CHARACTERIZATION OF DIRECTIVITY EFFECTS OBSERVED DURING 1999 CHI-CHI, TAIWAN EARTHQUAKE

摘要

It has been observed in some recent strong earthquakes that ground motion amplitudes are often enhanced in the direction of fault rupture propagation. Damage pattern observations in the 1999 Mw=7.6 Chi-Chi Taiwan earthquake indicated that severe damage was concentrated near and along the fault rupture, particularly in the northern part of the fault. Directivity effects for this thrust earthquake would be expected to enhance ground motions up-dip of the hypocenter. There may also be some directivity to the north, as slip distributions have shown that displacements significantly increased from the south to the north along the fault as the rupture propagated in that direction. This paper aims to quantify the directivity effects on the peak ground acceleration (PGA) and 5%-damped response spectral accelerations (PSA) of strong-motions recorded during the Chi-Chi earthquake. The median relationships of ground motion amplitudes as a function of the closest distance to the fault and site category are determined in the first step, using 420 ground motion time histories recorded on rock and soil sites. Then the residuals (model errors) of these relationships as a function of selected geometrical parameters related to the fault are used to describe the directivity effects. The geometrical parameters include the fraction of the fault rupture that lies between the hypocenter and the site, and the angle between the fault plane and the path from the hypocenter to the site. Since the Chi-Chi earthquake is of thrust type, dip-slip directivity effects as well as along-strike propagation effects are studied. All sites are used to quantify along-strike directivity effects, while only sites within 50 km of the fault rupture are used to quantify dip-slip directivity effects. The dipslip component of the directivity effects has a strong influence, increasing ground motion amplitudes up dip of the hypocenter by a factor of approximately 2.7. The along-strike effects are also significant: ground motion amplitudes are approximately 1.5 times higher for stations in which rupture propagated strongly towards the site than those for which rupture propagated away from the station. Directivity effects are observed for PGA and PSA at all periods.