The model’s map

Not what the ground did, what the study says it would do. In November 2019 Tonkin & Taylor graded 600 areas of the district, about 43,269 hectares, for liquefaction in future shaking. 186 areas carry the top grade. A screening read for planning and consents, drawn from the deposits and the groundwater, the study itself says a site’s own analysis decides an address.

Areas assessed

600

Ground graded

43,269 ha

High vulnerability

186

the model expects damage

Free-face flags

142

lateral spreading possible

The graded district, drawn

Every assessed area over the suburb outlines, red is High, amber Medium, pale amber "damage is possible", grey Low and "unlikely". A heavier edge means the notes flag lateral spreading near a riverbank or coast. Hover for the class and the ground it rests on.

By the model’s verdict

high High Liquefaction Vulnerability · 123 flagged for lateral spreading 186

medium Medium Liquefaction Vulnerability 182

possible Liquefaction Damage is Possible · 19 flagged for lateral spreading 193

low Low Liquefaction Vulnerability 23

unlikely Liquefaction Damage is Unlikely 16

The suburbs the model grades worst

Each suburb by its dominant class, the share is the fraction of its outline that grade covers.

Sampled join, a grid of points inside each suburb outline asks which assessed area holds it; the class shares are fractions of the outline and the dominant class is the verdict. A class is the model’s read on a patch of ground, not a finding about an address.

The free-face flags

High Liquefaction VulnerabilityRiver flood basin and estuary plain deposits, observed performance variable and uncertainty in liquefaction performance due to effects of layered silty soil profiles · 65 ha · the ground →
Liquefaction Damage is PossibleDune related deposits which are usually well characterised by observed performance, CPT investigations and simplified liquefaction analyses · 41 ha · the ground →
High Liquefaction VulnerabilityRiver plain and channel deposits, observed performance variable and uncertainty in liquefaction performance due to effects of layered silty soil profiles · 34 ha · the ground →
Liquefaction Damage is PossibleRiver flood basin and estuary plain deposits, observed performance variable and uncertainty in liquefaction performance due to effects of layered silty soil profiles · 25 ha · the ground →
High Liquefaction VulnerabilityDune related deposits which are usually well characterised by observed performance, CPT investigations and simplified liquefaction analyses · 24 ha · the ground →
Liquefaction Damage is PossibleRiver plain and channel deposits, observed performance variable and uncertainty in liquefaction performance due to effects of layered silty soil profiles · 24 ha · the ground →
High Liquefaction VulnerabilityRiver plain and channel deposits, observed performance variable and uncertainty in liquefaction performance due to effects of layered silty soil profiles · 23 ha · the ground →
High Liquefaction VulnerabilityRiver plain and channel deposits, observed performance variable and uncertainty in liquefaction performance due to effects of layered silty soil profiles · 22 ha · the ground →

Areas the study’s notes flag for lateral spreading, the ground close enough to a riverbank or coastline that shaking could slide it sideways. The study says a site’s own analysis should confirm the extent.

Questions, answered briefly

What is the liquefaction vulnerability study?

The model’s map of the next big one. In November 2019 Tonkin & Taylor delivered the council a district-wide liquefaction vulnerability assessment, 600 areas, about 43,269 hectares: graded under the 2017 national guidance for liquefaction-prone land. Each area is classed High, Medium, "damage is possible", Low, or "damage is unlikely", 186 areas carry the top grade. It exists to screen planning, subdivision and building consents.

Is this what happened in the earthquakes?

No, that record is /liquefaction, which draws what the ground actually did in February 2011. This page is the other half: a model’s verdict on what the same ground would *likely* do in future shaking, drawn from the deposits underneath, the groundwater, and the observed performance the study weighed. Where the two disagree, the disagreement itself is the record.

Does a High rating mean my house is unsafe?

The study doesn’t answer for an address, it says so itself. This is a district-scale screening layer, level-C detail: a class is the model’s verdict on a patch of ground, meant to decide where site-specific analysis is needed, not to replace it. The notes flag 142 areas close to a free face, a riverbank or coastline, where lateral spreading is possible and the study says a site’s own analysis should confirm it.

What does "damage is possible" mean?

The study’s hedged middle class, the ground whose deposits could liquefy but where the outcome is genuinely uncertain, often because the soil profile is layered silt that behaves inconsistently. It sits between the Medium and Low grades: not cleared, not condemned, the model’s honest shrug.

The record behind this

Christchurch City Council’s open land layers, the LiquefactionVulnerability layer of OpenData/LandCharacteristic: the Christchurch Liquefaction Vulnerability Study, Tonkin & Taylor for the council, November 2019, assessed under the MBIE/MfE 2017 guidance for liquefaction-prone land. A class is the model’s verdict on a patch of ground, not a finding about an address. What the ground actually did in February 2011 is /liquefaction; the grades the rebuild drew on the same ground are /technical-categories; a point’s own answer is at /before.

This page as markdown: /vulnerability.md · the record as data: /vulnerability.json · /vulnerability.geojson · /suburb-vulnerability.json · on the live map: /map?layer=vulnerability.