Kiehwa Lee

Historical Seismicity of Korea

The Korean peninsula lies in the Eurasian plate, neighboring seismically active China and the Japanese Islands. Instrumental observation of earthquakes began in Korea in 1905, and about one thousand events of mostly small magnitude, less than 4.0, have been detected to date in and near the peninsula. Strain release due to these earthquakes is almost negligible compared to that due to about 2186 historical earthquakes that occurred from 2 to 1904 a.d. in the peninsula. In this study, these historical earthquakes are cataloged from Korean historical documents, and their epicenters and intensities are estimated as best as possible. Temporal variations of the historical seismicity of Korea clearly reveal a very irregular strain release pattern over the nineteen centuries, with low to moderate seismicity most of the time except for the unusually high seismicity from the fifteenth to the eighteenth centuries. This highly irregular strain release pattern clearly indicates the characteristics of intraplate seismicity of the peninsula. Epicenters of large earthquakes appear to be well associated with the major faults and the boundaries of major geological units in the peninsula. It is to be noted that the major faults associated with large historical earthquakes were created by the Mesozoic tectonic activities that severely disrupted crustal layers of the peninsula. Historical earthquakes occurred all over the peninsula; however, the northeastern part appears relatively less seismic, probably due to the area being less disrupted during the Mesozoic. The b value in the magnitude– frequency relation determined from historical Korean earthquakes is 0.71, which is between those of the Japanese Islands and Eastern China and is lower than in many stable intraplate regions. Online material: Catalog of historical seismicity of Korea 2–1904 a.d.

A Study of High-Frequency Q (super -1) Lg in the Crust of South Korea

Knowledge of the value of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(Q_{Lg}^{-1}\) \end{document} is very important not only for scientific research but also for practical field research. We first measured the \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(Q_{Lg}^{-1}\) \end{document} of South Korea by applying the coda normalization method to long-period recorded seismograms at four stations deployed by the Korea Institute of Geoscience and Mineral Resources. We analyzed 345 seismograms of 50 earthquakes that occurred between January 1999 to May 2002. The hypocentral distances range between 155 and 400 km. The regression of the plots of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(Q_{Lg}^{-1}\) \end{document} shows frequency dependence, which decreases from (1.4 ± 1.0) × 10–3 at 1.5 Hz to (0.3 ± 0.1) × 10–3 at 24 Hz. If we fit a power law depending on frequency, the best-fit line for \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(Q_{Lg}^{-1}\) \end{document} is 0.0018 f –0.54. This value generally agrees with those of seismically inactive regions.

Crustal in South Korea Using the Source Pair/Receiver Pair Method

![Graphic][1] has been studied extensively because its regional variations reflect geological structure: values around 1 Hz show close correlation with tectonic seismicity. We analyze ![Graphic][2] in South Korea by applying the source pair/receiver pair (sprp) method to 39 Korean earthquakes and 32 far-regional earthquakes. We obtain nearly 1500 source-receiver pairs with the combined difference of epicentral distance, D , ranging from 16 to 657 km. Seismograms are filtered by using an eight-pole bandpass Butterworth filter with five-center frequencies. 0.375, 0.75, 1.5, 3, and 6 Hz. The obtained ![Graphic][3] for all frequencies are less than 0.002, which is a reasonable value for a seismically inactive region. In spite of the short range of D , the results of the sprp method show reliable values, especially at 1.5 and 3 Hz. [1]: /embed/inline-graphic-2.gif [2]: /embed/inline-graphic-3.gif [3]: /embed/inline-graphic-4.gif

Intensity Attenuation in the Sino-Korean Craton

Intensity attenuation formulas are proposed for strong earthquakes of modified Mercalli intensity (MMI) VIII-X in the Sino-Korean craton. Isoseismals of 11 earthquakes that occurred in the craton during the twentieth century are used in the analysis. New attenuation formulas are expressed as \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[I\_{0}-I=\frac{1}{\mathrm{ln}{\phi}}\mathrm{ln}\left[1+\frac{{\phi}-1}{{\phi}\_{0}}\left(\frac{D}{D_{0}}-1\right)\right]\] \end{document} where I and I are the epicentral intensity and intensity at an epicentral distance D; φ and φ are constants having some geometrical implications of the attenuation pattern; and D is the reference value for the isoseismal I . Constants φ and φ have values of 1.31 and 1.77, respectively. Values of D for I = X, IX, and VIII are 9.3, 6.5, and 5.2, respectively. New attenuation relations fit the observations better than the conventional relations such as \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[I\_{0}-I=a\_{1}+b\_{1}{\ }\mathrm{ln}D+c\_{1}D\] \end{document} \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[\mathrm{ln}I\_{0}-\mathrm{ln}I=a\_{2}+b\_{2}{\ }\mathrm{ln}D+c\_{2}D.\] \end{document} It appears that the new formulas represent intensity attenuation of Korean earthquakes well to the limits of felt areas. New attenuation formulas using the relation between the total felt area and I may be used in the estimation of epicentral intensities of strong Korean historical earthquakes for which only felt places are provided in documents without information on earthquake damages or phenomena. The formulas may also be used in more realistic evaluation of the seismic hazard of the Korean peninsula.

Spatial Distribution of Coda Q in South Korea

Analyzing spectral attenuation of coda waves, we first mapped coda Q- values in the crust over the whole of South Korea. During the period from 1995 to 2004, we selected 574 north-south-component seismograms with epicentral distances less than 100 km from 328 earthquakes with magnitudes between 1.4 and 5.2. We estimated coda Q-values using the single isotropic scattering model at center fre- quencies of 1.5, 3, 6, 9, 12, 15, and 18 Hz, and found significant spatial variation over all frequency ranges and strong frequency dependence of coda Q in the region. The estimated coda Q-value at 1 Hz (Q0) and the g-value ranges are 80-300 and 0.4-1.1, respectively. The values have strong correlation with the regional geology in the Korean peninsula. Low Q0-values are mainly obtained in the regions com- prising sedimentary strata in southeastern South Korea, whereas granite regions in the northern part of South Korea show high Q 0 . The Q 0 -values in the study area agree well with those of the eastern China and Kyushu, western Japan. Furthermore, our g-values are also in good agreement with those of Japan. Online material: Locations of seismometers, list of event-station pairs, and linear regression of QC measurements.

Response of a multilayered earth with layers having exponentially varying resistivities

The resistivity kernel function for calculating the apparent resistivity of a multilayered earth with layers that have exponentially varying resistivities is derived using a recurrence relation. This relation is applicable to general cases in which layers have either constant or exponentially varying resistivities. A FORTRAN subroutine that computes the apparent resistivity of Schlumberger, Wenner, and pole-to-pole array soundings for layers of exponentially varying resistivities is presented. Apparent resistivities of some models computed by the new kernel function are compared with those of the same models approximated by a number of constant resistivity layers. Apparent resistivities computed by both methods show excellent agreement.

Earthquakes in Korea from 1905 to 1945

From 1905 to 1945, earthquakes occurring in and in the vicinity of the Korean peninsula were monitored by the Meteorological Observatory of the Government General of Tyosen with one to six seismological stations. Seismological data of 323 earthquakes of the Meteorological Observatory during the period are analyzed in this study. Epicenters of earthquakes are estimated by reports in bulletins of the Meterological Observatory. Epicenters and magnitudes of events that occurred from 1926 to 1943 were determined previously based on the crustal structures of the peninsula and Japanese islands and Tsuboi9s formula, respectively (Lee and Jung, 1980; Kim, 1980). The b -value determined from these events turned out to be about 0.8, which is between those values for earthquakes of the Japanese islands and China. The epicenter distribution of earthquakes during the period indicates rather diffuse seismicity over the peninsula not well related to major faults and much lower seismicity in the northeastern part of the peninsula compared with other regions. Previous studies of historical seismicity of the Korean peninsula showed that epicenters of destructive large earthquakes are well associated with major faults in the peninsula formed during Mesozoic tectonic activities (Lee, 1987) and lower seismicity in the northeastern part of the peninsula. The seismic-wave energy release during the period was rather steady, except for the period 1936–1938, during which the 1936 M 5.1 Ssanggyesa earthquake, the largest event during the period, occurred in the southern part of the peninsula. The source mechanism of the Ssanggyesa earthquake indicates a thrust fault with a considerable strike-slip component (Shimazaki, 1984); one of the nodal planes trending north-northeast coincides with the strikes of adjacent faults. Reliable source mechanisms of earthquakes in the vicinity of the Korean peninsula indicate that the Korean peninsula is subject to compressive stresses parallel to the great circles from the Himalayas to the Japan trench. It appears that the earthquakes in the Korean peninsula occur along the faults formed during the Mesozoic by stresses transmitting from plate boundaries between the Eurasian plate and the Indian plate along the Himalayas in the southeast and the Pacific and Philippine plates along the Japan and Ryuku trenches in the east, respectively. The source mechanisms of large earthquakes in the peninsula are likely to be thrust faults with considerable strike-slip components in the trends of faults.

Paleomagnetic investigation of seamounts in the vicinity of Ogasawara Fracture Zone northwest of the Marshall Islands, western Pacific

Nine seamounts located northwest of the Marshall Islands near the Ogasawara Fracture Zone were inverted for their uniform magnetization using total field magnetic anomaly and detailed bathymetric data. The paleomagnetic poles of most of the seamounts in our study area generally cluster around the Pacific Apparent Polar Wander Path (APWP). However, those that deviate significantly from the APWP are located south of the fracture zone. The seamounts in our study area can also be divided into two groups on the basis of complexity of the observed magnetic field anomaly. In general, simple conical seamounts exhibit a dipole-like field anomaly pattern with a paired anomaly low and high, and can be explained to a large extent by a uniformly magnetized source. On the other hand, those with complex morphology are larger in size, show multiple magnetic lows and highs and lie very close to or within the fracture zones, suggesting that they were formed by multi-stage volcanism.

Geophysical investigation of seamounts near the Ogasawara fracture zone, western Pacific

This paper provides an analysis of multi-channel seismic data obtained during 2000–2001 on seamounts near the Ogasawara Fracture Zone (OFZ) northwest of the Marshall Islands in the western Pacific. The OFZ is unique in that it is a wide rift zone that includes many seamounts. Seven units are delineated on the basis of acoustic characteristics and depth: three units (I, II, and III) on the summit of seamounts and four units (IV, V, VI, and VII) in basins. Acoustic characteristics of layers on the summit of guyots and dredged samples indicate that the seamounts had been built above sea level by volcanism. This was followed by reef growth along the summit margin, which enabled deposition of shallow-water carbonates on the summit, and finally by subsidence of the edifices. The subsidence depth of the seamounts, estimated from the lower boundary of unit II, ranges between 1,550 and 2,040 m. The thick unit I of the southern seamounts is correlated with proximity to the equatorial high productivity zone, whereas local currents may have strongly affected the distribution of unit I on northern seamounts. A seismic profile in the basin around the Ita Mai Tai and OSM4 seamounts shows an unconformity between units IV and V, which is widespread from the East Mariana Basin to the Pigafetta Basin.

Geoelectric study on the probable seismic segment boundaries of Yangsan fault zone

Geoelectric surveys are made in areas of probable seismic segment boundaries along the Yangsan fault zone (between 36.2°N and 35.5°N) in order to investigate resistivity structure. These segment boundaries are proposed on the basis of reduced seismicity recorded in historical earthquake data. Wenner horizontal profilings reveal resistivity of about 300 ohm-m with electrode separation of 30 m, which compares with that of 100 ohm-m in other parts of the Yangsan fault zone. Schlumberger vertical soundings indicate that the fault fracture zone diminishes in depth from about 100 m (north) to 20 m (south). Previous vertical soundings in other parts of the Yangsan fault zone show that the fault fracture zone extends to a depth greater than 100 m. Electrical profilings and soundings appear to indicate poor development of fault fractures in the study area. Geoelectrical surveys support the existence of boundaries between the northern and the central segments as well as between the central and the southern seismic segments in the Yangsan fault zone.