Estimation of Semarang Fault Zone Using Magnetic Method

According to the geological structure of Semarang area, there are several faults. They are Kaligarang fault and Semarang fault. Semarang fault stretches from west to east in Semarang. It separates upper and lower Semarang morphologically. Because of its existence, there are so many damaged roads and problems in stability of soil. There haven’t been many researches concern with the existence of faults. So, in this opportunity, the researcher was encouraged to conduct the research to map Semarang fault area using Magnetic Method. Fault mapping needed to be done to draw up the instructions of disaster mitigation, especially in the area around the faults. The basic principle of Magnetic Method is by measuring the Earth’s magnetic field totally with Proton Precession Magnetometer (PPM) at certain areas on the Earth surface. The research was conducted in the location suspected as Semarang fault area geologically. It was approximately 15 km×16 km, in UTM coordinate of easting 429000–445000 and northing 9227000. In this research, the faults were successfully measured. There are 110 measurement points distributed almost in every Semarang faults area which had been predicted geologically. Data of total magnetic field which had been measured was reduced to get the components of IGRF and daily variation components. Therefore, the measurement obtained the anomaly of total magnetic field. This anomaly data showed total magnetic field that is differentiated horizontally in east-west direction and north-south direction. Then it continued by carrying out the pseudo gravity transformation. The location of anomaly corresponded to the maximum point of horizontal gradient. However, in the data, pseudo gravity was between the maximum and minimum point. From these criteria, the area or location of Semarang faults was estimated in the southeast part to Kalipengkol between Kalikayen and Jabungan village. It continued to the north direction until JurangBlimbing and SendangKenongo village. From SendangKenongo, it turned to the west direction through Trangkil village, Ngesrep. It means that it continued from the west direction to the north through south Karangrejo village and still continued to the west until Ngelosari, Sadeng village.