Surface Waves Recorded by Downhole Arrays
Abstract
Downhole, or subsurface geotechnical arrays have been installed at La Cienega in Los Angeles, near the Vincent Thomas Bridge in Long Beach, and at Meloland near El Centro as well as at several other locations. More than 25 low amplitude recordings of a few percent g have been recorded at the La Cienega, El Centro and Long Beach arrays, from earthquakes with magnitudes ranging from 2.4 to 7.1. Data from the M7.1 Hector Mine earthquake recorded at these arrays, at epicentral distances of 200-220 km, deserve special attention. The acceleration (short period motion) at the surface is amplified up to 3 times relative to the motion at depth, but the velocity and displacement show practically no near-surface site amplification. Displacements with relatively large amplitudes (up to 10 cm) were recorded at long-periods (up to 8 sec). These large motions are associated with surface waves which have long duration, more than 40 seconds, and are generated in the deep sedimentary basins. Use of downhole array data provides an opportunity to identify Rayleigh waves at the La Cienega and Long Beach Vincent Thomas Bridge arrays. Particle motion variations of ground displacement at different depths are studied in time windows from the beginning to the end of the records. Various windows of the radial component of ground displacement show clear Rayleigh wave type particle motion. In the case of La Cienega array retrograde rotation is observed at the ground surface and at depths of 18 m and 100 m, which shifts to prograde rotation at depth of 252 m. Similarly, in the case of the Vincent Thomas Bridge array in Long Beach, Rayleigh wave particle motion with certain predominant period are identifiable. The phase and group velocity of these short period Rayleigh waves are discussed. The observed response of deep sedimentary sites to distant large earthquakes clearly demonstrates the necessity of taking surface waves into consideration. These long-period waves with long duration are important for certain structures. Since the rate of amplitude decay with distance for these surface waves is less than that for body waves, strong-motion attenuation relationships may benefit from including the consideration of surface wave amplitudes separately. This is particularly important for sites distant from earthquakes and overlying deep sedimentary basins.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.S42C0680S
- Keywords:
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- 7212 Earthquake ground motions and engineering;
- 7255 Surface waves and free oscillations;
- 7260 Theory and modeling