### Earthquake FAQ

What causes earthquakes in New Zealand?
The Earth's entire outer surface is like a hard shell, which is broken into smaller pieces called 'tectonic plates'. New Zealand is on the boundary between two of these plates, called the Australian Plate and the Pacific Plate. These plates are constantly grinding into each other, which causes stresses to build up in the brittle, upper layers of the plates. An earthquake is a sudden motion in the crust caused by the abrupt release of accumulated stress along a fault, a break in the Earth’s crust. Under New Zealand, the Pacific Plate is moving to the west-south-west at about 50 mm a year - about the same rate that your fingernails grow. The entire plate interaction zone is potentially a source of moderate-to-large earthquakes in New Zealand. For more information, refer to GNS Science's plate motion and deformation page.
How many earthquakes happen in New Zealand each year?
GeoNet locates around 20,000 earthquakes in and around New Zealand each year; most of these are small but around 250 are big enough to be felt. View the Earthquake Statistics page for New Zealand region earthquakes based on observations since 1960, and the GeoNet Quake Statistics page for earthquake statistics in the last year.
What was the biggest earthquake in New Zealand's history? How about the world?
The biggest known earthquake in New Zealand was the 1855 magnitude 8.2 in the Wairarapa; around 5,000 km2 of land was shifted vertically during the earthquake. The largest earthquake recorded in the world in the last 100 years was the 1960 magnitude 9.5 earthquake in Chile. It caused thousands of casualties and created a large tsunami.
What is an earthquake swarm?
A sequence of earthquakes that strike in a short period of time, located in close vicinity to each other.
Does a swarm mean the 'big one' is coming?
No. Although large earthquakes are sometimes preceded by smaller associated earthquake swarms (called foreshocks), swarms of seismic activity do not necessarily indicate that a large earthquake is to follow.
Where can I find information on New Zealand earthquakes that occurred over a period of time or in a certain region?
The most recent earthquakes are posted on the website. You can also search our entire earthquake catalogue using Quake Search.
Why is there a discrepancy between the USGS and GeoNet magnitudes/locations?
The locations and magnitudes are determined mathematically by using readings from seismographs and modelling the most likely source of the observations. These will vary if the stations used or model of the crust are different. In general, most agencies have their own methods, models and selection of stations for deriving earthquake information.
Why do the magnitudes of posted earthquakes sometimes change?
Earthquakes are posted to the website as soon as data start to come in. They are now located automatically without being reviewed by a duty officer or analyst. The magnitude and precise position of the quake may change when more data become available or if they are reviewed by the duty officer or analyst.
Why is there activity on the drums when no earthquake is occurring?
The seismometers that we use are very sensitive instruments and can sometimes be triggered by unrelated phenomena like weather, traffic, or movement of livestock. This signal is recorded as 'noise' on the seismograph. See examples here.
What do the characters on the top left of a seismic drum? For example, what about MRZ/10-HHZ/NZ?

This uniquely identifies the seismic trace you are viewing:

The first part MRZ shows the international seismic station code, in this case MRZ is the code for 'Mangatainoka River'.

The second part 10 shows the location code of the seismometer at Mangatainoka River.

The third part HHZ is defined by the SEED convention and indicates the characteristics of the seismometer in use.

The fourth part NZ shows the network code, in this case 'New Zealand'.

What is the difference between the 'dark blue' and 'purple' data traces on the drum plots?

There is no difference in these data; the traces are coloured such on alternate lines so it is easier to see which data corresponds to each line. The seismogram is coloured red if the data is clipped, i.e. the largest parts of the signal are not shown.

Information on the instruments in the GeoNet geophysical network, including their location, can be found on the website through the Delta database. Maps of our instruments can be viewed in the Networks and Equipment section.

Can I use GeoNet images and data in my presentation/website?

All GeoNet images and data are freely available. Please refer to our data policy and cite us when you use us.

Where can I get data?

All data is made freely available through the GeoNet website in the Data section.

Is GeoNet hiring staff?

Please check out the 'Vacancies' link on the GNS Science website for any job vacancies in GeoNet.

If your question is not answered above, visit the GNS Science Earthquake Learning page.