## 5 Observation of Time-Averaged Shear Wave Velocity  
V\({}_{530}\) indicates the rigidity of the soil. This measure assumes a particular relationship with site amplification, which is included in the ground motion prediction equation [26]. The average spectral amplification factor also corresponds to V\({}_{530}\). There are many data collection methodologies. The method with the highest accuracy for observing V\({}_{530}\) is borehole logging, which can directly examine the soil characteristics in each sediment layer. However, drilling a borehole to a depth of 30 m is costly and resource intensive. To cover the entire study area, with limited time and workforce, the microtremor utilizing technique was applied in this study. The term \"microtremor\" refers to the small waves that propagate through the ground. These originate from several sources near ground zero such as human activities, wind, and ocean tides. In total, observations from 73 stations in the districts of Chiang Rai, Chiang Mai, and some parts of Lamphun (see Figure 1) were considered in this study. PAC2016 (red dots in Figure 1) represent data obtained from 23 stations during the current research. The observations from previous studies include 41 stations from NKN2014 and 6 stations from NKN2015 (blue and yellow dots, respectively, in Figure 1) [51]. For the PAC2016 dataset, the SPAC method was applied to analyze microtremors [4; 5; 6; 7; 8]. In this method, four sensors were placed in an array to detect the phase velocity from tremors that arrive from every direction. The array of the sensors consisted of one sensor in the middle and three sensors at the 1 m radians (see Figure 5). This research took about one week to conduct 27 stations (PAC2016 dataset). There are some factors that can cause disturbance to the measured signal. However, the site that were located in remote areas had higher systematic noise due to insufficient tremors. Figure 4: Descriptions of classified geomorphology units from pixel-based classification (PBC) and object-based classification (OBC) [9; 50]. Four stations were eliminated. From sensor's configuration, the duration of the observation period was 20 min. For data treatment, the first 5 min and last 3 min were discarded. The waveform was split for every 20 s. The periods that had high amplitudes due to noise during the observation were removed. Noise refers to abnormal activity that creates higher than average amplitudes, for example, due to heavy vehicles passing. The V\({}_{\text{S30}}\) values collected in this study (PAC2016) were obtained using the concept of the phase velocity of the Rayleigh wave corresponding to a 40-m wavelength (C(40)). V\({}_{\text{S30}}\) can be obtained without creating a soil profile from the phase dispersion curve inversion. [PERSON] et al. (2010) also showed a high correlation between the V\({}_{\text{S30}}\) values and phase velocity C(40) [52]. This approach was proposed by [PERSON] et al. (2000), who examined 85 sites around Tokyo [53]. A method of V\({}_{\text{S30}}\) estimation was proposed in this study using the phase velocity of the Rayleigh wave fundamental mode obtained from microtremor array observations. The results from that study showed that V\({}_{\text{S30}}\) was nearly equal to the phase velocity at wavelengths ranging from 35 to 40 m in the small array. Validation in the current study is was not possible at this site owing to the lack of ground-truth data, which was the main obstacle and the reason we used the microtremor observation technique along with the existing methodology. However, the validation of V\({}_{\text{S30}}\) values from the past study showed that the regression coefficients of V\({}_{\text{S30}}\)-C(40) were 0.9204, 17.86, (\(\pm\)13.83) and the correlation coefficient was 0.982. It was concluded that V\({}_{\text{S30}}\approx\) C(35, 40) [53]. The obtained V\({}_{\text{S30}}\) values classify the sites based on NEHRP provisions [54; 55; 56].