Tephrochronology

Tephrochronology is the use of primary, characterized tephras or cryptotephras as chronostratigraphic marker beds to connect and synchronize geological, paleoenvironmental, or archaeological sequences or events, or soils/paleosols, and, uniquely, to transfer relative or numerical ages or dates to them using stratigraphic and age information together with mineralogical and geochemical compositional data, especially from individual glass-shard analyses, obtained for the tephra/cryptotephra deposits. To function as an age-equivalent correlation and chronostratigraphic dating tool, tephrochronology may be undertaken in three steps: (i) mapping and describing tephras and determining their stratigraphic relationships, (ii) characterizing tephras or cryptotephras in the laboratory, and (iii) dating them using a wide range of geochronological methods. Tephrochronology is also an important tool in volcanology, informing studies on volcanic petrology, volcano eruption histories and hazards, and volcano-climate forcing. Although limitations and challenges remain, multidisciplinary applications of tephrochronology continue to grow markedly

Impacts of surface fault rupture on residential structures during the 2016 Mw 7.8 Kaikōura earthquake, New Zealand

Areas that experience permanent ground deformation in earthquakes (e.g., surface fault rupture, slope failure, and/or liquefaction) typically sustain greater damage and loss compared to areas that experience strong ground shaking alone. The 2016 Mw 7.8 Kaikōura earthquake generated ≥220 km of surface fault rupture. The amount and style of surface rupture deformation varied considerably, ranging from centimetre-scale distributed folding to metre-scale discrete rupture. About a dozen buildings – mainly residential (or residential-type) structures comprising single-storey timber-framed houses, barns and wool sheds with lightweight roofing material – were directly impacted by surface fault rupture with the severity of damage correlating with both local discrete fault displacement and local strain. However, none of these buildings collapsed. This included a house built directly atop a discrete rupture that experienced ~10 m of lateral offset. The foundation and flooring system of this structure allowed decoupling of much of the ground deformation from the superstructure thus preventing collapse. Nevertheless, buildings directly impacted by surface faulting suffered greater damage than comparable structures immediately outside the zone of surface rupture deformation. From a life-safety standpoint, all these buildings performed satisfactorily and provide insight into construction styles that could be employed to facilitate non-collapse performance resulting from surface fault rupture and, in certain instances, even post-event functionality

First use of fragile geologic features to set the design motions for a major existing engineered structure

We document the first use of fragile geologic features (FGFs) to set formal design earthquake motions for a major existing engineered structure. The safety evaluation earthquake (SEE) spectrum for the Clyde Dam, New Zealand (the mean 10,000 yr, ka, return period response spectrum) is developed in accordance with official guidelines and utilizes constraints provided by seven precariously balanced rocks (PBRs) located 2 km from the dam site and the local active Dunstan fault. The PBRs are located in the hanging wall of the fault. Deterministic PBR fragilities are estimated from field measurements of rock geometries and are the dynamic peak ground accelerations (PGAs) required for toppling. PBR fragility ages are modeled from B10e cosmogenic isotope exposure dating techniques and are in the range of 24–66 ka. The fragility ages are consistent with the PBRs having survived at least two large Dunstan fault earthquakes. We develop a PGA‐based fragility distribution from all of the PBRs, which represents the cumulative toppling probability of a theoretical random PBR as a function of PGA. The fragility distribution is then used to eliminate logic‐tree branches that produce PGA hazard curves that would topple the random PBR with a greater than 95% probability (i.e., less than 5% survival probability) over a time period of 24 ka (youngest PBR fragility age). The mean 10 ka spectrum of the remaining hazard estimates is then recommended as the SEE spectrum for the dam site. This SEE spectrum has a PGA of 0.55g⁠, which is significantly reduced from the 0.96g obtained for a preliminary version of the SEE spectrum. The reduction is due to the combined effects of the PBR constraints and a substantial update of the probabilistic seismic hazard model. The study serves as an important proof‐of‐concept for future applications of FGFs in engineering design

A revised age for the Kawakawa/Oruanui tephra, a key marker for the Last Glacial Maximum in New Zealand

The Kawakawa/Oruanui tephra (KOT) is a key chronostratigraphic marker in terrestrial and marine deposits of the New Zealand (NZ) sector of the southwest Pacific. Erupted early during the Last Glacial Maximum (LGM), the wide distribution of the KOT enables inter-regional alignment of proxy records and facilitates comparison between NZ climatic variations and those from well-dated records elsewhere. We present 22 new radiocarbon ages for the KOT from sites and materials considered optimal for dating, and apply Bayesian statistical methods via OxCal4.1.7 that incorporate stratigraphic information to develop a new age probability model for KOT. The revised calibrated age, ±2 standard deviations, for the eruption of the KOT is 25,360 ± 160 cal yr BP. The age revision provides a basis for refining marine reservoir ages for the LGM in the southwest Pacific. © 2012

New Zealand Community Fault Model – version 1.0.

Fault models developed by the scientific community aim to provide a consistent and broadly agreed-upon representation of faults in a region for such societally important endeavours as seismic hazard assessment (e.g. national seismic hazard models), strong ground-motion predictions and physics-based fault systems modelling. The New Zealand Community Fault Model (NZ CFM) is a two- and three-dimensional representation of fault zones associated with the New Zealand plate boundary for which Quaternary activity has been established (or deemed probable) and are, in the main, considered capable of producing moderate- to large-magnitude earthquakes. The NZ CFM builds on the Active Fault Model of New Zealand (Litchfield et al. 2013, 2014), updates that model through science community engagement and input and extends the updated faults from the surface to seismogenic depths. A nominal compilation scale of 1:500,000– 1:1,000,000 was chosen to provide representative surface fault traces consistent with finer-scale representations, such as the New Zealand Active Faults Database. Faults in the model are defined based on criteria that include surface geology, seismicity, seismic reflection profiles, bores and geologic cross-sections. The first edition of the NZ CFM (v1.0) is populated from 36.0 to 49.8° S and from 163.5° E to 179.0° W and comprises two principal datasets. The first dataset is a two-dimensional (2D) map representation of active (or potentially active) fault zone traces. This 2D fault zone representation contains information on the geometric and kinematic attributes of each fault zone or fault zone segment as expressed on the ground surface. Generalised estimates of geometric (dip and dip direction), kinematic (sense of movement, rake and net slip rate) and slip rate timeframe parameters have been provided for most fault zones, along with assigned uncertainties. In addition, a ‘Quality Code’ provides an indication of the quantity and type of data available for each fault zone, weighted toward the quality of the slip-rate data. The second dataset is a three-dimensional (3D) triangulated mesh surface representation of the fault zones in the model. However, there are some important differences between the 2D and 3D fault zone models; in particular, major fault zone intersections and subduction plate interfaces. NZ CFM v1.0 is publicly available from the GNS website. The NZ CFM v1.0 data package includes ArcGIS and QGIS projects and shapefiles of the 2D fault zone model, a MOVE project and triangulated surfaces for the 3D fault zone model, tabulated fault zone parameters and documentation