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Abstract
Ground motions recorded in several earthquakes by the Hutt Valley strong-motion accelerograph array are analyzed to demonstrate and quantify microzone effects. Spectral amplifications in low to moderate ground shaking are calculated at sites with different soil conditions with respect to a reference site located on rock adjacent to the edge of the valley. It is shown that the response characteristics of various sites correlate well with the soil conditions underlying the sites. A shallow site located on 20 m of sediment adjacent to the edge of the valley produces short-period amplification, with the average amplification curve for the 5% damped acceleration response, spectrum peaking at 0.35 s with a value of 4. Deeper sites with up to 300 m of stiff sediments yield broad-band amplifications up to 2 s period, with typical maximum amplifications of 5-10. The greatest amplifications, exceeding 10, are obtained at two sites which are located on shallow very soft deposits.
Highlights
The Engineering S'.!hmology Section of the Institute of Geological & Nude·ar Sciences maintains a strong--m.otion accelerograph P.etwork for studying microzone effect~. i-n the Hutt Valley and the neighbouring Wainuiomata vaUey
Since 1966, when tile first strong-motion accelerograph Y.vas installed, there hav(: been more than 20 earthquakes wi'Ji r nagnitude greater than 4 which have triggered the strong-motion instruments in the Hutt Valley
The categories range from Zone 1 which consists of rock at the surface, or with up to 10 m of deeply weathered gravel on rock, through to Zone 5 which is underlain by more than 200 m depth of gravel with 10-30 m of soft, .flexible sediments at the surface in the lower Hutt valley, or up to 10
Summary
Records from three recently instrumented soil sites where accelerographs have been installed within the last 18 months, are discussed. The ground motions recorded in the Hutt Valley in three recent deep events that have occurred following our WRC study are investigated. These deep events, in July 1991, September 1991, and May 1992, give us an opportunity to verify the site characteristics established in the WRC study and to examine the nature of the response of the new sites. Strong-motion records obtained in the Hutt Valley from nine earthquakes are discussed in this paper. BULLETIN OF THE NEW ZEALAND NATIONAL SOCIETY FOR EARTHQUAKE ENGINEERING, Vol 25, No 4, December 1992
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