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Several geological studies have revealed that Southeast Asia is prone to earthquakes. This is inseparable from the fact that Southeast Asia is at the crossroads of three tectonic plates: the Pacific Ocean/Philippine Ocean Plate to the east, the Indo-Australian Plate to the south, and the Eurasian Plate to the northwest.

In addition, several areas traversed by the Opak Fault have high liquefaction potential. This is due to the geological environment in the form of alluvium deposits, groundwater table conditions, the path of the active Opak fault zone, and the Bantul basin (Bantul Graben).

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The 2006 Yogyakarta Earthquake had caused a disaster in Bantul area. Several institutions had reported different results for the epicenter location. However, aftershocks studies indicated that the rupture area was at about 10 km east of Opak Fault. Analysis of gravity anomaly, including several degrees of residual anomalies and tilt derivative, facilitated this regional tectonic study to determine the structural constraints on the main earthquake and its aftershocks. The Yogyakarta area was primarily characterized by several SW-NE faults; one of them is the Opak Fault. Among those faults,, there are a series of WNW-ESE faults. Several groups of these lineations indicated a presence of some pairs of parallel strike-slips faults that formed pull-a-part basins. The obtained structural pattern has signified the dynamic response of the force from the subduction of the Australian Plate toward Sunda (Eurasia) Plate. The subduction force produced the strike-slip fault in a parallel direction of subduction, and subsequently, the faults caused the formation of thrust structures that are perpendicular to them.

Gempabumi Yogyakarta pada tahun 2006 telah menyebabkan bencana di daerah Bantul dan sekitarnya. Lokasi episenter yang ditentukan oleh beberapa lembaga menunjukkan hasil yang berbeda. Tetapi analisa gempabumi susulan telah menunjukkan daerah pegerakan hingga 10 km ke sebelah timur dari Sesar Opak. Analisa anomali gayaberat yang terdiri dari perhitungan anomali sisa dan turunan kemiringan (tilt derivative) diharapkan dapat membantu studi tektonik regional dalam menentukan batasan struktur yang menyebabkan kejadian gempabumi di daerah Yogyakarta. Daerah ini dicirikan oleh sesar-sesar berarah BD (Barat daya)-TL (Timur laut), yang salah satunya adalah Sesar Opak. Di antara sesar-sesar tersebut, terdapat pula deretan sesar-sesar berarah BBL (Barat barat laut)-TTG (Timur tenggara). Beberapa kelompok kelurusan-kelurusan membentuk kemungkinan adanya cekungan pull-a-part, yang terbentuk karena adanya deretan sesar-sesar strike-slip. Pola struktur yang diperoleh menunjukkan respon dinamik dari subduksi Lempeng Australia terhadap Lempeng Eurasia (Sunda). Tekanan dari gaya subduksi menyebabkan terbentuknya sesar-sesar strike-slip. Kemudian sesar-sesar tersebut menyebabkan adanya struktur sesar naik yang tegak lurus terhadapnya.

Kegiatan konferensi tentang geologi, sumberdaya geologi dan rekayasa ini juga dirangkai dengan kunjungan ke kampus Institut Teknologi Kambodia untuk meninjau laboratorium-laboratorium yang ada di lembaga pendidikan tinggi tersebut.

The geological structures in the study area are dominated by faults. The rest directions of the faults are northeast-southwest (16 faults) and north-south (14 faults). The remainder of faults are trending in northwest-southeast (3 faults) and west-east (3 faults) directions. Generally, the northeast-southwest and north-south trending faults are dominated by sinitral faults and a few of them are reactivated as normal faults. Whereas the northwest-southeast and west-east trending faults are mainly dextral and or normal faults.

The Pleret Sub District is predominantly located on two major geological units. The main part of Pleret Sub District (the capital sub district) is situated in a low-relief alluvial deposit while the East area of Pleret consists of an ancient volcanic deposit which is collectively referred to as the Semilir Formation and Nglanggran Formation. According to the geological map of Yogyakarta (scale 1:100,000) (Rahardjo 1995), those two geological units can be subdivided into several smaller formation units as follows (see Fig. 2).

There are many geological faults formed in the research area, as a result of the plate movements along the subduction zone in the south part of the Java islands. One of them is known as Opak Fault which is often associated with the Opak River. Abidin, et al. (2009) concluded that this SW-NE normal fault is an active one. Another normal fault which has the same orientation with Opak Fault lies in the middle of the research area. This normal fault is known as Bawuran Fault. Both of them have the same movement, i.e., the west part of the fault line is moving downward while the east part of the fault line is moving upward. There are also two major strikes-slip faults that located in the research area, namely Bawuran-Cinomati (centre part) and Becucu-Tekek Fault (north part) (see Fig. 2). Both of them were formed later after the development of Opak Fault. In the post-stage of uplifting movement of Opak Fault, the strike-slip faults were created and trimmed horizontally the research area into north and south area. The north part is moving eastward while the south part is moving westward (Sanjoto 2004).

Three main analyses were applied in this study; first, the visual interpretation of geological features and land use; second, the probabilistic analysis of building damage; and third, the population distribution analysis. The Landsat 7 ETM+ and Quickbird 2012 satellite imagery were utilized to generate the geology and land use map in the areas of interest. The probabilistic analysis was done by using the building damage dataset which is obtained from the rapid survey of earthquake damage after the Yogyakarta earthquake, 2006. In addition, to disaggregate the numbers of population into particular land use units, this study used the data of local livelihood obtained from the local government at village level. The final results of this study is the multi vulnerability model which was combination between the damage probability of building block and the population distribution model. The multi-vulnerability model was visualised in a map form with the scale of 1: 30,000.

The next step is to identify the geological characteristics through visual interpretation of Landsat 7 ETM+. The visual interpretation elements of colour/tone, shape, pattern, texture, and association were used to extract the geological information. Moreover, a 1: 100,000 geology map of Yogyakarta and a topographic map with a 12.5 m contour interval were also used to support the interpretation process. The geological features were obtained from the visual interpretation process of Landsat 7 ETM+ imagery with RGB colour composite of 4,5,7. The Band 4 or near infrared (0.7-0.9m) is good for determining water or land surfaces because almost all radiation in this wavelength range is absorbed by water. The band 5 or middle infrared (1.55-1.75 m) is very sensitive to moisture and very suitable to monitor vegetation. The band 7 or middle infrared (2.08-2.35 m) is good for soil and geological mapping. By using the elements of interpretation and topographic information such as relief and slope, a detailed unit of geological unit can be produced to complement the geology map. Finally, a fieldwork has been conducted to justify qualitatively the accuracy of geological interpretation. At least 53 observation locations have been identified and verified in term of the geological unit characteristic. A physical characteristic identification of the outcrops or surface sediment have been conducted in the field. The rock and formation classification referred to the Geological map of Yogyakarta scale 1: 100,000.

There are some fundamental differences between the geological units derived from visual interpretation and the 1:100,000 geological map of Yogyakarta. Based on the latter, the border between Tmse and Tmn is located along the foot slope of Baturagung Escarpment near the Guyangan Villages, but we found that the border is located in the upper slope of Baturagung Escarpment near the Dlingo Villages. Then, we also found that there are some isolated hills in the north part of research area which is belong to the Tmn units. The main difference was also found in the extents of Qmi. Based on the visual interpretation, Qmi is spread along the left and the right side of Opak River, while the alluvium only occupies the small part of flat area near the hilly area of Semilir Formation. The summary of interpretation results and the differences between both geological maps are shown in Table 7 and Fig. 9.

The rapid field identification was conducted to determine the border between Qmi and Qa. Two approaches were used to determine both geological units. The first approach was to locate the boulder location. Qmi is dominated by fluvial process and produces very well sorted sediment. The upper layer consists of very fine sediment, while the lower layer consists of rough sediment. The boulder indicate that the particular area is influenced by the colluvium sediment which is characterized as bad sorted or mixture sediment from the hilly area nearby. The second approach is the clay content of both Qmi and Qa. Qmi tends to have less clay content being the Merapi Volcano sediment, while the Qa tends to have more clay content as it is the denudation material of the weathering process in the hilly surrounding areas. Therefore, one of the objectives of the rapid field investigation was to locate the traditional brick factory along the Opak River. The brick makers tend to use the best soil which has less clay rather than soil which has more clay. The reason is that the brick will crack during the heating process if there are a lot of clay contents inside the soil. Therefore, the location of traditional brick makers can be used to indicate the Qmi area. 2351a5e196