Characteristics of response spectra of free-field vertical ground motion recorded during the 1994 Northridge earthquake are examined. Dependence of vertical and horizontal response spectra, and their ratio, on the site-to-source distance is investigated through development of attenuation relationships for vertical and horizontal spectral ordinates. The database includes 123 response spectra of the motions recorded at 41 alluvial sites. Vertical-to-horizontal (V/H) response spectral ratio is found to be strongly dependent on period and distance of site to the seismic source. V/H spectral ratio largely exceeds the commonly assumed value of 2/3, at short periods in the near-field region. The main characteristics of V/H spectral ratio for the Northridge earthquake are found to be qualitatively similar to those observed in the 1989 Loma Prieta, California, and in several other earthquakes recorded over the SMART-1 array in Taiwan. These characteristics are very likely to be universal.

The method gives the slope of the travel time curve directly, the paramecer required for a velocity-depth determination. For this distance range it is found that although the slope of the P travel time curve decreases substantially with dirtance, i.e., increasing apparent velocity, the variation is not uniform. Two relatively abrupt changes are observed at distances of about 17 and 24 degrees. These are most readily interpreted as two second order discontinuities in the region of the mantle above 1000 km.Assuming an average crustal structure as derived from seismic refraction measurements and taking the uppermost 200 km of the mantle given by Gutenberg or Jeffreys, various possibilities are explored for the form of variation of the P wave velocity with depth. The discontinuities in the rate of change of velocity with depth which cause the observed breaks in the apparent velocity curve are found to be at depths of about 320 and 640 km. Although no absolute times are required in the method the travel times for the various derived models are computed and compared with standard tables. f J /C

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SUMMARYDigitally recorded acceleration time histories of the 1979 Imperial Valley earthquake by the five elements of the El Centro Differential Array are used to estimate the induced torsional and rocking vibrations of long structures in the near field of strong earthquakes. The underlying assumptions in these estimates are high rigidity of the foundation relative to the surrounding soil and its negligible mass. Based on these analyses, the peak rotation amplitude of an 18 m long foundation within 5 km of the fault is approximately 1 x lo-' rad, or nearly 0.06 degrees, and decreases rapidly for larger foundation dimensions for either the rocking or torsional mode. The strongest impulses of rotations coincide with arrival of the S phase. This paper examines the sensitivity of the resulting peak displacement and particle velocity amplitudes to the foundation dimension. It is inferred that the base averaging effects of displacement and velocity fields at frequencies contributing to peak values (f< 1 Hz) are negligibly small. Rocking and torsional amplitudes for the specific site and geometry considered here show a tendency to reduce with the increase of foundation dimensions.

SUMMARYOver 700 accelerograms recorded from 12 earthquakes in northeast Taiwan have been analysed for investigating the behaviour of the vertical and horizontal peak and spectral ground motion in the near-source region. Pseudo-relative spectral velocities (PSV), at 5 per cent critical damping for 23 frequencies in the range of engineering interest have been subjected to non-linear regression procedures in terms of magnitude and hypocentral distance. Predicted response spectra for several discrete distances and magnitudes are presented. The results show that the shape of response spectra for both vertical and horizontal components of ground motion is magnitude-as well as distance-dependent.The 2/3 ratio of vertical to horizontal ground motion, commonly used in engineering applications, appears unconsei-uatioe in the very near field for high frequency ground motion. However, it falls below 1/2 at distances greater than 50 km. The same ratio for peak ground velocity (FGV) and peak ground displacement (PGD) tends to increase with distance-the latter at a faster rate.