Preliminary Analysis for Downhole Arrays

Preliminary Analysis for Downhole Arrays

In order to understand site amplification of seismic wave propagation in the near surface layers, National Center for Research on Earthquake Engineering(NCREE) of National Applied Research Laboratories(NARL) installed a downhole seismic array near the edge of the Taipei Basin. Five seismometers were installed at the surface and at depths of 20, 30, 58, 78 m. The depth of the interface between sediment and bedrock is 58 m. Therefore, this array is able to analyze the seismic site effect from the bedrock to the surface inside the Taipei Basin. Moreover, one downhole array includes a surface seismometer and three downhole seismometers at depths of 20, 30, and 70 m. It was installed at the Observatory Experiment for Earthquake Precursors of the National Dong Hua University(NDHU), in order to observe and analyze the characteristics of near source seismic waves. The two sites were investigated and logged prior to array installation. Most of the recordings belong to zero grade with Peak Ground Acceleration(PGA) < 0.8 gal. The largest one is only three grade(PGA between 8 and 25 gals) at both arrays because the time of observations is short and the resolution of the seismometers is high(24 bits). Although strong motion recording is not currently available, we used weak motion data to analyze the attenuation of PGA with increasing depth.