The faults under the plains

The Greendale fault appeared on no hazard map before it tore open at 4.35am on 4 September 2010. Afterwards the surveyors walked the paddocks and drew where the ground had broken, 1,832 mapped trace lines across 3 faults near the city, about 91.3 km of drawn rupture and deformation. This is what the instruments-only pages can’t show: the actual scar.

Mapped faults

Trace lines

1,832

Drawn length

91.3 km

The opener

Greendale

mapped 2011

The plains, drawn

Suburb outlines in grey; the traces as the survey drew them. Greendale, the September 2010 rupture, 1,762 segments of break and deformation west of the city · Ashley, the mapped zone under the northern plains. A trace is where the survey drew it, not a promise about any one address.

Greendale Fault mapped 2011

The surface rupture of the 4 September 2010 M7.2 — the fault that appeared on no hazard map until it opened.

Trace lines: 1,762
Drawn length: 68.6 km
As the survey labelled it: fault ×1290, fault scarp ×330, suspected fault ×123, broad scarp ×11

Ashley Fault Zone mapped 2014

The mapped zone under the northern plains, between the city and Rangiora — the active fault Christchurch knew about.

Trace lines: 44
Drawn length: 17.8 km

Hanmer Fault mapped 2004

The trace through the Hanmer basin, North Canterbury.

Trace lines: 26
Drawn length: 4.9 km

Questions, answered briefly

What fault caused the September 2010 Christchurch earthquake?

The Greendale fault, a fault that appeared on no hazard map before it ruptured on 4 September 2010. GNS and ECan mapped its surface trace afterwards: the drawn record holds 1,762 rupture and deformation lines across about 68.6 km of ground west of the city.

Are there other active faults near Christchurch?

Yes, the Ashley Fault Zone under the northern plains between the city and Rangiora is the closest mapped one, and the Hanmer Fault through the Hanmer basin is in the same record. The plains hide most of what is down there: Greendale itself was invisible until it moved.

Does a mapped fault line mean an address will shake?

A trace is where the survey drew the fault or the deformation it left, the record of what is mapped, not a prediction about any one address. Faults rupture along their length, not at a point, and shaking reaches well past the drawn line: February 2011’s epicentre sat under the Port Hills and the damage ran kilometres in every direction.

Where does this record come from?

Environment Canterbury’s open active-fault layers, the GNS and council mapping published as Public/EarthquakeFaults. Lines are kept as the survey drew them, simplified only for size. The shakes themselves are counted at /sequence and the story is the earthquakes explainer.

Also in the earthquakes

The sequenceevery shake since 2010, counted
Earthquakes, livethe ones that just landed
Liquefactionthe ground that moved
The land gradesTC1, TC2, TC3
The red zonethe land the Crown wrote off
The rebuild’s receiptsSCIRT’s photograph archive
The earthquakes, explainedwhat happened and why
Liquefaction, explainedwhy the east failed
The rebuild, explainedthe machinery that put it back

The record behind this

Environment Canterbury’s open active-fault layers, the mapping GNS Science and the district councils published as Public/EarthquakeFaults, kept as the survey drew it and simplified only for size. A line is a mapped trace or deformation mark: the record of the fault, not a forecast and not a finding about any one address. The shakes these faults made are counted at /sequence; what the shaking did to the ground is at /liquefaction; the verdicts it left the foundations at /technical-categories.

This page as markdown: /faults.md · the traces as data: /faults.json · on the live map: /map?layer=faults · the story in plain language: the earthquakes explainer.