| Abstrak/Abstract |
The Mw 7.5 September 2018 of Palu earthquake created a 67 km-long rupture zone along the Palukoro Fault in Central Sulawesi, Indonesia. Despite the clear and sharp rupture in most places along the fault, with a spectacular 3-5.5 m strike-slip sinistral offsets displacing many roads, fences, houses and other artificial alignments, finding suitable sites for paleoseismological trenching was challenging. Beside it is difficult to find a site that was suitable for excavation using backhoe, the ground water level in the area along the ruptures are generally very shallow. The selected trench site is located about 6 km to the south of the Palu Bay, where the rupture shows a ~3.6 m prominent sinistral offset at Palupi village. The site was chosen based on the following reasons: (1) it is located in an open field that is being constructed for a new housing complex, only few hundred meters away from the existing housing complex that was largely destroyed during the 2018 earthquake, (2) the groundwater level at this site is at 14 m depth, measured using geoelectrical survey that would prevent flooding to our trench, and (3) the site is located on the young alluvial fan unit that consists of predominantly sands alternated with clays and silts with some gravels layers. Therefore, it will ensures that the trench walls will have good layering for analysis. The size of the trench is 50 m long, 7 m wide, and 5 m depth and it was opened for a week. We found multiple fault traces on the trench walls through observation of offsets of the sedimentary layer units. In addition to the 2018 rupture, we were able to identify another 2 to 4 faulting events prior to the 2018 event, documented through a detailed logging. We collected a total 30 of detrital charcoal samples that will be analyzed using radiocarbon dating method, to reconstruct the timing of these events. The fault zone revealed in the trench has a 20-m wide damage zone. Results from this study will allow us to reconstruct the earthquake history and recurrence interval of earthquake along the fault, which is important for the earthquake hazard analysis of the area. Further, this study will provide an example of how to appropriately determine and regulate the restricted buffer zone along a fault to minimize future earthquake hazards. |
