Site Resonance from Strong Ground Motions at Lucerne, California, during the 1992 Landers Mainshock

Abstract

Analog station LUC near Lucerne, California, recorded strong motions from the nearby (1.25 km) fault rupture during the 28 June 1992 Landers mainshock. The records illustrate general issues that can arise at near‐field stations. In the area of the station, weathered granite regolith with 400 m/s S ‐wave velocity overlies intact granite with 3000 m/s S ‐wave velocity. A strongly reverberating signal persisted for several seconds after near‐field velocity pulse passed. Recovery of the vertical record in digital form provided calibration of resonant site properties and qualitative separation of site and source effects. The dominant signal on the vertical spectrum arises from vertically reverberating P waves in an ∼13.5‐m‐thick layer. Horizontal spectra marginally resolve the analogous resonance for vertical S waves and coupling of vertical P waves into horizontal motion. Here, resonant amplification of a broad high‐frequency band ∼5–40 Hz by a factor of a few over lower frequencies sufficed to dominate acceleration records and to make velocity records jittery. Conversely, the amplitude damping times of these resonances are much less than 1 s, indicating that the time‐domain decay of the acceleration signal over ∼8 s is an incident‐wave effect. Overall, the raw seismograms represent incident incoming high‐frequency 5–40 Hz signals, but poorly resolved directional effects preclude straightforward determination of the incident body waves.