Evaluation of Wave Propagation Characteristics Using Dense Array Observation and Deep Borehole Observations

Abstract

We investigated the wave propagation characteristics of the seismic waves from small earthquakes near the site, using the dense surface array, vertical arrays, and deep-borehole observations in the Kashiwazaki-Kariwa nuclear power station. We confirmed that the initial P- and S-wave velocities pass through the site and that the time it takes to pass through the vertical array is related to the angle of incidence on the site. In the M2 class event’s waveforms in deep-depth records, the reflected waves from the ground surface are unclear, and no clear later arrivals can be found. This is because of the upper layers' scattering and absorption of high-frequency waves. For the M4 class events, in addition to reflected waves from the ground surface, there were clear later arrivals after 6 to 7 s from initial S-waves in event traces. Since these waves propagated as body waves in upper layers, we assumed that these waves were waves reflected from seismic bedrock. Using the surface array and deep borehole data, we also recognized the locally trapped surface waves in the upper layers. Three-dimensional earthquake observation is effective for understanding the propagation characteristics of seismic waves.