Shear wave velocity model using HVSR inversion beneath Bandar Lampung City

Abstract

The horizontal-to-vertical spectral ratio (HVSR) method has been used to characterize site-effect parameters that are indispensable in seismic hazard and risk-reduction studies in urban areas and rapid land-use planning. This method is widely used because it is the cheapest and simplest geophysical method for the acquisition and processing stages. In subsequent developments, the HVSR method has been widely used to determine elastic rock parameters, particularly shear wave velocity (v_S), through the HVSR curve inversion process. Furthermore, the v_S structural model can be used to delineate the presence of complex geological structures, particularly faults and sedimentary basins. Bandar Lampung is a city in Lampung Province with many fault structures and groundwater basins to the south. There are 83 HVSR measurement points around Bandar Lampung for delineating the presence of fault structures and groundwater basins. We produced the HVSR curve from the measurement results and then performed an inversion process using the particle swarm optimization algorithm to obtain v_S for the depth profile. Subsequently, from this profile, we produced a two-dimensional (2D) lateral and vertical model. The mean v_S value was calculated from all the measurement points, and we found stiff soil layers reaching depths of approximately 5 m, with a value of v_S < 330 m/s. A bedrock layer with a velocity exceeding 1250 m/s was visible at a depth of 100 m. Based on the 2D model, the v_S structure shows that the city of Bandar Lampung is divided into two zones, with a NW-SE boundary. The north-middle-eastern part of the city consists of harder rocks. This harder rock is characterized by extremely high v_S values, starting from a depth of 50 m. In contrast, the south-middle-west exhibits a low-moderate v_S anomaly associated with groundwater basins SW of the city. From the 2D v_S structural model, fault structures can be found along the city, characterized by a contrast of v_S values from low to medium and from medium to high.