New constraints on seismicity in the Wellington region of New Zealand from relocated earthquake hypocentres

Abstract

The Wellington region of New Zealand overlies a strongly coupled portion of the shallow subduction interface between the Pacific and Australian plates. To better understand the active deformation in the area, we perform a double-difference (DD) relocation of 6825 local earthquakes that occurred between 1990 and 2001. Using both cross-correlation (CC) and bispectrum (BS) methods, we calculate high-quality waveform-based differential times (WBDTs) for event pairs. We manage to more than triple the S-wave differential time measurements after reasonably estimating many of the unpicked S arrivals. After relocation, the image of the double seismic zone beneath this region is greatly sharpened. We find several northeast-striking linear seismic features near Lake Wairarapa on the North Island, which may result from slip along normal faults within the subducting Pacific Plate. We further search for repeating events that might define creeping patches along the interplate thrust. Using a 20-s data window for CC, we find 287 event clusters with very high waveform similarities. Most of these clusters contain just a doublet that occurred within a short time period. The failure to find repeating earthquakes along the plate boundary in this 12-yr time period is consistent with the hypothesized strong coupling between the Pacific and Australian plates under the Wellington region. © 2004 RAS.