pertama, (4) menginterpretasi jalur sesar sekitar Danau Toba berdasarkan liniasi gempa swarm, dan (5) mengidentifikasi penyebab gempa swarm Wilayah Toba. Tim Laboratorium Gempa Bumi TDMRC USK memasang 26 temporary fairfield nodal seismometer di sekitar Danau Toba dan Pulau Samosir sejak Mei hingga Juni 2021 untuk merekam gempa swarm tersebut. Di sekitar Danau Toba juga sudah terpasang 14 seismometer permanen BMKG. Gelombang seismik yang terekam digunakan untuk melokalisasi gempa bumi menggunakan program HYPO71 sehingga diperoleh 336 kejadian gempa swarm yang terdistribusi di sisi barat Pulau Samosir. Gempa bumi kemudian diseleksi dengan kriteria azimuthal gap angle ≤ 180° sehingga terseleksi 317 gempa untuk direlokasi menggunakan metode coupled velocity hypocenter (VELEST) sehingga diperoleh solusi lokasi baru dan model kecepatan 1D. Relokasi lanjutan menggunakan metode double difference menghasilkan enam klaster gempa swarm yang lebih tajam dari 304 kejadian gempa. Distribusi gempa hasil relokasi membentuk sesar atau patahan aktif dan tidak membentuk pola terpusat (centralized). Interpretasi dilakukan berdasarkan distribusi gempa hasil relokasi yang dikombinasikan dengan data mekanisme fokal sehingga didapatkan enam liniasi sesar. Perhitungan mekanisme fokal menunjukkan pola normal pada Sesar Samosir – Pangururan, pola normal dan strike-slip pada Sesar Samosir – Palipi, pola strike-slip dengan komponen normal pada Sesar Toba – Silimapuluh, pola normal dan strike-slip pada Sesar Toba – Sitiotio, pola strike-slip dan thrust pada Sesar Toba – Balige, dan pola strike-slip pada Sesar Toba – Simanindo. Kami menginterpretasikan bahwa gempa swarm Toba 2021 bukan merupakan gempa vulkanik, tetapi gempa tektonik yang terjadi di sepanjang secondary fault yang teraktivasi kembali.

Kata Kunci: Sesar Sumatera, gempa swarm, relokasi, coupled velocity hypocenter, double difference, mekanisme fokal, secondary fault

The Toba region is characterized by the geologically complex structure and surrounded by active seismic zones with the Great Sumatra Fault (GSF) in the west, that causes seismic activity to continue to occur. In 2021, a swarm earthquake occurred in the Lake Toba Region which the cause is unknown. The objectives of this study are (1) to localize the swarm earthquakes that occurred in the Toba-Samosir Region based on the 2021 seismic data, (2) to relocate the 2021 Toba swarm earthquakes using two methods, namely the coupled velocity hypocenter and the double difference, (3) to determine the focal mechanism of the earthquake based on the first polarity, (4) to interpret the fault lines around Lake Toba based on the lineation of the swarm earthquakes, and (5) to identify the cause of the swarm earthquakes in Lake Toba. The USK TDMRC Earthquake Laboratory team installed 26 temporary seismometer consisting of the farfield seismic nodes surrounding the Toba Lake and Samosir Island from May to June 2021. Around the Toba region, BMKG has also installed 14 permanent seismometers. The recorded seismic waves were used to localize the earthquakes with the HYPO71 program to obtain 336 swarm earthquake events distributed on the west side of Samosir Island. The earthquakes were then selected with the criteria of azimuthal gap angle ≤ 180° so that 317 earthquakes were selected to be relocated using the coupled velocity hypocenter (VELEST) method and simultaneously updated the 1D velocity model. The double difference method was used for further analysis resulting in six sharper clusters of 304 seismic events. The distribution of relocated earthquakes forms active faults and does not form a centralized pattern. Interpretation is based on the distribution of relocated earthquakes combined with focal mechanism data to obtain six fault lineations. The focal mechanisms show normal patterns in Samosir – Pangururan fault, normal and strike-slip patterns in Samosir – Palipi fault, strike-slip patterns with normal components in Toba – Silimapuluh fault, normal and strike-slip patterns in Toba – Sitiotio fault, and strike-slip and thrust patterns in Toba – Balige and strike-slip patterns in Toba – Simanindo fault. We interpret that the 2021 Toba swarm earthquake is not a volcanic earthquake, but a tectonic earthquake that occurred along a reactivated secondary fault. Keywords: Sumatera fault, swarm earthquake, relocation, coupled velocity hypocenter, double difference, focal mechanism, secondary fault