Earthquake Observation and Seismotectonics
Data Acquisition and Prospecting Methods
Appendix 1. The recent events recorded at the Youngwol station
Appendix 2. Seismograms recorded at the Youngwol station
Appendix 3. Earthquake Catalogue near Youngwol Area(1404-1998)
We have surveyed geophysical seismic prospecting near the area of Youngwol Dam site. Youngwol, Pyongchang, Chungsun areas are widely distributed formation of Chosun Supergroup which consists of a Great Limestone Group of Cambro-Ordovician Periods. These formations have been thrusted up from NE to SW. The surface geology of dam site consists of sandstone and shale, the upper and lower boundaries of dam site covered with limestone of Chungsun Formation, dolomite and limestone of Yonghung Formation.
We carried out geophysical seismic prospecting in these areas using various seismic sources (hammer, seisgun). From the result, we have found that thickness and velocity of overburden are 7-9 m and 0.61-0.63 km/sec, respectively, and the second layer consists of sandstones with a thickness of 29-40 m with a velocity of 3.19-3.70 km/sec, and the third layer consists of quartz veins or conglomerates with a high velocity(Vp>5.0 km/sec) at a depth of 38-49 m. Our findings are well consistent with the drilling data of Korea Water Resources Corporation Report (KOWACO, 1997). We have also found the time reduction due to faults and/or fracture zones from the travel time curves at the middle part (60-120 m). The fracture zones and/or faults start below 8 m deep at the dam embankment site.
We may infer that recent microearthquakes in the Youngwol area must be associated with tectonic forces corresponding to many faults in this region. We should also take into account the induced seismicity in the Youngwol area because there are many dams like Soyanggang.
Therefore, it is necessary to perform detailed geophysical prospecting for the sake of safety analysis of the Youngwol Dam construction. Furthermore, the seismic activities of Youngwol, Pyongchang, Chungsun areas are high as compared to other areas in the Kangwon province. In order to find clearly fracture zones and underground blind faults, we propose a high resolution seismic tomography.
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