In this study, we presented the result of the geophysical survey first (MASW, SPAC, and HVSR results), followed by the discussions on the Medan City environment, geotechnical and environmental analysis, and finally discussions on the potential environmental damage in Medan City due to earthquake shaking. In addition to the figures, the results are also presented as Tables in the supplementary material. The comparison of the different methods have been including discussions regarding each model.
MASW result
In this study, the used assessment to determine the classification of soil types using the value of shear wave velocity at a depth of 30 m (Vs30) with the Multi-Channel Analysis of Surface Wave (MASW) method based on the SNI 1726:2019 classification because it is considered not to contribute vertical waves from ambient noise in the dominant period microtremor (TDOM) with the Horizontal to Vertical Spectral Ratio (HVSR) method (Irsyam et al. 2020).
The measurements result from 198 points in 21 districts in Medan City produced shear wave velocity data up to 30 m depth (Vs30). The obtained value vary from 61.86 to 307.22 m/s. The distribution table for the Vs30 value is presented in Additional file 1: Table S14.
Generally, Medan city in the central and southern parts is composed of medium soil (SD), and theVs30 value ranges from 180.29 to 307.22 m/s (orange to light green). Whereas the northern part of Medan City is composed of soft soil (SE). with Vs30 values ranging from 61.86 to 179.81 m/s (red color). This medium soil (SD) and soft soil (SE) dominate almost all Medan City areas, it can be seen in Fig. 2a.
SPAC result
Measurement of engineering bedrock depth in the Medan City area using the Spatial Autocorrelation (SPAC) method (Kita et al. 2011) was done three times. From field measurements and data processing, the results of the dispersion curve and shear wave velocity profile (Vs) concerning the depth of each point were obtained. The dispersion curve shows the frequency relationship to the varying velocity phase caused by different lithological conditions. Velocity profiles higher than 750 m/s were considered as velocity in engineering bedrock.
The results of measurements using the SPAC method show that the bedrock values varied with the dominant bedrock at 250–450 m depth (Fig. 2b). That depth is on the west edge of Medan city. The deepest part of bedrock is at 750 m depth, i.e., the central Medan, which are Medan Deli, Medan Timur, and Medan Perjuangan. Detailed bedrock depth in each district is presented in Additional file 1: Table S15.
HVSR result
The results of the dominant period (TDOM) measurement in Medan City produced the Amplification (A) and Period (T) seismic data. Interpolation was done to be used as the contour of the TDOM value in the survey area. The TDOM contour was made based on the closeness between the measurement points.
Dominant period values vary from 0.09074 to 8.30724 s. This value can reflect the character of the soil type based on the Zhao equation (Zhao et al. 2006). This TDOM value shows the character of soft soil types in almost all areas of Medan City (Fig. 2c). The detailed distribution of the dominant period values is presented in Additional file 1: Table S16.
The distribution of seismic vulnerability (KG) of 21 districts in Medan City is clearly shown in Additional file 1: Table S16. From 198 measurement sites, as many as 136 measurement points produced a seismic vulnerability index value of less than five, indicated by the dominance of light brown color, which indicates that area has a low seismic vulnerability scale (Fig. 2d).
Otherwise, the 56 measurement sites show orange color, which means the seismic vulnerability has ranged from 5 to < 50, so it is included in the medium classification. High vulnerability ranges from 50 to 1486.38, which is marked as red in the northern and central parts of Medan City. Based on the seismic vulnerability index scale by Nakamura (2008), Medan city has a range of vulnerability index values on all scales from low and medium to high, with values ranging from 0.07427 to 1486.38.
Medan city environment
Medan City is an area with a dense population. The most densely populated areas are concentrated in the central part of Medan city (Medan Perjuangan, Medan Tembung, and Medan Denai) (Fig. 3a). The land cover of the city of Medan is quite varied but is dominated by settlements (Fig. 3b). The Medan city area has a flat slope, only between the Districts of Medan Petisah (Fig. 3d), Medan Maimun, and Medan Area where there are steep areas. The population density of the city of Medan compared to the area occupied, the highest is in the Medan Perjuangan area with more than 200 people/ha. Then followed by Medan Denai District and Medan Area, with a population density of more than 150 people/ha. Meanwhile, the other 18 Districts in Medan City have an average population density of fewer than 150 people/ha. So that the potential impact of a disaster is very significant if it occurs in the Medan Perjuangan area. Rainfall in the Medan city area is generally classified as medium rainfall (class 2) with an average rainfall range of 190–220 mm (Fig. 3c). The highest level of rainfall in the city of Medan is in the southern region, followed by the central area and then north of the city of Medan.
Geotechnical and environmental analysis
Based on the Compilation of Environmental Parameters of the Medan City Map, the areas with the highest risk parameter values are in the central to the southern region of Medan City, and the lowest areas are in the northern region (Fig. 4a). The area that has the potential to have a significant impact on landslides is the downtown area of Medan because the slope is quite steep (15–45%) and land cover in the form of dense settlements is in the area. The population density in the downtown area of Medan is also quite dense, namely 7000–24,000 people/km.
The Compilation Map of Geotechnical Parameters for Medan City (Fig. 4c) was obtained from the combination of geotechnical parameters, Site Class (MASW/Vs30), Seismic Vulnerability (KG), and Dominant Period (TDOM) with a weight of each parameter, i.e., 52% of Site Class (MASW) parameters, 30% Seismic Vulnerability (KG), and 18% Dominant Period (TDOM). Based on the Geotechnical Parameter, the area with the highest geotechnical parameter value is in the northern region, and the lowest area is in the southern region.
Map Compilation of Geotechnical and Environmental Parameters of Medan City (Fig. 4b) were obtained from a combination of geotechnical parameters, Site Class (MASW/Vs30), Seismic Vulnerability (KG), and Dominant Period (TDOM) as well as environmental parameters such as Land Cover, Slope, and Weather Conditions (Rainfall Class) with the weight of each parameter, namely 38% Site Class (MASW), 22% Seismic Vulnerability (KG), 13% Dominant Period (TDOM), 13% Land Cover, 7% Slope, 7% Weather Conditions (Rainfall Class).
Based on geotechnical and environmental parameters (Fig. 4b), the northern to central Medan area has the potential for significant impact due to earthquakes, especially the northern part of the Medan Belawan district, Medan Marelan, Medan Deli, West Medan, and Medan Area which are in the southern region.
Potential environmental damage due to earthquake in Medan city
The Zoning Map of Potential Environmental Damage in Medan City was obtained from a combination of population density parameters, geotechnical parameters, and environmental parameters with the weight of each parameter, namely 28% Population Density parameter, 28% Soil Class parameter (MASW), 16% Seismic Vulnerability (KG), 9% Dominant Period (TDOM), 9% Land Cover, 5% Slope, and 5% Weather Conditions (Rainfall Class).
The results of the Zoning analysis of Potential Environmental Damage Due to the Earthquake in Medan City show that the areas that have the potential to have a high enough impact are the Medan Perjuangan, Medan Area, Medan Tembung, and Medan Denai areas. The potential medium impact is in the areas of Medan Maimun, Medan Helvetia, Medan Deli, Medan Petisah, Medan Johor, Medan Amplas, Medan Baru, Medan Belawan, Medan Kota, Medan Labuhan, Medan Marelan, Medan Barat, Medan Polonia, Medan Sunggal, Medan Timur, and Tuntungan Field. Then, areas with low impact potential are only found in the Medan Selayang sub-district. The detailed zoning weighting of potential environmental damage due to the earthquake for each Medan City district is shown in Additional file 1: Table S17.
In this study, possible verification of the results has been obtained by comparing the result from various methods, such as the MASW, SPAC, and HVSR, as well as by performing the Analytic Hierarchy Process (AHP). The result is also verified by empirical information from the damage due to the recent moderate-magnitude earthquake on 16 January 2017 (Fig. 1a) (Prasetyo et al. 2018).