Euan G. C. Smith

Subduction and back-arc activity at the Hikurangi convergent Margin, New Zealand

The Hikurangi Margin is a region of oblique subduction with northwest-dipping intermediate depth seismicity extending southwest from the Kermadec system to about 42°S. The current episode of subduction is at least 16–20 Ma old. The plate convergence rate varies along the margin from about 60 mm/a at the south end of the Kermadec Trench to about 45 mm/a at 42°S. The age of the Pacific lithosphere adjacent to the Hikurangi Trench is not known.The margin divides at about latitude 39°S into two quite dissimilar parts. The northern part has experienced andesitic volcanism for about 18 Ma, and back-arc extension in the last 4 Ma that has produced a back-arc basin onshore with high heaflow, thin crust and low upper-mantle seismic velocities. The extension appears to have arisen from a seawards migration of the Hikurangi Trench north of 39°S. Here the plate interface is thought to be currently uncoupled, as geodetic data indicate extension of the fore-arc basin, and historic earthquakes have not exceededMs=7.South of 39°S there is no volcanism and a back-arc basin has been produced by downward flexure of the lithosphere due to strong coupling with the subducting plate. Heatflow in the basin is normal. Evidence for strong coupling comes from historic earthquakes of up to aboutMs=8 and high rates of uplift on the southeast coast of the North Island.The reason for this division of the margin is not known but may be related to an inferred increase, from northeast to southwest, in the buoyancy of the Pacific lithosphere.

A seismic velocity profile across the central South Island, New Zealand, from explosion data

Abstract Data from a seismic experiment across the central South Island, New Zealand, were used to model the crustal structure of the Australia‐Pacific plate continent‐continent boundary to a depth of c. 40 km. Large explosions were detonated off each coast and in Lake Tekapo and recorded on 19 seismographs deployed in a line from Timaru to just south of Fox Glacier, and on the Pukaki Seismograph Network. In the upper crust, a clear difference was found between the seismic velocities in Permian and younger rocks, in agreement with findings from other studies. Within the Permian schists and greywackes, the P‐wave velocity is 5.8 km/s to 2.7 km depth and 6.0 km/s below, contrasting with 5.4 km/s in the Triassic greywackes which form the basement near Lakes Pukaki and Tekapo in the central part of the survey area. Strong second arrivals from a layer 24 km deep at the east coast, and dipping inland at 3.4°, were seen from both offshore shots. The true velocity of the arrivals was determined to be 7.2 km/s. Ar...

Adaptive Kernel Estimation and Continuous Probability Representation of Historical Earthquake Catalogs

To develop spatially continuous seismicity models (earthquake probability distributions) from a given earthquake catalog, the method of kernel estimation has been suggested. Kernel estimations with a global (spatially invariant) bandwidth deal poorly with earthquake hypocenter distributions that have different spatially local features. For example, a typical earthquake catalog has several areas of high activity (clusters) and areas of low-background seismicity. An alternative approach is adaptive kernel estimation, which uses a bandwidth parameter that is spatially variable. Its performance compared to kernel estimations with spatially invariant bandwidths suggests that (discrete) earthquake distributions require different degrees of local smoothing to provide useful spatial seismicity models. Using adaptive kernel estimation, the (local) indices of temporal dispersion of any earthquake probability distribution can be estimated and used to model the spatial probability distribution of mainshocks. The application of these methods to New Zealand and Australian earthquake catalogs shows that the spatial features (earthquake clusters) in which the mainshocks occurred have been reasonably stable throughout the observation period. The observed regions of higher activity persisted throughout the observation period, and none of these regions decreased to background activity during that time. This suggests that these regions will continue to represent higher risk for the occurrence of moderate to large earthquakes within the next few years or decades. Furthermore, it has been observed that shallow earthquakes are mostly part of temporal sequences (e.g., aftershocks or swarms), whereas the earthquakes within the subduction zones in New Zealand showed only small temporal variation during the observation period.

Seismic studies of the crust under the hydrothermal areas of the Taupo Volcanic Zone, New Zealand

Abstract Observations of relative P-wave travel-time residuals for local mantle earthquakes and P/S amplitude ratios for more distant events have been used to investigate the nature of the crust under the hydrothermal areas of the central Taupo Volcanic Zone. In contrast to some other regions of similar activity, such as the Yellowstone and Geysers geothermal areas, U.S.A., there is no evidence in the data for an extensive region of semi-molten or molten rock in the crust. Thus the large amounts of mafic intrusive rock and associated crustal melt inferred to be present on geological grounds must be now largely at temperatures below the appropriate solidus. The data do provide further evidence for previously suspected lateral inhomogeneity in the underlying mantle.

The Edgecumbe earthquake sequence: 1987 February 21 to March 18

Abstract The Edgecumbe earthquake sequence consisted of more than 600 shocks of M L ≥ 3. 0 that occurred in an area of about 70 × 50 km in the Bay of Plenty, New Zealand, between 1987 February 21 and March 18. About 130 of these occurred prior to the main shock (M L 6. 3) on March 2. The number, distribution, and variable sensitivity of the stations of the National Seismograph Network has meant that the accuracy of the epicentres computed from data from these stations alone is excellent: by comparison with epicentres determined from temporary station data, the absolute (systematic) uncertainty of epicentres in the vicinity of the i s is estimated to be about 2 km, and the relative uncertainty of epicentres is estimated to be ± 4 km. The analysis of variance technique was applied to the magnitude data to obtain a set of self-consistent values across the whole mnge of magnitudes observed. A large discrepancy was evident between the apparent magnitudes at KRP and those at the east coast stations TUA and GNZ,...

Joint hypocentre determination of intermediate depth earthquakes in Fiordland, New Zealand

Abstract Relocation of well observed, intermediate depth earthquakes in the Fiordland region by the method of joint hypocentre determination has revealed some fine structure in the Benioff zone. The earthquakes occur in three groups. The central group is the largest and occupies a planar volume less than 15 km thick striking N40°E and dipping at 80°. The deepest events in the region, at depths of 150 km, occur at the northeast end of this group. The two smaller groups lie to the northeast and to the south of the main group. The focal mechanism of the majority of the main group is that of thrust faulting. We suggest that the main group lies within a section of Indian plate lithosphere which has been broken off and rotated into its observed position and that the northern edge of the unbroken subducted Indian plate is indicated by the southern group. We suggest that the small northeastern group has quite a different tectonic origin and is similar to a group of earthquakes further north which are at a similar distance from, and presumably related to, the Alpine Fault. Use has also been made of the travel-time information which is a by-product of the joint hypocentre method to construct upper mantle velocity models for P and S waves in the South Island. The features of this model are a high-velocity region in the vicinity of the Benioff zone, and a subcrustal zone of high seismic velocities running east-west across the center of the South Island in an otherwise normal mantle.

Comparison of Seismicity Models Generated by Different Kernel Estimations

Different forms of kernel estimation have been used for the construction of spatially continuous earthquake occurrence representations. A method has been developed to quantify the difference between such representations. The method compares seismicity models generated by kernel estimation for an earlier and a later time period to measure the ability of the models to forecast the locations of future earthquakes. Further, the (earlier) models are compared to the distribution of the original earthquake catalog to measure the closeness of representation and data. The method has been applied to historical earthquake catalogs of New Zealand and Australia. We found that kernel estimations with spatially varying bandwidths performed better than kernel estimations with spatially constant bandwidths. The use of a temporal sequence filter improved the match between the earlier and later models. The Gaussian kernel provided better results than the inverse-biquadratic kernel, due to the longer tails of the inverse-biquadratic kernel, which did not describe the distribution of earthquakes in space accurately. A bandwidth of about 1.0 (in area units) for the Gaussian kernel provided the best results, whereas a bandwidth of about 0.4 provided the best results for the inverse-biquadratic kernel.

Continent‐scale strike‐slip on a low‐angle fault beneath New Zealand's Southern Alps: Implications for crustal thickening in oblique collision zones

New Zealands Southern Alps lie adjacent to the continent-scale dextral strike-slip Alpine Fault, on the boundary between the Pacific and Australian plates. We show with a simple 2-D model of crustal balancing that the observed crustal root and erosion (expressed as equivalent crustal shortening) is up to twice that predicted by the orthogonal plate convergence since ∼11 Ma, and even since ∼23 Ma when the Alpine Fault formed. We consider two explanations for this, involving a strong component of motion along the length of the plate-boundary zone. Geophysical data indicate that the Alpine Fault has a listric geometry, flattening at mid crustal levels, and has accommodated sideways underthrusting of Australian plate crust beneath Pacific plate crust. The geometry of the crustal root, together with plate reconstructions, requires the underthrust crust to be the hyperextended part of an asymmetric rift system which formed over 500 km farther south during the Eocene—the narrow remnant part today forms the western margin of the Campbell Plateau. At ∼10 Ma, the hyperextended margin underwent shallow subduction in the Puysegur subduction zone, and then was dragged over 300 km along the length of the Southern Alps beneath a low-angle (<20°) section of the Alpine Fault. We speculate that prior to 10 Ma, more distributed lower crustal shortening and thickening occurred beneath the Southern Alps, accommodating southward extrusion of continental crust in the northern part of the plate boundary zone, providing a mechanism for clockwise rotation of the Hikurangi margin.

The relationship between adolescents' well-being and their wireless phone use: a cross-sectional study

BackgroundThe exposure of young people to radiofrequency electromagnetic fields (RF-EMFs) has increased rapidly in recent years with their increased use of cellphones and use of cordless phones and WiFi. We sought to ascertain associations between New Zealand early-adolescents’ subjective well-being and self-reported use of, or exposure to, wireless telephone and internet technology.MethodsIn this cross-sectional survey, participants completed questionnaires in class about their cellphone and cordless phone use, their self-reported well-being, and possible confounding information such as whether they had had influenza recently or had a television in the bedroom. Parental questionnaires provided data on whether they had WiFi at home and cordless phone ownership and model. Data were analysed with Ordinal Logistic Regression adjusting for common confounders. Odds ratios (OR) and 95% confidence intervals were calculated.ResultsThe number and duration of cellphone and cordless phone calls were associated with increased risk of headaches (>6 cellphone calls over 10 minutes weekly, adjusted OR 2.4, CI 1.2-4.8; >15 minutes cordless use daily adjusted OR 1.74, CI 1.1-2.9)). Texting and extended use of wireless phones was related to having a painful ‘texting’ thumb). Using a wired cellphone headset was associated with tinnitus (adjusted OR 1.8, CI 1.0-3.3), while wireless headsets were associated with headache (adjusted OR 2.2, CI 1.1-4.5), feeling down/depressed (adjusted OR 2.0, CI 1.1-3.8), and waking in the night (adjusted OR 2.4, CI 1.2-4.8). Several cordless phone frequencies bands were related to tinnitus, feeling down/depressed and sleepiness at school, while the last of these was also related to modulation. Waking nightly was less likely for those with WiFi at home (adjusted OR 0.7, CI 0.4-0.99). Being woken at night by a cellphone was strongly related to tiredness at school (OR 3.49, CI 1.97-6.2).ConclusionsThere were more statistically significant associations (36%) than could be expected by chance (5%). Several were dose-dependent relationships. To safeguard young people’s well-being, we suggest limiting their use of cellphones and cordless phones to less than 15 minutes daily, and employing a speaker-phone device for longer daily use. We recommend parental measures are taken to prevent young people being woken by their cellphones.

Models of the upper mantle beneath the central North Island, New Zealand, from speeds and anisotropy of subhorizontal P waves (Pn)

[1] The central North Island of New Zealand shows significant variations in Pn wave speeds over small distances, ranging from 8.5 ± 0.2 km/s to 7.4 ± 0.1 km/s over a distance of 150 km. A combination of national network seismometers, local volcanic seismic monitoring networks, and temporary deployments are used to collect arrival times from local events, during the period of 1990-2006. The data set consists of approximately 11,200 Pn observations from 3000 local earthquakes at 91 seismograph sites. We have created a method that allows us to model the predominant wavelength features of P wave speeds in the uppermost mantle, as well as estimating values of mantle anisotropy and irregularities in the crust beneath stations, using least squares collocation. The resulting model shows distinct variations in uppermost mantle Pn velocities. Velocities of less than 7.5 km/s are found beneath the back-arc extension region of the Central Volcanic Region, and under the Taranaki Volcanic Region, indicating the presence of water and partial melt. The region to the east shows extremely high velocities of 8.3-8.5 km/s, where the P waves are traveling within the subducting Pacific slab. Slightly lower than normal mantle velocities of 7.8-8.1 km/s are found in the western North Island, suggesting a soft mantle. Pn anisotropy estimates throughout the North Island show predominately trench-parallel fast directions, ceasing to nulls in the west. Anisotropy measurements indicate the strain history of the mantle. Null anisotropy measurements suggest an undisturbed mantle, suggesting that mantle beneath the western North Island is young.