Estimated horizontal acceleration time series for the two-layer model using aftershock 1 as the input wave at GOK1, GOK2, and GOK4 for four different incidence angles and 20°). Estimated spectral acceleration in the horizontal direction for the two-layer model using aftershock 1 as the input wave at GOK1, GOK2, and GOK4 for four different incidence angles. Estimated time series of accelerations in the horizontal direction for the two-layer model using aftershock 2 as the input wave at GOK1, GOK2 and GOK4 for four different incidence angles and 20°).
Estimated horizontal spectral acceleration for the 2-layer model using Aftershock 2 as input wave at GOK1, GOK2 and GOK4 for four different incident angles. Estimated acceleration time series in horizontal direction for the 4-layer model using Aftershock 1 as input wave at GOK1, GOK2 and GOK4 for four different incidence angles and 20°). Estimated horizontal spectral acceleration for the 4-layer model using Aftershock 1 as input wave at GOK1, GOK2 and GOK4 for four different incident angles.
Estimated horizontal acceleration time series for the 4-layer model using aftershock 2 as the input wave at GOK1, GOK2 and GOK4 for four different incidence angles and 20°). Estimated horizontal spectral acceleration for the 4-layer model using AfterShock 2 as input wave at GOK1, GOK2 and GOK4 for four different incident angles. PGA for the estimated acceleration of the two-layer model using aftershock 1 as input in the horizontal direction at location GOK1, GOK2 and GOK4 for four different incident angles and 20°).
PGA for estimated acceleration of a 4-layer model with Aftershock 1 as input in the horizontal direction at a location of GOK1, GOK2 and GOK4 for four different angles of incidence and 20°).
Introduction
Pohang earthquake and study area
Study area and damage pattern
The Gokgang-ri study area, located about 3.5 km east of the main earthquake epicenter as shown in Figure 2 , is a small town with 65 households. With a visual inspection, a series of damages were investigated on all 65 houses in the city. damage was classified into five levels of damage based on the criteria established in Kang et al. (5) damage level 5: completely destroyed or abandoned houses, as shown in Figure 3. On the other hand, there was severe damage such as building collapse in the northern district of Gokgang-ri. the southern district had only minor damage as shown in Figure 4a.
Both districts are at a similar distance from the mainshock and aftershocks and the same geological formation. The two topographic profiles in Figure 4b (i.e. Line-1 and Line-2) in a west-east direction show the two blue lines in Figure 4a where the temporary seismic stations are installed. The mainshock, and two aftershocks, propagated from the west of Gokgang-ri as assumed in Figure 4b.
2019b) conducted Downhole tests at BH-1 and BH-2 near GOK1 and GOK2, respectively, to measure shear velocity (VS) profiles as shown in Figure 5. VS profiles obtained from MASW conducted near GOK1 and GOK2 correspond with those from borehole tests at BH-1 and BH-2, respectively as shown in Figure 5. a) Gokgang-ri damage pattern and location of temporary seismic station (GOK1, GOK2 and GOK4) (b) topographic profiles in the west-east direction according to the three blue lines shown in (a).
Aftershock ground motion records
Locations of epicenters of the mainshock and two aftershocks (circles), as well as the three temporary seismic stations. Acceleration time series of (a) Aftershock 1 (ML 3.5); and (b) Aftershock 2 (ML 2.8) recorded at three temporary seismic stations in east-west (EW), north-south (NS), and up-down (UD) directions. 5% damped spectral acceleration of the ground motions in EW, NS and UD directions recorded at temporary seismic stations (GOK1, GOK2 and GOK4) during Aftershocks 1 and 2.
Numerical analysis
Previous studies
Assimaki and Gazetas (2004) performed 2D seismic site response analysis for the model describing the Kifisos River Gorge and the Adames region. Two methods were used for 2D wave propagation analyses: ABAQUS based on the finite element code and the AHNSE based on spectral element code. The idealized cross-section of the Kifisos River Gorge and the Adames region is shown in Figure 12 as a configuration of a layered structure.
It has homogeneous layer (ie VS1 = VS2) over bedrock (VS3) and has three different soil properties.
Methodology
This includes independent dashpots attached to the boundary in the normal and shear directions (Itasca, 2011). Free field boundaries are used on lateral boundaries to preserve their non-reflective properties and prevent energy leakage. The lateral boundaries are connected to the free-field grid by viscous dashpots to simulate a quiet boundary (see Figure 16), and the unbalanced forces from the free-field grid are applied to the main grid boundary (Itasca, 2011).
The ground motions of Aftershock 1 and Aftershock 2 in the EW direction measured at the temporary station GOK2 were used as input waves of the incident SV (Figure 18,h). The input waveforms were scaled so that the estimated PGAs of Aftershock 1 and Aftershock 2 in GOK2 agree with the measured PGAs.
Topography model
The amplitudes of input movements used in this study are not large and the surface layers are stiff enough. Therefore, we perform linear analyzes with ground damping ratios of 5% and 1%, as shown in Table 2 and Table 3. Free field profiles (without surface topographies) for both Line-1 and Line-2 of the two different models were also simulated and estimated motions of the ground surface of the free field were used as a reference for amplification factors.
Free field boundary and quiet boundary in the x and y directions were applied to the left and right sides and the baseline of the model, respectively. The ground motion of Aftershock 1 measured at the GOK2 temporary station in the EW direction for a duration of 3 s was used as the input SW wave. Linear elastic material properties with a damping ratio of 5% and 1% were used as shown in Table 3.
The acceleration time series and spectral acceleration from both software programs are similar as shown in Figure 15. Layered structure of 1D simulation model. a) Acceleration time series and (b) Spectral acceleration of 1D response results simulated by DEEPSOIL and FLAC 2D. Schematic diagram of (a) critical angle and (b) topographic effect of the wave propagating at a critical angle to the slope surface. a) Dimension of 2D model used in numerical simulation.
Result
PGA amplification factor for the estimated acceleration of the two-layer model using AfterShock 1 as input in the horizontal direction at location GOK1, GOK2 and GOK4 for four different angles of incidence and 20°). Amplification factor Sa at 0.1 s for the estimated acceleration of the two-layer model using aftershock 1 as input in the horizontal direction at location GOK1, GOK2 and GOK4 for four different incidence angles and 20°). Amplification factor Sa at 1 s PGA for the estimated acceleration of the two-layer model using aftershock 1 as input in the horizontal direction at location GOK1, GOK2 and GOK4 for four different incident angles and 20°).
PGA for the estimated acceleration of the 2-layer model using Aftershock 2 as input in a horizontal direction at a location of GOK1, GOK2 and GOK4 for four different incidence angles and 20°). As in 1s for the estimated acceleration of the 2-layer model using Aftershock 2 as input in a horizontal direction at a location of GOK1, GOK2 and GOK4 for four different incidence angles and 20°). PGA amplification factor for the estimated acceleration of the 2-layer model using Aftershock 2 as input in a horizontal direction at a location GOK1, GOK2 and GOK4 for four different.
Amplification factor of Sa at 0.1s for the estimated acceleration of the 2-layer model using Aftershock 2 as input in a horizontal direction at a location of GOK1, GOK2 and GOK4 for four different incidence angles and 20°). Amplification factor of Sa in 1s PGA for the estimated acceleration of the 2-layer model using Aftershock 2 as input in a horizontal direction at a location GOK1, GOK2 and GOK4 for four different incidence angles and 20°). As in 1s for the estimated acceleration of the 4-layer model using Aftershock 1 as input in a horizontal direction at a location of GOK1, GOK2 and GOK4 for four different incidence angles and 20°).
Amplification factor Sa at 0.1 s for the estimated acceleration of the 4-layer model using aftershock 1 as input in the horizontal direction at location GOK1, GOK2 and GOK4 for four different incident angles and 20°). Amplification factor Sa at 1 s PGA for estimated acceleration of 4-layer model using aftershock 1 as input in horizontal direction at location GOK1, GOK2 and GOK4 for four different incident angles and 20°). PGA for the estimated acceleration of the 4-layer model using AfterShock 2 as input wave in horizontal direction at location GOK1, GOK2 and GOK4 for four different incident angles and 20°).
Sa at 0.1s for Estimated acceleration of 4-layer model using Aftershock 2 as input wave in a horizontal direction at a location of GOK1, GOK2 and GOK4 for four different incidence angles and 20°). Sa at 1s for Estimated acceleration of 4-layer model using Aftershock 2 as input wave in a horizontal direction at a location of GOK1, GOK2 and GOK4 for four different incidence angles and 20°). Amplification factor of PGA for Estimated acceleration of 4-layer model using Aftershock 2 as input wave in a horizontal direction at a location of GOK1, GOK2 and GOK4 for four different incidence angles and 20°).
Amplification factor of Sa at 0.1 s for estimated acceleration of a 4-layer model using Aftershock 2 as input wave in horizontal direction at a location of GOK1, GOK2 and GOK4 for four different angles of incidence and 20°). Gain factor of Sa at 1s PGA for estimated acceleration of a 4-layer model using Aftershock 2 as input in horizontal direction at a location of GOK1, GOK2 and GOK4 for four different angles of incidence and 20°).
Discussion
Conclusions
