Seismic site characterization of Malang Regency (Indonesia) using HVSR inversion

Based on Indonesia's active fault map, this study focuses on the Eurasian and Indo-Australian plate subduction zone in the southern sea of Java, an area that is highly susceptible to earthquake risk and soil vulnerability. This study aims to determine soil vulnerability based on average shear wave velocity of the top 30 m (Vs30) and sediment thickness (z 1.0 ) parameters to map areas at risk of earthquake in the study area. From both parameters, the most vulnerable and safest area in Purwodadi Village, Indoensia will be determined. The method used in this research is microtremor measurement with a portable miniature broad band (MBB-2) seismograph. Measurements were conducted at 19 points spaced approximately 500 m apart. Horizontal vertical spectral ratio (HVSR) inversion was applied to obtain shear wave velocity profiles at each point. Soil assessment was performed using Vs30 parameters, classified according to the American society of civil engineers (ASCE) standards. The results show that areas with relatively high earthquake vulnerability, indicated by low Vs30 values, are located near measurement points RKI08 and RKI18 in residential areas and RKI04 on slopes with Vs30 values are 219.0 m/s, 248.9 m/s, and 230.2 m/s respectively. Besides that, these points have high z 1.0 value, these values are 388.0 m, 517.8 m, and 46.6 m respectively. Meanwhile, areas with lower vulnerability are found at RKI10 and RKI09 in residential areas and RKI05 on slopes, with Vs30 values are 502.0 m/s, 481.1 m/s, and 416.0 m/s respectively. These points have Vs30 values classified as “very dense soil” and have low z 1.0 values (60.3 m, 109.6 m, and 43.9 m respectively). Based on data processing, there is a fault that stretches straight from RKI04, RKI11, to RKI14. However, more research needs to be conducted using alternate methods to confirm the existence of the fault. This mapping is expected to provide local authorities valuable input for disaster mitigation planning. • Active faults in southern Java make the area vulnerable. • This research used microtremor; HVSR inversion derived shear wave velocity profiles. • Soil vulnerability mapping using Vs30 & sediment thickness (z1.0). • High-risk zones with low Vs30 & sediment thickness found at RKI04, RKI08 & RKI18.