Assessment of the Relationship between ESR Signal Intensity and Grain Size Distribution in Shear Zones within the Atotsugawa Fault System, Central Japan

Abstract

For the first time, a relationship between ESR signal intensity and grain size distribution (sieve technique) in shear zones within the Atotsugawa fault system have been investigated using fault core rocks. The grain size distributions were estimated using the sieve technique and microscopic observations. Stacks of sieves with openings that decrease consecutively in the order of 4.75 mm, 1.18 mm, 600 μm, 300 μm, 150 μm and 75 μm were chosen for this study. Grain size distributions analysis revealed that samples further from the slip plane have larger d 50 (average gain size) (0.45 mm at a distance of 30 - 50 mm from the slip plane) while those close to the slip plane have smaller d 50 values (0.19 mm at a distance of 0 - 10 mm from the slip plane). This is due to intensive crushing that is always associated with large displacement during fault activities. However, this pattern was not respected in all shear zones in that, larger d 50 values were instead observed in samples close to the slip plane due to admixture of fault rocks from different fault activities. Results from ESR analysis revealed that the relatively finer samples close to the slip plane have low ESR signals intensity while those further away (coarser) have relatively higher signal intensity. This tendency however, is not consistence in some of the shear zones due to a complex network of anatomizing faults. The variation in grain size distribution within some of the shear zones implies that, a series of fault events have taken place in the past thus underscoring the need for further investigation of the possibility of reoccurrence of faults.