Abstract   Geothermal manifestations on Bangka Island are found in the villages of Terak, Pemali, Sungailiat/Pelawan, Dendang, Permis, and Nyelanding. The manifestation of hot water in Terak Village, Central Bangka Regency is in the form of 3 hot springs with a surface temperature of 55ᵒC this research is to be carried focus on the structure of the subsurface rock layers using the geophysical method, namely the gravity method. The data used are topography and Free Air Anomaly. The data processing is in the form of Bouguer Correction and Terrain Correction to obtain the Complete Bouguer Anomaly (CBA) value. Then the CBA value is separated from regional anomalies and residual anomalies using the upward continuation method, as well as 2D modeling interpretation (forward modeling). From the research results, it is known that the subsurface rock structure of the non-volcanic geothermal system in Terak Village in the form of sandstone (2.28 – 2.49 gr/cm3) at a depth of 0 – 1.44 km is estimated as caprock, granite (2.77 – 2.78 gr/cm3) at a depth of 0 – 1.8 km is estimated as reservoir rock, and diorite rock (2.87 – 2.99 gr/ cm3) at a depth of 0 – 2 km is estimated as basement rock.  

Geological structures on Pariangan is influenced by the tectonic activities of the Sumatera Fault System, volcanic arc of Complex Mt. Marapi and Central Sumatera Basin, stratigraphy of this area was dominated by andesitic-basaltic lava and pyroclastic rocks (Quaternary). This study purpose to determine of geological structures, and heat source location of geothermal system in this area using Magnetotelluric Method (MT). 2-D resistivity model from MT method was combined with geological map for the conceptual models of geothermal system. Based on conceptual model, geothermal system in area was controlled by a couple of main fractures. These fractures are associated with faults in the northwest-southeast and southwest-northeast directions on geological map. It’s also influenced the surface manifestation (e.g hot springs) and alteration mineral zone. Heat source was composed by metamorphic rock (slite rock and granit rock) at 1500-4000 musl as based rock. This heat source associated with activities of MT. Marapi.

Non-volcanic geothermal is a heat system that is not related to volcanism such as in Nyelanding and Permis areas. In previous studies, hot springs in the Nyelanding area were spread from West to Northeast, with the constituent rocks of sandstone, dry gravel and granite. In the Permis area, the manifestation is influenced by shear faults trending Northwest to Southeast and the geothermal source is granite. Therefore, this study was conducted to determine the strenght of local tectonic structures using the gravity method. The data used in this study are the FAA (Free Air Anomaly) and topography data obtained through the TOPEX website. The gravity anomaly is obtained by performing spectral analysis using the Fourier Transform and moving average filter. 2D modeling is performed using inverse modeling with the smoothness constraint equation. The result of this research is that the Permis and Nyelanding geothermal systems are controlled by local tectonic structures in the Northwest and Southeast of the study area, in the form of fractures (possibly faults) with a Northeast – Southwest direction

Cubadak area was located in depression zone of great Sumatra fault zone which affected by normal faults. Geothermal in this area was characterized by the appearance of hot springs type chloride-bicarbonate on Cubadak, Sawah Mudik and Talu. The manifestation temperature on this area is about 37,1° - 74,8° C. This study purpose to recontruct model of subsurface geothermal system in Cubadak area based on MT data. Model 1-D was processed by apply Bostick inversion and NLCG (Nonlinear Conjugate Gradient) for model 2-D. The result showed that lowest resistivity was on Southwest to Northeast research area. Highest resistivity spread at West to Northeast. Geothermal system was composed by caprock (2-10 ohm), reservoar (20-300 ohm) and heat source (>300 ohm).

Jawa barat mempunyai aktivitas kegempaan yang sangat tinggi. Aktivitas kegempaan di wilayah Jawa Barat dikontrol oleh struktur geologi regional dan keberadaan sesar-sesar lokal. Aktivitas sesar-sesar lokal dan zona keberadaan zona subduksi dapat dipelajari dari sebaran hiposenter pada daerah tersebut. Sebaran hiposenter yang diperoleh dari katalog BMKG biasanya berasal dari quick count dengan mengabaikan beberapa parameter. Oleh karena itu, lokasi hiposenter tersebut harus direlokasi untuk menentukan hubungan hiposenter gempa dengan aktivitas tektonik daerah tersebut. Metode relokasi hiposenter yang digunakan adalah metode Double Difference. Metode ini digunakan untuk mengidentifikasi zona patahan berdasarkan distribusi gempa yang terjadi. Prinsip metode Double Difference adalah jarak persebaran hiposenter antara dua gempabumi sangat kecil dibandingkan dengan jarak antara stasiun gempabumi, maka raypath dan waveform kedua gempabumi dapat dianggap mendekati sama. Data yang digunakan dalam penelitian ini adalah data gempabumi yang terjadi pada periode Juli 2017-Juni 2018 di Jawa Barat. Hasil relokasi hiposenter menunjukkan bahwa kegempaan di Jawa Barat Sebagian besar berasosiasi dengan zona subduksi. Penunjaman yang terjadi relatif landai. Kedalaman aktivitas gempabumi pada zona subduksi tersebut sekitar 0-70 km. Aktifitas kegempaan pada Sesar-sesar lokal relatif sangat dangkal dan tersebar hampir di setiap segmen sesar lokal.