31 



as the production wells continued operation. It should be stressed that 

 the rate of increase in relative sea level in this instance was much 

 greater than any expected rates due to eustatic or neotectonic changes. 



Japan - Several regions of Japan have experienced large rates of 

 subsidence due to compaction, generally caused by overpumping of 

 groundwater. Ground elevations in Niigata Prefecture and the cities of 

 Osaka and Tokyo have dropped as much as 4 m in the past 40 years, sometimes 

 reaching rates as high as 15 cm/yr (Takeuchi et al., 1970). Fig. 3.3 

 displays the isolines of the total amount of land subsidence in Osaka from 

 1935 to 1968. The subsidence is greatest near the coast (280 cm) and small 

 (40 cm) in the hilly region in the center of the city where the compactible 

 stratiom is thin. Fig. 3.4a displays monthly measurements of groundwater 

 elevation and Fig. 3.4b shows the corresponding monthly rates of 

 compaction. The two are clearly correlated. The period where subsidence 

 stopped is due to destruction of the city during the bombing of World War 

 II when pumping of groundwater ceased. The installation of an industrial 

 water system and the reduction in pumping started in 1961 have since raised 

 the groundwater table and arrested the subsidence. In Niigata the most 

 severe subsidence has also occurred right on the coast. In all of these 

 cases, regulations controlling groundwater pumping have since been enacted, 

 plus recharge has been practiced in several of the regions where the 

 subsidence is particularly acute. These measures have always proved 

 successful in at least slowing the rate of compaction. In the Tokyo region 

 however, 253 km of embankments, 41 sluice units, and 9 pumping stations 

 were required to protect against typhoon flooding and extreme tides, and to 

 provide drainage for rainwater (Ukena et al., 1970; Tagami et al . , 1970). 

 These are precisely the types of measures that may be required in many 

 coastal cities within the next century. 



3.6 RESEARCH NEEDS 



One important aspect of compaction that requires investigation is its 

 effect on the tide gage measurements used to determine sea level rise, as 

 noted in section 2. Although gage elevations are often surveyed in 

 relation to bench marks that are anchored to bedrock, the error inherent in 



