Glossary and FAQs

Glossary for seismic hazard work. Frequently asked questions about the NSHM.

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Glossary of terms used in seismic hazard work

A BC D E F G H I J K L M N O P Q R S T U V W X Y Z

Acceleration – The change in velocity (speed in a certain direction) in one unit of time. The ground experiences acceleration during an earthquake when the ground is shaking.

Aleatory Uncertainty – Uncertainty dependent on chance, caused by the intrinsic variability of nature. It is used in Probabilistic Seismic Hazard Analysis. (For more: http://www.opensha.org/sites/opensha.org/files/PSHA_Primer_v2_0.pdf)

Background source or Distributed seismicity model – This model is meant to account for faults of all sizes that we don’t know about on the surface and at depth. It is a multi-layered (depth) grid of seismicity rates developed using the GeoNet earthquake catalogue. It is one of two components of the NSHM source model and is largely being developed by the SRM working group.

Basin Effects – Amplified earthquake ground shaking within a geologic basin due to its geometry and the presence of softer soils (i.e. a ‘sedimentary basin’). The update of the NSHM is looking to include modelling of basin amplification for the Wellington region. The Wellington basin has been an area of key concern since the 2016 Kaikōura earthquake, however, this issue applies to many basins around the country.  

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CSHM - Canterbury Seismic Hazard Model – A regional time-dependent model for the greater Christchurch area developed following the mainshocks of the ongoing Canterbury earthquake sequence. The CSHM is time-dependent because it accounts for the decrease in aftershock activity over time.

Deaggregation/disaggregation (deag/disag) – The process of splitting up the contributions to probabilistic seismic hazard results by specified criteria. This is done to help understand what earthquake sources may impact and are important for a particular region. The criteria can be combinations of (i.e. magnitude, distance, location, and tectonic region type (i.e. active shallow crust, volcanic, subduction interface, subduction slab).

Deterministic Seismic Hazard Analysis - Using only a single and important potential earthquake to estimate the ground shaking for a particular location or region. It may or may not include uncertainty in the calculation of the ground shaking. This is typically used to supplement probabilistic seismic hazard analysis.

Directivity – The variability in earthquake shaking depending on the direction a fault ruptures. Ground motion in the direction of the rupture propagation (the direction the energy of the earthquake is released) is more severe than in other directions (Ref: USGS Earthquake Glossary; and https://earthquake.usgs.gov/data/rupture/directivity.php

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Epistemic Uncertainty – The uncertainty in our knowledge of how earthquakes occur and shake the ground. Time and additional observation can reduce these uncertainties. It is used in Probabilistic Seismic Hazard Analysis.  (For more see: Probabilistic Seismic Hazard Analysis at Regional and National Scale: State of the Art and Future Challenges; and  http://www.opensha.org/sites/opensha.org/files/PSHA_Primer_v2_0.pdf;

GMM Ground Motion Model – Mathematical/statistical models used to estimate the amount of ground shaking at a given site, or at a given series of points. Sometimes also referred to as an “Attenuation relationship”. New Zealand-specific GMMs include McVerry et al. (2006) and Bradley (2013). There are suites of other international GMMs from the US-led NGA West-1, NGA West-2 projects, and NGA Subduction. The GMCM working group is tasked with developing and determining which GMMs will be included in the NSHM.

GMCM - Ground Motion Characterisation Model – This model combines multiple methods of estimating the ground shaking using different GMMs and other factors such as Basin Effects.

Ground motion – The movement of the earth’s surface caused by earthquakes. Earthquakes produce seismic waves that travel through and along the surface of the earth. (See USGS Earthquake Glossary.)

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Hazard curve – A set of values (usually looking like a curve, when plotted) representing the probabilities of exceedance (vertical axis) of given acceleration values (horizontal axis). In short, the values of this curve represent the probability of exceeding a specific acceleration value.  These plots are for a single spectra period (i.e. PGA, 0.5, 1).

KSHMKaikōura Seismic Hazard Model. A regional time-dependent model developed for central New Zealand (i.e. northern/north-eastern South Island and lower North Island) following the 2016 Kaikōura earthquake. This model includes significant updates to the fault source model, including the Hikurangi subduction zone sources, and to the distributed seismicity model: both components include time-dependent earthquake rates which change through time. The KSHM also includes a suite of ground motion models (GMMs) rather than a single model as in the 2010 NSHM.

New Zealand Site Class – It is used to convey an understanding of soil conditions at a site and is currently used in New Zealand building standards and guidelines (e.g. New Zealand Standard 1170). Site classes range from A to E and are defined by a range of soil conditions including, but not limited to, shear wave velocities, measurement of other geotechnical properties, and evaluation of the period of the site. Most sites in New Zealand correspond to site class B, C, or D. The quick descriptions of the site classes are as follows:

-          A: Strong Rock
-          B: Rock
-          C: Shallow Soil
-          D: Deep or Soft Soil
-          E: Very Soft Soil

New Zealand Strong Motion Database (NZSMD) – a database of strong motion records recorded in New Zealand. This database was produced by GNS (Van Houtte et al. 2017), including data up to 2016. It is being updated as part of the NSHM revision.

NSHMNational Seismic Hazard Model. This is the official seismic hazard model for New Zealand, which estimates the likelihood and strength of earthquake shaking across New Zealand.

NSHMP-haz – The United States Geological Survey National Seismic Hazard Mapping Project (NSHMP) code for seismic hazard analyses. The platform is coded using java and is open-source (freely available and can be developed on by its users).

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OpenQuake – An end-to-end software platform for earthquake hazard and risk calculation, developed by the Global Earthquake Model Foundation. The platform is coded using python and is open-source (freely available and can be developed on by its users).

Period (T) – The time interval required for one full cycle of an earthquake wave. (See diagram in USGS Earthquake Glossary.)

PGAPeak Ground Acceleration. The largest increase in velocity recorded in an earthquake by a particle on the ground, or that can be expected based on the estimated ground motion at a specific site (i.e. as shown in a site-specific spectrum). In PSHA, PGA is usually represented at spectral period of zero.

PSHAProbabilistic Seismic Hazard Analysis. A numerical/analytical approach to quantifying the probability of exceeding various ground motion levels at a site, given all possible earthquakes. (For more: http://www.opensha.org/sites/opensha.org/files/PSHA_Primer_v2_0.pdf)

Probability of Exceedance (PoE, PE) – The PoE for a ground motion provides the probability that a ground motion will be exceeded in a time-window of interest. This is a fundamental output of PSHA.

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Return Period, Recurrence Interval and Probability of Exceedance

Return Period and Recurrence Interval are two different things. Return Period is used to describe PSHA outputs. Recurrence Interval is used to described behaviour of earthquakes on a particular fault. However, the regularity of earthquake occurrence is highly debated topic. For example, we do not necessarily model individual earthquakes to occur on a regular interval and therefore the use of Return Period for ground shaking can be misleading and ground motions with a particular probability of exceedance (PoE) is the correct term. For example, we should describe ground motions with a 10% probability of exceedance in 50 years, rather than ground motions with a 475-year return period.

Seismicity Rate Model – Representations of known and unknown fault sources for the purposes of estimating earthquake hazard. This model produces estimates of where earthquakes occur, what rate they occur and what magnitudes they can be. It includes data on known faults and estimates of unknown faults. Both components are constrained by multiple data sources including earthquake geology and the Community Fault Model, the GeoNet earthquake catalogue, and the GeoNet GNSS network. The final Seismicity Rate Model is made of multiple component models that allow us to include epistemic uncertainty.

Spectrum / Spectra – a curve showing amplitude (in the case of hazard analyses, this would be acceleration) on the y axis, versus period on the x axis. So, a spectrum overall should show the amount of shaking there is at each period (how much of each type of shaking there is). This is produced for horizontal ground motions. (Ref: USGS Earthquake Glossary for a diagram.)

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UHS – Uniform Hazard Spectrum/Spectra commonly referred to as “spectrum” or “spectra”.

Vs30 – Average shear wave velocity to 30 metres depth. Often used alone or in conjunction with Z1.0 and/or Z2.5 to represent site characteristics in ground motion models (GMMs).

Z1.0 – Depth at which shear wave velocity reaches 1km/s (1000 m/s). Often used in conjunction with Vs30 to represent site characteristics in ground motion models (GMMs).

Z2.5 – Depth at which shear wave velocity reaches 2.5km/s (2500 m/s). Often used in conjunction with Vs30 to represent site characteristics in ground motion models (GMMs).

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FAQs about the NSHM

What is the National Seismic Hazard Model?

Damage to the railway at Tirohanga following rupture of the Kekerengu Fault during the 2016 Kaikoura Earthquake - Image by Tim Little

Who uses the NSHM?

What’s happening with the model? 

Who is involved in revising the NSHM?

What is the Core NSHM?

What is the Seismicity Rate Models Working Group (SRM)? 

What is the Ground Motion Characterisation Models Working Group (GMCM)?

What is the End-User Delivery Working Group?

What is the Technical Advisory Group (TAG)?