Soil Classification Mapping by Vs30 MASW Measurements in Gorontalo City

Abstract The city of Gorontalo, in the north of Sulawesi Island, has a relatively high potential for earthquake events. This condition is influenced by the meeting of three plates, namely the active subduction zone of the Sulawesi Sea trench, which is subducting to the south, and the Maluku Sea trench (Sangihe Trench), which is subducting to the west. Apart from that, the Gorontalo Fault on land, which is thought to pass through the city of Gorontalo, contributes to the potential danger of earthquakes. Therefore, microzonation studies are needed to analyze site effects in Gorontalo City. To obtain site effect characteristics in Gorontalo City, shallow seismic methods can be applied. In previous research for Sulawesi Island, there was a Vs30 map based on a geomorphological model. The map has a less detailed resolution because it is based on available lithological and topographic maps with limited resolution. In this study, Vs30 measurements were carried out through field geophysical surveys using the Multichannel Analysis of Surface Wave (MASW) method. The MASW survey principle is based on the Rayleigh surface wave propagation theory, where the waves are generated from the interaction of shear waves with the surface soil layer. 130 MASW measurement locations have been used for Gorontalo City to identify soil classifications in the area. The results of Vs30 measurements from a field geophysical survey using the MASW method show conformity with the results of previous research based on geomorphological classification. The spatial mapping results from this research even show a more detailed pattern than the results of previous research. In general, the entire city of Gorontalo mainly in the low land is composed of soft soil with Vs30 values ranging from 131.4 m/s to 173.6 m/s, especially in the area near Lake Limboto, in accordance with the ground formation formed by lake sediments. Apart from that, the area is classified as medium to hard, especially in the southern mountains, with a Vs30 of 1187.5 m/s, which indicates rock as the base of the ground surface.