Seismic Structure in the Northwestern Margin of the Okinawa Trough

오키나와트러프 북서 주변부의 탄성파 구조

  • 선우돈 (한국 지질자원연구원 석유해저자원연구부)
  • Published : 2003.12.01

Abstract

The Okinawa Trough is a rift basin formed by extension. Analysis of multichannel seismic reflection profiles from the northwestern margin of the northern Okinawa Trough reveal that the trough is characterized by a series of tilted fault blocks bounded by listric normal faults and half-grabens developed between blocks, showing typical rifted structures. The trough display three kinds of sedimentary sequences with different seismic reflection characteristics: prerift, synrift and postrift sediments. The prerift sequence develops parallel to the dip direction of tilted fault blocks. The synrift sediments, mostly deposited in the half-grabens between tilted fault blocks, are generally well characterized by divergence of the reflectors towards the blocks indicating contemporaneous deposition during tilting. The postrift sediments are featured by continuous and parallel reflectors. The width of the half-graben and the throw-displacement rate of the basin bounding fault are closely connected. The throw-displacement rate is the maximum when the rifting event is the most active and the width of the half-graben is proportional to the rate.

오키나와트러프는 인장력에 의해 형성된 열개 분지이다. 한반도 남부해역에 위치하는 북부 오키나와트러프의 북서 주변부에서 취득한 다중 채널 탄성파 자료 해석에 의하면 오키나와트러프의 지하 지질 구조는 전형적인 열개 분지에서 나타나는 경사진 단층 블록, 단층 블록들 사이에 발달한 반지구, 그리고 반지구의 경계부에 발달한 점완단층으로 특징 지워진다. 오키나와트러프에 퇴적된 퇴적층은 퇴적 시기에 따라 선열개, 동시열개 및 후열개 퇴적층으로 나뉘며 이들은 서로 다른 탄성파 반사 특성을 나타낸다. 선열개 퇴적층은 경사진 단층 블록의 경사 방향에 평행하게 발달한다. 동시열개 퇴적층은 주로 경사진 단층 블록 사이에 발달한 반지구에 퇴적되며 경사진 단층 블록을 향하여 반사면들이 경사를 이루며 발산(divergence)하는 특징을 나타낸다. 이는 동시열개 퇴적층이 단층 블록의 기울어짐(tilting)과 동시에 퇴적이 이루어졌음을 시사한다. 후열개 퇴적층은 반사면의 연속성이 양호하며 평행하게 발달한다. 열개 분지에서 반지구의 폭과 분지 경계 단층의 낙차 변위율은 밀접한 관계가 있다. 낙차 변위율은 열개 작용이 가장 활발한 시기에 최대이며 반지구의 폭은 분지 경계 단층의 낙차 변위율에 비례한다.

Keywords

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