- © 2010 by the Seismological Society of America
The 2008 Iwate-Miyagi Nairiku earthquake (Mw 6.9, Mjma 7.2) occurred in southwestern Iwate Prefecture, Japan (39.03°N, 140.88°E, depth 8 km), on 13 June, at 23:43:45 GMT (Japan Meteorological Agency 2008). The surface accelerometer of KiK-net station IWTH25, located 3 km southwest of the epicenter, produced one of the largest strong-motion values of peak ground acceleration (PGA) ever recorded (4278 cm/s2 for the vector sum of the three components [Kik-net 2000]) and exhibited an asymmetric amplification in the vertical component (Aoi et al.). This asymmetric amplification has been interpreted to be a result of separation of the surface layer (Aoi et al. 2008; Yamada et al. 2009b). Subjected to extremely strong vertical ground motions, a near-surface soil layer is separated from a sublayer and the motion of the separated layer is controlled by the gravity load (Eisler and Chilton 1964). When this layer returns, striking the separation surface, high acceleration in the positive direction is produced (Yamada et al. 2009b). However, according to the field reconnaissance (Yamada et al. 2009a), there is no obvious physical evidence of layer separation, and the effects of the local soil properties are not yet clear.
The National Research Institute for Earth Science and Disaster Prevention (NIED) obtained a velocity profile from the log at station IWTH25 (see Figure 1). However, transfer functions of the observed ground motions suggest that a slower seismic velocity model than that obtained from the logging data may be appropriate (Yamada et al. 2010). We have conducted spatially dense microtremor array measurements in the area surrounding station IWTH25. The objective of this paper is to estimate the shallow velocity structure around the station using the microtremor measurements and to provide information on the spatial variation of the subsurface velocity structure, which is related to the …