Seismic Zone New Zealand — NZS 1170.5:2004 · Z and kh by region
NZS 1170.5:2004 (Structural design actions Part 5: Earthquake actions) defines the hazard factor Z mapped across New Zealand, combined with MBIE Module B7 guidance on retaining walls and revisions to Canterbury Z values after the 2010-2011 earthquake sequence. New Zealand straddles the boundary between the Pacific and Australian plates: subduction along the Hikurangi margin under the east coast of the North Island, the transform Alpine Fault along the Southern Alps, and the Puysegur subduction zone south of Fiordland. Typical Z values range from 0.13 in Auckland and Northland up to 0.40 in Wellington and 0.45-0.60 in Canterbury and the Southern Alps after the post-2011 revisions. This tool returns regional Z values and the pseudo-static horizontal coefficient kh for retaining walls, slopes and foundations.
What it is and when it applies
NZS 1170.5 applies to the design of structures in New Zealand under the Building Act 2004 framework. The hazard factor Z represents the 500-year return period peak ground acceleration on site subsoil class C. The standard is supplemented by the NSHM 2022 (National Seismic Hazard Model released by GNS Science), which MBIE is progressively incorporating through Module B1, B7 and the 2025 revision of the code. For geotechnical design, Z is used directly to build the pseudo-static horizontal coefficient: kh = 0.5·Z for retaining walls and slopes with acceptable Newmark displacement (10-25 cm per Hynes-Griffin & Franklin 1984), consistent with MBIE Module B7 recommendations for walls supporting a road or building, and kh = Z for rigid structures. Application is mandatory for buildings, bridges (NZTA Bridge Manual), industrial facilities, port structures, MSE walls and dams (NZSOLD 2015 guidelines).
Governing equations
Design ground acceleration (NZS 1170.5 Clause 3.1):
C(0) = Ch(0) · Z · R · N(T,D), where R is the return-period factor and N is the near-fault factor
Pseudo-static horizontal coefficient (MBIE Module B7):
kh = 0.5·Z → retaining walls and slopes with acceptable deformation
kh = 1.0·Z → rigid structures with no tolerated deformation
kv = 0 (typical) or kv = ±0.5·kh (near-fault projects, N(T,D) > 1)
Surface acceleration with subsoil factor (NZS 1170.5 Clause 3.1.3):
amax = Ch(0) · Z, with Ch(0) by subsoil class: A=0.90, B=1.00, C=1.33, D=1.12, E=1.12
Mononobe-Okabe seismic earth pressure:
θ = arctan(kh/(1−kv)), then Kae follows the classic formulation
Worked example
| Parameter | Value |
|---|---|
| Location | Wellington CBD — very high hazard, near Wellington Fault |
| Z | 0.40 |
| Subsoil class C (shallow soil) | Ch(0) = 1.33 |
| Use | Retaining wall with acceptable deformation |
| Pseudo-static kh | 0.5 × 0.40 = 0.20 |
| Surface amax | 1.33 × 0.40 = 0.53 g |
With kh = 0.20 the Mononobe-Okabe seismic earth pressure gives θ = arctan(0.20/(1−0)) = 11.3°. That Kae multiplies the backfill weight above the wall and adds to the static Rankine or Coulomb thrust, matching the MBIE Module B7 2014 guidance for Wellington retaining walls. Required checks per NZS 1170.5 and the NZ Geotechnical Society Guidelines: FSoverturning ≥ 1.5 static / 1.1 seismic, FSsliding ≥ 1.5 static / 1.1 seismic. Near the Wellington Fault within 2 km, the near-fault factor N(T,D) adds further amplification at long periods that must be checked for tall walls.
Result: Wellington · Z = 0.40 · kh = 0.20 · amax = 0.53 g · apply in Mononobe-Okabe with NZGS / MBIE B7 checks
Regional hazard table (NZS 1170.5 + post-2011 revisions)
| Z | Region | Notes |
|---|---|---|
| 0.13 | Auckland, Northland, Bay of Islands | Lowest Z in NZ; still above most Australian cities |
| 0.20-0.30 | Hamilton, Tauranga, Rotorua, New Plymouth | Taupo Volcanic Zone influence |
| 0.35-0.40 | Wellington, Lower Hutt, Napier, Hastings | Wellington Fault, Hikurangi subduction, Hawke's Bay 1931 |
| 0.45-0.60 | Christchurch (post-2011), Kaikoura, Hanmer Springs, West Coast | Canterbury sequence 2010-2011 led to Z increase from 0.22 to 0.30-0.45; Kaikoura 2016 M7.8 reinforced Southern Alpine Fault hazard |
Interpretation of results
New Zealand has some of the most active seismicity in the world. The Alpine Fault along the west of the South Island is a transform fault with a slip rate of 27-38 mm/year and a paleoseismic record showing M8+ earthquakes roughly every 250-300 years; the last rupture was in 1717. The Hikurangi subduction margin along the east of the North Island hosts a locked interface capable of M8-9 events plus numerous upper-plate faults (Wellington, Wairarapa, Ohariu). The Canterbury sequence 2010-2011 (Darfield M7.1, Christchurch M6.2) killed 185 people, triggered widespread liquefaction across the eastern suburbs and prompted the Canterbury Earthquakes Royal Commission to revise Z upward for Christchurch from 0.22 to 0.30, and later to 0.45 within MBIE guidance. Kaikoura 2016 (M7.8) ruptured 12 faults simultaneously, extending the understanding of multi-fault ruptures. For geotechnical work, projects with Z ≥ 0.30 require specific liquefaction triggering analysis (Boulanger & Idriss 2014, Robertson & Wride CPT method), Mononobe-Okabe or numerical analysis for walls, and Newmark displacement analysis for slopes. Dams follow NZSOLD 2015 with site-specific PSHA regardless of the mapped Z value.
Reference standards
- NZS 1170.5:2004 — Structural design actions Part 5: Earthquake actions
- NZS 1170.5 Supp 1:2004 — Commentary to the earthquake actions standard
- MBIE Module B7 (2014, revised ongoing) — Retaining walls under seismic loading
- NZGS 2016 — Earthquake geotechnical engineering practice, Modules 1 to 6
- NSHM 2022 — GNS Science National Seismic Hazard Model
- NZTA Bridge Manual v4 2022 — Seismic design of bridges
- NZSOLD 2015 — Dam Safety Guidelines
- Hynes-Griffin & Franklin (1984) — Rationalizing the seismic coefficient method
Frequently asked questions
Why is New Zealand so seismically active?
New Zealand sits on the boundary between the Pacific and Australian plates. Three regimes converge: subduction of the Pacific Plate under the North Island along the Hikurangi margin, transform motion along the Alpine Fault in the South Island (one of the most significant on-land faults on Earth), and Puysegur subduction south of Fiordland. The combined plate motion is 35-45 mm/year, comparable to California. Large historical events include Hawke's Bay 1931 (M7.8), Murchison 1929 (M7.3), Inangahua 1968 (M7.1), Christchurch 2011 (M6.2), and Kaikoura 2016 (M7.8).
What changed after the Canterbury sequence 2010-2011?
The Canterbury sequence exposed shortcomings in the original NZS 1170.5 Z value for Christchurch (0.22) and the design for liquefaction. MBIE issued Module 1-6 guidance through the NZGS, revised Z upward to 0.30 for Christchurch (and up to 0.45 in some MBIE workflows for post-2011 design), introduced foundation design criteria in Technical Categories TC1/TC2/TC3, and reinforced the need for site-specific liquefaction assessment using CPT-based methods (Boulanger-Idriss 2014).
How does the Alpine Fault affect West Coast design?
The Alpine Fault has a slip rate of 27-38 mm/year and ruptures in M8+ events roughly every 250-300 years. The last event in 1717 puts the elapsed time near the average recurrence, so the conditional 50-year probability of an Alpine Fault M8 is around 30%. Z values along the West Coast and near Haast, Franz Josef and Fox Glacier are typically 0.45-0.60 in post-2011 assessments. Near-fault factor N(T,D) adds significant long-period amplification within 20 km of the fault trace.
Is NZS 1170.5 enough for geotechnical design?
NZS 1170.5 gives Z and the subsoil class framework. Geotechnical design complements it with MBIE Module B7 for retaining walls, NZGS Modules 1-6 for earthquake geotechnical practice, NZTA Bridge Manual for bridge foundations and abutments, NZSOLD 2015 for dams, and project-specific analyses: Mononobe-Okabe for walls, Boulanger-Idriss for liquefaction, Newmark for slope displacement, and nonlinear FEM for dams and port structures. For standard projects with acceptable deformation, kh = 0.5·Z is adequate per MBIE B7.