Above the focal area of the 2008 Iwate-Miyagi-Nairiku earthquake (Mj7.2), we discuss the activity of the blind reverse fault by studying the spatial variation in the relative heights of fluvial terraces. We revised the correlation and chronology of the fluvial terraces along the Iwai river. To obtain the spatial variation of uplift in the Pleistocene, we measured the relative heights of fluvial terraces. Crustal movements of the 2008 earthquake were also obtained by comparing airborne LiDAR DEM data sets before and after the earthquake. The geological structure of the Pliocene strata, estimated from the seismic profile and the spatial variation of uplift from the relative height of fluvial terraces, can be explained well by the accumulation of crustal deformation caused by the earthquakes along the fault. From these results, it can be concluded that the relative height distribution of the fluvial terraces is an effective index for investigation of the activity of a blind fault.
Volcanic glass shards in muddy sediments of the late Pleistocene Nohbi Formation of central Japan are considered to be derived from multiple tephras, although the shards have petrographic properties similar to those of the Aira-Tn tephra. Hierarchical cluster analysis of concentration data for 58 elements obtained by laser ablation-inductively coupled plasma-mass spectrometry was used to distinguish glass populations in muddy sediment samples from the Nohbi Formation. Cluster analysis of volcanic glass shards from a sample in the lowermost horizon (Nohbi 38-54) of the formation identified the Aira-Tn (44%), Ontake-Pm1 (33%), and Ebisutouge-Fukuda (23%) tephras. Glass shards from another sample in the middle horizon (Nohbi 60-2) of the formation were identified as being the Aira-Tn (35%) and Ebisutouge-Fukuda (65%) tephras. The combination of multi-element data and cluster analysis demonstrates that the muddy sediment layers contain several tens of percent of Aira-Tn glass shards, as suggested by previous studies. The Aira-Tn tephra is the youngest one within the tephras identified from the Nohbi muddy sediments. As suggested by the previous studies, this evidence indicates that the Nohbi Formation was deposited during and/or after the AT ash fall stage and before the deposition of the Nan’yo Formation.