Indonesian Journal on Geoscience

15

Authors: Bambang Setiawan

A topic of seismic hazard analysis (SHA) is briefly elaborated. A probabilistic seismic hazard analysis (PSHA) is commonly used to assess the ground motion level expected with different likelihood at a rock site during a future seismic event. The Incorporating Monte Carlo method into PSHA in an intraplate region (i.e. Adelaide region) is an interesting topic to explore. The result of the analysis using this method is able to characterize the likelihood of seismicity in a targeted region. Furthermore, the results clearly display the seismic ground motions in term of peak ground acceleration and peak ground velocity in Adelaide region. The de-aggregation of the analysis suggests two expected severe events for the Adelaide City. The first expected event is an earthquake M5.2 from a distance of 15 km and 25 km from the city. The second expected one corresponds to an earthquake M6.6 occurring 85 km away from the Adelaide City. However, the results of this analysis must be treated carefully due to dubious seismic data catalogue for a relatively large seismic event in Adelaide region.

8

Authors: Lina Handayani, Maryati Maryati, Kamtono Kamtono, M. Ma’ruf Mukti, Yayat Sudrajat

The characteristic of Cimandiri Fault Zone has not been completely defined despite plenty of studies had already been accomplished. Therefore, an audio-magnetotelluric modeling was carried out. An audiomagnetotelluric survey was conducted at two parallel lines (N166oE) that intersected Cimandiri Fault Zone in Cibeber area, Cianjur. The distance between those two lines was 4.5 km and each line consisted of twenty-one stations with the distance between stations was 500 m. From the acquired forty-two apparent resistivity curves, inversion was executed to obtain two models. The models indicate layers with resistivity value of > 1000 ohm.m at about 500 m depth at both lines, which are associated to the basement layer. Columns of low resistivity zones in about the middle of each model represent fault zones as the weak zones of the area, and both models displayed them slightly dip southward as thrust faults.

9

Authors: Ovinda Ovinda, Joseph J Lambiase

Several outcrops were studied to identify sedimentary facies and to analyze permeability distribution, through which an outcrop analogue for upper shoreface reservoirs can be established. Four facies were identified: upper shoreface, lower shoreface, offshore transition, and tidal ones. Stratigraphic correlation of eleven outcrops indicates that the upper shoreface sandstone is generally clean, well sorted, parallel, and planar cross laminated. The sand becomes thinner and pinches out to the northwest where the mud proportion increases within the sand. Muddier sand was deposited in a relatively low energy upper shoreface setting. The thickness of the upper shoreface reservoir sand generally is 5 m. It decreases to zero over approximately 1.3 km as the sand pinches out to the northwest. To the northeast, the thickness also decreases to 4 m over approximately 4 km. Permeability values are more variable laterally than vertically. The permeability distribution has an obvious relationship to the sedimentary facies and is mainly controlled by the proportion of mud and bioturbation. As the sand pinches out in the northwest, permeability decreases from 590 md to 97 md over 1 km. To the northeast, permeability also decreases to 152 md over approximately 4 km where the sand becomes highly bioturbated. These values indicate that the sands are of good to very good reservoir quality. It appears that there are no major barriers to the lateral flow of fluid within the upper shoreface sandstone.