Observation of Seismic Ground Motion and Pore Water Pressure in Lineated Valley Fill of Wakayama, Southwest Japan

We performed in-situ observations of seismic ground motion, pore-water pressure, and ground-water level at lineated valley fill (LVF) in Wakayama city, southwest Japan, from 2004 to 2008. In Wakayama city and its surrounding area, felt earthquakes often occur with epicenters in the northwestern part of the Kii Peninsula and the Nankai Trough off the coast of the Kii Peninsula. We obtained simultaneous observation records of surface ground motion and pore-water pressure from several small- to medium-sized earthquakes. The excess pore-water pressure was clearly observed during a medium-sized earthquake with magnitude Mj 4.5. From the detailed analysis of surface ground motion, we detected a long-period pulse, although it was usually hidden behind oscillating short-period components with large amplitudes. The strong shaking induced ground tilting motion, causing a step-like acceleration change in the seismometer pendulum. Although no ground deformation could be confirmed through visual observation during the earthquake, the in-situ instrumental observation revealed ground tilting motion. Theoretical long-period pulses were synthesized assuming models of ground tilting motion during an earthquake. From comparison of the theoretical model with the observed long-period pulse, the permanent tilt is of an order of 10−6 degree down nearly toward the direction of the valley line.