The behaviour of fluids in the crust is key to understanding earthquake occurrence due to the effect of fluid behaviour on fault strength. The attenuation of seismic waves may be locally high in fault zones as fluids are intensely distributed in these zones. This study uses a novel, simple approach to examine near-source attenuation in the focal region of intense swarm activity in the Yamagata-Fukushima border region, Japan. Near-source attenuation was estimated by determining the decay of amplitude ratios of nearby earthquake pairs with travel time differences precisely quantified using a waveform correlation. In the initial ~50 d, Q^(-1) was high, then it significantly decreased to become almost constant for the subsequent period. This pattern is similar to those independently observed for background seismicity rate, b-value, stress drop, and fault strength. These patterns can be attributed to the hypothesis that the swarm was triggered by fluid movement following the 2011 Tohoku-Oki earthquake, and the source and seismicity characteristics and the seismic attenuation were altogether affected by the temporal change in pore pressure. The method demonstrated in this study may be a useful tool to detect high pore pressure anomaly at depth and understand its relationship with earthquake occurrence.