Ken Gledhill

Upper mantle anisotropy in the New Zealand Region

Shear-wave splitting parameters of fast polarization direction (Φ) and delay time (δt) are determined using data from the Southern Alps Passive Seismic Experiment (SAPSE), on the South Island of New Zealand and in the surrounding region. Our results clearly show that Φ are subparallel to trends of the Alpine and Marlborough Faults, and to the Pacific-Australian plate boundary. The δt values range from 0.6–2.2 s with an average value of 1.6 s; the largest values are from the central South Island. The main source of the observed shear-wave splitting is an anisotropic region between 40–400 km. The width of the zone is approximately 200 km. We attribute the coincidence of surface structural trends with the measured Φ, and the large δt values, to significant shear deformation in a 200 km thick zone along the plate boundary extending from the surface to deep within the upper mantle.

The MW 6.2 Cass, New Zealand, earthquake of 24 November 1995: Reverse faulting in a strike‐slip region

Abstract On 24 November 1995 an earthquake of moment magnitude MW 6.2 struck near the small settlement of Cass in the Southern Alps, South Island, New Zealand. Body‐wave modelling using teleseismic arrivals gives an oblique reverse focal mechanism for the mainshock, with the fault plane striking approximately north‐south, and a shallow centroid depth of 3–6 km. Aftershock recordings at the station SNZO near Wellington were used as empirical Greens functions to estimate a source time function duration of 7 s. A joint inversion for velocity and location of 169 selected events was used to derive a one‐dimensional velocity model with station terms, and this velocity model was then used to relocate all recorded aftershocks. A subset of the best 803 events was then selected for further analysis. The apparent trend of the aftershock zone is NNW‐SSE, with the mainshock near the centre. However, projections of the aftershocks on north‐south and east‐west cross‐sections show a band of activity shallowing to the south and dipping to the west. The north‐striking, west‐dipping nodal plane of the mainshock focal mechanism is therefore most likely to be the fault plane. Early aftershocks occurred mainly to the south of the mainshock location, suggesting rupture to the south, a feature supported by the mainshock modelling. The aftershock focal mechanisms are mixed but reflect the regional stress field (NW‐SE compression).

A Detailed Noise Characterization and Sensor Evaluation of the North Island of New Zealand Using the PQLX Data Quality Control System

In this paper we analyze five years of recordings (2005-2009) from the National Seismograph Network in the North Island of New Zealand using the power spectral density probability density function (PDF) method of McNamara and Buland (2004). At each station the ambient noise is characterized and the stable noise model is then represented with the modes of the corresponding PDFs over all periods. Obtaining such an accurate long-term noise baseline for each station provides a reference model that should serve to prioritize maintenance issues for the network operators. The PDF mode low-noise model (MLNM) for the North Island is then obtained from the minimum of all the noise modes at each period. The maximum and minimum differences between the North Island MLNM and the noise mode model periods are then calculated at each station as a quick assessment tool. The daily and seasonal variations of the noise mode model are then characterized, and the horizontal and vertical mode noise models are then compared at each station. The applied tech- nique is practical for evaluating the cultural noise condition and the earthquake detec- tion capability, as well as the installation design against unwanted tilt and temperature variation.

The Ormond, New Zealand, earthquake of 1993 August 10: Rupture in the mantle of the subducted Pacific plate

Abstract Data from temporary seismographs installed immediately after the Ml 6.3 Ormond earthquake of 1993 August 10 have been used to determine the nature of faulting which took place during the event. The rupture began at 37 km depth, within the mantle of the subducted Pacific plate, and aftershocks extended from near the base of the subducted crust to c. 20 km into the subducted mantle. Aftershocks in the mantle decayed exceptionally rapidly compared with those in the crust of the subducted plate. This may reflect a hotter, more ductile mantle and/or relatively homogeneous rupture within the mantle during the mainshock. Aftershocks within the mantle show a variety of thrusting mechanisms. Focal mechanisms of aftershocks within the subducted crust indicate that compression along strike dominates over slab pull, and that the down‐dip stress has a similar magnitude to the vertical stress. This suggests that, at least after the Ormond earthquake, the tectonic stress coupled across the plate interface is ra...

Applying Haar and Mexican Hat Wavelets to Significantly Improve the Performance of the New Zealand GeoNet P‐Phase Picker for the 2008 Matata Region Swarm

Abstract The New Zealand GeoNet P ‐phase picker’s performance can be significantly improved, by up to 70%, when Mexican hat and Haar wavelet scale thresholding (WST) is implemented in the preprocessing stage of manually picked events. In total, 811 events with the required spatial and quality criteria from the 2008 Matata swarm were studied. The WST was applied to manually processed waveforms recorded by key stations, including 48 waveforms recorded by URZ; 47 by stations LIRZ, MARZ, and OPRZ; 64 by EDRZ; 33 by KARZ; and 47 by OPRZ. The signal‐to‐noise ratios of these key stations were dominated by additive noise rather than weaknesses introduced by the radiation pattern. We found that the Mexican hat WST produced the largest amount of retrieved and revised P onsets for station EDRZ (70.3%). The next best station was KARZ, which had 66.7% of picks showing improvement and only 3% of picks adversely affected. The main best scales (i.e., the wavelet’s degree of compression) for stations EDRZ and KARZ were scales 1 and 2, respectively. Haar WST for the OPRZ recordings resulted in a total improvement greater than 50%, with the lowest adversely affected picked onsets of 2.1%. Similarly, the Haar WST retrieved a considerable number of missed onsets at the TGRZ and LIRZ stations (25.5% and 20%, respectively), with a minor effect on the performance. The results confirmed that WST served to significantly enhance the performance of the GeoNet P picker for the chosen Matata earthquakes due to the frequency localization of the WSR compared to the current Butterworth filtering scheme. Online Material: Figure and table of quality of picks.

PQLX noise model for the URZ station in the Matata region of New Zealand

This paper includes the main topics associated with seismic noise modeling. First part will describe the seismic noise modeling bases and its corresponding mathematical approaches. The second part will review the main approaches taken in practice to find the noise models. Two main research performed by Peterson [1] and McNamara [2] will be studied. The third part reviews the software tool developed in this regard. The PASCAL Quick Look eXtended (P.Q.L.X) package developed by Boaz [3–5], will be introduced and applied to build up a noise model for the URZ station in Matata, New Zealand [6]. It is shown that the noise model is consistent with the URZ sensor installation and geological features in the URZ region. Such a model serves to reproduce accurate characteristics of the noise sources within the URZ region which are of great importance to improving earthquake detection in the URZ region.

Iterative coupling of standardised earthquake detection & wavelet thresholding to determine simplified earthquake event waveforms (SEEW)

This paper introduces a new method to determine simplified earthquake event waveforms (SEEW) of regional earthquakes from their seismological signals which are heavily plagued with seismic noise. This method iteratively couples a standardised earthquake detection procedure with a wavelet thresholding procedure in order to cut through the seismic noise floor and determine the SEEW. The suggested method is implemented and tested for 3 events from a deep borehole station URZ in the Matata region of New Zealand. These preliminary results show that it is possible to dramatically simplify the earthquake event and hence reveal important underlying dynamical information of the system.