While investigating features of Quaternary displacement along the Great Sumatran fault zone in the Padang Highlands of Sumatra, Indonesia (Figure 1), the authors incidentally came across heretofore unreported evidence of significant surficial displacements along the fault during the Ms = 6~ earthquake of 28 June 1926 and the Ms = 7.6 earthquake of 9 June 1943. In both cases, eyewitness reports by long-term local residents allowed unequivocal documentation of the location and nature of the surficial fault displacements associated with the earthquakes, although it was only through the use of vertical aerial photographs to locate the active fault trace that it was possible to concentrate the questioning in promising areas. In every case, it was thus possible to locate the primary active fault trace to within a few hundred meters before commencing the search for local eyewitnesses.The 1700-km-long Great Sumatran fault has been recognized by geologists for many years as a profound regional tectonic feature, and it has usually been thought to be associated with oblique subduction along the Sunda arc (Fitch, 1972). Although its recent activity has been summarized by Katili and Hehuwat (1967), the detailed neotectonic trace has for the most part not been mapped or investigated. Katili and Hehuwat (1967) present inferential evidence for right-lateral displacement during several historic earthquakes, mainly based on geodetic observations (e.g., Muller, 1895) or reported directions of strong ground motion (e.g., Visser, 1927), but the present report is, to our knowledge, the first documentation of surficial fault rupture during individual historic earthquakes.Best documented is the fault rupture associated with the 1943 earthquake, which was described to us with remarkable memories by long-term residents in 13 selected localities along a 50-km-long segment of the fault near the town of Solok, between Danau (Lake) Singkarak and Danau Diatas (Figure 2). Since the rupture clearly extended into Danau Singkarak on the northwest, and it continued into an area of steep terrain and landsliding southeast of Danau Diatas, we estimate the total rupture length was at least 60 km. Ground cracking parallel to the fault was also described by local residents at least as far south as Surian, 15 km south of the southern border of Figure 2, but we judged this to be more likely related to landsliding than to true fault rupture. Vertical displacements of up to 2 m were described by many residents, varying in sense ("scissoring"), but virtually always in the same sense as that represented by the pre-existing Quaternary scarp. Particularly in the mid-section of the fault, numerous residents gave clear descriptions of strike-slip displacements (usually without prompting by us), and there was general agreement that the 1943 movement had been dextral and not sinistral-in keeping with the regional tectonic framework (Katili and Hehuwat, 1967;Hamilton, 1979). Although local residents .estimated strike-slip displacements in 1943 of up to 5 m, the large...
Exploration field geology mapping and acquisition of gravity data has been conducted on approximately 650 line kilometres of26 surveyed traverse lines across the Onin and Kumawa Peninsulas of western Irian Jaya. Karstified New Guinea Limestone with a maximum thickness of 2,150 m is the predominant surface outcrop, and precludes use of seismic. However, integrated use offield geology data, balanced cross-sections, and gravity modelling has enabled us to identify two giant hydrocarbon prospects.The Onin and Kumawa Peninsulas lie at the margin of Jurassic age faulting associated with the Australian Northwest Shelf. Jurassic rift sands of the Lower Kembelangan Formation are the primary reservoir target. During the Plio-Pleistocene, collision of the Australian plate. and the Banda Arc inverted sections of the rift system, including the Onin and Kumawa Peninsulas.A better understanding of the regional structure was gained by integrating the Mobil Oil gravity data (of 1992) and that collected by Shell Oil (in the 1950's) in the structurally less deformed Bomberai region east ofOnin and Kumawa. Bouguer reduction was carried out using 2.4 glcm 3 density, GRS 1967 and IGSN 1971. A sequence of gravity maps were generated, including Bouguer, regional, residual, downward continued, and second derivative. Spectral analyses indicate that basement is about 3 km depth at Onin and about 6 km in the Bomberai area. The Bomberai and Onin-Kumawa regions are separated from one another by a steep gravity gradient which has a SE-NW strike direction. This gravity gradient may represent a change of lithology.Prospect definition was obtained by evaluating traverse profile data. Balanced cross-sections were constructed, using detailed biostratigraphy to determine formation thicknesses and amount of fault offsets. Where possible, onshore balanced section profiles were tied to offshore seismic profiles and wells. Two dimensional forward gravity models were calculated, using formation densities from well data. The calculated profiles were then compared to profile observed values.Differences between calculated and observed profiles were resolved by adhering to the exploration model, which required that the main thickness changes would be in the Lower Kembelangan Formation rift section. Density values for the seven formations were held constant.Constraining the variables to one (Lower Kembelangan thickness) in the gravity profile modelling maintains credibility of the technique. Errors inherent in the structural maps derived from the crosssections are likely to be vertical shift which would be approximately constant across the prospects. The interactive work between geologists and geophysicists was effective in producing a logical representation of subsurface geology, which in turn allowed selection of drill sites with some degree of confidence. Subsequent to completing the modelling, offshore seismic data across the western plunge of the Onin Peninsula was obtained. The structural style demonstrated in the modelling and the seismic data are comp...
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