Document library

Geographic Area Covered 

The report covers Study Area 3, the Porirua Basin, which includes Porirua City and the suburbs of Tawa, Linden, Kenepuru, Titahi Bay, Porirua East, Whitby, Paremata, Pauatahanui, Maria, and Plimmerton. 

Summary of the Report 

This report evaluates the potential risks and impacts of earthquakes on the Porirua Basin’s buildings, critical facilities, and population. It forms part of a broader strategy by the Wellington Regional Council to manage geological hazards. The study uses two earthquake scenarios: Scenario 1 involves a large distant earthquake with moderate shaking, while Scenario 2 considers a severe local earthquake on the Wellington Fault. The assessment includes ground shaking, liquefaction, slope failure, and fault rupture hazards. 

The study area includes a mix of residential, commercial, and industrial zones. Building vulnerability varies based on construction type, age, and ground conditions. Residential buildings, mostly timber-framed, show lower vulnerability compared to commercial and industrial buildings, which include older and more varied construction types. Buildings on soft or reclaimed land are more susceptible to damage due to amplified shaking and liquefaction. 

Estimated repair costs for Scenario 1 are $27 million in 1995 dollars, which equates to approximately $49 million in 2025. Scenario 2 estimates reach $201 million in 1995 dollars, or approximately $4364 million in 2025. Residential damage costs are lower than commercial/industrial due to construction type and location. Casualty estimates for Scenario 1 include 3 fatalities and 13 injuries during daytime, while Scenario 2 could result in 33 fatalities and 133 injuries. Nighttime casualties are significantly lower due to occupancy patterns. 

Critical facilities such as hospitals and emergency services buildings are assessed for their likelihood of sustaining damage, with some showing moderate to extensive risk.   

The following table lists the estimated probabilities of critical facilities in the Porirua Basin being in each damage state under Scenario 2. These estimates are based on the original earthquake risk assessment report and reflect the likelihood of structural damage ranging from none to complete. 

Estimated Damage to Critical Facilities in Porirua Basin (Scenario 2) 

The report concludes that while the Porirua Basin is at risk, especially in a severe earthquake, the findings help inform emergency planning and mitigation strategies.