Agency: GTR | Branch: NERC | Program: | Phase: Research Grant | Award Amount: 196.01K | Year: 2016
Subduction zones are located where one of the Earths tectonic plates slides beneath another - this motion is controlled by the plate boundary fault. These plate boundary faults are capable of generating the largest earthquakes and tsunami on Earth, such as the 2011 Tohuku-oki, Japan and the 2004 Sumatra-Andaman earthquakes, together responsible for ~250,000 fatalities. Although some plate boundary faults fail in catastrophic earthquakes, at some subduction margins the plates creep past each other effortlessly with no stress build-up along the fault, and therefore large earthquakes are not generated. Determining what controls whether a fault creeps or slips in large earthquakes is fundamental to assessing the seismic hazard communities living in the vicinity of plate boundary faults face and to our understanding of the earthquake process itself. In the last 15 years a completely new type of seismic phenomena has been discovered at subduction zones: silent earthquakes or slow slip events (SSEs). These are events that release as much energy as a large earthquake, but do so over several weeks or even months and there is no ground-shaking at all. SSEs may have the potential to trigger highly destructive earthquakes and tsunami, but whether this is possible and why SSEs occur at all are two of the most important questions in earthquake seismology today. We only know SSEs exist because they cause movements of the Earth that can be measured with GPS technology. Slow slip events have now been discovered at almost all subduction zones where there is a good, continuous GPS network, including Japan, Costa Rica, NW America and New Zealand. Importantly, there is recent evidence that SSEs preceded and may have triggered two of the largest earthquakes this decade, the 2011 Tohuki-oki and 2014 Iquique, Chile earthquakes. Therefore, there is an urgent societal need to better understand SSEs and their relationship to destructive earthquakes. We know little about SSEs because most of them occur at depths of 25-40 km: too deep to drill and to image clearly using seismic data, a remote method that uses high-energy sound waves to probe the Earths crust. The Hikurangi margin of northern New Zealand is an important exception. Very shallow SSEs occur here at depths of c. 5 km below the sea bed, and they occur regularly every 1-2 years. This SSE zone is the only such zone worldwide within likely range of modern drilling capabilities and where we can image the fault clearly with seismic techniques - this location provides us with an opportunity to sample and image the fault zone that slowly slips. This will allow testing of a number of different hypotheses proposed to explain SSEs. We can also compare the properties of these rocks with drilling and seismic data from other locations such as Japan, where the faults behave differently and generate very large earthquakes. Through this comparison we can get closer to understanding why some subduction margin faults fail in large earthquakes and others do not and what fault properties control the different slip processes. Before the drilling can take place we need 3D seismic data to characterise the drill site to highlight any potential risks and to allow us to learn more about how rock properties vary in three dimensions away from the drill sites. Even before or without drilling the seismic images will provide important details of the slow slip process and fault properties. We will use a new technique, called full-waveform inversion (FWI) that can produce high resolution models of the speed of sound waves through the Earths crust. Sound waves travel slower through rocks that contain a lot of fluids so we will look for low velocity anomalies signifying the presence of fluids, which models have suggested could allow generation of SSEs. The groundbreaking FWI imaging of the New Zealand subduction zone will be the first of its kind, providing information on fault zone properties at unprecedented resolution.
Reyners M.,Institute of Geological & Nuclear Sciences
Earth and Planetary Science Letters | Year: 2013
Recent work involving relocation of New Zealand seismicity using a nationwide 3-D seismic velocity model has located the subducted western edge of the Hikurangi Plateau. Both the thickness (ca. 35km) and the area of the plateau subducted in the Cenozoic (ca. 287,000km2) are much larger than previously supposed. From ca. 45Ma, the westernmost tip of the plateau controlled the transition at the Pacific/Australia plate boundary from subduction to the north to Emerald Basin opening to the south. At ca. 23Ma, curvature of the subduction zone against the western flank of the buoyant plateau became extreme, and a Subduction-Transform Edge Propagator (STEP fault) developed along the western edge of the plateau. This STEP fault corresponds to the Alpine Fault, and the resulting Pacific slab edge is currently defined by intermediate-depth seismicity under the northernmost South Island. Alpine STEP fault propagation was terminated at ca. 15Ma, when the western edge of the plateau became parallel to the trench, and thus STEP fault formation was no longer favoured. Wholesale subduction of the plateau at the Hikurangi subduction zone began at ca. 10Ma. The development of a subduction décollement above the plateau mechanically favoured deformation within the overlying Australian plate continental crust. This led to inception of the Marlborough fault system at ca. 7Ma, and the North Island fault system at 1-2Ma. At ca. 7Ma, the western edge of the converging plateau again became more normal to the trench, and there is evidence supporting the development of a second STEP fault beneath the Taupo Volcanic Zone until ca. 3Ma. Both episodes of STEP fault development at the plateau edge led to rapid slab rollback, and correspond closely with episodes of backarc basin opening to the north in the wider Southwest Pacific. The Cenozoic tectonics of New Zealand and the Southwest Pacific has been strongly influenced not only by the resistance to subduction of the buoyant Hikurangi Plateau, but also by the shape of its western edge and changing angle of attack of this edge at the plate boundary. © 2012 Elsevier B.V.
Mortimer N.,Institute of Geological & Nuclear Sciences
Journal of Structural Geology | Year: 2014
Most of the South Island of New Zealand lies within an Eocene-Recent continental shear zone related to Pacific-Australia plate motion. Macroscopic finite strain in this shear zone has, in the past, been tracked through the deformation of the Dun Mountain Ophiolite Belt. This paper identifies additional sub-vertical basement strain markers including: Buller-Takaka Terrane boundary, Darran Suite and Jurassic volcanic belt within the Median Batholith, Taieri-Wakatipu-Goulter Synform axial trace, Esk Head Melange and bedding form surfaces within the Buller, Takaka and Torlesse terranes. An analysis of the oroclinal bend over the entire Zealandia continent shows that it is a composite feature involving pre- as well as post-Eocene bending of basement structures. Satisfactory paleogeographic reconstructions of Zealandia cannot be made without the use of substantial regional scale, non-rigid intracontinental deformation. © 2013 Elsevier Ltd.
Zhao J.X.,Institute of Geological & Nuclear Sciences
Bulletin of the Seismological Society of America | Year: 2010
Attenuation models derived from recorded ground motions are still important elements of probabilistic seismic hazard studies. Engineers use empirical attenuation models to derive the displacement demand for a site of interest from an earthquake at a given location. Many attenuation models have been published for different parts of the world and for different types of earthquakes. Most models have a simple function of constant or magnitude-dependent geometric spreading, and seldom consider well-known seismological effects such as Moho reflection for shallow crustal earthquakes, multiple travel paths and constructive interference for subduction earthquakes, and special characteristics of volcano zones. The reason for not accounting for such effects may be the desire for simplicity in the attenuation functional forms for engineering applications and a lack of records from which to reliably identify these effects quantitatively. In this article, a large set of strong-motion records obtained from dense recording networks in Japan is used to derive geometric attenuation functional form and a possible manner to model the effect of volcanic zones. A liberal approach is taken to introduce a relatively large number of parameters that can account for known seismological effects while retaining a fairly simple attenuation functional form, based on analyses of residuals from simple models similar to those published previously. Preliminary results are reported here, together with the proposed geometric attenuation function forms and plausible explanation of the physical process that leads to the proposed geometric attenuation functions. The proposed model shows a large increase in the maximum likelihood from the random effects methodology, the elimination of bias in the distribution of residuals with respect to source distance, and much improved fitting for well-recorded earthquakes.
Bradley B.A.,Institute of Geological & Nuclear Sciences
Bulletin of the Seismological Society of America | Year: 2010
Acceleration spectrum intensity (ASI), defined as the integral of the pseudospectral acceleration of a ground motion from 0.1 to 0.5 sec, was originally proposed as a ground-motion intensity measure (IM) relevant for the seismic response of concrete dams over two decades ago. ASI may be a desirable IM in emerging performance-based earthquake engineering frameworks because its consideration of a range of spectral periods makes it useful for concurrent prediction of acceleration and displacement demands in individual structures and also for regional loss estimation where short-period structures are typically prevalent. This article presents a theoretical basis for predicting ASI, based on prediction equations for spectral acceleration, both for individual sites and spatially distributed regions. ASI is found to have a better predictability than conventional ground-motion IMs such as elastic pseudospectral acceleration at a specific period. Furthermore, for site-specific applications conditional response spectra are derived, which can be considered as the correct target response spectra for ground-motion selection, and the features of these conditional spectra as a function of earthquake magnitude, source-to-site distance, and epsilon are examined. For spatially distributed applications, the intraevent correlation of ASI as a function of the separation distance of two sites is derived and compared to that of other common IMs.
De Pascale G.P.,University of Canterbury |
Langridge R.M.,Institute of Geological & Nuclear Sciences
Geology | Year: 2012
The dextral-reverse Alpine fault is the major onshore plate-boundary structure between the Australian and Pacific plates in New Zealand. No previous study of the central portion of the 200-km-long central segment has provided on-fault evidence for the most recent event (MRE). Using lidar (light detection and ranging) data coupled with field mapping, we recognized the main trace of the Alpine fault north of Gaunt Creek (South Island) as a north-striking fault scarp. We enhanced a natural exposure that revealed evidence for repeated late Holocene thrust fault movement. The north-northwest-striking fault zone is characterized by a distinct 5-50-cm-thick clay fault-gouge layer juxtaposing hanging-wall bedrock (mylonites and cataclasites) over unconsolidated late Holocene footwall colluvium. The bedrock is cut by a strath terrace and overlain by mid-Holocene (ca. 5400 calibrated 14C yr B.P.) alluvial terrace, which has been faulted repeatedly and is conformably overlain by undeformed late Holocene colluvium and alluvium. An unfaulted peat at the base of the scarp is buried by post-MRE alluvium and yields a calibrated 2σ radiocarbon age of A.D. 1710-1930, which dates the MRE as post-1709. Our data are consistent with sparse on-fault data, and validate earlier off-fault records that suggest an A.D. 1717 MRE. The 1717 event had a moment magnitude of M w 8.1 ± 0.1, based on the 380-km-long surface rupture. Because the fault has not ruptured for ∼300 yr, it is likely approaching the end of its seismic cycle and poses a significant seismic hazard to New Zealand. © 2012 Geological Society of America.
Rhoades D.A.,Institute of Geological & Nuclear Sciences
Bulletin of the Seismological Society of America | Year: 2013
Optimal mixtures of three space-time-magnitude earthquake likelihood models are found for forecasting earthquakes with magnitudes of 5.0 and greater in the New Zealand and California catalogs, with forecasting time horizons ranging from 0 to 3000 days. The models are the Epidemic-Type Aftershock (ETAS) short-term clustering model, the Every Earthquake a Precursor According to Scale (EEPAS) medium-term clustering model, and the Proximity to Past Earthquakes (PPE) quasi-time-invariant smoothed seismi city model. The ETAS model is by far the most informative of these models for short time horizons of a few days, but even with a zero time horizon, an optimal mixture of the three models, here called the Janus model, outperforms it with an information gain per earthquake (IGPE) of about 0.1. For time horizons of 10-3000 days, the Janus model outperforms the most informative of its component models with IGPEs ranging from 0.2 to 0.5. As the time horizon lengthens beyond six months in New Zealand and two years in California, the EEPAS model becomes the most informative of the individual models and the major component of the optimal mixture. Changes in the Janus model parameters with the forecasting time horizon reveal features of time-and-area scaling of precursory seismicity. The results suggest that both cascades of triggering and the precursory scale increase phenomenon contribute to earthquake predictability and that these contributions are largely independent.
Institute of Geological & Nuclear Sciences | Date: 2013-12-17
A magnetometer 100, for determining an external magnetic field, comprises a magnetoresistive material forming, an electrode arrangement 104, and a processor. A resistive response of the magnetoresistive material comprises a decreasing response for a first range of increasing applied external magnetic fields, and an increasing response for a second range of increasing applied external magnetic fields. The electrode arrangement 104 measures the resistive response of the magnetoresistive material to the applied external magnetic field. The processor is configured to determine if the external magnetic field applied to the magnetoresistive material is in the first range or in the second range. The processor is configured to determine the external magnetic field based at least partly on the resistive response of the magnetoresistive material to the external magnetic field and whether the external magnetic field is in the first range or in the second range.
Institute of Geological & Nuclear Sciences | Date: 2013-04-12
A magnetometer (100) for measuring an external magnetic field has at least one core (102), two excitation coils (106a), (106b), and a pick-up coil (104). The at least one core (102) has a magnetoresistance property measurable in response to the external magnetic field (111). Each excitation coil (106a), (106b) is near or around opposite ends of the core (102) or near or around a respective core. The excitation coils (106a), (106b) are configured to be driven by an alternating current to partially saturate a magnetisation of the core during part of the AC cycle. The pick-up coil (104) is near or around at least a portion of the core (102) and the excitation coils (106a), (106b). The pick-up coil (104) is configured to carry a signal induced at least in the presence of the external magnetic field (111). The induced signal is measurable in response to the external magnetic field (111).
Stewart M.K.,Institute of Geological & Nuclear Sciences
Journal of Hydrology | Year: 2012
Knowledge of the sources and flowpaths of water in the Christchurch groundwater system will be vital to future management of the system. To gain such knowledge, oxygen-18 ( 18O), tritium ( 3H), carbon-14 ( 14C), and chemical concentrations have been measured on deep and shallow groundwaters since 1970. 18O measurements show that seepage from the Waimakariri River is the dominant source of the groundwater. Early 3H measurements (in the 1970s) showed non-zero concentrations in the deep groundwaters, but these were discounted at the time as due to "at most a few percent of very young water" However, reinterpretation in light of the 14C ages in this work has revealed much younger ages for the deep waters than was previously believed, with average ages of 38years in 1971, 71years in 1976, 98years in 1985, greater than 120years in 1986, and greater than 150years in 1993-1994.Because the Waimakariri River was identified as the major source of the deep groundwater, the river's 14C concentration between 1986 and 2006 was modelled by combining the records of its two carbon sources (biogenic carbon and atmospheric CO 2). This allowed the initial 14C concentrations of the groundwaters to be unequivocally determined and their mean 14C ages estimated using the same flow model as was applied to the 3H measurements. The resulting mean 14C ages are in the range 5-1500years. The long sequence of measurements reveals that the mean ages of the deep Christchurch groundwater have changed markedly during the study. The pre-exploitation rate of turnover of water in the system is not known, but was probably quite slow. By the 1970s, ages in the deep system (Aquifers 4 and 5) had become relatively young right across Christchurch (with mean ages of 60-70. years) indicating mainly lateral inflow of young Waimakariri River water because of groundwater abstraction. Mean ages measured since have gradually increased showing increasing upflow of much older water from depth - this water has 10-15% rainfall recharge and is sourced from the inland plains region. There is now (in 2006) a steep gradient in age from west to east across Christchurch (from 300. years to 1400. years) showing that a large body of much older, deeper water is stored on the seaward side of the system where the deep aquifers are blind. This body will yield good quality water for many years, but eventually it is likely to be replaced or bypassed by younger (a few hundred years old), Waimakariri River-dominated but surface recharge-bearing, water from inland. © 2012 Elsevier B.V.