Pemetaan Daerah Rawan Gempabumi di Wilayah Sorong Provinsi Papua Barat berdasarkan Metode Probabilistic Seimic Hazard Analysis (PSHA)

Pemetaan daerah rawan gempabumi di wiayah Sorong Provinsi Papua Barat telah dilakukan berdasarkan analisis data sekunder sejarah gempabumi wilayah Sorong bersumber dari ANSS Catalog. Pemetaan wilayah ini didasarkan pada distribusi nilai percepatan tanah maksimum (Peak Ground Acceleration) menggunakan pendekatan Probability Seismic Hazard Analysis (PSHA) dengan probability of excedance 2% terkomputasi pada program komputer EQRISK dan analisis pohon logika. Hasil penelitian menunjukkan bahwa nilai percepatan tanah maksimum wilayah Sorong yang diperoleh mengikuti model seismisitas sumber seismik dan faktor geologi berkisar antara 300 – 1700 Gal atau 0.3 – 0.7 g dan dipetakan menggunakan program komputer ArcGIS 9.2, nilai kisaran ini dibagi kedalam beberapa kategori tingkat kerawanan gempabumi. Berdasarkan distribusi nilai percepatan tanah maksimum, wilayah Sorong dikategorikan kedalam daerah dengan tingkat kerawanan besar satu dan besar dua. Nilai tingkat kerawanan gempabumi besar satu berkisar antara 300 – 600 Gal meliputi daerah distrik Seget, distrik Beraur, sebagian distik Makbon dan Kota Sorong sedangkan tingkat kerawanan gempabumi besar dua berkisar antara > 600 Gal meliputi distrik Samate, distrik Sausapor, distrik Sorbar dan sebagian kecil distrik Makbon. Wilayah Sorong di Provinsi Papua Barat termasuk daerah dengan tingkat kerawanan gempabumi yang tinggi hal ini disebabkan karena wilayah Sorong merupakan daerah dengan banyaknya kejadian gempabumi  ≥ 5 SR dengan kedalaman dangkal ≥ 30 km serta keberadaan jalur Sesar aktif gempabumi Sorong-Yapen akibat pertemuan lempeng Eiruasia, lempeng Indo-Australia dan lempeng samudera Pasifik. Kata Kunci : Percepatan tanah maksimum, PSHA, Sorong, gempabum

RANCANGAN PEMODELAN INVERSI NON-LINIER 2-D DAN GRADIEN HORISONTAL ANOMALI GRAVITASI BUMI BERBASIS MATLAB (STUDI KASUS: MODEL SEMI-INFINITE HORIZONTAL SHEET DAN FAULTED VERTICAL SHEET)

Abstrak Penelitian ini dilakukan untuk merancang suatu pemodelan ke depan (forward modeling) dan pemodelan inversi (inverse modeling) untuk mendapatkan respon anomali gravitasi dan parameter model bawah permukaan bumi berdasarkan beberapa model benda anomali yaitu model semi-infinite horizontal sheet dan faulted vertical sheet menggunakan perangkat lunak MATLAB. Perancangan pemodelan ini dibuat nantinya diharapkan dapat diterapkan untuk mendeteksi keberadaan patahan atau sesar dibawah permukaan bumi berdasarkan data anomali gravitasi baik data primer ataupun sekunder. Inversi anomali gravitasi dilakukan menggunakan pendekatan metode inversi non-linier untuk menyelesaikan model matematika benda anomali bawah permukaan pada penelitian ini inversi model inversi dilakukan menggunakan model inversi bebas dan inversi terkonstain untuk mencari beberapa parameter model seperti densitas batuan, kedalaman dan ketebalan benda anomali. Kata kunci:Anomali Gravitasi; pemodelan ke depan ; pemodelan Inversi Abstract This study was conducted to design a forward and inverse modeling to obtain the response of gravity anomalies and subsurface model parameters based on some models of anomalous object such as semi-infinite horizontal sheet and vertical faulted sheet models using matlab software. The design of this models are expected to be applied to detect the presence of faults below the earth's surface by gravity anomaly data in this case primary and secondary data. Gravity anomaly inversion performed using the non-linier inversion approach to solve the mathematical model of the subsurface anomalous object. In this research inversion models performed using unconstrain inversion and constrain inversion to look for some model parameters such as rock density, depth and thickness of anomalous object. Keywords: Gravity anomaly; forward modeling; inverse modelin

PEMODELAN TIGA DIMENSI (3D) STRUKTUR BAWAH PERMUKAAN BUMI DI PROVINSI PAPUA BARAT BERDASARKAN ANALISIS DATA GRAVITASI

Manokwari and Bintuni bay regions in West Papua province as the main route of Sorong, Ransiki, and Lengguru faults was dominated by clastic sedimentary rocks of Australia continental crust and Pasifik ocean crust. Included: limestone, especially coral limestone, conglomerate, sandstone, and clay was estimated from moicene to pleistocene and early holocene in Autralia plate, mid miocene to end Pliocene in Pasifik plate. Igneous rocks as lavas, basalt, and andesite especially in Arfak mountain from eocene to early miocene. Three dimensional (3D) modeling of subsurface in West Papua province especially in Manokwari and Bintuni bay regions has been done based on gravity data analysis. The models proceed with Grablox and bloxer computer programs based Graphical User Interface (GUI). This research was utilized a secondary data of complete Bouguer anomaly gravity data with techniques processing use singular value decomposition inverse method and Occam inverse method. The result shows that a rock density in Manokwari and Bintuni bay regions have estimated to 2.68 g/cm3. With an average density of each layer is 2.66 g/cm3 for first layer, 2.65 g/cm3 for second layer, 2.66 g/cm3 for third layer, 2.70 g/cm3 for fourth layer, and 2.74 g/cm3 for fifth layer. The average thickness estimated using the model is 6 km

The Study of Geological Structures in Suli and Tulehu Geothermal Regions (Ambon, Indonesia) Based on Gravity Gradient Tensor Data Simulation and Analytic Signal

In early 2017, the geothermal system in the Suli and Tulehu areas of Ambon (Indonesia) was investigated using a gravity gradient tensor and analytic signal. The gravity gradient tensor and analytic signal were obtained through forward modeling based on a rectangular prism. It was applied to complete Bouguer anomaly data over the study area by using Fast Fourier Transform (FFT). The analysis was conducted to enhance the geological structure like faults as a pathway of geothermal fluid circulation that is not visible on the surface because it is covered by sediment. The complete Bouguer anomaly ranges of 93 mGal up to 105 mGal decrease from the southwest in Suli to the northeast in Tulehu. A high gravity anomaly indicates a strong magmatic intrusion below the Suli region. The gravity anomalies decrease occurs in the Eriwakang mountain and most of Tulehu, and it is associated with a coral limestone. The lower gravity anomalies are located in the north to the northeast part of Tulehu are associated with alluvium. The residual anomaly shows that the drill well TLU-01 and geothermal manifestations along with the Banda, and Banda-Hatuasa faults are associated with lowest gravity anomaly (negative zone). The gravity gradient tensor simulation and an analytic signal of Suli and Tulehu give more detailed information about the geological features. The gzz component allows accurate description of the shape structures, especially the Banda fault associated with a zero value. This result will be useful as a geophysical constraint to subsurface modeling according to gravity gradient inversion over the area