Introduction

The first geological map of Korea, with a scale of 1:1,000,000, was published in 1928. In 1974, geological maps with a scale of 1:250,000 covering the whole of the Republic were published. The Geological Survey of Korea started to publish geological quadrangles with a scale of 1:50,000 from 1961, and by 1996, 83% of the landmass was covered. The Geological Society of Korea was established in 1964, the Korea Institute of Mining Geology in 1968, and the Paleotological Society of Korea in 1984. The Geological Survey of Korea was renamed the Korea Institute of Geology, Mining and Materials (KIGAM) in 1991.

Geological Overview

Physiographically, Korea is a mountainous peninsula extending south-southeast from the northeastern part of the China mainland. The north-northwest, south-southeast trend forms the Taebaeksan Range, which is close to the east coast. The east coast is of an uplifted topography, showing a relatively straight shoreline, whereas the west coast shows the features of a submerging shoreline. The mountains are not high, rarely exceeding 1,200 meters, but they are found almost everywhere. As a consequence, the terrain is rugged and steep. Only near the west and southwest coasts are there extensive flat alluvial or deluvial plains and more subdued rolling hilly lands.

Being a mountainous peninsula, Korea is of a diverse geologic make-up. It is composed largely of Precambrian rocks, such as granite gneisses and other metamorphic rocks. Two separate blocks of Paleozoic Strata are found in South and North Korea. The one in the South covers the Taebaeksan Range, and the one in the North is near Pyeongyang. Mesozoic Strata are found in the southeastern part of the peninsula and Cenozoic Strata are limited to some small areas scattered around the peninsula. Jurassic and Cretaceous granites intrude through the older rocks in a northeastward-southwestward direction in some places, but show no specific trend in others.

Unlike nearby Japan, Korea is a stable landmass with no active volcanoes and rare earthquake shocks, although the islands of Ulleungdo and Jejudo are of volcanic origin. Mt. Baekdusan in the North is capped with a caldera lake, and Mt. Hallasan on Jejudo island has a small crater lake.

For more details about Korea's geological make-up, please refer to the appendix II located at the end of this chapter.

Earthquakes

Nearly 1,800 earthquakes are recorded in various historical documents of past dynasties from A.D. 2 to 1907 and more than 200 have been scientifically recorded since 1905 on the Korean Peninsula. Thus, the total number of recorded earthquakes in Korea is a little more than 2,000 of which only 48 were destructive - far fewer than those recorded in Japan but more than in Manchuria. Frequency and intensity of earthquakes in these areas are directly related to their proximity to the Circum-Pacific Earthquake Belt. Japan is located on the belt, while Korea and Manchuria are located some distance from it. Thus, seismicity in Korea is much stronger than in Manchuria, but much weaker than in Japan.

 In Korea, earthquakes occur mainly on faults or tectonic plates, which, viewed on the surface, are river courses. However, during last century, earthquakes had occurred frequently along mountain ranges such as Mt. Jirisan. South Korea is a comparatively stronger seismic area than North Korea, and the west half of the Korean Peninsula has shown stronger seismicity than the eastern half.

The downstream basins of each of the main rivers flowing into the Yellow Sea show a much denser distribution of earthquake epicenters than the middle and upper stream basins. However, both the Geumgang and Nakdonggang rivers show a higher seismicity in both the lower and middle stream basins. The southwestern edge of the Gyeonggi Massif, located in the central part of the Korean Peninsula, or the northern margin of South Korea, has shown much stronger seismicity than the inland and northeastern coastal areas. Almost the entire area of the Okcheon Zone has shown higher seismicity.

The northeastern part of the Yeongdong-Gwangju, however, has shown a relatively higher seismicity than the southwestern region. The southwestern region of the Taebaeksan Block of the Yeongnam Massif has shown a higher seismicity than the northeastern region, which is located in the eastern coastal area. The northeastern edge of the Jirisan Block of the Yeongnam Massif, a southwestern continuity of the Taebaeksan block, and the central zone of the southern half along which Mt. Jirisan runs, have shown relatively higher seismicity than other areas. The entire area of the Gyeongsang Basin, occupying the southeastern edge of the Korean Peninsula, has shown high seismicity, and particulary both the eastern and western edges have historically shown an intensive seismicity with the southwestern edge of the Gyeonggi Massif. The Chugaryeong Graben running through the central zone of the Gyeonggi Massif in the north-northeast to south-southwest directions, has shown a higher seismicity in both past and present times.

Tectonic and Geological Provinces

The Korean Peninsula lies within the Korea-China Heterogen. Generally speaking, the Precambrian basement of the peninsula is tectonically related to that of Manchuria and China. The Pyeongbuk-Gaema Massif forms the southern part of Liao-Gaema Massif of southern Manchuria, and the Gyeonggi and Mt. Sobaeksan massifs of the peninsula can be correlated with the Shandong and Fujian Massifs of China.

The Paleozoic sediments, lying on the Precambrian Massifs in the Pyeongan and Okcheon basins, can be correlated with those in the Yellow River and Yangtze basins. On the other hand, the Mesozoic rocks of the Gyeongsang Basin in the southeastern part of the peninsula can be said to extend toward the Kwanmon Basin in the southwestern tip of Japan across the Korea Strait. The Pohang Tertiary sedimentary rocks, distributed mainly in the southeastern corner of the land, lie on Mesozoic rocks.

Precambrian Geology

The Precambrian geology of Korea is yet to be fully explored. The 1:250,000 scale geological maps published in 1974 offer a more detailed Precambrian stratigraphy of South Korea.

In general, the Gyeonggi and Yeongnam Massifs belong to a low-pressure facies series, and the Okcheon Metamorphic Belt to an intermediate pressure series.

Yeoncheon System

This system, belonging to the Gyeonggi Massif, is distributed around Seoul extending to Yeoncheon-gun in a northeasterly direction. The system is divided into upper and lower parts. The lower part is composed of biotite-quartz-feldspar schist, marble, lime-silicate, quartzite, and graphite schist, and the upper part is of mica-quartz-feldspar schist, mica schist, quartzite, augen gneiss, and garnet-bearing granitic gneiss.

Precambrian Granite Gneiss

This gneiss is also known as grey granite gneiss or Goguryeo granite. Outcrops of it cover one-third of the surface of Korea, and are especially prevalent in Pyeongan-do and Hamgyeongnam-do Province. Intrusions of Goguryeo granite are also found in the Yeoncheon System in the Gyeonggi Massif.

Sangwon System

This system was first observed in the northern part of Korea, Sangwon, Pyeongannam-do province, and Hwanghae-do province in central Korea, the main area of its distribution. Collenia fossils have been found in the limestone interbedded in the system. The presence of Collenia suggests that the system belongs to the Proterozoic.

Paleozoic Erathem

Paleozoic sediments in Korea are represented by the lower Paleozoic Joseon Supergroup and the upper Paleozoic Pyeongan Supergroup. The Joseon System comprises the Cambrian to Middle Ordovician sedimentary rocks, the Pyeongan System, the Middle Carboniferous to Permian, and the Unidentified Triassic sedimentary rocks. Lower Silurian limestone was discovered from conodont fossils in 1980.

The Upper Ordovician, Devonian and Lower Carboniferous systems are not known to exist.

Cambro-Ordovician Strata

The Cambro-Ordovician Joseon Supergroup is widely distributed in the limestone plateau in northwest Korea. There are many other localities with scattered patches of the Joseon Supergroup. The Joseon Supergroup has been divided into two parts according to lithology, the Samcheok Group and the Sangdong Group. These groups are conformable to each other. The Samcheok Group overlies the Precambrian rocks unconformably.