Indonesian Journal on Geoscience

8

Authors: Mahfud Arifin and Rina Devnita

Micromorphological characterization has been studied on six pedons of soils developing in volcanic materials in West Java. The pedons represent deposits of different volcanoes (Mount Tangkuban Perahu, Mount Patuha and Mount Papandayan) with different ages (Pleistocene, Holocene) within two types of volcanisms (andesitic, basaltic), and three agroclimatic zones (A, B1, B2). Undisturbed soil samples were taken from each identifiable horizon for thin section preparations. Observations were carried out by means of a magnifying lens, binocular stereomicroscope, polarization microscope, and scanning electron microscope (SEM). The result demonstrates that micromorphological characteristics of volcanic soils developing from different ages, types of parent material, and climate were different through their c/f related distribution patterns, c/f2μ ratios, sorting, infillings and coatings of voids, and microstructure. Keywords: Micromorphology, soil, volcano, pedogenetic

10

Authors: Yudhicara and K. Budiono

Tsunamigenic is a natural phenomena which is potential to generate a tsunami, such as water disturbance due to the presence of activities of volcanism, earthquakes, coastal and sub marine landslidse, or other causal factors . Historically, the Sunda Strait has experienced several tsunami events recorded in the tsunami catalog. Those tsunamies were caused by some geological phenomena such as eruptions of Krakatau submarine volcano in 416, 1883, and 1928; earthquakes in 1722, 1852, and 1958; and other causes which were suggested as a mass failure of coastal and submarine landslide in 1851, 1883, and 1889. Tectonic condition of the Sunda Strait is very complicated, because this region is located at the boundary of Indian-Australian and Eurasian Plates, where a unique island arc system occurs with its association such as trench, accretionary zone, volcanic arc and back-arc basin. Sunda trench as a plate boundary is the most potential region to produce big earthquakes. Existence of a seismic gap in the region can cause a stress accumulation and store energy, then it will be released any time as a big earthquake to generate a tsunami. Along eruption history, Krakatau volcanic arc has four stages of reconstruction and three stages of destruction, and every destruction stage produces tsunami which is suggested to be potentially repeated in the future in a period between 2500 to 2700. Seafloor of the Sunda Strait has an unstable geological condition due to geological structure development, which creates grabens and also enable to produce submarine landslides triggered by earthquake. Coastal condition around the Semangko and Lampung Bays consisting of steep topography with high intensity of weathering, is another factor to contribute landslide, particularly in the case of triggering be heavy rainfall between December to Februari. Furthermore, if landslide materials tumble into the water, even very small and locally, could create a potency of tsunami. Keywords: tsunamigenic, earthquake, submarine volcano, landslide, tsunami

13

Authors: A. Soehaimi

A seismogenetic study shows the Jawa Island Arc and its subduction zone system belong to a highly active seismotectonic arc unit (west Jawa and Sumatera) and an active seismotectonic arc unit (western part of West Jawa – Central Jawa – East Jawa). In general, these regions are part of the Indonesian Earthquake Hazard Zones No. VI, VII, VII and IX. The regions are characterized by the presence of rare earthquake of magnitude > 8.5 Richter Scale (western part of Java), frequent magnitude of 7 Richter Scale and common 5 - 6 Richter Scale (Southern part of Java). The potential hazardous earthquake in Jawa that is > 5,6 Richter Scale of magnitude and shallow depth (< 30 km) is due to a subduction zone earthquake. Epicenter distance, magnitude, geological site conditions, population, and infrastructure are the index of earthquake hazard and risk in these regions. The earthquake hazard mitigation programme in the near future is a risk assesment based on macro and microzonation of earthquake hazard and risk. These macrozonation and microzonation assessments are essentially needed for provinces, districts, and cities. Keywords: Seismotectonic, seismogenetic, macro and micro zonation, potential hazard and risk