The Mexico Earthquake of September 19, 1985—Relationships between Soil Conditions and Earthquake Ground Motions

Abstract

Comparisons are presented between the characteristics of ground motions at five sites underlain by clay at which ground motions were recorded in Mexico City in the earthquake of September 16, 1985 and for which analyses of ground response have been made, based on the measured properties of soils and the motions recorded on hard formations at the National University of Mexico. It is shown that the ground response in areas of Mexico City underlain by clay is extremely sensitive to small changes in the shear wave velocity of the clay and it is suggested that a probabilistic approach which allows for uncertainties in shear wave velocity measurements and in the characteristics of the motions on the hard formations is desirable to assess these effects. Based on the results of such an approach it is concluded that simple ground response analyses can provide very useful data for engineering assessments of the effects of local soil conditions on the characteristics of ground motions likely to develop at sites underlain by soft clays, and that the use of these procedures also provides a useful basis for estimating the general nature of the ground motions in the extensive heavy damage zone of Mexico City in the 1985 earthquake.

The 1997 NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures

“…In Mexico City in 1985, the maximum rock acceleration was amplified four times by a soft clay deposit that would have been classified as S whereas the spectral amplitudes were 4 about 15 to 20 times larger than on rock at a period near 2 sec. In other parts of the valley where the clay is thicker, the spectral amplitudes at periods ranging between 3 and 4 sec also were amplified about 15 times, but the damage was less due to the low rock motion intensity at these very long periods (Seed et al, 1988). Inspection of the records obtained at some soft clay sites during the 1989 Loma Prieta earthquake indicates a maximum amplification of long-period spectral amplitudes of the order of three to six times.…”

Strong Ground Motion Estimation

“…Borcherdt et al, 1979;Joyner et al, 1981;Seed et al, 1988). Williams et al (1999) showed that significant resonances can occur for impedance boundaries as shallow as 7-m depth.…”

Soil responses near Delhi ridge and adjacent regions in Greater Delhi during incidence of a local earthquake

“…Layer resonances and near-surface impedance gradients are the main factors that cause soil amplification in a simple horizontally layered structure (Haskell 1960;Murphy et al 1971;Shearer and Orcutt 1987). The importance of soil amplification has clearly been demonstrated for the Mexico earthquake of 19 September 1985 by Seed et al (1988). Significant damage during the 17 October 1989 Loma Prieta (California) earthquake occurred in areas of San Francisco and Oakland underlain by poor soil Fig.…”

“…The nonlinear characteristic of the soil is usually called dynamic deformation characteristic. Seed and Idriss (1970) and Hardin and Drenevich (1972a, b) expressed nonlinear characteristic of soil is caused by cyclic loading. Thus, earthquake response analysis widely done on computer code SHAKE (Schnabel et al 1972), which based on multiple reflection theory and nonlinearity of soil behavior, considered as equivalent linear method where the stress-strain relationship must be linear in solving the equation of motion in frequency domain.…”