Data analysis · case study
New Zealand earthquakes
The question: where in New Zealand do earthquakes concentrate — and is the answer the same for the strong ones as for all of them? I took one year of the public GeoNet catalogue, from 3 November 2022 to 3 November 2023, and worked through it the way I would for a client: the answer first, then the evidence, then how far it can be trusted.
19,887
earthquakes analysed
157
of magnitude 4 or more
6.0
strongest, on 15 February 2023
509
in one day, on 18 March 2023
The short answer
Most earthquakes are in the North Island.
Two neighbouring regions, Taupō, Bay of Plenty, Gisborne and Lower North Island, hold 67% of everything recorded.
The strong ones are somewhere else.
Offshore north-east has 9% of the earthquakes but 39% of those of magnitude 4 or more — 61 of 157.
So don't rank regions by count alone.
A raw count mostly shows where the instruments can pick up small earthquakes. To compare regions fairly, compare the strong ones.
Where they happened
Loading the map…
| Region | Earthquakes | Magnitude 4+ | Median depth |
|---|---|---|---|
| 7644% | 25 | 7 km | |
| 9785% | 16 | 5 km | |
| 3,08015% | 17 | 25 km | |
| 6,58433% | 10 | 21 km | |
| 6,74734% | 28 | 14 km | |
| 1,7349% | 61 | 49 km |
Dot size shows magnitude
Point at the map, or tap it, to see a single earthquake. Select a region in the table to show it on its own; select it again to bring the others back.
The data, and what was removed
The source is GeoNet's public earthquake catalogue. Before any analysis, two kinds of rows came out:
| Events downloaded | 21,627 |
|---|---|
| Not earthquakesmarked by GeoNet as outside its area of interest, not locatable, quarry blasts, explosions or landslides | −1,278 |
| Outside the mapmostly out to sea to the north-east | −462 |
| Earthquakes analysed | 19,887 |
How sure are the locations
An earthquake is located from the stations around it. When they all sit to one side, the position is less exact. Share of each region's earthquakes located that way:
First look: when
Earthquakes per week. A typical week has about 355; the week from 16 Mar had 1,287.
First look: how strong
Earthquakes by magnitude. Half are below 2.0; only 157 reach 4.
Why six regions
The regions come from k-means clustering on location only — it never sees an earthquake's size, depth or date. The method needs the number of groups up front. The chart shows how much of the spread in location is accounted for as groups are added.
What to keep in mind
- One year is short. A single burst changes the picture.
- Counts favour places with many stations nearby.
- k-means draws compact, rounded groups. Faults are long and thin, so the borders show where the method cuts, not where the geology changes.
What I would do next
- Compare regions using only earthquakes above a magnitude that every region records reliably.
- Use several years and check whether the regions hold.
- Try a density-based method that can follow long, thin shapes.
About this page
A script downloads the catalogue, cleans it, runs the clustering and writes the numbers this page shows; the map and charts are drawn in the browser. I first did this analysis in 2023 as a master's assessment, using R, QGIS and Python. This page redoes it from the same public data.