Subagyo Pramumijoyo

Neogene and quaternary fault kinematics around the Sunda Strait area, Indonesia

Abstract The Sunda Strait corresponds to the transition zone from the orthogonal subduction off Java to the oblique subduction off Sumatra. In this last region, the roughly N-S convergence between the Indoaustralian and Eurasian plates appears to be accommodated by dextral strike-slip displacement along the Sumatran Fault System. The opening of the Sunda Strait has been considered as a consequence of the right lateral movement of the Sumatran Fault System. This paper reports structural data on the fault kinematics in the Sunda Strait area. The result of this structural study shows that two main fault kinematics are seen on the faults located around the Sunda Strait area: dextral strike-slip movement and normal movement. Strike-slip deformations are observed within Miocene or older rocks, while Pliocene and younger formations are only affected by normal faulting. The geological evolution of faults near the Sunda Strait region suggests that dextral slip on the Sumatran Fault System began during the Middle Miocene and that normal faulting has prevailed in the Sunda Strait since 5 Ma. This extensional tectonics appears to control the peculiar bathymetry of the Sunda Strait.

Weak Tectono-Magmatic Relationships along an Obliquely Convergent Plate Boundary: Sumatra, Indonesia

The tectono-magmatic relationships along obliquely convergent plate boundaries, where strain partitioning promotes strike-slip structures along the volcanic arc, are poorly known. Here it is unclear if and, in case, how the strike-slip structures control volcanic processes, distribution and size. To better define the possible tectono-magmatic relationships along strike-slip arcs, we merge available information on the case study of Sumatra (Indonesia) with field structural data. The Sumatra arc (entire volcanic belt) consists of 48 active volcanoes. Of these, 46% lie within 10 km from the dextral Great Sumatra Fault (GSF), which carries most horizontal displacement on the overriding plate, whereas 27% lie at >20 km from the GSF. Among the volcanoes at <10 km from GSF, 48% show a possible structural relation to the GSF, whereas only 28% show a clear structural relation, lying in pull-aparts or releasing bends; these localized areas of transtension (local extensional zone) do not develop magmatic segments. There is no relation between the GSF along-strike slip rate variations and the volcanic productivity. The preferred N30 •-N40 • E volcano alignment and elongation are subparallel to the convergence vector or to the GSF. The structural field data, collected in the central and southern GSF, show, in addition to the dextral motions along NW-SE to N-S striking faults, also normal motions (extending WNW-ESE or NE-SW), suggesting local reactivations of the GSF. Overall, the collected data suggest a limited tectonic control on arc volcanism. The tectonic control is mostly expressed by the mean depth of the slab surface below the volcanoes (130 ± 20 km) and, subordinately, local extension along the GSF. The latter, when WNW-ESE oriented (more common), may be associated with the overall tectonic convergence, as suggested by the structural data; conversely, when NE-SW oriented (less common), the extension may result from co-and post-seismic arc normal extension, as supported by the 2004 mega-earthquake measurements. Overall, the strike-slip arc of Sumatra has intermediate features between those of extensional and contractional arcs.

Investigation to the Local Site Effects During Earthquake Induced Ground Deformation Using Microtremor Observation in Yogyakarta, Central Java-Indonesia

Local site effects have an enormous influence on the character of ground motion and play an important role in seismic damage to human being made structures. Several powerful earthquakes have struck Yogyakarta, Indonesia during recent years, one of the destructive which was an Mw 6.3 event that occurred on May 27th, 2006 and caused more than 5,700 fatalities and over 37,000 injuries. Following this event, the single observations of microtremors were densely performed at 274 sites in Yogyakarta City. To provide a good coverage of the study area, the single microtremor observations are designed with a grid spacing of 500 m to the main path of the north–south and east–west of the research area to known in detail the configuration soft rocks and contact hard rock. The predominant periods due to horizontal vertical ratio (HVSRs) are in the range of 0.15–4.00 s. The amplification factor (Ag) and the soil natural frequency (Fg) are generally ranging between 0.70–5.56 and 0.4–3.3 Hz according to results derived from HVSR analysis. It is obviously observed that the thickness of local sediments has great influence on amplification characteristics. In general, the local geology effect is more dominant than the topography effect. The level of soil damage due to the local site effects at the ground deformation location was indicated by the vulnerability index (Kg) and the effective shear strain (γ). Actually, vulnerability index (Kg) varied from 0.6 to 51.3 and the effective shear strain (γ) had values ranging from 300 × 10−6 to 25,000 × 10−6 which for γ > 10,000 × 10−6 catastrophic landslide or very large deformation will be occurred. This study illustrates how the vulnerability index and the effective shear strain could be used to make preliminary assessment of the research area deformed during the earthquake.

Designed microtremor array based actual measurement and analysis of strong ground motion at Palu city, Indonesia

In this study, we investigated the strong ground motion characteristics under Palu City, Indonesia. The shear wave velocity structures evaluated by eight microtremors measurement are the most applicable to determine the thickness of sediments and average shear wave velocity with Vs ≤ 300 m/s. Based on subsurface underground structure models identified, earthquake ground motion was estimated in the future Palu-Koro earthquake by using statistical green’s function method. The seismic microzonation parameters were carried out by considering several significant controlling factors on ground response at January 23, 2005 earthquake.

Estimation of underground structure at Prambanan area, Yogyakarta, Indonesia

In this study, we investigated the underground structure at Prambanan area, Yogyakarta. We performed single observations of microtremor at 124 sites in Prambanan area. The results enabled us to estimate the site-dependent shaking characteristics of earthquake ground motion. We also conducted 2-site bore holes investigation to gain a representative determination of the soil condition of subsurface structures in Prambanan. From the SPT of borehole observations, the prambanan area corresponds to relatively soil condition with Vs ≤ 298 m/s, the predominant periods due to horizontal vertical ratios (HVSRs) are in the range of 0.48 to 1.19 s and the frequency are in the range of 0.95 to 1.92 Hz. By making these observations, we can obtain a relationship between the predominant periods, frequency and distribution of the first layer thickness of the sediment.