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All Journal IJOG : Indonesian Journal on Geoscience Journal of Mathematical and Fundamental Sciences Journal of Engineering and Technological Sciences Jurnal Geologi dan Sumberdaya Mineral (Journal of Geology and Mineral Resources) Jurnal Geofisika Indonesian Journal on Geoscience Indonesian Journal on Geoscience Jurnal Geologi dan Sumberdaya Mineral (Journal of Geology and Mineral Resources) Jurnal Meteorologi dan Geofisika
Afnimar
Faculty Of Mining And Petroleum Engineering, Institut Teknologi Bandung,
Astronomy Chemistry Earth & Planetary Sciences Education Energy Engineering Environmental Science Mathematics Physics

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Stress Drop Variation di Sumatra Rahmat Setyo Yuliatmoko; Afnimar Afnimar; Indra Gunawan
Jurnal Geofisika Vol 15 No 3 (2017): Jurnal Geofisika
Publisher : Himpunan Ahli Geofisika Indonesia (HAGI)

Show Abstract | Download Original | Original Source | Check in Google Scholar | Full PDF (1512.952 KB) | DOI: 10.36435/jgf.v15i1.14

Abstract

Sumatera merupakan daerah dengan kondisi tektonik yang rumit dimana terjadi konvergensi lempeng India terhadap lempeng Eurasia dan terdapat beberapa sesar aktif. Untuk melihat keaktifan daerah Sumatera penulis menghitung stress drop di Sumatera dan membahas bagaimana kaitanya dengan tektonik secara regional. Stress drop diperoleh dengan menganalisis spektrum displacement observasi menggunakan metode inversi nonlinier Nelder Mead Simplex. Dari penelitian ini didapatkan bahwa stress drop pada daerah crustal segment Siulak menghasilkan nilai terbesar yaitu 39.26 Mpa dikarenakan daerah tersebut memiliki slip rate yang kecil jika dibandingkan dengan segment lainnya di Sumatera. Zona Ninety East Ridge menghasilkan nilai stress drop yang kecil 2.6 - 4.3 Mpa dikarenakan daerah ini merupakan daerah rekahan serta patahan. Daerah accretional wedge menghasilkan nilai stress drop tinggi 20.65 Mpa, merupakan lapisan sedimen tebal sehingga mampu menahan stress. Zona Forearc basin menghasilkan nilai stress drop kecil 0.49 Mpa karena merupakan lapisan sedimen yang mudah mengalami deformasi. Stress drop di Sumatera dipengaruhi oleh tipe tektonik, material batuan, serta slip rate masing-masing region di Sumatera.
Shallow S-wave Velocity Profile Estimation using Surface Velocity and Microtremor HVSR with a Linear Velocity Increase Approach Andi Muhamad Pramatadie; Hiroaki Yamanaka; Afnimar Afnimar
Journal of Mathematical and Fundamental Sciences Vol. 54 No. 3 (2023)
Publisher : Directorate for Research and Community Services (LPPM) ITB

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.5614/j.math.fund.sci.2023.54.3.4

Abstract

We propose a simple method for 1D S-wave velocity (Vs) profile estimation using a measured surface S-wave velocity (V1) and peak frequency of the observed microtremor horizontal-to-vertical spectral ratio (HVSR). In this method, the S-wave velocity profile is presented as linear velocity increase with depth in sediments over a bedrock layer that has a given constant S-wave velocity. Thus, the profile can be parameterized with the measured surface S-wave velocity and the velocity gradient. The gradient can be estimated based on the agreement of the peak frequencies of the observed microtremor HVSR and the theoretical ellipticity of the fundamental mode of the Rayleigh wave. We examined the applicability of the proposed method using numerical tests as well as application to actual data at five sites in the Bandung Basin, Indonesia, where observed Rayleigh wave phase velocities from microtremor array surveys were available. The applicability was confirmed in numerical tests using sample models of soil profiles in the basin. Actual application indicated the appropriateness of the estimated S-wave velocity profiles due to the similarity of their theoretical Rayleigh wave phase velocities with the observed Rayleigh wave phase velocities. Since the proposed method needs prior confirmation of the linear increase of the S-wave velocity, it is suitable for use in spatial interpolation of shallow S-wave velocity profiles with simple data acquisition.
MIKROZONASI BAHAYA GEMPA BUMI DI WILAYAH KOTA BANDUNG BERDASARKAN DATA MIKROTREMOR Marjiyono Marjiyono; Afnimar Afnimar
Jurnal Geologi dan Sumberdaya Mineral Vol. 21 No. 1 (2011): Jurnal Sumber Daya Geologi
Publisher : Pusat Survei Geologi

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.33332/jgsm.geologi.v21i1.134

Abstract

Bandung is one of the metropolitan city which rapidly developed and tectonically located in the vicinity of active tectonic lines. The surface geology condition of this region vary from very soft sediments to hard volcanic rocks. The soft sediment area has potential to experience amplification when an earthquake occure. Characterization of surface geology is necessary to identify vulnerability level in order to mitigate the earthquakes hazard in this region. In this regard, we have conducted microtremor measurements in 97 sites which are spread throughout the city of Bandung. Data processing is based on the Nakamura method (HVSR) to obtain the value of amplification factor. The results show the value of amplification factor ranged from 2.1 to 17, with the highest amplification factor distribution in the southeastern of the research area. Distribution of this very high amplification factors are broadly corresponded with lacustrine sediment (lake sediment). The amplification factor scores were then used as the basis in determining the level of relative vulnerability to earthquake hazards.Keywords : microtremor, effect of surface geology, amplification factor, Nakamura methods.
The Drop of Relative Velocity Variation and Coherence Values Prior to Sinabung 2013 Eruptions Yasa Suparman; Afnimar Afnimar; Devy Kamil Syahbana
Indonesian Journal on Geoscience Vol. 8 No. 1 (2021)
Publisher : Geological Agency

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.17014/ijog.8.1.109-117

Abstract

DOI: 10.17014/ijog.8.1.109-117The cross-correlations of ambient seismic noise at Sinabung Volcano were analyzed from February 2013 to February 2014. Many eruptions occurred during these periods, started on 15 September 2013. Looking at the variations in the coda of the correlations, two types of measurements can be distinguished associated to two types of changes: relative velocity variation and waveform decoherence. The drop of relative velocity variations and waveform decoherence were observed for each station pair of Sinabung one to two months before the first eruption. These changes in accordance to the deformation of the Sinabung edifice were estimated from geodetic measurements, since an analysis of baseline change between GPS stations indicated an inflation of the volcanic edifice prior to September 2013 eruption. The monitoring of relative velocity variations and decoherence provides insights into the ongoing processes in the volcanic edifice to assist in determining the level of volcanic activity.
KATEGORISASI STASIUN SEISMIK DAN PENGARUHNYA DALAM PENENTUAN PARAMETER MAGNITUDO GEMPABUMI BMKG Muhammad Fahmi Nugraha; Afnimar Afnimar; M. Taufik Gunawan; M. Ramdhan; Iman Fatchurochman; Nova Heryandoko
Jurnal Meteorologi dan Geofisika Vol. 24 No. 1 (2023)
Publisher : Pusat Penelitian dan Pengembangan BMKG

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.31172/jmg.v24i1.886

Abstract

The responsibility to send information within five minutes causes the magnitude disseminated by BMKG only from limited seismic records. The result shows that the magnitude produced in the first five minutes can fluctuate and cause a difference in the final magnitude. In the SeisComP system at BMKG, the event magnitudes of each type of magnitude MLv, mb, mB, and Mwp, are the result of the station magnitude average using trimmed mean, so the largest or smallest station magnitudes will become outliers and are eliminated in event magnitude calculation. However, the drawback of the trimmed mean is seismic stations that always tend to be outliers have the potential to be still involved in determining the event magnitude in the early minutes so that it can disrupt the magnitude calculation. This study aims to reduce the fluctuations in determining the magnitude in the first five minutes by identifying seismic stations that are often eliminated by the trimmed mean method and classifying them. We validate them with the site quality of the station and create two main categories of seismic stations. The first category is primary stations to determine the location and magnitude of earthquakes. The second category is secondary stations used only at the earthquake site, then tested using SeisComP playback by replaying 256 earthquake events. The results show a correlation where good site quality will also produce a good magnitude value, indicated by 285 seismic stations, and can be categorized as primary stations. The remaining 126 seismic stations are categorized as secondary stations. The playback results show that the fluctuation of magnitude determination in the first five minutes using the primary station can be reduced, as indicated by the mean residual and the deviation to the final magnitude.
Estimasi Potensi Panas Bumi Daerah Seram Bagian Barat dan Pulau-Pulau Sekitar, Provinsi Maluku Berdasarkan Data Gaya Berat dan Aeromagnetik Mutiah, Siti; Wawan Gunawan, A.K; Afnimar; Aji Suteja; Santia Ardi M
Jurnal Geologi dan Sumberdaya Mineral Vol. 25 No. 2 (2024): JURNAL GEOLOGI DAN SUMBERDAYA MINERAL
Publisher : Pusat Survei Geologi

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.33332/jgsm.geologi.v25i2.755

Abstract

Penelitian ini difokuskan pada estimasi kedalaman titik Curie (Curie Point Depth/CPD) dan pemodelan kedepan (forward modelling) menggunakan data aeromagnet dan gaya berat residual Pulau Seram Bagian barat dan pulau sekitar (Ambon, Haruku, Saparua, Nusalaut, Kelang, Boano, dan Manipa) untuk mengetahui potensi panas bumi. Penelitian dimulai dengan delineasi struktur bawah permukaan menggunakan metode total horizontal gradien data aeromagnet. Tahap selanjutnya yaitu pembuatan peta estimasi CPD menggunakan metode analisis spektral data aeromagnet resolusi tinggi yang di reduksi ke kutub (RTP) dan dibagi menjadi 30 blok area berukuran 50 x 50 km untuk memperkirakan kedalaman sumber panas. Peta CPD yang dihasilkan selanjutnya digunakan untuk menghitung gradien geotermal dan aliran panas. Proses selanjutnya yaitu pemodelan kedepan (forward modeling) menggunakan data gaya berat residual dan total intensitas magnet untuk mempelajari struktur bawah permukaan. Estimasi CPD menghasilkan nilai berkisar 18 – 59.4 km, gradien geotermal berkisar 11.8 - 26 ˚c/km, dan aliran panas berkisar 29 – 64 mW/m2. Nilai CPD dangkal terletak di Pulau Ambon, Haruku, dan Saparua, sedangkan nilai terdalam terdapat pada Pulau Seram bagian tengah. Sebaran kedalaman CPD dan kelurusan struktur jika dikaitkan dengan data titik panas bumi menunjukkan keselarasan, kecuali di Pulau Seram yang berada pada CPD bernilai besar. Kemungkinan terdapat dua tipe potensi panas bumi daerah penelitian yaitu terkait vulkanisme (volcanic-hosted) di Pulau Ambon, Haruku, Saparua; dan terkait sesar (fault-hosted) seperti di Pulau Seram (Tehoru dan Banda Baru). Potensi panas bumi berikutnya berdasarkan CPD dan kelurusan struktur tersebar di Pulau Seram (Asilulu, Kairatu, Taniwel), Haruku dan Saparua yang perlu untuk diteliti lebih lanjut. Kata kunci: data aeromagnet, gayaberat, kedalaman titik Curie, total horizontal gradien, panas bumi.
Co-Authors Ahmad Syahputra Aji Suteja Andi Muhamad Pramatadie Andri Dian Nugraha Danny Hilman Natawidjaja Devy Kamil Syahbana Gunawan Ibrahim Hiroaki Yamanaka Hiroaki Yamanaka Iman Fatchurochman Indra Gunawan M. Ramdhan M. Taufik Gunawan Marjiyono Marjiyono Marjiyono Marjiyono, Marjiyono Masyhur Irsyam Mohamad Ridwan Muhammad Fahmi Nugraha mutiah, siti Nanang T. Puspito Nova Heryandoko Rahmat Setyo Yuliatmoko Reiji Kobayashi Santia Ardi M Sri Widiyantoro Sugeng Pribadi Suparman, Yasa Syahbana, Devy Kamil Wawan Gunawan, A.K Yasa Suparman