Ground Motion of Subduction Earthquakes for Seismic Design

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1. Site investigation at strong-motion sites in the central part of Chile.At 9 strong-motion sites where strong-motion records have been obtained in the 1985 Central Chile earthquake, microtremor array observations were conducted to estimate the shear-wave velocity profiles at the sites. The microtremors were also observed in Santiago where the strong-motion array have been developed. The results contributed to complete information about site characteristics at strong-motion sites in the central part of Chile.2. Construction of strong-motion databaseThe database of the strong-motion records from Chilean and Japanese earthquakes were constructed. About 100 Chilean data and more than 500 Japanese data were compiled in the database. In the database, imformation about site characteristics of the strong-motion sites were also included.3. Analysis of strong-motion records from subduction earthquakesGround motion parameters such as peak ground acceleration, velocity and response spectra were computed for strong-motion records in the database. The source, propagation path and site effects on the ground motion parameters were analyzed. The results indicate that (1) site effects are significant in longer periods, (2) the amplitude of ground motion increases with increase of the source depth, (3) earthquakes in the ocean plate tend to generate stronger seismic waves than those in the continental plate, and (4) attenuation of seismic waves from large earthquakes in the ocean plate is smaller.4. Evaluation of ground motion characteristics from large subduction earthquakesEmpirical relationships for predicting peak acceleration and velocity were proposed in terms of earthquake magnitude, fault distance, source depth, and average shear-wave velocity of ground. It was also pointed out that ground motion from large subduction earthquakes is considered to have longer-period contents, smaller attenuation decay, and larger amplitudes from deeper events.