Estimating seismic site response in Christchurch City (New Zealand) from dense low-cost aftershock arrays

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The Mw 7.1 September 2010 Darfield earthquake, New Zealand, produced widespread damage and liquefaction ~40 km from the epicentre in Christchurch city. It was followed by the even more destructive Mw 6.2 February 2011 Christchurch aftershock directly beneath the city’s southern suburbs. Seismic data recorded during the two large events suggest that site effects contributed to the variations in ground motion observed throughout Christchurch city. We use densely-spaced aftershock recordings of the Darfield earthquake to investigate variations in local seismic site response within the Christchurch urban area. Following the Darfield main shock we deployed a temporary array of ~180 low-cost 14-bit MEMS accelerometers linked to the global Quake-Catcher Network (QCN). These instruments provided dense station coverage (spacing ~2 km) to complement existing New Zealand national network strong motion stations (GeoNet) within Christchurch city. Well-constrained standard spectral ratios were derived for GeoNet stations using a reference station on Miocene basalt rock in the south of the city. For noisier QCN stations, the method was adapted to find a maximum likelihood estimate of spectral ratio amplitude taking into account the variance of noise at the respective stations. Spectral ratios for QCN stations are similar to nearby GeoNet stations when the maximum likelihood method is used. Our study suggests dense low-cost accelerometer aftershock arrays can provide useful information on local-scale ground motion properties for use in microzonation. Preliminary results indicate higher amplifications north of the city centre and strong high-frequency amplification in the small, shallower basin of Heathcote Valley.

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Publication type Conference Paper
Publication Subtype Conference Paper
Title Estimating seismic site response in Christchurch City (New Zealand) from dense low-cost aftershock arrays
Year Published 2011
Language English
Publisher Incoporated Research Institute for Seismology
Contributing office(s) Earthquake Science Center
Description 11 p.
Larger Work Type Book
Larger Work Subtype Conference publication
Larger Work Title 4th International IASPEI/IAEE symposium on the effects of surface geology on seismic motion
Conference Title 4th International IASPEI/IAEE Symposium on the Effects of Surface Geology on Seismic Motion
Conference Location Santa Barbara, California
Conference Date August 23-26 2011
Country New Zealand
City Christchurch
Online Only (Y/N) N
Additional Online Files (Y/N) N
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