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com
Seismic attenuation parameters of seismic
stations in the Korean Peninsula: Kappa and
quality factor
Byeong Seok Ahn*, Hyun Jae Yoo, Tae-Seob Kang
Division of Earth Environmental System Science, Pukyong National University, Busan, Republic of Korea
PRESENTED AT:
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INTRODUCTION
Kappa (κ): A high-frequency spectral decay parameter (Anderson and Hough, 1984)
κ=κ0+κR*R, Eq. 1 (Ktenidou et al., 2013)
- κ0: site-specific κ
- κR: distance-dependency of κ
Quality factor (Q): The inelastic attenuation parameter defined by fractional energy loss during wave propagation
Q=1/β•κR, Eq. 2 (Ktenidou et al., 2015)
- β: An average crustal shear-wave velocity
Estimation of κ0 of stations and Q value for the southern part of the Korean Peninsula
Applying estimated κ0 and Q to the ground-motion simulation (EXSIM; Boore, 2003) of the 912 Gyeongju Earthquake.
Y(f)=S(f)*Z(R)*exp(-πfR/Q*β)*A(f)*exp(-πκ0f)
- Y(f): Ground-motion at a site
- S(f): Source term
- Z(R): Geometrical spreading term
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DATA AND METHODS
Data
60 inland events (M ≥ 2.7) occurred in the southern part of the Korean Peninsula from Jan 2020 to Feb 2019
Acceleration data of 155 stations operated by Korea Meterological Administration and Korea Institute of Geology And
Resource
The number of surface staions: 99
The number of borehole stations: 56
Figrure 1. Distribution of stations and events. Red circles denote epicenters of events. Yellow triangles and squares indicate
surface and borehole stations, respectively. The propagation paths between the events and the stations are presented by black
solid lines.
κ0 estimation
1. Using transverse component rotated from horizontal components of acceleration time series
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2. Manual measure a range of linear decay slope (∆f) in natural log acceleration spectrum
3. Calculating the slope using ordinary linear regression (OLS) of natural log amplitude on frequencies and κ (κ=-λ/π)
4. Estimating κ0 and κR of a station using iteratively reweighted linear regression of κ on distance (Eq. 1)
5. Criteria for estimation: ∆f ≥ 10 Hz, κ ≥ 0, the number of κ ≥ 4, κ0 ≥ 0, and κR ≥ 0
Q estimation
1. Estimating κR using OLS of κ values of all stations on distance
2. Calculating Q value from κR using Eq. 2 (β=3.5km/s)
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Κ₀ AND Q ESTIMATION
The variation pattern of estimated κ0 is consistent with the surface geology and tectonic setting of the Korean Peninsula,
though those of CHR and CHC2 show incosistent values.
κ0 and κR estimated from the surface stations appear relatively higher values than those from the borehole stations.
Figure 2. Examples of κ0 estimation for surface station (top) and borehole station (bottom). Open circles denote κ corresponding
to the event. A black line indicates the result of linear regression of κ on hypocentral distance.
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Figure 3. All κ values of all events verse hypocentral distance. Red and blue open circles denote κ for surface and borehole
stations, respectively. Red, blue and black lines indicate results of linear regression of κ on hypocentral distance for datasets of
the surface, the borehole, and all stations, respectively.
Table 1. Estimated κ0, κR, and Q values. The Q values were calculated from estimated κR.
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Figure 4. A distribution map of residuals between κ0 values of station (κ0STN) and representative value (κ0KR) estimated from
this study. The representative value is 0.0163 sec (Table 1). Black solid line denote boundaries of tectonic setting in the Korean
Peninsula (Gyeongsang Basin, GB; Gyeonggi Massif, GM; Hupo bank and basin, HUB; Imjingang Fold Belt, IFB; Okcheon
Fold Belt, OFB; Pyeongnam Basin, PB; Yeonil Basin, YIB; Yeongnam Massif, YM).
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GROUND-MOTION SIMULATION
Comparison of observed and simulated ground-motions of the 912 Gyeongju Earthquake at 74 surface stations and 54
borehole stations.
Using ground-motion corrected by HVSR method (Nakamura, 1989) in case of the surface stations
Applying HVSR method (Nakamura, 1989) to the surface stations to correct site responses
Table 2. Parameters used in the simulation
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Figure 5. Distribution of observed and simulated ground-motion for 912 Gyeongju Earthquake. (Top) The log-scale plot of
ground-motions. Black and blue dots denote the observed ground-motions at the borehole and surface stations, respectively. Red
dots and vertical lines at the red dots represent the mean and the one standard deviation of the simulated ground-motions.
(Middle and bottom) Contour maps of the observed and the simulated ground-motions, respectively. A black star denotes the
epicenter of the 912 Gyeongju Earthquake.
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DISCUSSION
Relatively lower κ0 founded around CHR and CHC2 station in this study were also identified by a previous study (Jo
and Baag, 2007).
Thoese lower κ0 estimations are inconsisent with the surface geology and the tectonic setting of the Korean Peninsula.
Q values estimated in this study are comparable to the results of previous sutdies (Park et al., 2000; Junn et al., 2002; Jo
and Baag, 2007).
Figure 6. The geological map of the souther part of the Korean Peninsula from KIGAM.
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Figure 7. A distribution map of κ0 estimated from Jo and Baag (2007).
Table 3. Estimated κ0, κR, and Q for the southern part of the Korean Peninsula in the previous studies.
The residuals between the observed and the simulated ground-motions seem to depend on the propagation distace.
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The simulations slightly underestimate the observations to 100 km distance and overestimate beyond 100 km (Top of
Figure 5)
However, residuals in some region do not seem to follow the trend with distance.
It might be due to insufficient site response correction and could imply that site response also exist at borehole station
unlike assumtion that borehole station has no amplification of ground-motion.
Figure 8. A distribution map of log residuals between the observed and the simulated ground-motions for the 912 Gyeongju
Earthquake.
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SUMMARY AND CONCLUSION
We esimated κ0 and κR using the accelerograms from 155 stations in the southern part of the Korean Peninsula.
The values of κ0, κR, and Q estimated for the southern part of the Korean Peninsula are 0.0163 s, 1.33•10-4 s/km, and
2152, respectively.
The distribution pattern of κ0 is consistent with the surface geology and the tectonic setting of the Korean Peninsula,
altough some estimations (around CHR and CHC2 stations) show site-specific characteristics.
The simulated ground-motions, applied κ0 and Q estimated in this study, of the 912 Gyeongju Earthquake show
overestimates beyond about 100 km propagation path.
Thoese discrepancies may resulted from other parameters (the geometrical spreading function and the site response
correction, etc.) used in the ground-motion simulation.
In addtion, the site response correction, for the surface stations, by using HVSR method could be incorrect.
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AUTHOR INFORMATION
Mail: 11196abs@gmail.com
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ABSTRACT
Kappa (κ) and quality factor (Q) are parameters describing attenuation of ground motions by characteristics of media in
propagation path; Q indicates anelastic attenuation along whole propagation path, and κ means decay of high-frequency
amplitude due to subsurface geological conditions. These parameters can be used to consider attenuation of ground motions
in the stochastic ground-motion simulation . Accelerograms recorded at seismic stations administered by Korea
Meteorological Administration and Korea Institute of Geoscience and Mineral Resources from 2003 to 2019 were used to
estimate site-specific κ (κ0) and distance-dependency of κ (κR) of each station. Data were corrected for the and trend, and
then the instrumental responses were deconvolved. We applied 5% Hanning taper at ends of them, and transferred to a
spectrum using fast Fourier transform. We measured κ by dividing slope of decay in natural-log amplitude spectrum of an
event at a station into -π. κ0 and κR of the station were estimated by linear regression of all κ values on hypocentral distances.
The stochastic ground-motion simulation was performed using estimated κ0 for MW 5.5 earthquake occurred in Gyeongju in
2016. The simulated ground motions using site-specific κ0 apparently look closer to observed ground motions at the stations
than a case using uniform κ0 value (0.023 sec) in the simulations. Some underestimated results are found in closer
hypocentral distance. It might indicate that a set of path parameters used in the simulation is inappropriate to describe
characteristics of attenuation for the Korean Peninsula. Meanwhile, Q can be estimated from relation between κR and Q. We
checked and verified the attenuation models of Korea using the high-frequency decay parameter (κR).
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REFERENCES
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southern korea. Geosciences Journal, 11(2), 165-174.
Junn, J., Jo, N., & Baag, C. (2002). Stochastic prediction of ground motions in southern korea. Geosciences Journal, 6(3),
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Acknowledgements
This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of
Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20171510101960).
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