The Earthquake Engineering Online Archive

Estimation of seismic wave coherency and rupture velocity using the SMART 1 strong-motion array recordings

Abrahamson, Norman A.

UCB/EERC-85/02, Earthquake Engineering Research Center, University of California, Berkeley, 1985-03, 134 pages (305/A27/1985)

The Jan. 29, 1981, Taiwan earthquake (M subscript L = 6.7) was recorded digitally by 27 triaxial force-balanced accelerometers in the SMART-1 strong-motion array centered 30 km north-northwest of the epicenter. The seismic source of this event had a reverse mechanism with unilateral rupture from east to west. The array recordings are used to make direct measurements of the wave coherency and the time-dependent rupture velocity and results of a frequency-wavenumber analysis are used to demonstrate the coherent energy levels of P and S waves across the 4 km array. Variations in rupture velocity over a fault length of 14 km are noted. Engineering implications of coherent nonvertically propagating waves are explored by measuring the dynamic response ratio, which indicates the relative amplitude of the dynamic response of the structure compared with the traditional response spectrum. Phase shifts in the ground motion across the array caused by nonvertically propagating waves produce up to a 25 percent reduction in the response spectrum at 5 Hz for rigid structures spanning a distance of 200 m. The out-of-phase dynamic response ratio of a flexible structure with a period of 1 sec and a support spacing of 200 m is measured.

Available online: http://nisee.berkeley.edu/documents/EERC/EERC-85-02.pdf (8 MB)