Distribution of ground-motion intensity inferred from questionnaire survey, earthquake recordings, and microtremor measurements—A case study in Christchurch, New Zealand, during the 1994 arthurs pass earthquake

Abstract

The areal distribution of seismic ground-motion intensity in the city of Christchurch, New Zealand, during the 1994 Arthurs Pass Earthquake (ML 6.6) was evaluated using an intensity questionnaire together with local site amplifications inferred from seismic recordings and microtremors. In order to estimate the intensity in parts of the city where no intensity data were available, intensity data were compared to relative levels of shaking determined from both weak-motion and microtremor recordings. Weak ground-motion amplification factors were determined using ratios of ground accelerations at five sediment sites with respect to a rock site. Microtremor amplification factors were determined from horizontal-to-vertical spectral ratios at a 1-km spacing throughout the city. A positive correlation between weak-motion and microtremor amplification factors allowed extrapolation of microtremor amplification to estimated MM intensity (EMMI). EMMI ranged from 3 to 6 and was consistent with the questionnaire intensity and geological conditions and showed detailed information on the areal distribution of ground-motion intensity in the city.

The influence of multiple impedance contrasts on mHVSR site period estimates in the Canterbury Plains of New Zealand and implications for site classification

“…These 12-min-long records were collected using a Micromed Tromino at a sampling frequency of 128 Hz. Mucciarelli (2011) observed mHVSR periods ranging from 0.15 to 1.0 s, which agree with the observations of Toshinawa et al (1997). A general trend of decreasing period moving westward across the city was noted.…”

“…The previous mHVSR studies (Mucciarelli, 2011; Toshinawa et al, 1997) reported site periods ranging from 0.2 to 1.0 s. Due to short time records or the limitations of instrumentation used, the long-period response of the deep basement was not identified. Thus, the reported periods likely correspond to either the period above the Riccarton Gravel or the BPV.…”

“…Several studies have collected recordings of microtremors, both in urban Christchurch and the surrounding Canterbury Plains, in efforts to improve the understanding of ground shaking in the region using results from mHVSR and/or other geophysical testing methods (Mucciarelli, 2011;Toshinawa et al, 1997;Wotherspoon et al, 2015Wotherspoon et al, , 2018. The details and experimental parameters for each of the microtremor datasets are summarized in Table 1 and were each considered for use in the present study.…”

“…The details and experimental parameters for each of the microtremor datasets are summarized in Table 1 and were each considered for use in the present study. Toshinawa et al (1997) recorded microtremors at 130 locations on a 1 km 3 1 km grid area in urban Christchurch using a portable seismometer with a flat instrument response down to a period of 10 s and digitized at a sampling frequency of 100 Hz. However, only short records (2 min long) were taken, limiting the ability of the measurements to resolve the low-frequency response.…”