Geologic interpretation of artificial strata in urbanized areas

It is most important that the geological significance of artificial strata and the stratigraphic relation between artificial and natural strata are clarified and the characteristics common to artificial strata are extracted and sorted out. These are the basic data for considering their effects on environment and natural disasters. The northern part of the Boso Peninsula is selected as the study area in order to clarify the above problems. Stratigraphic analyses of artificial strata in the study area were carried out on the basis of geologic profiles prepared from drill logs. In addition, outcrops of artificial strata by trenching were observed and sketched in detail. Based on the results of these studies, sedimentary conditions of artificial strata were compared with those of natural strata. The following are the main results of this study. The concept of unconformity should not be applied to the discontinuity between artificial strata and natural strata. However superposing relations of the both strata have significant meaning in geo-environmental problems. The concepts in stratigraphy can be applied to hydraulic fills. The sedimentary conditions affect the occurrences of land subsidence and liquefaction and fluidization during earthquakes. The concepts in stratigraphy cannot be applied to filled strata, waste strata and surplus soil strata, because their formation processes are strongly affected by human activities. The artificial strata are products of human activities in the Quaternary Period, also called Anthropogene. The idea that the strata are similar to bioturbation (Kumai, 1991) is most reasonable.