Focusing on variation of peak period of horizontal to vertical spectral ratios (HVSRs) of microtremors due to time and space at flat engineering bedrock sites, we evaluate experimental variation coefficients and discuss the reason of the variation. The continuous microtremor measuments resulted in significant daily variation of peak periods of HVSRs, which are shorter in daytime of weekday than those in nighttime or weekend. Through a numerical simulation, it implies that the peak period are affected not only by subsurface structures but also by other factors such as the relative locations from stations to sources.
