Earthquakes


Tectonic History Of Japan

By Siobhan Kenny.

 

The tectonic history of Japan

 

The islands of Japan are positioned on an extremely unstable region of the earth's crust where the Philippine Sea Tectonic Plate and the Pacific Tectonic Plate subduct under the Eurasian Continental Plate and the North American Continental Plate near East Asia.(1)

The subduction of the Pacific Plate and Philippine Plate under the Eurasian Plate not only causes the volcanoes in Japan, but also makes it one of the most earthquake-prone regions of the world. There are numerous faults running across Japan, that are associated with the subduction zones causing many major and a vast number of minor earthquakes.(2)

 

 

 

Japan sits atop four tectonic plates.

http://jove.geol.niu.edu/students/mdare/VirtualFieldTrip/Mt_Fuji/japan_trenchs.jpg 22/04/08

 

 

Japan lies on the edge of the Pacific-Philippine-Eurasian triple plate junction, where the complex interactions of three tectonic plates is unpredictable and loaded with potential activity. Due to being older and more dense, the Pacific plate is sub ducted beneath the younger Philippine plate, and results in the formation of a deep ocean trench. The Marianas Trench is 36,000 feet below sea level and is known as the deepest region of the worlds oceans.(3)  

 

Japans dangerous positioning in the subduction zone at the boundaries of these plates, results in Japan living with more earthquakes than any other country on earth. Up to7,500 earthquakes may occur in Japan and its surrounding areas in any given year.  

About a quarter of these can be felt by humans. In the area of Tokyo alone, about 40 to 50 of these earthquakes occur. Destructive earthquakes occur somewhere in Japan approximately once in every two years. Because of its rocky foundation, weak or strong, near or far, earthquakes in this part of the world are capable of causing considerable damage.(1)

 

Two major earthquakes that occurred in Japan, due to plate tectonics were the Kobe/Hanshin earthquake and the Kanto earthquake.

 

 

 

 The Kobe/Hanshin earthquake.

On January 17th 1995, at 5.46 a.m., an earthquake measuring 7.2 on the Richter Scale struck the Kobe region of south-central Japan. The region of Kobe is the second most populated and industrialized area after Tokyo, with a total population of about 10 million people. The ground shook for only about 20 seconds but in that short time, over 5,000 people died, over 300,000 people became homeless and damage worth an estimated £100 billion was caused to roads, houses, factories and infrastructure. The cause of this destruction was due to the shallow depth of the focus which was only about 16 kms. below the surface and also the fact that the epicentre occurred quite close to a very heavily populated area. Seismic shockwaves travelled from Awaji Island (the epicentre) along the Nojima Fault to the cities of Kobe and Osaka.(4)

 

 

 

http://aphs.worldnomads.com/dan_in_japan/259/slide.jpg 15/04/08

 

 

 

The Kanto earthquake.

The worst known earthquake to occur in the history of the Japanese islands was the great Kanto earthquake. On September 1, 1923, a devastating earthquake hit the densely populated area of Tokyo and Yokohama. The earthquake measured an unbelievable 7.9 on the Richter scale. The death toll was atrocious, up to 140,000 people lost their lives and   58,000 of them in were from Tokyo. The region of Tokyo and Yokohama were destroyed to 70 and 80 percent.(5) The source of the terrible 1923 Kanto earthquake is due to a mega thrust between the Philippine Sea plate and Honshu plate. The great 1923 Kanto earthquake was a typical inter-plate event by the subduction of Philippine-sea plate beneath Kanto region. The subduction of these two plates created a complex structure, in which the plates abut beneath the southern Kanto region. Several active faults are also known to exist in the central part of the Kanto Plain.(6)

 

 

 

                        Aftermath of the Kanto earthquake

http://tokyology.files.wordpress.com/2007/10/kanto-earthquake.jpg 23/04/08

 

 

The main reason for the occurrence of a vast number of earthquakes in Japan is the tectonic activity of the Eurasian, the Philippine, the Pacific Ocean, and the North American plates, which surround Japan. The relative movement of these plates is the main source of strain build-up in Japan.(7)

 

 

 

 

        

                     Historical earthquake events in Japan from 1978 to 2007.

 

 

 

DATE

LOCATION

MAGNITUDE*

Jul. 16, 2007

NEAR THE WEST COAST OF HONSHU, JAPAN

M 6.6

Mar. 25, 2007

NEAR THE WEST COAST OF HONSHU, JAPAN

M 6.7

Nov. 28, 2004

Hokkaido

M 7.0

Oct. 23, 2004

Niigata Ken Chuetsu

M 6.6

Sept. 5, 2004

Western Honshu

M 7.4 & 7.2

Sept. 25, 2003

Hokkaido

M 8.3

Jul. 25, 2003

Honshu

M 6.1

May 26, 2003

Miyagi Oki

7.0

Jan. 17, 1995

Kobe

M 6.9

July 12, 1993

Hokkaido-Nansei-Oki

M 7.8

January 15, 1993

Kushiro Oki

JMA 7.8

Dec. 17, 1987

Tokyo

M 6.6

Sept. 14, 1984

Nagano-Ken Seibu

M 6.9

May 26, 1983

Nihan-Kai-Chubu

M 7.7

Mar. 21, 1982

Urakawa-Oki

M 7.3

June 12, 1978

Miyagi-Ken-Oki

M 7.5

 

   

 

 

 

                                http://www.eeri.org/lfe/japan.html 20/04/08 

 

 

 

 

 

 

                                          

                                    

REFERENCES: 

 

(1)Caraway M.W.(1997) Korean history project

http://www.koreanhistoryproject.org/Jta/Jp/JpGEO1.htm  22/04/08

 

(2)Johnson A.D. The geology of Japan, Earthquakes

 http://www.seinan-gu.ac.jp/~djohnson/natural/quakes.html 20/04/08

 

(3)Heezen B & Tharpe M. Deep Ocean Trenches and The Ring of Fire

 http://www.platetectonics.com/oceanfloors/japan.asp 22/04/08  

  

(4)Rayner D.(2002) Georesources, Kobe earthquake            

 http://www.georesources.co.uk/kobehigh.htm 23/04/08 

 

(5)The great Kanto earthquake,

 http://www.artelino.com/articles/kanto_earthquake.asp 23/04/08

                      

(6)Characteristics of seismic activity in the Kanto region, 

http://www.hp1039.jishin.go.jp/eqchreng/5.htm 23/04/08  

                         

(7)Ghasemi H, Otsuka H, Cooper D.J, Nakajima H.(1996) Aftermath of the Kobe earthquake,

http://findarticles.com/p/articles/mi_m3724/is_n2_v60/ai_19562457 14/04/08