Japonya’nın Altında Yeni Bir Tektonik Plakanın Olduğu Düşünülüyor
Sık sık büyük depremlerle sarsılan Tokyo bölgesinin, Avrasya, Pasifik ve Filipin plakaları arasında sıkışmış bir fay hattı üzerinde bulunduğunu ortaya çıkardılar.
Nature Geoscience dergisinde yayımlanan makaleye (Toda, S. Stein, R. S. Kirby, S. H. and Bozkurt, S. B. 2008. November, A slab fragment wedged under Tokyo and its tectonic and seismic implications) göre, bilim adamlarının ortaya çıkardığı kırık plaka, 100 km kadar uzunluğunda 25 km derinlikte bulunuyor. 2 veya 3 milyon yıl önce Pasifik plakasından, Tokyo’nun 200 km doğusundaki Japon Çukuru’nda denizaltındaki iki dağ silsilesinin çarpışması sonucu ayrılan bu fay hattı, jeologların Tokyo’da 30 milyondan fazla insanın yaşadığı Kanto Ovası’nın altında meydana gelen 300 bin küçük depremi analiz etmesiyle ortaya çıkarıldı.
Kırığın önce Filipin plakasının bir bölümü olduğunu düşünen, ancak yer sarsıntısına yanıtlarının Pasifik plakasına ait olduğunu gören araştırmacılar, fay hattının batıya doğru uzandığını ve Filipin plakasını kuzeye iterken, Avrasya plakasının altına girdiğini belirlediler.
Bilim adamları, fay kırığının, “yutulamayan bir hap” gibi Pasifik ve Filipin kırıkları arasında sıkıştığını belirttiler. 1703, 1855 ve 1923′te çok yıkıcı depremlerin meydana geldiği ve ülke nüfusunun dörtte birine karşılık gelen 30 milyondan fazla insanın yaşadığı başkent Tokyo, üç büyük tektonik plakanın buluştuğu noktanın 300 km kuzeybatısındaki Kanto Ovası’nda bulunuyor.
Japon hükümeti, Tokyo’da 140 bin kişinin ölümüne ve 600 binden fazla konutun yıkılmasına yol açan 7,9 büyüklüğündeki 1923 depremiyle aynı şiddette bir depremin bugün meydana gelmesi durumunda en az 1 trilyon dolarlık hasar meydana geleceğini ve yarım milyon binanın tahrip olacağını tahmin ediyor.
(USGS) A Slab Fragment Wedged Under Tokyo and Its Tectonic and Seismic Implications
Tokyo lies between a volcanic front and the sea. The 400-year-old capital city and the surrounding Kanto plain are home to one-quarter of Japan’s population. The region was shaken by destructive earthquakes in 1703, 1855 and 1923, the last of which claimed 105,000 lives; reoccurrence of any of these today is expected to cost about $1 trillion (100 trillion Yen). Just 300 km (200 mi) northwest of Tokyo lies an undersea ‘triple junction’, where three of the two dozen or so moving tectonic plates that comprise the earth’s surface meet. Tokyo sits atop the Eurasian plate; beneath it the Philippine Sea plate descends (or ‘subducts’) from the south, and the Pacific plate subducts from the east. Subduction is not steady but instead occurs in a stick-slip manner that gives rise to infrequent great earthquakes.
To reinterpret the features of the descending plate ‘slabs’ on which the large earthquakes strike, Shinji Toda and his U.S. colleagues examined 300,000 earthquake hypocenters in a 3D viewing system, and carried out a tomographic ‘brain scan’ of the slabs. In addition to the paper, they produced a 5-minute narrated movie in which the viewer flies through the earthquakes as they are rotated, spun, and interpreted.
The authors find a distinct 25-km-thick and 100-km-wide (15 x 60 mi) body beneath the Kanto basin that since 1992 has been considered to be part of the Philippine Sea slab. Toda et al find that several of the body’s characteristics―such as the speed of seismic waves that pass through it, the presence and type of microearthquakes that surround it―suggest that it is a fragment of the descending Pacific slab. They argue that the fragment was detached partly as a result by the collision of two chains of Pacific seamounts into the Japan trench 2－3 million years ago. The authors infer that the fragment is now jammed between the Pacific and Philippine Sea slabs like a pill that can’t be fully swallowed.
Their proposed Kanto fragment would subject Tokyo to a persistent source of deep thrust earthquakes, such as the 1855 magnitude~7.3 Ansei-Edo shock, which are highly destructive because of their location beneath the Kanto basin and their proximity to the city. The authors suspect that the fragment is not unique but is instead is uniquely well illuminated by the unprecedented high-quality national seismic instrument network in the Kanto region.
Perspective views of the Kanto fragment with the Pacific and Philippine Sea plate slabs, volcanoes and seismicity. The left panel shows view of southwest Japan (the coastline is grey), together with the volcanic front (yellow pyramids), their presumed magma conduits (red vertical lines) tend to lie along the 120-140 km (75-85 mi) depth contour (blue dashed lines) of the Pacific plate. The blue ball is Tokyo. The right panel shows a close-up view of the proposed Kanto fragment with microseismicity colored by depth. Toda et al argue that the Kanto fragment broke off the Pacific plate 2-3 million years ago, and is being pushed northwestward under Tokyo. Note the seismicity streak on the upper surface of the proposed fragment, perhaps indicative of its motion with respect to the overlying crust.
Anonymous contemporary woodcuts of Tokyo (formerly, Edo) before and after the great 11 November 1855 magnitude~7.3 Ansei-Edo earthquake. Toda et al contend that this quake struck as a result of the movement of the Kanto fragment against the Pacific plate slab below, about 70 km (45 mi) deep. The Japanese government estimates that a repeat of this quake today would cost $1 trillion, destroy about 500,000 buildings and take up to 10,000 lives. Woodcut source, Documenting Disaster: Natural Disasters in Japanese History, 1703-2003, Foundation for Museums of Japanese History, Chiba, 2003.
Topography and bathymetry of the Kanto tectonic ‘triple junction’ viewed from the south. Topography and bathymetry are vertically exaggerated by a factor of four. The Kanto plain marks the land surface of the deep Kanto basin. The main Japanese island of Honshu is in the upper left. The Philippine Sea plate descends west-northwestward beneath Tokyo the along the Sagami and Suruga troughs. The broad Izu volcanic ridge and the young age of the Philippine Sea plate make the plate buoyant, uplifting of the peninsulas south of the Kanto Plain, which form the southern margin of the Kanto Basin. The much older Pacific plate descends to the northwest along the Japan and Izu trenches. A chain of Pacific seamounts is visible entering the Japan Trench on the right, forming the east margin of the Kanto basin. The authors argue that the buoyant seamounts interfere with the descent of the Pacific plate and so distort the trench into a broad arc.
İlk ağızdan haberler için sicarius.wr.usgs.gov/tokyo
Bu haber, Cumhuriyet, NTVMSNBC ve USGS sitelerinden derlenmiştir.
