• Journal of the Earthquake Engineering Society of Korea
• /
• v.13 no.5
• /
• pp.23-29
• /
• 2009

On December 13, 1996, an earthquake with local magnitude (M$_L$) 4.5 occurred in the Yeongwol area of South Korea. The epicenter was 37.2545$^{\circ}$N and 128.7277$^{\circ}$E, which is located inside the Okcheon Fold Belt. The waveform inversion analysis was carried out to estimate source parameters of the event according to the filtering bandwidth of seismic data. Using 0.02$\sim$0.2 Hz filtering bandwidth, focal depth and seismic moment were estimated to be 6 km and 1.3$\times$10$^{16}$ N$\cdot$m, respectively. This seismic moment corresponds to the moment magnitude (M$_W$) 4.7. The focal mechanism by the waveform inversion and P wave first motion polarity analysis is a strike slip faulting including a small thrust component, and the direction of P-axis is ENE-WSW. The moment magnitude estimated by spectral analysis was 4.8, which is similar to that estimated by waveform inversion. Average stress drop was estimated to be 14.3 MPa.

• Journal of the Korean Geophysical Society
• /
• v.1 no.1
• /
• pp.23-30
• /
• 1998

Fault-plane solutions of the December 13, 1996 Yeongweol earthquake with magnitude 4.5 is obtained using the grid test technique. Thirty polarities of P waves recorded at KMA, KIGAM, KSRS and JAPAN stations are used for the event. The obtained fault plane solution shows strike-slip motion with significant amount of thrust component. The orientation of the fault is 180±20° in strike, 50±5° in dip and 150±5° in rake, or 292±3° in strike, 65±5° in dip and 30±10° in rake. These solutions are similar to those of earthquakes occurred at Sagju (Jan. 7, 1980), Pohang (Apr. 15, 1981) and offshore Gunsan (Oct. 6, 1976). The compressional axis of the stress field is trending from ENE to WSW, which is consistent with the previously defined typical regional tectonic stress orientation in and around Korean Peninsula.

• The Journal of Engineering Geology
• /
• v.6 no.3
• /
• pp.165-184
• /
• 1996

The earthquake(M=4.5 by KMA), which occurred in the northeastern part of Yeongweol($37^{\circ}$ 15.75' N, $128^{\circ}$ 42.13' E) on December 13, 1996, shows shallow focal depth(about 8.0km) and relatively large felt area covering most of the southern peninsula. This is the first medium-size earthquake in inland region of the southern peninsula since 1978. It did not trigger foreshock but 13 aftershocks($M{\;}{\geq}{\;}2.5$) for a month. The intensity based on the felt area estimation of about 400 places shows MIMI III-VII in inland region, II on Cheju Island and I on ulreung Island. The isoseismal of MIMI VII shows an elongated circle in the direction of NE-SW and covers some parts of Jungdong-myon, Yeongweol-kun, Sindong-eup and Nam-myun, Jeongseon-kun. There occurred quite strong shaking, numerous cracks on the walls of buildings, falling and movement of slate and tiles on the roofs, falling of tiles from the wall and falling of materials from desks, rock falling from mountain and collapse of gravel layers on the river side. Mainshock and aftershocks occurred condensely between Yemi and Hwaam areas. The felt area due to the Yeongweol earthquake is Quite larger than those of similar size earthquakes in the Korean Peninsula.

• Journal of the Earthquake Engineering Society of Korea
• /
• v.1 no.1
• /
• pp.19-24
• /
• 1997

The seismic of the main shock and two after shocks of the Yeongwol Earthquake are studied based on the phase analysis. The travel time curves and two point ray tracing with 12 different possible phases are used to analyze the phases of the records, which were provided by KIGAM seismic network. The results of phase analysis of the Yeongwol Earthquake show the characteristics as follows 1) The main shock ($M_s$=4.5) clearly shows Pn phase but two after shocks ($M_s$=3.8 and $M_s$=2.5) do not show Pn phase. 2) The Pg PmP looks as first arrival phase in the after shock records whose epicentral distance is smaller or larger than 145 km, 3). It is very difficult to identify the phases in the seismic records, which ae related to the Conrad discontinulty, even if the Conrad discontiulty exists. 4) The record of GRE station located outside of the Kyeongsan Basin shows different arrival time of Pn phase, P-S duration time and frequency comared with those of the other stations located within the Kyeongsan Basin.

• Journal of the Korean Geophysical Society
• /
• v.2 no.4
• /
• pp.235-240
• /
• 1999

From the seismic network data of the Korea Institute of Geology, Mining & Materials during 1995-1996, we derived a composite fault-plane solution of the microearthquakes occurred in the Yangsan fault area. The composite fault-plane solution of nine events shows the orientation of fault 15 ± 3°in strike, 60 ± 8°in dip and 140°in rake or 128 ± 3°in strike, 56 ± 8°in dip and 37°in rake. The compressional axis of the stress field trends ENE to WSW, and this field suggests strike-slip motion with thrust component. The result is consistent with the 1996 Yeong-weol event and the stress field in and around the Korean Peninsula, previously reported.

• Proceedings of the Earthquake Engineering Society of Korea Conference
• /
• 1997.04a
• /
• pp.21-26
• /
• 1997

For 262 locations throughout the southern part of Korean peninsula, intensities of 13 December 1996 Yeongweol earthquake are estimated to make an isoseismal map and investigate attenuation properties in the southern part of Korea. Due to the inherent uncertainties in the estimation of intensities, obtained intensity map show quite scattered pattern of intensity distribution. Estimated intensities range from III to possibly Ⅷ. In case of intensity larger than Ⅵ, considerable damages such as fracturing of walls are reported one of the most significant feature of the intensity map is, considering its magnitude 4.7 reported by KMA, the felt area is appeared to be unusually large covering most of the Korean peninsula except Cheju island. This result indicates ether the magnitude is underestimated or the focus of this earthquake is much deeper. Assuming shallow(less than 10km) intraplate earthquake, we obtained average magnitude 5.6 by using the area encircled by isoseismal contour lines from intensity IV to intensity Ⅶ. This ambiguity can be clarified if more reliable focal depth is estimated by using teleseismic earthquake records in the future.

• Journal of the Earthquake Engineering Society of Korea
• /
• v.1 no.1
• /
• pp.69-77
• /
• 1997

An intensity survey on the 13 December 1996 Yeogweol earthquake has mode for 262 locations throughout southern part of Korean peninsula, then we investigated attenuation properties in the south Korean region as well as intensities distribution. In this study, intensities are estimated to be from II to possibly VIII. The iso-seismal intensity map we obtained shows general pattern of intensity distribution in the south korean region quite clearly despite the inherent uncertainties included in the process of intensity estimation. In case of intensity larger than VI, considerable damages such as fracturing walls are frequently reported. One of the significant feature of this intensity map is, considering its magnitude 4.5 reported by KMA, the felt area is unusually large covering most of the Korean Peninsular except Cheju island. This result indicates either the magnitude is under estimated or the focal depth is much deeper than expected. Assuming indicates either the magnitude is under estimated or the focal depth is much deeper than expected. Assuming shallow earthquake whose focal depth is by iso-seismal contour lines for intensity IV to VII, respectively. To resolve this ambiguity, more reliable estimation of focal depth and magnitude by using telesesmic instrumental records should be made in the future.

• Proceedings of the Earthquake Engineering Society of Korea Conference
• /
• 1997.04a
• /
• pp.15-20
• /
• 1997

Source parameters of the December 13, 1996 Yeongweol earthquake are estimated using the grid test technique. Thirty polarities of P waves recorded at KMA, KIGAM, KSRS and JAPAN stations are used for the event. The obtained fault plane solution shows predominantly strike-slip motion with small amount of thrust component. The orientation of the fault is 180$\pm$10$^{\circ}$in strike, 50$\pm$5$^{\circ}$in dip and 150$\pm$5$^{\circ}$in rake, or 292$\pm$3$^{\circ}$in strike, 65$\pm$5$^{\circ}$in dip and 30$\pm$10$^{\circ}$ in rake. These solutions are very similar to those of earthquakes occurred at Sagju, Pohang and offshore Gunsan. The compressional axis of stress field is trending from ENE to WSW, which is consistent with the previously defined typical regional tectonic stress orientation in and around Korean Peninsula.. From the result of this study and other source mechanisms around the Korean Peninsula, we are of opinion that tectonic stress around the Korean Peninsula may be more attributed to the collision of Indian plate with the Eurasian plate than subduction of Pacific and Philippine plates.

• Proceedings of the Earthquake Engineering Society of Korea Conference
• /
• 1997.04a
• /
• pp.77-87
• /
• 1997

The earthquake data(M$\geq$4.0) for post-1900 in the Korean Peninsula show temporal variation with active and quiet periods. The pattern is quite similar to northeastern China and Inner Zone of Southwest Japan. Yeongweol earthquake occurred in the seismic gap region of the Korean Peninsula. This is the first medium-size earthquake in inland region of the southern peninsula since 1978. The intensity based on the felt area estimation of about 400 places shows MMI III-Ⅷ in inland region. IIon Cheju Island and Ion Ulreung Island. The isoseismal of MMI Ⅶ shows an elongated circle in the direction of NE-SW and covers some parts of Jungdong-myon, Yeongweol-kun, Sindong-eup and Nam-myun, Jeongseon-kun. There occurred quite strong shaking, numerous cracks on the walls of buildings, falling and movement of slate and tiles on the roofs, falling of tiles from the wall and falling of materials from desks, rook falling from mountain and collapse of gravel lauers on the river side. The least square fitting of the intensity data of the Yeongweol earthquake by a popular intensity attenuation relation yields the following : I=Io＋1.82249 - 0.65295*InR - 0.00707*R