Additional residuals between measured VS30 values and estimated VS30 values, such as between-site residuals and within-site residuals and mean pooled bias, were considered to assess the availability of the method in the South Korea region. After adjusting the estimated VS30 results, more than 72% of the measured VS30. and estimated VS30 are within ±50% of each other. Using the revised procedure, this study evaluated 122 VS30. values at stations where VS30 is not available. a) Location of damaged structures and occurrence of liquefaction with faults and fault zones, and (b, c) sand volcanoes formed by liquefaction near the epicenter of the earthquake (Kang et al. Fig. 2.
Histogram of the number of estimated VS30 records after application of SNR lower limit for KMA and KIGAM stations..21. The number of estimated VS30 for each station is expressed in brackets next to station code name..28 Figure 18. The number of estimated VS30 for each station is expressed in brackets next to station code name..29 Figure 19.
The number of VS30 estimated for each station is expressed in parentheses next to the station code name..30 Figure 20. Map of measured VS30 and estimated VS30 values from this study with the color-coded bar for free-field seismic stations. KMA and KIGAM. 36.
INTRODUCTION
2016a) proposed a method to estimate VS30 using the initial peaks of the compression wave (P) seismogram and validated the method for the Central and Eastern North American region using data from 31 seismic stations. 2018) applied the mentioned P-wave method to estimate VS30 using 298 seismic stations connected to the Kiban-Kyoshin network (KiK-net) in Japan. 2020) validated the P-wave seismogram method at 50 seismic stations and suggested 118 VS30 values for seismic stations where VS30 values were not measured in South Korea.
The 101 of 152 open-field seismic stations operated by the Korea Meteorological Administration (KMA) and 34 of 44 open-field seismic stations operated by the Korea Institute of Geoscience and Mineral Resources (KIGAM) have no measured VS30 values. To cope with the potential seismic hazard, VS30 values should be prefixed for a wide area. 2020) has already validated the P-wave method in South Korea using the 359 P-wave seismograms from January 2015 to February 2019, and then estimates the VS30 for the stations where the VS30 values were not measured. This study validates the P-wave method in the South Korea region using 53 free-field seismic stations that have available measured VS30 and estimates 122 VS30.
The evaluation performance of the P-wave method in Korea is compared with previous research and performance in other countries. Distribution maps of color-coded seismic stations based on (a) mean values of estimated VSZ and (b) mean values of estimated VS30, and interpolated maps based on (c) mean values of estimated VS30 and (d) USGS DMR geologic maps of Texas, Oklahoma, and Kansas ( Zalachoris et al., 2017).
DATA
Maps of seismic stations of KMA and KIGAM color-graded with measured VS30 next indicator bar and earthquake epicenters.
METHODOLOGY
V SZ estimation
2016a) calculated the radius parameter p for all seismic stations by assuming the ground as two simplified ground layers with regional crustal P-wave velocity profiles. The P-wave velocity profile model used in this study was from the model proposed by Hong et al. From the P-wave velocity profile model, the 1 km from the ground surface was considered as the first layer.
Thus, D2 is calculated as the value of the focal depth minus 1 km, while VP2 is determined as the average P-wave velocity taking into account the total time required during propagation between a depth of 1 km and the focal depth and using the iteration of equation 5.
Correlation between V SZ and V S30
Clarifying initial peaks of P-wave seismograms and the amplitudes
Zoomed P-wave seismogram with (a) raw acceleration time series and (b) acceleration time series with baseline correction and Butterworth digital filtering. The examples of P-phase picking algorithm proposed by Kalkan (2016) apply to the P-wave velocity seismograms recorded in South Korea. Histogram of the number of estimated VS30 records after applying SNR lower limit for KMA and KIGAM stations.
The velocity time series of (a) EW, NS and vertical directions, (b) radial and vertical directions in the time window near the tP arrival with the timeline of tP arrival (green dashed line) and 𝑡𝑈̇𝑍 (blue dashed line), (c) the same as (a) but zoomed in for the time window near tP arrival, and (d) vertical and rotated horizontal velocity time series with 𝑈̇𝑅𝑜𝑡50 recorded at Uiseong station during the 2012 ML 3.1 earthquake. To calculate the radial velocity time series two approaches were attempted to calculate the radial component, taking into account the change in P-wave propagation due to erratic subsurface conditions. The velocity amplitudes for the velocity time series of EW and NS directions at 𝑡𝑈̇𝑍 were identified as 𝑈̇𝐸𝑊 and 𝑈̇𝑁𝑆.
The geometric mean of the two horizontal components (𝑈̇𝐺𝑀) calculated using equation (7) is used instead of 𝑈̇𝑅.
VALIDATION
Comparison between measured V S30 and estimated V S30
Statistical analysis for residuals
The mean overall bias could also be easily compensated, and the standard deviation of the total and within-site residuals depended on the local condition of each seismic station and the number of recorded seismograms. Through this consideration, it is suggested that the estimate with the approach of using 𝑈̇𝑅𝑜𝑡50/𝑈̇𝑍, which resulted in between-site residual (𝜏) of 0.3227, was slightly better than the other two approaches. The standard deviations of residuals between sites from these studies range from 0.34 to 0.41, which is comparable to the value obtained in this study.
The average overall bias (a) and standard deviations of total residuals (𝜎𝑌), between-site residuals (𝜏), and within-site residuals (𝜙) calculated by three approaches proposed in this study (i.e., 𝑈̇𝑅/𝑈̇𝑍, 𝑈̇𝐺𝑀/ 𝑈̇𝑍, and 𝑈̇ 𝑅𝑜𝑡50/ 𝑈̇𝑍), and calculated values from other studies. The estimated VS30 count for each station is shown in parentheses next to the station code name. The estimated VS30 count for each station is shown in parentheses next to the station code name.
APPLICATION
Map of measured VS30 and estimated VS30 values from this study with color graded bar for KMA and KIGAM free-field seismic stations.
CONCLUSION
This study estimated VS30 values for the 122 free-field seismic stations operating in Korea and finally proposed 183 VS30 values, including measured VS30 and estimated VS30 values. Due to the harshness of operational field tests and the highly varied subsurface conditions in Korea, the collection of VS30 values is important for use in performing seismic hazard assessments and constructing ground motion prediction models. This study suggested the possibility of collecting VS30 with the P-wave method for the sites with unavailable field test condition.
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