The seismic response of a steep slope: High-resolution observations with a dense, three-component seismic array

We deployed an array of 35, three-component seismographs to measure site effects associated with a 50-m high slope that is the highwall of an abandoned surface coal mine near Sullivan, Indiana. The strip mine pit had filled with water to within 10 m of the top of the highwall. Our array was deployed over this slope and recorded 10 underwater explosions detonated in the lake 800 m away. The raw data show a site resonance revealed by ringy waveforms observed at stations near the slope crest. The ringing is seen on the vertical component immediately following the P arrival and on the horizontal components immediately following what we interpret as a secondary P arrival formed by a direct wave propagating through the water. Three-dimensional particle-motion analysis demonstrates that the P-wave signal is linearly polarized on the flat ground behind the slope but becomes strongly elliptical for stations on the slope. The direct wave through the water is nearly horizontally polarized and shows a pronounced amplification of about a factor of 2 for stations at the top of the slope. We quantify the variations in the site resonance by using spectral ratios. We observe a factor of 2 amplification at about 9 Hz that we interpret as a fundamental mode of the slope face.