Effects of an alluvial basin on strong ground motions

Abstract

It has been reported that structures on loose soil sustain more damage during earthquakes than those on solid rock. This phenomenon has long been ascribed to the influence of local site geology. Gutenberg (1956) observed stronger shaking and longer duration at sites on alluvium than at those on crystalline rock, and large variations in ground shaking even at sites only 1000 feet apart. During the recent Michoacafi earthquake of 1985, most of the damages occurred in Mexico City more than 300 km from the epicenter (Anderson et al., 1986). The city is situated atop very soft sediments that caused dramatic amplification of strong ground motion, severe damage to the structures, and considerable loss of life within the area of the city located on top of the soft sediments. On 20 May 1986 a magnitude M L = 6.5 earthquake occurred near Hualien in eastern Taiwan. The epicenter was located at 24°4.9'N and 121o35.49 ' E, and the focal depth was 15.8 kin. This earthquake caused some damage in the Hualien and Taipei areas (Tsai et al., 1986). Accelerographs at 37 sites of the Strong Motion Accelerographs Array in Taiwan phase 1 (SMART1 array, Fig. 1; Bolt et al., 1982) were triggered. The Lotung Large Scale Seismic Test Array (LSST Array, Figs. 1 and 2; Wen et al., 1986) is situated within the southwestern quadrant of the SMART1 array. Surface recordings and two groups of downhole recordings of the LSST array were also obtained. In this study, we use the frequency-wavenu mber (f-k) analysis to study the variations in wave propagation direction and velocity. The change in peak acceleration, velocity, and displacement with respect to depth and the frequency-dependent ground motion amplification are studied by using the downhole and surface recordings of the LSST array.