Sun-Cheon Park

Crustal deformation on the Korea peninsula based on GNSS data during 2004–2015

Since the 2011 Tohoku megathrust earthquake had been occurred, there were strong demands to monitor and mitigate the geological hazards such as an earthquake and volcano in South Korea. This study focused on the crustal deformation, which indicates the earths dynamics on or below the surface, for the long periods or by the transient events using the GNSS data and the Bernese software v5.2. The Korea peninsula has moved to the south-east direction with 3.3cm per a year during 2004-2015 when the GNSS data have been available. Also, it was testified that the transient displacement had been appeared on the Korea peninsula by the 2011 Tohoku earthquake. Therefore, we conclude that there needs to accumulate GNSS observations over the longer duration and to periodically analyze the crustal deformation as a phenomenon related to the seismic activity.

Development of the Global Tsunami Prediction System using the Finite Fault Model and the Cyclic Boundary Condition

A global tsunami prediction system was suggested for a distant tsunami using a finite fault model and a cyclic boundary condition. The possibility of the suggested system as a distant tsunami response system was checked by applying it into the case of 2014 Chile tsunami. A comparison between the numerical results(tsunami height and arrival time) with different conditions (boundary condition, governing equation, grid size and fault model) and measured data (DART buoy, tide station) showed the importance of the finite fault model and the cyclic boundary condition.

Did the 12 September 2016 Gyeongju, South Korea earthquake cause surface deformation?

An earthquake with a local magnitude (ML) of 5.8 occurred on 12 September 2016 near Gyeongju, South Korea. This earthquake was the largest event on record in Korea since 1978. A relatively large (ML 5.1) foreshock preceded the main shock by about 50 min, and numerous aftershocks followed. In this study, we performed seismological and geodetic analyses to determine the possibility of the occurrence of surface deformation. Estimated surface deformation using seismological analysis was less than 1 cm, and that observed by geodetic (GNSS and InSAR) data was within the range of error. These results indicate that no surface deformation occurred due to this earthquake. This may have been due to relatively small size of the fault plane (4 km × 4 km), moderate moment magnitude (Mw 5.5) or deep focal depth (15.4 km) of the earthquake.

Numerical Simulations of the 2011 Tohoku, Japan Tsunami Forerunner Observed in Korea using the Bathymetry Effect

The 2011 Tohoku, Japan Tsunami, which occurred on March 11, 2011, reached the Korean Peninsula and was recorded at numerous tide stations. In the records of the north-eastern tide stations, tsunami forerunners were found in only about a few minutes after the earthquake, which was much earlier than the expected arrival time based on a numerical simulation. Murotani et al. (2015) found out that the bathymetry effect is related to the tsunami forerunners observed in Japan and Russia. In this study, the tsunami forerunners observed in Korea were well reproduced by a numerical simulation considering the bathymetry effect. This indicates that it is important to consider the bathymetry effect for a tsunami caused by an earthquake on shallowly dipping fault plane(e.g. 2011 Tohoku, Japan Earthquake). However, since the bathymetry effect requires additional computation time, it is necessary to examine the problems that results from applying the bathymetry effect to the tsunami warning system.

Near-surface Shear-wave Velocities Derived from Microtremors and Teleseismic Data at the Hwacheon Seismic Station

We estimated near-surface shear-wave velocity (v s ) at the Hwacheon seismic station using a geologic log of a well, microtremors recorded during a period of 56 days, and records of three teleseismic events (M w ≥ 6.0). The v s of the 10-m thick soil layer ( = 296 m/s) was determined from horizontal-to-vertical spectral ratios of microtremors recorded at the surface. The average v s ( = 1,309 m/s) from the surface to the 96-m depth of a borehole sensor, was computed using spectral coherence analyses of data recorded by surfaceand borehole-sensors for the three teleseismic events. Using these calculated values of and , the computed bedrock v s is 2,150 m/s and the time-averaged v s to a 30-m depth is 696 m/s. Accordingly the Hwacheon seismic station is regarded as a relatively good site. The deduced near-surface v s can be used for further quantitative evaluation of site amplification and earthquake hazard.

Source parameters of the 2011 Yellow Sea earthquake ( M L 5.3): Different features from earthquakes on the Korean Peninsula

A moderate earthquake of magnitude ML 5.3 occurred in the Yellow Sea on January 12, 2011. We estimated the source parameters and found that the quake was a shallow strike-slip fault event with a moment magnitude of 4.6. The stress drop of this event, 1.2–2.0 MPa, is lower than that of moderate earthquakes inland and at the eastern offshore of the Korean Peninsula, and also that of the typical value for shallow intraplate earthquakes. A stronger event (M ∼ 6) in the southern Yellow Sea in 1984 was previously reported to have a low stress drop. Therefore the low stress drop is probably characteristic of earthquakes in the Yellow Sea region. We found that aftershocks of the 2011 Yellow Sea event, with magnitudes greater than 2, occurred for about 5 days, while similar-sized aftershocks of some inland earthquakes of the Korean Peninsula continued for several hours only. The lower stress drop and greater active aftershocks in the Yellow Sea region might reflect a different tectonic setting from that on the Korean Peninsula.