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Geographic area covered. Porirua and State Highway 58, Wellington Region, New Zealand. 

This report, prepared by P.A. Kingsbury for the Wellington Regional Council, outlines the potential for earthquake-induced slope failures in Porirua and State Highway 58. It accompanies a hazard map that categorises areas based on their susceptibility to slope failure during seismic events. The study is part of a broader regional initiative to understand and mitigate earthquake hazards, which also includes assessments of fault rupture, ground shaking, liquefaction, and tsunami risks. 
 
The report identifies five slope failure susceptibility zones—ranging from very low to very high—and evaluates how these zones might respond to three earthquake scenarios. These scenarios include a distant magnitude 7 earthquake (Scenario 1), a regional earthquake of moderate intensity (Intermediate Scenario), and a local magnitude 7.5 earthquake on the Wellington Fault (Scenario 2). The likelihood of these events occurring within the next 50 years varies from very high (Scenario 1) to low (Scenario 2). 
 
Slope failures can occur immediately during shaking or be triggered later by aftershocks or heavy rainfall. The most vulnerable areas are steep slopes, modified slopes such as road and quarry cuts, and locations with historical landslides. The report explains that slope angle and modification are the most critical factors influencing susceptibility. Other contributing factors include geology, slope height, groundwater conditions, and the presence of existing landslides. 
 
The methodology used to assess hazard involved compiling factor maps, assigning weighted values to slope characteristics, and integrating these into a susceptibility rating. These ratings were then mapped to show potential slope failure zones. The report emphasises that while the maps are useful for regional planning, they are not a substitute for site-specific geotechnical investigations. 
 
Limitations of the study include the generalised nature of the geological data, the indicative boundaries of hazard zones, and the exclusion of liquefaction-related slope failures, which are covered in a separate study. The report also notes that susceptibility ratings are specific to the Wellington Region and should not be compared directly with hazard maps from other regions. 

Porirua and State Highway 58 context: Porirua has significant slope modifications, especially in areas like Camborne, Granada North, and along SH1, SH58, and the North Island Main Trunk railway. The geology includes weathered greywacke and ancient erosion surfaces with deeply weathered materials, including plastic clay. These conditions can reduce slope stability, especially when disturbed. Notable landslides were identified at Goat Point and near Granada North. These features, combined with steep terrain and human modifications, contribute to higher susceptibility in parts of the area.