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Geographic Area: 

This report covers the Porirua and Tawa areas in the Wellington Region, New Zealand.

 
Summary: 

This report, prepared by the Wellington Regional Council in October 1992, is part of a broader project initiated in 1988 to evaluate and reduce earthquake risks in the Wellington Region. It focuses specifically on the hazard of ground shaking in the Porirua and Tawa areas. The report is one of six in a series and accompanies a seismic hazard map for the region. 
 
The Wellington Region is located on the boundary between the Pacific and Australian tectonic plates, making it seismically active. Four major active faults cross the region, and earthquakes of varying magnitudes and depths are common. The report uses two earthquake scenarios to estimate potential ground shaking effects. Scenario 1 involves a large, distant, shallow earthquake (magnitude 7 at about 100 km), while Scenario 2 considers a large, local earthquake on the Wellington Fault (magnitude 7.5). 
 
Three hazard zones are identified based on geological conditions and expected seismic response. Zone 1, the least hazardous, is underlain by bedrock. Zone 2 has firm ground with gravel and compact sand, while Zone 5, the most hazardous, consists of soft, flexible sediments up to 30 meters thick. Each zone is evaluated for expected ground motion parameters such as Modified Mercalli (MM) intensity, peak ground acceleration, duration of shaking, and amplification factors. 
 
In Scenario 1, MM intensities range from V–VI in Zone 1 to VIII–IX in Zone 5. In Scenario 2, intensities range from VIII in Zone 1 to X–XI in Zone 5. Peak ground acceleration and duration of shaking increase significantly from Zone 1 to Zone 5. Amplification of shaking is also much higher in Zone 5, with factors up to 20 times greater than in Zone 1. 
 
Expected ground damage varies by zone. Zone 1 may experience minor slope failures, Zone 2 could see localized failures and minor liquefaction, and Zone 5 is at risk of widespread liquefaction, lateral spreading, and ground settlement. 
 
The report notes several limitations, including the generalization of site-specific conditions, poorly defined boundaries in some areas, and the lack of data for near-source ground motion in Scenario 2. Despite these limitations, the report provides valuable guidance for understanding and mitigating earthquake risks in the Porirua and Tawa areas.