Basic analysis of measurement records
Seismic motion waves were measured in three directions:
E
,
N
and
Z
(x, y, and z-axis). Figures?
3
a and b show the wave time-series for the horizontal component of 9.12 EQ#2 and PH EQ#1 recorded at the PHT station, respectively. The peak ground acceleration level can be examined on seismic motion records for each EQ. The 9.12 EQ originated 15?km beneath the surface. It is a deeper than the average depth of EQs with a magnitude 5.0 or greater in Korea, which is 8.16?km. The strong motion duration was also measured for 1 to 2?s in the form of an impact wave in which energy was concentrated
34
. In the case of PH EQ#1, which was relatively close to the epicenter, the vector summation of three components measured at the PHF station reached maximum of 454.839 gal, as summarized in Table
4
. In addition, the strong motion duration was quite short, as shown in Fig.?
3
b.
Table 4 Peak ground acceleration according to the station.
Figures?
4
and
5
show the seismic motion data in the frequency domain through Fourier spectrum analysis for all EQs used in the analysis at the target station, PHT. The vertical axis represents the Fourier amplitude on acceleration values, and the unit corresponds to [gal·s]. Figures?
4
a and
5
a show the
E
-component in a horizontal direction for each seismic motion, Figs.?
4
b and
5
b are the
N
-component in the horizontal direction, and Figs.?
4
c and
5
c show are the
Z
-component in the vertical direction in the FFT result. The 9.12 EQ#2 had a large high-frequency energy, and, for the PH EQ#1, relatively low-frequency energy predominated. In particular, in the band below1.0?Hz, the amplitude of the horizontal components (
E
,
N
) of PH EQ#1 was approximately twice as large as that of the 9.12 EQ#2. In addition, for 9.12 EQ#2 and PH EQ#1, the peak frequency band of the
Z
-component was different from that of the
E
- and
N
-components. The high-frequency (short-period) component of the
Z
-component was predominant in the PH EQ#1 compared to the 9.12 EQ#2.
Figures?
6
and
7
show the response spectra of the measured data from seismic accelerometers at the PHT station. Here, a damping factor was applied as 5%. Figures?
6
a, b,
7
a, and b graph the result of horizontal components. Figures?
7
c and
8
c indicate the vertical component. The result of the response spectrum of the 9.12 EQs is shown in Fig.?
6
. The period components were predominant in range of 0.07?0.7?s. In the case shown in Fig.?
7
for the PH EQs, the peak periods were also diversely distributed as 0.06?0.7?s for each component (
E, N, Z
). In particular, for the vertical direction of the PH EQs, shown in Fig.?
7
c, the acceleration had larger amplitude than that of the 9.12 EQs, as shown in Fig.?
6
c. Using the analyzed results, it can evaluate the structural condition compared with the natural frequency characteristics of the structure applied to the seismic design. For reference, the natural frequency bands of the gravity-type caisson and the pier-type quay wall, which are representative port structures, are 1.29?1.63?Hz (0.61?0.78?s) and 0.66?3.73?Hz (0.27?1.52?s), respectively
35
.
Site-specific response of Pohang Yeongil New Port based on seismic records
The local site effect may cause amplification of seismic waves induced by EQs and plays an important role in site-specific ground motion predictions and seismic hazard assessments. To evaluate dynamic response characteristics considering structure-soil Interaction, seismic sources, and an attenuation in crusts, the site effect should be considered when deliberating how to do it. In this study, the three calculating methods for the site-specific response to determine the site effect were selected as follows
31
: (1) the first method (#1:
\({\varvec{H}}_{{\varvec{t}}} /{\varvec{V}}_{{\varvec{r}}}\)
), using horizontal components of the PHT (the target station) to the vertical component of the PHF (the reference station), (2) the second method (#2:
\({\varvec{V}}_{{\varvec{t}}} /{\varvec{V}}_{{\varvec{r}}}\)
), using only vertical components of the target station and the reference station, and (3) the third method (#3:
\(({\varvec{H}}_{{\varvec{t}}} /{\varvec{V}}_{{\varvec{t}}} )/({\varvec{H}}_{{\varvec{r}}} /{\varvec{V}}_{{\varvec{r}}}\)
), using the H/V ratio of the target station to the H/V ratio of the reference station. Figures?
8
and
9
show the results of the H/V ratio at the Pohang International Container Terminal in Pohang Yeongil New Port for the S-wave phase of the 9.12 and the PH EQs when these methods were used. First, the amplification ratio of the site had a similar trend according to the all analyzed EQs and calculation methods. In addition, the epicenters of PH EQs were relatively close to the measuring stations as shown in Tables
2
and
3
, so a larger H/V ratio could be obtained than that of 9.12 EQs because to the hypocentral parameter.
The peak frequency (
\(f_{p} )\)
refers to the frequency band with a maximum value between 1 and 20?Hz in the H/V ratio graph, and it is also interpreted as the resonance frequency. In this study, the
\(f_{p}\)
of the PHT is as follows. The
\(f_{p}\)
obtained by Method 1 in Figs.?
8
a and
9
a was in the range of 6?12?Hz (0.083?0.167?s), and the
\(f_{p}\)
obtained by Method 2 ranged from 7 to 18?Hz (0.056?0.143?s), as shown in Figs.?
8
b and
9
b. For Method 3 in Figs.?
8
c and
9
c, it was in the range of 5?8?Hz (0.125?0.200?s). In the deep or soft soil, the peak frequency is generally observed in the lower frequency band. Therefore, the average peak frequency and natural period of the ground at PHT can be predicted as about 0.9?Hz and 0.11?s respectively. This is a result similar to that estimated the natural period for the inland areas of Korea as the 0.5?s range or less
14
.
There are two methods for time histories of a design ground motion to evaluate the seismic design and performance: a method using actual measurement records or an artificially synthesizing method compatible to the design response spectra. Recently, there has been a trend to use actual measurement records as the seismic acceleration time history to consider the impact on the site environment fully. However, in the case of Korean records, there are problems in that the intensity of the ground motion does not meet the design purpose or that the response spectra of the record are different form the shape of the design response spectra. Thus, to maintain the seismic wave characteristics and site characteristics and to examine appropriately the intensity and shape required for the seismic design and response spectra, it was compared with recorded data of the time history of the 9.12 EQs and the PH EQs. Here, the seismic performance objectives of structures in the target station were defined according to the domestic seismic design standards
36
as a combination of a design ground motion with an average return period of 500?years of seismic Zone I and a seismic performance level requiring collapse prevention of a Grade II. According to the drilling results conducted by the Pohang Regional Office of Oceans and Fisheries
37
, in the site around PHT, the soil stratification was distributed in the order of a reclamation layer, sediment layer, weathering ground, and bedrock. The average shear wave velocity (
\(V_{s} )\)
, according to the suspension P-S logging test was, exceeded 260?m/s. Finally, the soil type can be classified with S2 as shallow and hard soil.
Based on this, the acceleration standard design response spectrum was obtained using the site coefficient according to the effective horizontal peak ground acceleration (0.105?g) on the bedrock of Pohang Yeongil New Port. In Fig.?
10
, the standard design response spectrum and the acceleration response spectra are compared using the measured data in Figs.?
6
and
7
. Here, the measurement data are the result of analyzing the acceleration response spectrum of the vertical and the horizontal components with 5% damping ratio. In the case of the 9.12 EQs, as shown in Fig.?
10
a and c, the horizontal and vertical spectra showed responses within the design response spectrum. However, in case of PH EQ#1, which was measured very close to the epicenter, the seismic motion had a level exceeding 1.3 times the design response spectrum of the site as shown in Fig.?
10
b and d. The response was clearly noticeable, ranging from 0.1 to 0.5?s, which can affect the port structure. Compared with the 9.12 EQs, the response to the vertical component was more pronounced in the very short period (high frequency) of 0.01?s, as shown in Fig.?
10
d. For the PH EQs identified as injection-induced EQs
38
, the vibration characteristics were different from those of the 9.12 EQs, which were natural EQs. As Fig.?
3
b shows, casualties were not heavy because of the extremely short strong motion duration. This can be confirmed in the H/V Fourier spectra ratio calculated by Method 2?see Fig.?
9
b?and the
Z
-component response spectra in Fig.?
7
c.
For the seismic wave from the El-centro Observatory on the 1940 Imperial Valley EQ, which is typically used for seismic design and response analysis, the strong motion duration was approximately 24.44?s, whereas the EQs used in this study had the form of shock waves with a short strong motion duration. It can be observed that the seismic wave characteristics recorded for ground motion are different and not compatible with the design response spectrum. Therefore, to generate the ground motion using these measuring records, it should be converted as a virtual ground motion appropriately according to synthesis conditions. In order to adequately model the standard deviation of the surface response spectra, at least 10 motions (and preferably 20) are required to obtain stable average spectral results
39
.
In the recent study of the response spectrum based on acceleration measurement data, it was found that the natural frequency, epicenter distance, and soil type of the site strongly influenced on the characteristics of the response spectrum, and the fault movement type of the seismic source, magnitude of the EQ, and depth of the sedimentary layer were less affected
40
. In addition, the engineering characteristics of vertical ground motion had emphasized in the seismic analysis and design
41
.
Figure?
11
shows the H/V response spectral ratios of the site using only vertical components corresponding to Method 2, mentioned previously. Here, the H/V response spectral ratios were based on the method proposed in
42
using an acceleration response spectrum with a damping ratio of 5%. The vertical axis represents the amplification ratio of the normalized response spectrum with respect to the maximum acceleration. The measured acceleration values of the seismic ground motion were calculated using geometric mean values. In the case of the 9.12 EQs, the response spectral ratio occurred up to three times in the short period range at approximately 0.1?0.3?s, and in the case of the PH EQs, it also occurred up to seven times in a short period of approximately 0.06?0.2?s. It was similar to the peak period derived from the H/V Fourier spectra ratio in Figs.?
8
b and
9
b of 0.056?0.111?s.
For reference, background noise sources in the sub-1.0-s band include natural vibration sources, such as wind and waves, and artificial vibration sources, such as transportation and factories. In the period band of 1.0?10.0?s (0.1?1.0?Hz), the main source is a microseism such as the wave activity on the coast and tides
43
. Therefore, the measured data of the seismic accelerometers located in the port area may have been reflected in greater amplification.