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Road to earthquake mitigation: Lesson learnt from the Yogyakarta earthquake 2006 Subagyo Pramumijoyo
Journal of Applied Geology Vol 1, No 2 (2009)
Publisher : Geological Engineering Department Universitas Gadjah Mada

At early in the morning of May 27, 2006, people of Yogyakarta was stroke by earthquake and mostly heavily damaged building are in lowland or Yogyakarta depression where is occupied by the Young Merapi sediments. The magnitude of earthquake is Mw = 6.2 and USGS rapid moment tensor shows that this earthquake was due to strike-slip fault movement.Seismic history of Yogyakarta area shows that Yogyakarta was stroke by several earthquakes with different epicenter location. At least two earthquakes stroke the area, that is in 1876 and 1943. The damages are similar to the damages of actual earthquake. Yogyakarta depression is mostly covered by Young Merapi sediments that consist of tuff, volcanic ash, breccias, agglomerate and lava with Quaternary in age. The thickness of this sediment is up to 100 m.Our reactive work was to establish firstly zone of damage. For this purpose, we made aerial photograph along the most damaged area. In the same time one of our teams go to the field to measure the cracks, and the other teams to observe liquefaction, hydro geologic measurement, and observation on landslide induce by earthquake. Secondly, we must understand the soil properties and its thickness, because in seismic history it was a similar damage on the same area due to earthquakes however the earthquake epicenters were different. For this purpose we utilize the method of micro-tremors. We also made some drilling until 60 m each, measuring seismic velocity on bore hole, and magneto telluric measurement. We also have helped by Kyushu University in installing micro seismic net work. The research was followed by either undergraduate and graduate students. Fortunately our research was financed by AUN/Seed Net – JICA. Some of the results were published in a book entitled The Yogyakarta Earthquake of May 27, 2006. Another outcome is the Maps of Microzonation and Earthquake Hazard of Bantul Area that dedicated to Bantul people.Based on aerial photograph observation and field observation on Bantul Regency, especially along the Opak River, and to Wonosari to the East, there was no surface ruptures, so there is no fault on surface. Interpretation of aftershock data was showing the difference cluster. There is still open problem in determining either epicenter or aftershock location. The damage building was interpreted as due to its geologic setting, non engineered building, and close to epicenter of earthquake. This heavily damaged building are located on the Young Merapi sediments at Bantul Regency and lake deposits at Gantiwarno and Bayat area where it can amplify the surface seismic wave. It implies that Peak Ground Acceleration according to Indonesian National Standard should be modified in Yogyakarta area.Keywords: Earthquake, seismic, epicenter, micro-tremor, microzonation

The mineralogy of gold-copper skarn related porphyry at the Batu Hijau deposit, Sumbawa, Indonesia May Thwee Aye; Subagyo Pramumijoyo; Arifudin Idrus; Lucas Donny Setijadji; Akira Imai; Naoto Araki; Johan Arif
Journal of Applied Geology Vol 3, No 1 (2011)
Publisher : Geological Engineering Department Universitas Gadjah Mada

Clacic gold-copper bearing skarn in the Batu Hijau porphyry deposit is located in the western part of Sumbawa Island, Indonesia. Skarn mineralizations were found at the deep level of the deposit (-450m to -1050mL) by drilling program 2003. No evidence around Batu Hijau has limestone although most skarn are metasomatiz ed from carbonate-rich rock as limestone or marble. Most skarn-type metasomatic alteration and mineralization occurs at the contact of andesitic volcanic rock and intermediate tonalite porphyry intrusion and within intermediate tonalite in some. Although both endoskarn and exoskarn can be developed, it has no clear minerals to known the endoskarn. Exoskarn is more principle skarn zone. The formation of skarn occurred two min stages: (1) prograde and (2) retrograde. The prograde stage is temporally and spatially divided into two sub-stages as early prograde (sub-stage I) and prograde metasomatic (sub-stage II). Sub-stage I begin immediately after the intrusion of the tonalite stock into the calcium rich volcanic rocks. Then, sub-stage II originated with segregation and evolution of a fluid phase in the pluton and its invasion into fractures and micro-fractures of host rocks developed during sub-stage I. The introduction of considerable amount of Fe, Si and Mg led to the large amounts of medium- to coarse-grained anhydrous calc-silicates. From the texture and mineralogy, the retrograde metasomatic stage can be divided into two sub-stages: (a) early retrograde and (sub-stage III) and (b) late retrograde (sub-stage IV). During sub-stage III, the previously formed skarn zones were affected by intense multiple hydro-fracturing phases in the gold-copper bearing stocks. Therefore, the considerable amounts of hydrous calc-silicates (epidote), sulfides (pyrite, chalcopyrite, sphalerite), oxides (magnetite, hematite) and carbonates (calcite) replaced the anhydrous calc-silicates. Sub-stage IV was coexisting with the intrusion of relatively low temperature, more highly oxidizing fluids into skarn system, bringing about partial alteration of the early-formed calc-silicates and developing a series of very fine-grained aggregrates of chlorite, clay, hematite and calcite.

Study on mineralogy and chemistry of the saprolitic nickel ores from Soroako, Sulawesi, Indonesia: Implication for the lateritic ore processing Sufriadin Sufriadin; Arifudin Idrus; Subagyo Pramumijoyo; I Wayan Warmada; Akira Imai
Journal of Applied Geology Vol 3, No 1 (2011)
Publisher : Geological Engineering Department Universitas Gadjah Mada

An investigation of mineralogy and chemistry of saprolitic nickel ores developed on ultramafic rock with different serpentinization degree from Soroako, Sulawesi has been conducted using X ray diffraction, thermal analysis, FTIR, and ICP-AES. The implication for the processing of these ores under acidic media was also studied. Weathering of unserpentinized peridotite in the Soroako west block produces saprolitic ore containing minerals such as relict olivine, goethite, quartz, talc with minor serpentine and smectite; whereas the weathered materials overlaying serpentinized peridotite in the Petea area are mainly composed of residual serpentine with lesser chlorite, maghemite, and remnant pyroxene and amphibole. Chemical analysis determined by ICP-AES demonstrates that west ore is higher in Si, Mg, and Ni, as compared to that Petea ore. Conversely, Fe and Al concentrations are higher in Petea ore than in west block ore. SEM-EDX examination reveals that olivine, talc, serpentine and goethite are the Ni-bearing phases occurring in west block ore; while serpentine is the principal host for Ni in the Petea ore. Chemical leaching under sulfuric acid reveals that olivine has highest dissolution rate in the west ore followed by serpentine; while talc, pyroxene, and iron oxides have slow dissolution rates. In contrast, serpentine in Petea ore is easily dissolved and is followed by chlorite; whereas amphibole, pyroxene, and maghemite are difficult to leach. Quartz is present in both ores and it seems to be undissolved during the chemical leaching. It is shown that Ni recovery from Petea saprolitic ore is higher than that of West Block ore.

Mineral paragenesis and fluid inclusions of the Bincanai epithermal silver-base metal vein at Baturappe area, South Sulawesi, Indonesia Irzal Nur; Arifudin Idrus; Subagyo Pramumijoyo; Agung Harijoko; Akira Imai
Journal of Applied Geology Vol 3, No 1 (2011)
Publisher : Geological Engineering Department Universitas Gadjah Mada

The Baturappe prospect located at southern part of Sulawesi island, Indonesia, is a hydrothermal mineralization district which is characterized by occurrence of epithermal silver-base metal deposits. The mineralization is hosted in basaltic-andesitic volcanic rocks of the late Middle-Miocene Baturappe Volcanics. More than 20 units of quartz – base metal veins are distributed in the area, and one of the most significant is the Bincanai vein. This study is aimed to characterize the mineral paragenesis and to elucidate the physicochemical conditions of the formation of the deposit on the basis of mineral assemblage and fluid inclusion mictrothermometry. Sulphide assemblages in the vein indicate an intermediate sulfidation state epithermal; beside galena and sphalerite as the early stage minerals, chalcopyrite, tennantite, and tetrahedrite are also identified as the later stage. Microthermometric study of fluid inclusions in quartz indicates formation temperature of the vein ranges from about 230 to 280°C Histogram of homogenization temperature suggests that there are two generations of hydrothermal fluid responsible for the ore mineralization in the vein; the higher temperature range represents formation temperature of the base metal (galena, sphalerite), while the lower temperature range is correlate with the precipitation of the rest relatively lower temperature sulphides (chalcopyrite, pyrite, tetrahedrite, tennantite, polybasite, and Bi-Ag-Cu-Fe-bearing sulfide). The sequence is also consistent with the mineral paragenetic. The mean of salinity (2.0–2.5 wt.% NaCl eq.) indicates that fluid responsible for the mineralization in the Bincanai vein is relatively low-salinity fluid.

PRELIMINARY EVALUATION OF LOCAL SITE CONDITON IN YOGYAKARTA BASIN Tun Naing; Subagyo Pramumijoyo; Hiroshi Kawase
Journal of Applied Geology Vol 1, No 1 (2009)
Publisher : Geological Engineering Department Universitas Gadjah Mada

The Yogyakarta Basin can be defined as NE – SW elongated depression zone of 20 km in width and 45 km in length, trending from Yogyakarta City in north to the Indian Ocean in south. The local geologic and soil conditions have a great influence on the intensity of ground motion and earthquake damage. Local site effects play an important role in earthquake-resistant design and must be accounted for on a case by case basis. Local site conditions can profoundly influence all of the important characteristics such as amplitude (peak acceleration, velocity and displacement), frequency content and duration of strong ground motion. The extent of their influence depends on the geometry and material properties of subsurface materials. Therefore, subsurface investigation was carried out through nine boreholes throughout the basin. Based on drill-logs, collected samples and laboratory test results, the subsurface soil profiles of nine areas in the basin are simplified for future strong ground motion simulation. Five subsurface layers in Tampuran, Krajan areas and six subsurface soil layers in Watu, Jl. Parangtritis, Wijirejo areas and seven layers in Jl. Parangtritis, Karang Semut, Bambang Lipuro areas and eight layers in Pranti area, have been simplified and the geotechnical properties of each layer have been evaluated as well. Keywords: Basin, Yogyakarta, depression, ground motion, earthquake.

ESTIMATION OF S-WAVE VELOCITY STRUCTURES IN YOGYAKARTA BASIN, INDONESIA Tun Naing; Subagyo Pramumijoyo; Hiroshi Kawase
Journal of Applied Geology Vol 1, No 2 (2009)
Publisher : Geological Engineering Department Universitas Gadjah Mada

For the theoretical simulation or prediction of strong ground motion, it is prime importance to get information of underground structures, especially for sedimentary layers overlying on bedrock, like in Yogyakarta Basin. The Standard Penetration Test, Spectral Analysis of Surface Wave (SASW) and other geotechnical properties are used to estimate S–wave velocity structures in this basin. SPT tests were conducted at nine sites and SASW measurements were performed at seventeen sites. As a result, the S-wave velocity structures of top 30 m depth had been evaluated in each site. The average shear wave velocity v30 s had been successful estimated and the sites are classified into three types; soft soil, medium dense soil and hard soil. All sites where SPT performed are on soft soil according to their v30 s . However, according to v30 s from SASW measurements, 10 sites are located on medium dense soils type, 5 sites on dense soils and 2 sites on soft soils. The acceptable equivalent S-wave velocitystructure is observed by comparing the results from SASW and geotechnical approach in Imogiri, Bambang Lipuro, Pundong (Watu, Pranti) and Pandak (Wijirejo) areas. Keywords: Ground motion, underground structure, sedimentary layer, SPT, SASW, Pundong

ESTIMATION OF MAXIMUM EARTHQUAKE MAGNITUDE OF EARTHQUAKE POTENTIALS FOR YOGYAKARTA DEPRESSION AREA, INDONESIA Myo Thant; Subagyo Pramumijoyo; Heru Hendrayana; Hiroshi Kawase; Agus Darmawan Adi
Journal of Applied Geology Vol 2, No 1 (2010)
Publisher : Geological Engineering Department Universitas Gadjah Mada

Maximum magnitudes of earthquake potentials are estimated for Yogyakarta depression area by using the faultlength and earthquake magnitude relations for fault specific seismic sources. For estimation of maximum earthquake magnitude, the fault specific seismic sources are modeled as 18 normal faults and 6 strike-slip faults sources referring the geological map of McDonald, 1984 and Rihardjo et al., 1995. For the present area the subduction zone earthquakes are expected to happen in the offshore region regarding the study on the seismicity of the region and the focal mechanisms of the past earthquakes. So three area sources are also assumed for this region and the possible maximum earthquake magnitudes for these sources are determined by probabilistic approaches.

GEORADAR INVESTIGATION AT THE KEDULAN TEMPLE EXCAVATION SITE, KALASAN, YOGYAKARTA Salahuddin Husein; Saptono Budi Samodra; Subagyo Pramumijoyo; Wahyu Astuti
Journal of Applied Geology Vol 2, No 1 (2010)
Publisher : Geological Engineering Department Universitas Gadjah Mada

Kedulan Site is the buried and ruined 9th century Mataram Hindu Kingdom temple, located in Tirtomartani Village, Kalasan District, Sleman Regency, Yogyakarta Special Province. This temple was incidentally discovered by sand diggers on 24 November 1993 under several meter thick of fluvio-volcanic deposit of the modern Merapi. Several technical studies were needed to carefully excavate the temple, including geology and geophysical approaches. One of the geophysical method have been applied was ground penetration radar (georadar). This method uses radar technology to obtain a continuous profile of the shallow sub-surface and thus allows scientists to image soil substratums based on differing dielectric constants. Georadar investigation by Department of Geological Engineering, Faculty of Engineering, Universitas Gadjah Mada, was conducted on 4 December 2007. The main purpose was to identify the location of the outer stone fence as an estimation to define the temple site area to be excavated. About one line was chosen to cross the site in north-south direction in a distance of 328 m. Two runs were completed on the same line but different courses, i.e. forward and backward, where one was checked with another. The result indicates the presence of the outer stone fence was possibly buried in a depth of 7 m. It was located about 40 m distance outside the inner stone fence. Assuming the fences were quadrangle relative to the main temple, hence it is estimated that the site area to be excavated is about 13.830 m² and total 96.808 m³ gravels and sands to be removed.

Evaluation of strong ground motion for Yogyakarta depression area, Indonesia Myo Thant; Subagyo Pramumijoyo; Heru Hendrayana; Hiroshi Kawase; Agus Darmawan Adi
Journal of Applied Geology Vol 2, No 2 (2010)
Publisher : Geological Engineering Department Universitas Gadjah Mada

The probabilistic seismic hazard maps are developed for Yogyakarta depression area. The earthquake catalog of ANSS (1970-2007) is taken into account with the complement of NEIC (USGS, 1973-2007) and the records of BMG (2000-2004). On the basis of seismicity of the area, tectonics and geological information, the seismic source zones are characterized for this area. The seismicity parameters of each seismic source are determined by applying the classical Gutenberg-Richter recurrence model, regarding the historical records. The attenuation relation for Yogyakarta depression area cannot be evaluated since the sufficient strong ground motion records are not available for this region. Therefore the attenuation relations which were developed for other territories as Europe and Japan are used for the present hazard calculation by validating, using the aftershocks records, modeling the peak ground acceleration maps for the recent event, 27 May, 2006, Yogyakarta earthquake inserting the damage area distribution pattern. The probabilistic seismic hazard maps are finally developed by using the McGuire (1976) EQRISK computer program by modifying for the present purpose. The seismic hazard maps expressed in term of peak ground acceleration are developed for the recurrence intervals of 10, 50, 100, 200 and 500 years

PALEOSTRESS ANALYSIS TO INTERPRET THE LANDSLIDE MECHANISM: A CASE STUDY IN PARANGTRITIS, YOGYAKARTA Salahuddin Husein; Ignatius Sudarno; Subagyo Pramumijoyo; Dwikorita Karnawati
Journal of Applied Geology Vol 2, No 2 (2010)
Publisher : Geological Engineering Department Universitas Gadjah Mada

Paleostress analysis on the landslide boundary faults is able to explain the sliding mechanism. This method is particularly useful to study a paleolandslide. About 30 striated fault planes from the Parangtritis paleo-landslide, located in the Yogyakarta coastline, were analyzed to define their principle stress axes. The eastern boundary fault, named as the Girijati Fault, was the main fault responsible for the mass movement and leaving a considerable steep cliff. It moved normal in a left lateral sense with ENE – WSW extension and dragged the rockmass southward, creating a NNW – SSW extension along the Parangtritis Fault and turn it into the western boundary fault. The rockmass slided along the stratigraphic contact between the underlying Nglanggran Formation and the overlying Wonosari Formation, created a semi-circular crown cliff as the northern boundary and produced some isolated topographic highs of the thrust block near the toe. Keywords: Paleostress, landslide boundary, fault, paleolandslide

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