Tareas

A temporal-frequential analysis of seismic intensity

Silvia GARCIA*
Coordinación de Geotecnia, Instituto de Ingeniería, UNAM, Apdo. Postal 70-472, Coyoacán 04510, México D.F., e-mail: sgab@pumas.iingen.unam.mx,
and
Miguel ROMO
Coordinación de Geotecnia, Instituto de Ingeniería, UNAM, Apdo. Postal 70-472, Coyoacán 04510, México D.F., e-mail: mromo@pumas.iingen.unam.mx,

ABSTRACTScientists have assumed that local geologic conditions are the cause of the dramatic difference in shaking intensity experienced during earthquakes, but they have not been certain of the particular conditions that are most responsible, and the degree to which they affect the ground motions. This paper first provides the fundamentals of the energy-time-frequency representation of signals, theHilbert-Huang Transform HHT, and then uses it to analyze recordings of Mexico City soft-soil deposits and to review the manner in which local sites effects are treated in earthquake-resistant design. The HHT, integrated by the Empirical Mode Decomposition EMD and the Hilbert Transformation HT, enables engineers to analyze non-stationary oscillation systems and to obtain more detailed intensity descriptionson time-varying frequency diagrams. The advantage of the HHT over other representations, such as the wavelet and sliding window Fourier transform, is its sharp intensity-localization properties in the time-frequency plane. The results obtained in this investigation indicate that the HHT method is able to extract some motion characteristics useful in studies of seismology and geotechnicalengineering, which might not be exposed effectively and efficiently by other conventional data processing techniques. The HHT information would allow for better estimates of how intense shaking will be during future earthquakes.

Keywords: Hilbert-Huang Transform, seismic signal analysis, time series analysis, site effects, input motion, seismic ground response
*corresponding author
1. INTRODUCTIONEarthquake-resistant design is the first line of defense in improving structural safety and reducing losses from earthquakes. Geotechnical and structural engineers must take into account two fundamental characteristics of earthquake shaking: 1) how ground shaking propagates through the Earth, especially near the surface (site effects), and 2) how buildings respond to this ground motion. Becauseneither characteristic is sufficiently well understood, the seismic phenomenon is still a challenging research area.
Site effects play a very important role in forecasting seismic ground responses because they may strongly amplify (or deamplify) seismic motions at the last moment just before reaching the surface of the ground or the basement of man-made structures. Measurements of shaking areessential to understand the frequency nature of seismogenic processes, the radiation of seismic waves from the source (to establish the seismic input) and the effect of seismic loads applied to the physical environments (to determine the ground response -output-). Once this information is processed, the data are used also to develop empirical descriptions of the shaking and the seismic hazards.For much of the history of seismological research, site effects have received much less attention than they should, with the exception of Japan, where they have been well recognized through pioneering work by Sezawa and Ishimoto as early as the 1930’s (Kawase and Aki, 1989). The situation was drastically changed by the catastrophic disaster in Mexico City during the Michoacan, Mexico earthquake of1985, in which strong amplification due to extremely soft clay layers caused many high-rise buildings to collapse despite their long distance from the source. The cause of the astounding intensity and long duration of shaking during this earthquake is not well resolved yet even though considerable research has been conducted since then, however, there is no room for doubt that the primary cause...