Abstract
During the 2011 Tohoku Earthquake, the peak ground accerelation (PGA) with high value of above 1000 cm/s2 was observed at K-NET Hitachi and K-NET Hokota, located in the coastal region of Ibaraki prefecture which is close to the southern edge of the fault, and the PGA close to 1000 cm/s2 was also observed at KiK-net Iwase, located in the boundary between Ibaraki and Tochigi prefecture. However, few records were observed in the area between these stations, referred to as Higashi-Ibaraki Terrace. Moreover, we have less information about subsurface structures in the area. Accordingly we performed aftershock observation of the Tohoku Earthquake and microtremor measurements to evaluate ground motion characteristics in the area. Aftershock records mostly show large accelerations in the eastern part of the terrace, but it is also large at the middle of the terrace. We then estimated the S-wave velocity profiles from microtremor explorations and we found that the shallow soil layers are deep in the middle of the terrace, while seismic bedrock is deep in the eastern part. Thus, the soil amplification has a peak at short period in the western part of the terrace, while it has a peak at about 1 second in the middle of the terrace. It has two peaks at short and long periods in the eastern part due to the shallow and deep soil structures. These amplifications showed similarity with the spectral ratios of the aftershocks.
