Defining a reliable subsoil model is one of the most crucial points in the evaluation of the local seismic response, especially when the study area presents a complex geological setting. Performing a large number of investigations during and subsequently a seismic crisis (following a strong earthquake) is useful for collecting valuable data. By applying a multidisciplinary approach, these data can be used for constraining and test the model, as well as for directly evaluating the effects of strong earthquakes on the environment and the distribution of structural damages in heavily populated areas. In the present study, through a new multidisciplinary experimental-numerical approach, we investigated the role of local site conditions on the damage observed in the Central Business District (CBD) of Wellington (New Zealand) after the 2016 M7.8 Kaiko over bar ura event. Numerical 1D/2D site response analyses were carried out to explore the hypothesis of damage exacerbation due to basin effects. Our numerical models were then validated by comparing the corresponding numerical amplification functions with those derived from the application of the standard spectral ratio technique to a large accelerogram dataset. This dataset was obtained by GeoNet stations deployed in the study area, including those operative during the 2016 M7.8 Kaiko over bar ura event. Differences in ground motion for near- and far-field events were highlighted. Our results show that the site response in the Wellington CBD was controlled by complex 2D/3D valley effects (mainly edge effects), which were related to the local buried geomorphology. Overall, these findings suggest that the type and location of an earthquake influence the maximum distance at which 2D effects (generated at the basin edge) are still detectable. Comparing our dataset with those from other published studies would be useful for clarifying the factors leading to an exacerbation of the seismic response in alluvial basins. This study has also possible implications for seismic hazard mitigation in cities built on such basins. We conclude that robust numerical 2D models can provide and capture notable characteristics of the ground motion associated with basin effects. These characteristics are fundamental for understanding basin effects and should be considered when formulating building regulations.

© 2015. American Geophysical Union. All Rights Reserved. Determining the rates and distributions of microseismicity near major faults at different points in the seismic cycle is a crucial step toward understanding plate boundary seismogenesis. We analyze data from temporary seismic arrays spanning the central section of the Alpine Fault, New Zealand, using double-difference seismic tomography. This portion of the fault last ruptured in a large earthquake in 1717 AD and is now late in its typical 330 year cycle of Mw∼8 earthquakes. Seismicity varies systematically with distance from the Alpine Fault: (1) directly beneath the fault trace, earthquakes are sparse and largely confined to the footwall at depths of 4-11 km; (2) at distances of 0-9 km southeast of the trace, seismicity is similarly sparse and shallower than 8 km; (3) at distances of 9-20 km southeast of the fault trace, earthquakes are much more prevalent and shallower than 7 km. Hypocenter lineations here are subparallel to faults mapped near the Main Divide of the Southern Alps, confirming that those faults are active. The region of enhanced seismicity is associated with the highest topography and a high-velocity tongue doming at 3-5 km depth. The low-seismicity zone adjacent to the Alpine Fault trace is associated with Vp and Vs values at midcrustal depths about 8 and 6% lower than further southeast. We interpret lateral variations in seismicity rate to reflect patterns of horizontal strain rate superimposed on heterogeneous crustal structure, and the variations in seismicity cutoff depth to be controlled by temperature and permeability structure variations. Key Points: Seismicity is sparse near the Alpine Fault late in its typical seismic cycle Seismicity rates increase abruptly 9 km southeast of the fault trace This transition coincides with a strain rate peak and lateral velocity gradients

We utilise seismic data from the central section of the Alpine Fault to locate earthquakes and image crustal structure in three dimensions. Tomography results from c. 6500 sources reveal the fault as either a southeast-dipping low-velocity zone or a marked velocity contrast in different parts of the study region. Where our model is best resolved, we interpret the Alpine Fault to be listric in nature, dipping steeply in the upper crust (50-60 degrees) and flattening to 25-30 degrees in the lower crust. The base of the seismogenic zone shallows from c. 15km beneath the footwall and Alpine Fault to c. 6km beneath the Southern Alps Main Divide, and then deepens to c. 15km by c. 10km further southeast. The shallow brittle-ductile transition overlies a broad low-velocity zone, which together likely result from the presence of fluids and elevated temperatures brought about by enhanced exhumation rate in this section of the Alpine Fault.

Rotokawa geothermal field, Taupo Volcanic Zone (TVZ), New Zealand, has experienced persistent microseismicity since deep injection of condensate and brine began in 2005. Microseismicity is thought to be induced mostly by contraction due to injected fluid, c. 200 degrees C colder than natural reservoir temperatures, causing a reduction in the normal stress on pre-existing faults and fractures, which then slip. This mechanism is consistent with the ability to improve the permeability of Rotokawa injection wells over several years by stimulating them with cold water injection. Any wide-spread increase in reservoir pressure due to injection has been less than 1 bar (0.1 MPa) and has been restricted to areas close to injection wells; pressure change is therefore less likely to drive induced microseismicity.Since mid-2008 more than 1000 events of magnitude >= 0.8, 50 of magnitude >= 2, and 2 of magnitude >= 3 (largest M3.1) have been located in the field, over 70% in a sharply bounded zone approximately 1 km(2) in area and 1.5-3 km deep located between injection and production zones. This zone is believed to outline a compartment in the reservoir that influences the pressure temperature distribution during production and injection. A three-fold increase in deep injection rate accompanying the start-up of a second power station in early-2010 was accompanied by an almost equivalent increase in the rate of microseismicity. The location of microseismicity appears to be controlled by several factors, including: the presence of fractures, the locus of injection, injectate flow driven by pressure gradients, and by fault-controlled permeability and barriers to lateral fluid flow. The boundary of the main microseismicity zone closest to the production reservoir is controlled by the Central Field Fault (CFF), which acts as a partial barrier to lateral flow that slows injected fluid reaching the production zone. Many of the magnitude >= 2 events occur immediately adjacent to the CFF. The location of the CFF and the depth to the base of reservoir permeability, estimated from the depth of the deepest microseismicity, have been incorporated in numerical models of the field and have influenced decisions such as siting of new wells. Although two events of magnitude >= 3 occurred at Rotokawa in 2012, microseismicity of similar magnitude is common throughout the TVZ, so activity at Rotokawa has not caused any public concern. (C) 2014 Elsevier Ltd. All rights reserved.

New Zealand’s geothermal systems are mostly located in high-temperature, volcano-tectonic settings, but background levels of local natural microseismicity (micro-earthquakes) are quite variable. Triggered or induced seismicity effects from geothermal operations have also been very mixed. Understanding the reasons for these differences leads to better comprehension of the potential seismic risks and rewards (or opportunities) of different development options. Large-scale NZ geothermal projects started in the late 1950’s, but reinjection, the most commonly attributed cause of induced seismicity, only commenced in the mid 1980’s (at Wairakei). Here, initial injection trials were relatively shallow (~1.3 km) and triggered some local low-magnitude microseismicity, at high effective stimulation pressure (~5 MPa). Other, low-pressure, reinjection projects soon followed, with subsequent changes in injection strategy (depth, location, in-situ temperature, pressure and flow-rate). They included: Ohaaki (1988, deep to shallow), Kawerau (1992, shallow to deep), Rotokawa (1997, shallow to deep), Ngawha (1998, deep), Mokai (2000, shallow to deep), and Ngatamariki (2013, deep). A wide range of induced seismicity responses has accompanied these injection strategies and this paper provides an overview of these experiences. The overall objective of much of this research is to illuminate probable mechanisms, identify zones of potential fracture permeability enhancement from microseismic locations, thereby provide information on probable reservoir boundary conditions for simulation models, and help provide possible mitigation options, if (and when) induced seismicity magnitudes and felt event rates exceed acceptable values. At Ohaaki and Ngawha, natural seismicity rates are relatively low, and local induced seismicity (M>2) has not been detected, despite Ngawha’s 100% peripheral injection to ~1 km depth and Ohaaki’s 70% peripheral injection (<1km depth). At Kawerau, natural rates of seismicity are high (average 2 felt events/month), but there have been no obvious triggered events associated with production or injection changes, including an expansion and transition in 2008 from shallow (0.4 km) infield to mostly deep (2 km) peripheral injection. At Wairakei, Rotokawa, Ngatamariki and Mokai (adjacent systems), natural seismicity is moderate, but locally variable. Deep reinjection (increasing since 2006) has, in places, triggered moderate levels of microseismicity within inferred fault zones between injection and production sectors. The maximum magnitude recorded has been ML 3.5 (local network magnitude 3.1), but most are well below ML 2.5, and felt seismicity effects have not been an issue with the local rural inhabitants, who are familiar with similar-sized natural events. There is some evidence of cooling contraction increasing permeability with time, and microseismicity constrained by fault-controlled flow barriers. Our conclusion is that, in New Zealand, where examples of induced seismicity have occurred, the favoured mechanism is associated with the indirect effects of increased fluid-flow on pre-stressed, pre-existing, fracture networks. This flow induces stresses from cooling contraction, and is driven by pressure gradients through the fracture network, but triggers seismic failure only on favourably-oriented fractures, through thermal, chemical, or pressure transients, or by associated micro-stress perturbations, locally unlocking asperities on pre-stressed fractures. We propose several conditions that increase the likelihood that an operating geothermal field will experience reinjection-driven induced seismicity.

Sub-crustal earthquakes have been observed sporadically for ∼40 years in the central South Island of New Zealand. We report on 20 events recorded between December 2008 and February 2012 near the Alpine Fault in the continental collision zone between the Australian and Pacific plates. A subset of 18 events at depths of 47–74 km occurs south of Mt. Cook and together with recently reported tremor locations indicates along-strike variations in deformation behaviour along the plate boundary. The sub-crustal earthquakes south of Mt. Cook increase in depth, frequency and size southwards towards the Puysegur subduction zone. Focal mechanisms could be determined for 14 earthquakes and exhibit predominantly strike-slip and reverse faulting solutions. Stress inversion analysis of the focal mechanisms yields a stress field favouring oblique-reverse faulting. We interpret the geographic and vertical distributions of these sub-crustal events in relation to a previously proposed tectonic model of a remnant passive margin that formed south of New Zealand in the Eocene and was overridden when dextral strike-slip motion initiated on the Alpine Fault. We infer that sub-crustal earthquakes occur along the leading edge of this structure, which is attached to the continental Australian crust.

We are investigating bulk seismic properties and fault structure in the Rotokawa geothermal field, New Zealand, by accurately locating and characterizing microseismicity recorded by temporary seismometer arrays, deployed in 2006 and 2008-2009. Analysis approaches used in microearthquake location at Rotokawa and other similar fields need to allow for high background noise levels, complex sub-surface geology, and the high attenuation of near-surface volcanic deposits often experienced in New Zealand. The location problem is exacerbated by high subsurface temperatures, which currently negate the use of down-hole sensors for any length of time. Our focus in the analysis workflow is on increasing the signal-to-noise ratio of data recorded by surface seismometers, to enable higher accuracy for Pand S-wave arrival picks, dealing with data which occasionally has strongly polarized background noise. We can do so through the use of adaptive polarization and S-transform based filtering before slowness estimation, followed by rotation of 3-component data into ray coordinates (L-, T-, and Qcomponents) before S-wave picking. Waveform cross-correlation is then carried out on waveform data, before double-difference relocation of the microearthquakes, allowing for 3D heterogeneous velocity structure.

An 8.54 +/- 0.20 km s(-1) Pn speed is estimated on a line oriented ca. N5 degrees E from the Alpine Fault beneath the Southern Alps of South Island based on a refraction experiment that uses the Fiordland earthquake of August 2003 as a source. This high Pn speed results from both strong anisotropy in the mantle lid of 7-13 per cent and a high Pn speed average of 8.3 +/- 0.3 km s(-1). A maximum crustal thickness of 48 +/- 4 km is calculated for the southern South Island near the town of Wanaka. This represents a crustal root of about 18 km, compared to measured crustal thicknesses at the east and west coasts of the South Island. The average topography in the southern Southern Alps is of the order of similar to 1000 m, which is less than half that predicted by Airy isostasy for an 18 km crustal root. As recently proposed for the central South Island similar to 120 km to the north, we propose that thickened cold, and therefore more dense, mantle lithosphere exists beneath southern South Island, and that this excess of mass is an effective load that pulls down the overlying crust. The load is similar to that beneath the central Southern Alps, despite the predicted convergence across the Alpine Fault there being nearly twice that at Wanaka. Gravity modelling of crustal structure along a profile through Wanaka suggests that this mass excess has a minimum density contrast of 35 +/- 5 kg m(-3) between thickened mantle and asthenosphere, assuming an across-Moho density contrast of -300 kg m(-3). We speculate that the reason for the enhanced thickening beneath Wanaka is that the subducted Australian Plate at the southwestern corner of the South Island acts like a backstop onto which Pacific mantle collides at similar to 26 mm yr(-1), ca. 3/4 the full plate speed.

Over-thickened crust and fast, anisotropic mantle material are interpreted beneath South Island, New Zealand, from an earthquake refraction study along the Southern Alps foothills. An 8.54 ± 0.20 km/s Pn speed is estimated along the N60°E striking refraction profile and a maximum crustal thickness of 48 ± 4 km is inferred near Wanaka township, at the southern end of the profile. The crustal thickness represents an 18 km thick crustal root relative to a 30 km coastal average. Thus, the root is 2-3 times thicker than expected for Airy isostatic compensation of the mean ∼1000 m Southern Alps topographic load. This suggests that the underlying mantle plays an active role in depressing topography. Comparison of the 8.54 ± 0.20 km/s Pn-speed estimate with cross profiles suggests anisotropy arising from finite strain of the mantle lid rocks. The Pn anisotropy is estimated near Lake Tekapo, at the northern end of the profile, to be a minimum of 6.5 ± 3.5%. We predict a maximum Pn anisotropy of 7-13% and an average isotropic Pn speed of ∼8.3 km/s by adopting the fast polarization orientation from previous SKS splitting measurements done at the profile intersection. The Pn speed of 8.3 km/s is consistent with previous studies showing high average Pn speeds below the southern half of South Island and the presence of cold, dense mantle lithosphere.

From a cost point of view SNECMA has found DS columnar grain manufacturing technology to be highly attractive compared to single crystal. CM 186 LC alloy exhibits enhanced mechanical and environmental properties and temperature capability compared to MAR M 200 Hf alloy; these properties are close to first-generation single crystal alloys up to 982°C (1800°F). The alloy is shown to be amenable to various coating and brazing high-temperature processes. The longer term creeprupture/phase stability data base on the alloy has now been extended out to 8300 hours at 1038°C (1900°F). Castings for engine test have been produced using CM 186 LC alloy.