• The Journal of Engineering Geology
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• v.19 no.4
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• pp.509-515
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• 2009

Three earthquakes with local magnitude ($M_L$) greater than 3.0 occurred on April 24, June 2 and September 12 in 1999 nearby the Gyeongju area. Redetermined epicenters were located within the radius of 1 km. We carried out waveform inversion analysis to estimate focal mechanism of June 2 event, and P and S wave polarity and their amplitude ratio analysis to estimate focal mechanisms of April 24 and September 12 events. June 2 and September 12 events had similar fault plane solutions each other. The fault plane solution of April 24 event included those of other 2 events, but its distribution range was relatively broad. Focal mechanisms of those events had a strike slip faulting with a small normal component. P-axes of those events were ENE-WSW which were similar to previous studies on the P-axis of the Korean Peninsula. Considering distances between epicenters, similarities of seismic waves and sameness of polarities of seismic data recorded at common seismic stations, these events might occurred at the same fault. The seismic moment of June 2 event was estimated to be $3.9\;{\times}\;10^{14}\;N{\cdot}m$ and this value corresponded to the moment magnitude ($M_W$) 3.7. The moment magnitude estimated by spectral analysis was 3.8, which was similar to that estimated by waveform inversion analysis. The average stress drop was estimated to be 7.5 MPa. Moment magnitudes of April 24 and September 12 events were estimated to be 3.2 and 3.4 by comparing the spectrum of those events recorded at common single seismic station.

• Geophysics and Geophysical Exploration
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• v.13 no.2
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• pp.137-143
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• 2010

An earthquake with local magnitude $(M_L)$ 3.7 on December 9, 2000 occurred offshore Yeongdeok area, South Korea. In case of applying Chang and Baag (2006) crustal velocity model, the epicenter is $36.4462^{\circ}N\;and\;129.9789^{\circ}E$, which belongs to the inside of the Korean Peninsula Continental Shelf. Although we use the modified model reducing crustal thickness of Chang and Baag (2006) model by 5 km considering the transition from continental crust to oceanic crust in the East Sea, the epicenter was little changed. We carried out the waveform inversion analysis to estimate focal depth and focal mechanism of this event. The focal depth is estimated to be 11 ~ 12 km. The seismic moment is estimated to be $1.0{\times}10^{15}N{\cdot}m$, and this value corresponds to the moment magnitude $(M_W)$ 3.9. The offshore Yeongdeok event including May 29, 2004 offshore Uljin one show typical thrust faulting, and the direction of P-axis is ESE-WNW. The moment magnitude estimated by the spectral analysis is 4.0, which is similar to that by the waveform inversion analysis. Average stress drop is estimated to be 3.4 MPa.