Indonesian Journal on Geoscience

Number 3 vol 1, 2008

7
Karakterisasi sumber gempa Yogyakarta 2006 berdasarkan data GPS

Authors: Cecep Sulaeman, Lestari Cendekia Dewi and Wahyu Triyoso

Abstract

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The southern part of Central Jawa is one of earthquake hazard prone areas in Indonesia an earthquake occurred on May 27, 2006 and had the moment magnitude 6.3. Base on the GPS observation, the characterization of the epicenter and source of this Yogyakarta earthquake can be estimated using the displacement estimation and strain at the measurement point by using a simple kriging and sequential gaussian simulation method. The direction of the displacement and maximum shear strain anomaly in this research was shown by the fault of SW – NE direction and the displacement pattern shows that this fault is left lateral strike slip movement. The positive anomaly of the maximum shear strainis located about 10 km east of Bantul, which suggests as the position of Yogyakarta 2006 earthquake source, with the moment seismic and moment magnitude values are 8.1385 x 1025 dyne cm, and 6.5 respectively. Keywords: GPS data, simple kriging, sequential gaussian simulation, displacement, strain, earthquake source, moment seismic, moment magnitude

8
Amblesan di daerah Porong, Kabupaten Sidoarjo, Jawa Timur

Authors: Untung Sudarsono and Indra Budi Sudjarwo

Abstract

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On 29th May 2006, there was a huge mud outpouring in the Porong Sub-regency, Regency of Sidoarjo, East Jawa and for one year, it had flooded the area of more than 5 km2 including Porong, Tanggulangin and Jabon Sub-regencies. The mud known as Lumpur Sidoarjo. The impact of the mud outpouring was the presence of a subsidence around the main outpouring for the width of 6.3 km2 in ellipse stretching to the north. The subsidence area covered Tanggulangin Sub-regency: Kedungbendo Village, Porong Sub-regency: Siring, Jatirejo, Mindi, and Renokenongo Villages, and Jabon Sub-regency: Pejarakan and Besuki Villages. The rate of the subsidence is approximately 2 cm/day. Keywords: outpouring, Sidoarjo, subsidence, mud, Porong

11
Batubara Formasi Warukin di daerah Sampit dan sekitarnya, Kalimantan Tengah

Authors: Kusnama

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The Warukin Formation in Sampit area, occupying the Barito Basin, was conformably deposited on top of the Berai Formation, but unconformably lies on the Tanjung Formation, Undifferentiated Metamorphic Rocks and Sepauk Granitic Rock. The formation consists of sandstone, interbedded claystone-mudstone, conglomerate and intercalations of coal, having Early-Late Miocene in age. Coal of the Warukin Formation at the Sampit area occurs within sandstone and interbedded claystone-mudstone facies. Two main seams, A and B, was deposited within sandstone and interbedded claystone-mudstone facies respectively. Generally, the coal is bright banded with partly dull lithotypes; brownish black in colour and it has a medium to light in weight. Physically, the coal recognized in two seams, is generally banded with brittle to friable in hardness; and has partings of claystone and mudstone. The thickness of the coal deposit ranges from 80 cm to 200 cm. On the basis of calorific value and lithotype, and also ash, moisture, and sulphur contents, the coal includes to subbituminous C - A rank. The coal was deposited in a wet-forest swamp occupied by high plants and shrubs. Keywords: Coal, Sampit, Warukin Formation, Early-Middle Miocene, Barito Basin, wet-forest swamp

11
Gumuk gunung api purba bawah laut di Tawangsari - Jomboran, Sukoharjo - Wonogiri, Jawa Tengah

Authors: Gendoet Hartono, Adjat Sudrajat, and Ildrem Syafri

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This paper discusses the study on the basalt volcanic rocks and the volcano morphology indicating the existence of an ancient submarine volcano in Tawangsari-Jomboran sub-regency, Sukoharjo-Wonogiri, Central Java. In general, this basalt volcanic rocks were identified as andesite breccia which might be grouped into the Mandalika Formation of Oligosen-Miosen age (Surono et al., 1992). The origin of the Mandalika Formation in relation to the classic sedimentation process and the submarine volcanism is still needed to be evaluated. The present study was based on the detailed descriptions of the rocks both in the field and in the laboratory. The autoclastic basalt outcrops consisting of breccias show the characteristics of the igneous rock fragment component embedded in the groundmass with the same composition, namely igneous rock, dark grey to black in colour; porphyritic texture, rough surface, brecciated; pillow structures, massive, fine vesicularities, amygdaloidal filled with calcite, and radial fractures; calk-alkaline andesite composition ( SiO2 = 54.71% , K2O = 1.15% ). This rock body attains the dimension of 2 - 5 m length, and 40 cm - 1 m in diameter with the direction of the deposition varies following the direction of the eruption source. Brecciated structures on the surface was controlled by the high cooling rate and the low flow, while the interior of the rock is massive because it was not in a direct contact to the cooler mass outside. Autoclastic basalt breccias and or the pillow basalt lava was interpreted to be formed by the undulating low gradient of morphology with the average angle of <10o. On the other hand, the low basaltic magma viscosity produced the effusive eruption related to the formation of the low angle morphology. The distance between the hills generally composed of pillow basalt is between 500 m - 1 km. The typical pillow structure of the igneous rock as described above is interpreted to be the product of the lava flow related to the effusive eruption from a submarine volcano located under or close to the seawater surface. Keywords: submarine volcano, pillow lava, effusive eruption, autoclastic breccias

12
Petrologi - geokimia batuan Gunung Api Tampomas dan sekitarnya

Authors: Mesker H. J. Dirk

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Volcanic rocks of Mount Tampomas and its surroundings belong to low–K, calc–alkaline, and high–K series of basalt, basaltic andesite, and andesite suites containing 48.59 – 60.49 wt% of SiO2 and 0.31-1.67 wt% of K2O. The rocks are typically formed in an island arc environment characterized by the high content of Al2O3 (16.47 – 21.76 wt%), low to moderate content of MgO (2.37 – 8.84 wt%), low content of both titanium TiO2 (< 1 wt%) and high field strength elements (HFSEs), negative anomaly of Nb relative to K and La. The light rare earth elements (LREEs) enrichment but low abundances of heavy rare earth elements (HREEs) indicate fractionation process. The parent magma was an extensive removal of olivine and clino-pyroxene, plagioclase, apatite, and titanoferrousmagnetite from a basalt olivine magma of N-MORB character that was firstly enriched by LILEs such as Ba, Rb, Th, K, and LREE (La). The differentiation lasted, and the magma became water undersaturated at the crystallization phase of hornblende. The high content of incompatible elements such as Ba, Th, La, Ce, may indicate contamination by liquid derived from the subducted slab. Keywords: petrology, geochemistry, volcanic rocks, Mount Tampomas, picrite magma, N-MORB mantle