Abstract
We present the results of using seismic waveform data to show that the elliptical nature of isoseismal distributions in New Zealand is related to regional structural trends. The data also suggest that there are regional and azimuthal variations in the attenuation of coda waves, which may need to be considered in ground motion attenuation relations.
 We stacked over 20,000 waveforms from the New Zealand seismographic network. The data were filtered, normalized and stacked. Noisy or clipped records were down-weighted or removed. We also treated dense networks as a single station and generated a single stack for these networks. Stacks of shallow earthquake sources are presented by region and azimuth.
 Variations in coda length throughout New Zealand suggest regions of high scattering. Strong azimuthal dependence in the coda is observed for non-volcanic zone stations. NE-SW waveform stacks, which follow the strike of the subduction zone, contain significantly longer codas than those with NW-SE raypaths. Long coda trains are also observed in the volcanic and geothermal zones yet there is little or no apparent azimuthal variation. These coda are particularly strong throughout the records which explains the difficulty analysts have had in picking S waves.
Highlights
A common feature to New Zealand isoseismal distributions is a dominant elliptical pattern oriented roughly NE-SW as illustrated in Figure I by Downes (1995)
While these phases are clearly observed in the stacked seismograms (Figure 3) it is evident that there is a considerable difference in arrival times of these phases from the model
Seismograms of earthquakes from throughout New Zealand show azimuthal variation in coda waves which verifies prior work with isoseismal distributions
Summary
A common feature to New Zealand isoseismal distributions is a dominant elliptical pattern oriented roughly NE-SW as illustrated in Figure I by Downes (1995). Prior work has illustrated these strong ellipticities in most of the country with the exception of several volcanic zones and basins (Kozuch and Chadwick, 1997; Kozuch et al, 1996; VereJones et al, 1995; Smith, 1978, 1995) Some of these shapes were believed to be sampling artifacts due to population distributions or proximity to coastlines, yet Dowrick (1991) claims to find no systematic pattern to their shapes. The strong NE-SW trend of these ellipticities suggests that the elongation along strike with the subduction zone or structural trends might not just be a sampling artifact Modeling these isoseismals has often been difficult, given the nature of the data which heavily relies on the distribution of felt reports and the dimensions of New Zealand. The dimensions of isoseismals is dependent on magnitude (m) and epicentral distance (r): I= Io (m, r)
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