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All Journal Bulletin of Scientific Contribution : Geology Jurnal Penelitian Fisika dan Aplikasinya (JPFA) Eksplorium : Buletin Pusat Pengembangan Bahan Galian Nuklir
Suardi, Iman
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Astronomy Civil Engineering, Building, Construction & Architecture Earth & Planetary Sciences Education Energy Environmental Science Materials Science & Nanotechnology Physics Social Sciences

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Identification of Source Mechanisms for the August 5 2018 Mw 6.9 and the August 9 2018 Mw7.0 Lombok Earthquakes Priadi, Ramadhan; Perdana, Yusuf Hadi; Wijaya, Angga; Suardi, Iman
Jurnal Penelitian Fisika dan Aplikasinya (JPFA) Vol 10, No 1 (2020)
Publisher : Universitas Negeri Surabaya

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.26740/jpfa.v10n1.p44-55

Abstract

A series of earthquakes with magnitudes ranging from 5.8 to 7.0 occurred in Lombok in the period of July to August 2018. Two events occurred consecutively, the 6.9 on August 5, 2018 (11:46:38 UTC), and the 7.0 on August 9, 2018 (14:56:28 UTC). Those phenomena are rare because earthquakes usually require a relatively long time to accumulate their energies before being released. Because of those events, so an explanation is needed to explain what happened at the source. In this context, this study aims to determine the relations between the events based on the asperity zone and the slip distributions. Modeling was performed using teleseismic data and seismic inversion of body waves at low frequencies. The result shows that the asperity zone of 6.9 is at 0 km in a strike-direction and -18 km wide in a dip-direction with a maximum slip of 1.3 m, whereas, for the 7.0 event, the asperity zone is at -36 km in the direction of the strike and -7 km in the direction of the dip. Both events have the asperity in the up-dip section with an upward slip distribution towards the up-dip. The slip distribution of the first event and the second one has a relationship because the 6.9 earthquake slip leads to the 7.0 earthquake fault plane. The relation is suspected to be due to the weakening of rock conditions and an enlargement that is limited by space and time during the earthquake. As a result, those two earthquakes are closely related to stress distribution, forming a new asperity zone.
Tectonic Structure of Northern Sumatra Region Based on Seismic Tomography of P and S Wave Velocity Silitonga, Betrix Elisabet; Suardi, Iman; Firmansyah, Akmal; Hanif, Muhammad; Ramdhan, Mohamad; Sembiring, Andry Syaly
EKSPLORIUM Vol 44, No 1 (2023): May 2023
Publisher : Badan Riset dan Inovasi Nasional

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.55981/eksplorium.2023.6784

Abstract

The tectonic setting of Sumatra Island is strongly influenced by the oblique subduction of the Indo-Australian Plate, which subducts the Eurasian Plate at a speed of 52–60 mm/year. The movement of these plates resulted in the Northern Sumatra region having seismic sources from tectonic and volcanic activity. The data used in this study is in the form of seismic wave travel-time recorded by numerous seismic stations in the research area from January 2012 to December 2020. The data comes from 5,003 earthquakes recorded by the BMKG seismic network. The inversion is a simultaneous inversion between seismic velocity models (Vp and Vs) and hypocenter parameters by applying a double-difference seismic tomography algorithm. Tomogram results in parts of Aceh (Singkil and Subulussalam) and North Sumatra (Pakpak Bharat and Dairi) at a depth of 0 km show negative perturbations in Vp and Vs values and high Vp/Vs values. The anomaly is most likely related to cracks in fluid-saturated rocks. The tomograms in the south of Lake Toba at depths of 30 km and 40 km have high Vp and Vs perturbation values and low Vp/Vs values. This anomaly indicates a magma supply line that is no longer active or has cooled for a long time. Based on the seismic tomography modeling results, the subducted Indo-Australian Plate to the Eurasian Plate is visible in the study area.
PENENTUAN DISTRIBUSI SLIP DAN ASPERITAS GEMPABUMI MENGGUNAKAN METODE INVERSI GELOMBANG BADAN TELESEISMIK STUDI KASUS: GEMPABUMI LOMBOK Mw 6,9 (9 AGUSTUS 2018) Priadi, Ramadhan; Julius, Admiral Musa; Suardi, Iman
Bulletin of Scientific Contribution Vol 17, No 3 (2019): Bulletin of Scientific Contribution : GEOLOGY
Publisher : Fakultas Teknik Geologi Universitas Padjadjaran

Show Abstract | Download Original | Original Source | Check in Google Scholar | Full PDF (3.715 KB) | DOI: 10.24198/bsc.v17i3.24181

Abstract

Pada tanggal 9 agustus 2018 terjadi gempa signifikan dengan magnitude  6,9 (14:56:28 UTC). Gempa bumi tersebut merupakan salah satu gempa dari rangkaian gempabumi Lombok pada periode bulan Juli-Agustus 2019.  Diduga jika gempabumi Lombok disebabkan oleh adanya aktivitas back-arc thrust namun masih sulit untuk menginterpretasikan penyebab utamanya. Salah satu cara yang dapat dilakukan untuk mengetahui apa yang terjadi disumber dengan melakukan inversi moment tensor. Inversi moment tensor dilakukan untuk memisahkan waveform yang tercatat di seismogram dari fungsi raypath dan instrument sehingga diperoleh waveform di sumber. Metode yang digunakan dalam melakukan inversi moment tensor adalah metode teleseismic body wave inversion. Gelombang teleseismik dipilih karena gelombang teleseismic memiliki frekuensi gelombang rendah akibat dari penjalaran yang jauh sehingga frekuensi tinggi sudah teratenuasi selama penjalaran gelombang gempabumi. Komponen BHZ digunakan karena gelombang P terekam jelas di komponen Z. Stasiun seismik yang digunakan merupakan stasiun seismik dengan jarak antara stasiun ke sumber berkisar antara - sehingga diperoleh rekaman sinyal gelombang teleseismik. Dari hasil pengolahan diperoleh parameter sesar yaitu strike , dip , dan rake . Perbandingan antara sinyal observasi dan sinyal sintesis menghasilkan varian sebesar 0,1687. Luasan bidang sesar dari hasil inversi diestimasikan sebesar strike 48 km × dip 42 km. Pada gempabumi ini diperoleh momen seismik sebesar  0.428E+20 Nm. Magnitudo moment yang dihasilkan dari proses inversi adalah  7,02 dengan kedalaman 28 km. Dari hasil inversi diperoleh jika jenis patahan yang terjadi merupakan merupakan jenih patahan reverse (naik) dengan source duration selama 45 s. Slip maksimum yang dihasilkan dari proses inversi adalah 1,05 m. Dari hasil slip diketahui jika pada gempabumi Lombok  6,9 terjadi desakan dari arah selatan ke utara. Hal tersebut juga diperkuat dengan adanya asperitas tinggi di wilayah utara Lombok. Wilayah sekitar gunung Agung memiliki daerah-daerah dengan asperitas tinggi. Daerah dengan asperitas tinggi mengindikasikan jika wilayah tersebut merupakan wilayah dengan zona kuncian yang tinggi. Zona kuncian tersebut memiliki energi yang besar ketika release. Karena zona asperitas terbesar berada di utara Lombok dan menerima desakan dari arah selatan maka diindikasikan jika memang back-arc thrust merupakan penyebab utama gempa bumi Lombok.
Co-Authors Admiral Musa Julius, Admiral Musa Angga Wijaya, Angga Firmansyah, Akmal Muhammad Hanif Perdana, Yusuf Hadi Priadi, Ramadhan Priadi, Ramadhan Ramdhan, Mohamad Sembiring, Andry Syaly Silitonga, Betrix Elisabet