Variations of Standard Deviation of Gravity Anomalies in Chugoku District, Japan: Relationship with Distributions of Topographic Lineaments

Relationship between distribution of topographic lineaments and variation of standard deviation of gravity anomalies in Chugoku district, Japan is investigated. Tectonic movement may disturb lateral continuities of crustal structures at weak zones. Lateral discontinuities of the density structure cause undulations of gravity anomaly field over them. Therefore, complexities of the gravity anomaly field might be an indicator of the past crustal instability. On the other hand, topographic lineaments are formed along zones of inherent crustal weakness, and the gravity anomaly complexity relates to distribution of surface lineaments. In order to verify this conjecture, we investigated gravity anomaly complexities in relation to the spatial distributions of topographic lineaments.

As an index of complexity of gravity anomaly field, we employed standard deviation (SD) of Bouguer anomalies. We divided the survey area into a set of regular grid cells with a mesh size of 1 km x 1 km, to the centers of which we assigned a representative SD value calculated from Bouguer anomalies inside a given search area. In this study, the nodes are centered in the regular grid cells and we used a circled search area centering at each node. We also need to revise the array data in a point-registered file of topographic lineaments because the distance between two neighboring points along each lineament does not have a constant value. The average distance is 186 m, and the standard deviation is 294 m. These deviations of the distances between neighbor points may cause implausible results. In order to import this lineament data into a statistic analysis, we reproduced a new set of array data with the constant value of 10 m for the distance between them. Then, we made a statistical analysis by referring the Bouguer anomaly SD values to the numbers of the lineament data points within 3 km from each node. Thus, we repeated the present method while the search radii for the SD determination are 5, 10, 15 and 20 km.

Frequency distribution of the lineament data points to SD values showed that locations of the lineaments tend to overlap the high SD areas of gravity anomaly field. These results imply an applicability of SD of Bouguer gravity anomalies in order to discuss the crustal weakness and the instability. Furthermore, this index derived from gravity anomaly might be effective even over the area covered by thick sediments and/or volcanic products, where fault-like structures are hard to be detected.