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Comparison of 3-D Raytracing and Finite Frequency Tomography Sandy K Suhardja; Yosua Hotmaruli Lumban Gaol; Agus Abdullah; Andri Dian Nugraha; Z. Zulfakriza
Jurnal Geofisika Vol 17 No 1 (2019): Jurnal Geofisika
Publisher : Himpunan Ahli Geofisika Indonesia (HAGI)

We performed 3-D seismic tomography using teleseismic arrival time at Southwest Mexico. The Mexican subduction zone results from successive fragmentation events that affected the ancient Farallon plate as various segments of the East Pacific rise approached the paleo-trench off western North America. The complexity in this region is related to two subducting oceanic plates, the Rivera and Cocos plates, that have different ages, compositions, convergence velocities and subduction dip angles. In this study, we compared the 3-D raytracing tomography model with finite frequency tomography model. Final models show the differences in amplitude and pattern between the raytracing and finite frequency. 3D raytracing models produced sharper images of fast velocity structures in the mantle. The deeper slabs are more coherent and show less broadening with depth than using 1D finite frequency kernels. However, although the finite frequency and 3-D ray tracing models show some differences in amplitude and pattern, the overall agreement of the models supports the interpretation of Yang et al. (2009) that slab rollback is occurring in South Western Mexico. One possible different interpretation between the raytracing and finite frequency theory results concerns the deep structure of the Rivera slab. The finite frequency models show that the Rivera slab is clearly observable at a depth of about 300km but fades away at greater depths. However, the 3-D ray tracing model shows a clear fast velocity band down to a depth of 400 km and thus our model does not support a slab tear of the Rivera plate above 400 km depth

Analysis of H/V Ratio Curve to Estimate Seismic Hazard Vulnerability in Lombok Island, West Nusa Tenggara, Indonesia Muhammad Randy Caesario Harsuko; Zulfakriza Z.; Andri Dian Nugraha; Muzli Muzli; David Prambudi Sahara; Nanang T Puspito; Yayan M Husni; Billy S Prabowo; Achmad Fajar Narotama Sarjan
Jurnal Geofisika Vol 18 No 1 (2020): Jurnal Geofisika
Publisher : Himpunan Ahli Geofisika Indonesia (HAGI)

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.36435/jgf.v18i1.432

In 2018, Lombok Island was hit by a series of destructive earthquakes. According to Indonesian Meteo- rological, Climatological, and Geophysical Agency data, about 1,973 felt earthquakes (M > 3) which shaken Lombok were recorded during August 2018 with three earthquakes with the largest magnitude of 6.9 Mw, 6.8 Mw, and 6.2 Mw. National Board for Disaster Management reported about 555 deaths, 1,833 people injured, and 186,010 houses damaged as a result of Lombok earthquake on August 5th, 2018. A number of seismometers were placed on Lombok Island from August 3rd, 2018 to October 19th, 2018 to monitor the aftershock events. There are 17 stations that record seismicity in Lombok consisting of 10 broadband sensors and 7 short period sensors. In this study, we used the Horizontal-to-Vertical Spectral Ratio (HVSR) method to analyze the risk of earthquake in the Lombok region. The basic concept of this method is to do a comparison between the horizontal component spectrum and the vertical component spectrum of a wave, where theoretically the particle movement of the horizontal component is greater than the particle movement of the vertical component on soft ground, whereas on the hard ground both components (horizontal and vertical) will be similar. H/V curve obtained from earthquake record- ings (Earthquake Horizontal-to-Vertical Ratio/EHVR) and H/V curve obtained from microtremor recordings (Microtremor Horizontal-to-Vertical Ratio/MHVR) shows good agreement and high correlation. Empirical correction of EMR (Earthquake-to-Microtremor Ratio) managed to decrease the difference of estimation of predominant frequency and amplification factor between EHVR and MHVR. Predominant frequency, amplification, and seismic vulnerability map agree with the geological condition of Lombok Island, where high value of amplification and seismic vulnerability was found on soft and thick ground. This study conclude that the maximum ground acceleration and the construction of the building should also considered when one wants to investigate the effect of an earthquake to the damage occurred, beside the site effects.

PENGGUNAAN METODE SPECTRAL FITTING UNTUK MENDETERMINASI OPERATOR ATENUASI AKIBAT KANDUNGAN FLUIDA DI LAPANGAN PANAS BUMI Mia Uswatun Hasanah; Andri Dian Nugraha; Muhammad Rahmat Sule
PROSIDING SEMINAR NASIONAL FISIKA (E-JOURNAL) Vol 5 (2016): PROSIDING SEMINAR NASIONAL FISIKA (E-JOURNAL) SNF2016
Publisher : Program Studi Pendidikan Fisika dan Program Studi Fisika Universitas Negeri Jakarta, LPPM Universitas Negeri Jakarta, HFI Jakarta, HFI

Attenuation is the physical parameters of rock that can reflect the subsurface geological conditions. We conducted attenuation tomographic imaging in the geothermal field by using the Microearthquake (MEQ) data in the form of attenuation operator, t*. We applied a method of spectral fitting to invert for the t* value.. Keywords: Please write 3-5 keywords in english with font 9 point, italic.

Seismicity Pattern of the Great Sumatran Fault System from Hypocenter Relocation of Regional Seismic Network Ade Surya Putra; Andri Dian Nugraha; David Prambudi Sahara; Zulfakriza Zulfakriza; Antonius Nanang Tyasbudi Puspito; Faiz Muttaqy; Pepen Supendi; Daryono Daryono
Indonesian Journal on Geoscience Vol 10, No 1 (2023)
Publisher : Geological Agency

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.17014/ijog.10.1.83-95

The seismicity pattern along the Great Sumatran Fault (GSF) was analyzed during April 2009 - December 2017 period with magnitude of >3.0 and depth of < 30 km. Of 752 preliminary absolute locations, 695 were successfully relocated using double-difference method to provide an improved view of seismicity, sharpening locations and interpretations of seismogenic features throughout the region. The relocation results depict a pattern of significant increase on small to intermediate earthquakes occurring in a shallow part of northern Sumatra, i.e. the Aceh and Seulimeum segments, as well as in central Sumatra, i.e. the Toru and Barumun. This increase was interpreted due to indications of creeping that reduce or prevent stress build-up on these segments. Meanwhile, few segments, i.e. the Tripa segment in the northern part, then Suliti and Siulak segments in the central part, and Manna segment in the southern part of Sumatra show the least activities over the period. These segments were identified as lock asperity, which caused accumulating stress that could be released as an earthquake. The behaviour of these locking segments can be related to the seismic gap along the GSF which has not experienced major earthquakes (M>7) since 2000, making the densely populated area around these segments potentially have a great seismic hazard in the future.

Reducing Numerical Dispersion with High-Order Finite Difference to Increase Seismic Wave Energy: - Syamsurizal Rizal; Awali Priyono; Andri Dian Nugraha; Mochamad Apri; Mochamad Agus Moelyadi; David Prambudi Sahara
Journal of Engineering and Technological Sciences Vol. 55 No. 4 (2023)
Publisher : Directorate for Research and Community Services, Institut Teknologi Bandung

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.5614/j.eng.technol.sci.2023.55.4.5

The numerical dispersion of 2D acoustic wave modeling has become an interesting subject in wave modeling in producing better subsurface images. Numerical dispersion is often caused by error accumulation with increased grid size in wave modeling. Wave modeling with high-order finite differences was carried out to reduce the numerical error. This study focused on variations in the numerical order to suppress the dispersion due to numerical errors. The wave equation used in modeling was discretized to higher orders for the spatial term, while the time term was discretized up to the second order, with every layer unabsorbed. The results showed that high-order FD was effective in reducing numerical dispersion. Thus, subsurface layers could be distinguished and observed clearly. However, from the modeling results, the wave energy decreased with increasing distance, so the layer interfaces were unclear. To increase the wave energy, we propose a new source in modeling. Furthermore, to reduce the computational time we propose a proportional grid after numerical dispersion has disappeared. This method can effectively increase the energy of reflected and transmitted waves at a certain depth. The results also showed that the computational time of high-order FD is relatively low, so this method can be used in solving dispersion problems.

Seismicity Pattern of the Great Sumatran Fault System from Hypocenter Relocation of Regional Seismic Network Ade Surya Putra; Andri Dian Nugraha; David Prambudi Sahara; Zulfakriza Zulfakriza; Antonius Nanang Tyasbudi Puspito; Faiz Muttaqy; Pepen Supendi; Daryono Daryono
Indonesian Journal on Geoscience Vol. 10 No. 1 (2023)
Publisher : Geological Agency

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.17014/ijog.10.1.83-95

The seismicity pattern along the Great Sumatran Fault (GSF) was analyzed during April 2009 - December 2017 period with magnitude of >3.0 and depth of < 30 km. Of 752 preliminary absolute locations, 695 were successfully relocated using double-difference method to provide an improved view of seismicity, sharpening locations and interpretations of seismogenic features throughout the region. The relocation results depict a pattern of significant increase on small to intermediate earthquakes occurring in a shallow part of northern Sumatra, i.e. the Aceh and Seulimeum segments, as well as in central Sumatra, i.e. the Toru and Barumun. This increase was interpreted due to indications of creeping that reduce or prevent stress build-up on these segments. Meanwhile, few segments, i.e. the Tripa segment in the northern part, then Suliti and Siulak segments in the central part, and Manna segment in the southern part of Sumatra show the least activities over the period. These segments were identified as lock asperity, which caused accumulating stress that could be released as an earthquake. The behaviour of these locking segments can be related to the seismic gap along the GSF which has not experienced major earthquakes (M>7) since 2000, making the densely populated area around these segments potentially have a great seismic hazard in the future.

RELOKASI SUMBER GEMPA DI DAERAH SUMATERA BAGIAN UTARA MENGGUNAKAN HASIL INVERSI SIMULTAN RELOKASI DAN KECEPATAN GELOMBANG P TIGA DIMENSI Jajat Jatnika; Andri Dian Nugraha; Wandono Wandono
Jurnal Meteorologi dan Geofisika Vol. 16 No. 2 (2015)
Publisher : Pusat Penelitian dan Pengembangan BMKG

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.31172/jmg.v16i2.274

Penujaman miring antara lempeng Indo-Australia dan Eurasia memberikan pengaruh yang besar terhadap kondisi tektonik dan vulkanik di Sumatera bagian utara. Subduksi tersebut mengakibatkan terbentuknya deretan gunung api dan zona sesar yang terbentang di pulau Sumatera. Seismisitas yang tinggi di wilayah Sumatera bagian utara tidak hanya diakibatkan oleh pengaruh dari subduksi saja, namun dapat juga diakibatkan karena keberadaan sesar aktif dan aktivitas gunung api yang berada di darat pulau Sumatera. Oleh karena itu perlu dilakukan penentuan sumber gempa yang akurat dan presisi. Salah satu faktor yang mempengaruhi penentuan sumber gempa adalah model kecepatan yang digunakan. Dengan menggunakan program Simulps12 yang secara simultan menghitung kecepatan 3-D gelombang P dengan hasil relokasi gempanya, diharapkan dapat menentukan sumber gempa sesuai dengan kondisi tektonik sebenarnya. Data yang digunakan dalam penelitian ini adalah waktu tiba gelombang P dan parameter gempa dari katalog BMKG 2009-2012 dan katalog PASSCAL Februari-Mei 1995. Penjejakan sinar gelombang menggunakan metode pseudo-bending sedangkan metode LSQR teredam digunakan dalam teknik inversinya. Hasil penelitian menunjukan bahwa gempa hasil relokasi mengalami perubahan posisi baik secara horisontal maupun secara vertikal. Beberapa gempa menunjukan perubahan jarak horisontal yang besar yaitu sekitar 40-70 km. Sedangkan secara vertikal hampir setengah data mengalami perubahan kedalaman hingga 60 km. Setelah relokasi terlihat distribusi gempa dangkal di darat lebih berimpit dengan zona sesar Sumatera. Hal ini juga mengindikasikan bahwa zona sesar Sumatera sangat aktif dimana kedalaman gempa yang terjadi tidak lebih dari 50 km. The oblique subduction between the Indo-Australian plate and Eurasian plate in northern Sumatra gives a great influence on volcanic and tectonic conditions. The subduction resulted in the formation of a row of volcanoes and fault zones that lie on the island of Sumatra. The high seismicity in the northern Sumatra region is not only caused by the subduction alone but there are fault active and volcanoes. Then the precise determination of the earthquake source in accordance with the actual conditions needs to be done. One factor that affects the determination of earthquakes is the velocity model. By using the Simulps12 program that simultaneously calculates velocity models3-D and earthquake relocation, it was expected to determine the source of the earthquake following the actual conditions. The data used is the P wave arrival time and the parameters of the earthquake in a catalog of BMKG 2009-2012 and catalog of PASSCAL February-May 1995. Ray tracing in this study was using the pseudo-bending method, while the damped LSQR method was using inverse techniques. The results showed that the earthquake relocation results change positions either horizontally or vertically. Some earthquakes showed large changes in a horizontal distance of about 40-70 km vertically while almost half of the data changes to 60 km depth. After the relocation, the distribution of shallow earthquakes inland coincides with the Sumatra fault zone. It also shows that the Sumatra fault zone is highly active where the depth of the earthquake occurred not exceeding 50 km.

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