Norman Abrahamson
Pacific Gas and Electric Company

Incorporating Effects of Near Fault Tectonic Deformation into Design Ground Motions 


This presentation is hosted by the University at Buffalo Earthquake Engineering Research Institute Student Chapter (UB-EERI) and is sponsored by the EERI Friedman Family Visiting Professional Program.


There are two near fault effects on long period ground motions: directivity and fling. In earthquake engineering, the terms “directivity” and “fling” have been used interchangeably. Both effects result in large velocity pulses in the near fault ground motion, but they have very different causes. Directivity effects result from constructive interference of ground motions generated from different patches of slip located down strike for strike-slip faults or down dip for dip-slip faults (e.g. Somerville et al., 1997). Fling effects result from tectonic deformation at the site and are related to the slip on the fault near the site. Fling can lead to very large velocities and displacements.

Strong fling effects were observed in the near fault ground motions from the 1999 Turkey and Taiwan earthquakes. For example, the very large velocities (300 cm/s) recorded at the northern end of the Chilungpu fault during the Chi-Chi earthquake were due to fling effects. If the fling effect is separated out from these recordings, the peak velocity of the remaining ground motion due to transient displacement is reduced to about 90 cm/s.

Existing ground motion attenuation relations do not include fling effects. A separate ground motion model needs to be developed for the fling. The total ground motion is then computed by combining the ground motion from attenuation relations with the ground motion from the fling.

A preliminary model of the fling is developed based on a single sine-wave cycle to model the fling in acceleration. There are three parameters for this model: the amplitude of the sine-wave, the period of the sine-wave, and the arrival time of the fling. The amplitude is determined using empirical models of tectonic deformation based on geodetic data. The period is based on empirical observations of fling from the 1992 Landers, 1999 Chi-Chi and 1999 Kocaeli earthquakes. The arrival time of the fling pulse is just before the S-wave arrival. The parameters of this preliminary model are presented.

The fling is combined with the standard ground motion in the time domain. The time history of the fling is added to a time history based on standard attenuation relations. An example application of the method is shown for a magnitude 7.2 strike-slip earthquake at a distance of 5 km.

An important outstanding question is does the fling have a significant effect on the response of structures. Incorporating the fling into the ground motion adds complexity to the development of the ground motion. It has not yet been determined which classes of structures are affected by the fling that would justify the additional complexity in the development of the design ground motions.


Friday, October 26th, 2001


2:00 PM EST

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