November 27, 2003
The Ministry of the Environment put together a report about the ground subsidence in Japan, based on the leveling results and other information gathered by the national and local governments in FY 2002.
The area of ground subsidence (ground that has subsided by 2 cm or more in a year) has changed on a relatively small scale in recent years. However, in FY 2002, the subsided area expanded to 461 km2 in eight areas from 28 km2 in nine areas in FY 2001. The increases were largely due to the growth of a subsided area in Chiba Prefecture, from 27 km2 in FY 2001 to 455 km2 in FY 2002.
The greatest level of subsidence was the 3.3 cm that took place in the Kujukuri area (Sanbumachi, Sanbugun) of Chiba Prefecture. The greatest level of subsidence in recent years stayed generally at 3 cm without noticeable changes.
1. State of Ground Subsidence in Japan in FY 2002
The greatest level of annual ground subsidence in FY 2002 was the 3.3 cm that took place in Uekusa, Sanbumachi of Chiba Prefecture. As in FY 2001, no areas recorded annual ground subsidence over 4 cm. Eight areas recorded annual ground subsidence of 2 cm or more (9 areas in FY 2001), with the subsided area totaling 461 km2 (28 km2 in FY 2001) (Fig. 1).
2. Changes in the Area of Ground Subsidence
In terms of changes in the number of areas experiencing ground subsidence and changes in the total area of ground subsidence since the Ministry of the Environment started gathering data in FY 1978, areas where the ground subsided by 2 cm or more expanded largely in FY 2002 to 461 km2, with most of the subsided areas in Chiba Prefecture.
Although ground subsidence in Japan is generally on a downward trend, it can hardly be said that the trend has stabilized. As shown in Figure 2, there is the possibility of abrupt ground subsidence triggered by drought, as seen in FY 1994. The number of areas with 2 cm or more of ground subsidence has remained unchanged at nine or so since FY 1998, though the areas are not the same. (Fig. 2)
<Overview of Ground Subsidence Measures>
The following measures have been taken to prevent ground subsidence.
1. Regulation on the pumping of groundwater, etc.
Pursuant to the Industrial Water Law (under the co-jurisdiction of the Ministry of the Environment and the Ministry of Economy, Trade and Industry) and the Law concerning the Regulation of Pumping-up of Groundwater for Use in Buildings (under the jurisdiction of the Ministry of the Environment), the government regulates the pumping of groundwater for industrial use and for the heating and air-conditioning of buildings, designating areas subject to the regulation in ten prefectures and in four prefectures respectively.
In addition, local governments enacted ordinances to regulate the pumping of groundwater (25 prefectures and 310 municipalities as of September 2003).
2. Measures based on the Outline of Measures for Preventing Ground Subsidence
In order to implement comprehensive measures suitable to local conditions, the Ministerial Council concerned with Measures for the Prevention of Ground Subsidence formulated the Outline of Measures for Preventing Ground Subsidence for Nobi Plain, Chikugo-Saga Plain, and Northern Kanto Plain. In accordance with this outline, the national and local governments have implemented various measures.
3. Monitoring and studies
Local governments monitor ground subsidence and measure the level of groundwater using leveling and groundwater observation wells. The Ministry of the Environment provides subsidies to some of this monitoring and measurement.
4. Programs concerning ground subsidence
The national and local governments implement programs to secure and supply surface water as a substitute for groundwater, and undertake restoration programs to help areas recover from damages caused by ground subsidence, in addition to programs to prevent disasters such as flooding, high tides, and tidal waves.
5. Raising awareness for the prevention of ground subsidence by providing information
To raise awareness of the prevention of ground subsidence and to facilitate the work of national and local officials, the Ministry of the Environment posts the "National Ground Environment Information Directory" on its website to provide information on ground subsidence, groundwater levels, regulations regarding the pumping of groundwater and so on.
|*||Fig.1||State of Ground Subsidence in Japan in FY 2002|
|*||Fig.2||Total Area of Ground Subsidence in Japan (Changes by Fiscal Year)|