Neotectonic Movement and Subsidence Caused by Piezometric Decline

Abstract

Various forms of land subsidence can be divided into two major genetic groups according to origin: ENDOGENIC SUBSIDENCE being related to processes that originate essentially within the planet (tectonic movement and volcanism) and EXOGENIC SUBSIDENCE being related to processes that originate near the earth9s surface, including processes related to human activity (mining, etc.). Exogenic subsidence is basically a surface expression of compaction of deposits at depth and is caused by an increase in actual or effective loading. A classic example is subsidence due to an overdraft of confined ground water. The genetic classification is, however, only “a first approximation” because of the close interrelationship of natural processes. For example, piezometric head resulting from tectonic movement in a confined aquifer system can more or less compensate for the weight of subsequently deposited overlying sediments, limiting compaction of the aquifer. Reduction of piezometric head in such a system due to an overdraft will cause an increase in effective loading on the aquifer system and will result in subsidence. No subsidence will take place, however, due to decline in piezometric head which developed after deposition of overburden. Development of such “postdepositional” piezometric head is usually caused by neotectonic crustal movement. Recent studies of subsidence in the west-central San Joaquin Valley, California, indicate that the amount of “safe” piezometric decline which caused no subsidence (i.e., the magnitude of Holocene uplift) increased toward the Coast Range foothills and ranged from about 30 to 100 m. Future studies of “safe” piezometric decline may help to differentiate and date neotectonic movement in the Valley.