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Title Page

Abstract

Contents

Chapter 1. Introduction 14

1.1. Research background 14

1.1.1. Vₛ₃₀ estimation 14

1.1.2. Topographic ground motion amplifications 15

1.2. Outline of the dissertation 16

Chapter 2. Literature reviews 18

2.1. Estimation of site conditions 18

2.1.1. Proxy and HVSR method 18

2.1.2. P-wave seismogram method 20

2.2. Topographic ground motion amplifications 24

Chapter 3. Vs map for Pohang basin 28

3.1. Methodology 28

3.1.1. Signal processing 29

3.1.2. P-wave seismogram method 31

3.2. Ground motion records and processing 34

3.3. Vₛ₃₀ estimation 36

3.3.1. Example 36

3.3.2. All estimates 38

3.4. Validation of Vₛ₃₀ estimation 42

3.4.1. Vs measurements 42

3.4.2. Comparison between measured and estimated Vₛ₃₀ values 42

3.5. Final Vₛ₃₀ map 46

Chapter 4. Topographic ground motion amplifications 49

4.1. Data and processing 49

4.2. Topographic parameters 55

4.3. Regression analysis 55

Chapter 5. Conclusions 66

Chapter 6. Future works 68

References 69

Appendix 74

List of Tables

Table 1. Previous studies for proxy methods and HVSR method 20

Table 2. Previous studies for P-wave seismogram method 24

Table 3. Previous studies for topographic ground motion amplifications 27

Table 4. Measured Vₛ₃₀ values at the 54 sites, as well as the estimated Vₛ₃₀ values (i.e., Vₛ₃₀acc, Vₛ₃₀vel, and Vₛ₃₀dis) the at stations closer than 200 m from the measurement sites[이미지참조] 43

Table 5. Coefficient of regression model for PSA at period of 0.01-0.5 s. 56

Table 6. Standard deviation of Res1 to Res3 for PSA at period of 0.01-0.5 s. 62

List of Figures

Figure 1. Grouped Vₛ₃₀ values by geologic class 18

Figure 2. Correlations of measured Vₛ₃₀ versus topographic slope 18

Figure 3. Ratio of maximum values between horizontal and vertical components of seismic motions by observation site 19

Figure 4. HVSRs peak value versus measured Vₛ₃₀ 20

Figure 5. (a) Relationship between particle motions and incident P-wave, (b) transformation of ray path due to the subsurface layer, and (c) Examples of radial and vertical components (red and black lines,... 21

Figure 6. Estimated and measured Vsz for 31 seismic stations in CENA[이미지참조] 22

Figure 7. Vₛ₃₀ versus Vsz from Vₛ profiles for CENA with depth z of (a) 5, (b) 10, (c) 15, and (d) 20 m[이미지참조] 22

Figure 8. Maps of (a) Vₛ₃₀ estimates at K-net station and (b) both Vₛ₃₀ estimates at 989 K-net stations and Vₛ₃₀ measurements at the KiK-net stations by Kang et al. (2020). 23

Figure 9. Map of Vₛ₃₀ estimates and measurements for stations in Korea by Kim et al. (2020b). 23

Figure 10. Schematic of cliff topography 25

Figure 11. Topographic amplification against smoothed curvatures. 25

Figure 12. Site residuals for PSA at period of 1 s against relative elevations 26

Figure 13. Spectral accelerations (Sas) of ground motions measured at GOK1 and GOK4 (located on slope facing the epicenter) relative to those at GOK2 (located on the slopes facing the opposite direction)... 26

Figure 14. Flow chart for the procedures of Vₛ₃₀ estimation. 28

Figure 15. Examples of (a) raw velocity time series with only baseline corrected velocity time series, and (b) the baseline corrected velocity time series with baseline corrected and filtered velocity time... 29

Figure 16. Rotation of two horizontal components (EW and NS) into radial and tangential components. 30

Figure 17. Velocity time series recorded at B004 station on January 7th, 2019, in the radial and vertical directions near the arrival time of the P-wave. 30

Figure 18. (a) Schematic of transformation of the incident P-wave by the free surface and (b) simplified two-layer model with ray path 31

Figure 19. Vsz-Vₛ₃₀ relationships proposed by Kim et al. (2016) where the c₀ and c₁ are coefficients and ε is residual.[이미지참조] 33

Figure 20. Locations of (a) seismic stations and temporary seismic stations deployed in the two phases (Phase 1: January 3 to 25 and Phase 2: January 27 to February 14 in 2019) and earthquake epicenters,... 34

Figure 21. Histograms of (a) earthquake magnitudes, (b) focal depths, (c) epicentral distances, and (d) maximum PGV values of ground motions in the EW and NS directions used in this study. 35

Figure 22. Histograms of (a) pulse durations and (b) signal-to-noise ratios of 370 recordings. 36

Figure 23. (a) Velocity time series recorded at B042 station on January 7th, 2019, in the radial and vertical directions, and (b and c) velocity time series near the arrival time of the P-wave.[이미지참조] 37

Figure 24. (a) Velocity time series recorded at B091 station on February 2nd, 2019, in the radial and vertical directions, and (b and c) velocity time series near the arrival time of the P-wave.[이미지참조] 38

Figure 25. Vₛ₃₀ values estimated using velocity seismograms (Vₛ₃₀vel) versus those using acceleration seismograms (Vₛ₃₀acc) (a) for all 362 individual seismograms and (b) those averaged for the 102 stations....[이미지참조] 39

Figure 26. Signal-to-noise ratios of (a) acceleration seismograms (SNRacc) versus SNRvel, and (b) SNRdis versus SNRvel.[이미지참조] 41

Figure 27. Residuals between Vₛ₃₀acc and Vₛ₃₀vel versus (a) SNRacc, (b) SNRvel, (c) depth z for Vszacc and (d) depth z for Vszvel.[이미지참조] 41

Figure 28. Residual for (a) acceleration (Vₛ₃₀acc), (b) velocity (Vₛ₃₀vel), and (c) displacement seismograms (Vₛ₃₀dis).[이미지참조] 44

Figure 29. Measured Vₛ₃₀ versus estimated Vₛ₃₀ (bias-corrected) using (a) acceleration (Vₛ₃₀acc), (b) velocity (Vₛ₃₀vel), and (c) displacement seismograms (Vₛ₃₀dis).[이미지참조] 45

Figure 30. Maps of (a) the final VS30, (b) topography, and (c) geology produced by Sohn and Son (2004) in the study area. The faults identified (solid lines) and estimated (dashed lines) by Song et al. (2015)... 47

Figure 31. Photograph of landstreamer test with a weight-drop and rammer. 48

Figure 32. Vₛ₃₀ values by the landstreamer test and the final Vₛ₃₀ map along the landstreamer test line (A-A') presented in Figure 30. 48

Figure 33. (a) Locations of seismic stations and temporary seismic stations deployed from January 3 to February 14 in 2019 and earthquake epicenters; (b) relative elevation (radius: 400 m); (c) aspect... 49

Figure 34. Example of (a) velocity times series with P-wave arrival time (tP-arrival) and (b) FAS of signal and three times FAS of noise with fcHP for EW component of ground motion recorded at B002 station...[이미지참조] 50

Figure 35. Histograms of (a) earthquake magnitudes, (b) focal depths, (c) epicentral distances, and (d) PGA values of ground motions in the radial directions used in this study. 51

Figure 36. Cross-section lines of A-A' and B-B'. 52

Figure 37. (a) Cross-section of Line A-A' and three temporary stations along Line A-A', as presented in Figure 36; (b) acceleration time series and (c) spectral acceleration of ground motions in radial direction... 53

Figure 38. (a) Cross-section of Line B-B' and three temporary stations along Line B-B', as presented in Figure 36; (b) acceleration time series and (c) spectral acceleration of ground motions in radial... 54

Figure 39. PSA values of ground motions in a radial direction against earthquake magnitude for various periods (T): (a) 0.01; (b) 0.02; (c) 0.03; (d) 0.05; (e) 0.07; (f) 0.1; (g) 0.2; (h) 0.3; (i) 0.5 s. 58

Figure 40. Residual 1 of radial component against epicentral distance for various periods (T): (a) 0.01; (b) 0.02; (c) 0.03; (d) 0.05; (e) 0.07; (f) 0.1; (g) 0.2; (h) 0.3; (i) 0.5 s. 59

Figure 41. Residual 2 of radial component against Vₛ₃₀ for various periods (T): (a) 0.01; (b) 0.02; (c) 0.03; (d) 0.05; (e) 0.07; (f) 0.1; (g) 0.2; (h) 0.3; (i) 0.5 s. 60

Figure 42. Residual 2 of radial component against α for periods (T): 0.01; 0.02; 0.03 s by (a) low, (b) intermediate, and (c) high section based on hr⁴⁰⁰m.[이미지참조] 63

Figure 43. Residual 2 of radial component against α for periods (T): 0.05; 0.07; 0.1 s by (a) low, (b) intermediate, and (c) high section based on hr⁴⁰⁰m.[이미지참조] 63

Figure 44. Residual 2 of radial component against α for periods (T): 0.2; 0.3; 0.5 s by (a) low, (b) intermediate, and (c) high section based on hr⁴⁰⁰m.[이미지참조] 64

Figure 45. Measured PSA against estimated PSA for various periods (T): (a) 0.01; (b) 0.02; (c) 0.03; (d) 0.05; (e) 0.07; (f) 0.1; (g) 0.2; (h) 0.3; (i) 0.5 s. 65

초록보기

 The 2017 ML 5.4 Pohang, South Korea earthquake, which caused severe damage, raised attention to the importance of the characterization of the Pohang Basin structure and the identification of active faults. In chapter 3 ("VS map for Pohang basin"), dataset of 366 seismograms recorded at 102 densely deployed seismic stations during 18 microearthquakes with ML ranging from -1.11 to 1.70 was utilized. the P-wave seismogram method, which has been validated in various regions using seismograms for moderate earthquakes, was employed to estimate the time-averaged shear-wave velocity from the surface to a depth of 30 m (VS30) in and around the basin. Multichannel analyses of surface waves were performed at the 32 sites to obtain shear-wave velocity (VS) profiles, and an additional 22 existing VS profiles within the study area were acquired. It is observed that velocity seismograms predict VS30 more accurately than acceleration and displacement seismograms do. The estimated VS30 values are in good agreement with the measured VS30 values. Approximately 93.3% of the measured and estimated VS30 values are located between the ± 100% difference lines. A VS30 map for the vicinity of the Pohang Basin was proposed by geospatially interpolating both the measured and estimated VS30 values, which is consistent with the local topography and geology. The proposed map exhibits areas with strong VS30 contrast within the Quaternary and Tertiary sediments, which might be attributed to the existence of faults.

In chapter 4 ("Topographic ground motion amplifications"), an empirical model predicting ground motion amplifications was developed, considering topographic effects, for 5% damped pseudo-spectral accelerations at periods ranging from 0.01 to 0.5 s. A dataset of 498 ground motions recorded at densely deployed 118 temporary stations and one regular seismic station in Pohang, South Korea was used. The proposed model is dependent on magnitude and epicentral distance. In addition to the relative elevation, the angle of station-to-epicenter azimuth relative to slope aspect termed α was newly proposed as an important topography parameter. It is observed that ground motions are de-amplified when relative elevation is lower than -20 m and α angle is smaller than 90° (the slope is facing an epicenter). The ground motions are amplified when relative elevation is higher than 18 m and α angle is smaller than 90°, and de-amplified when relative elevation is lower than -20 m and α angle is smaller than 90°. The ground motions are not affected by α angle when the relative elevation ranges from -20 to 40 m. Finally, a predictive model is proposed as functions of earthquake magnitude, epicentral distance, relative elevation, and α angle.

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