Hikurangi response planning toolbox

The hazard:

The Hikurangi Subduction Zone, located off the east coast of Te Ika-a-Māui, the North Island, is Aotearoa's largest and most active fault.

Subduction Zones are known for producing the largest earthquakes in the world. In 2011, a magnitude 9.1 earthquake occurred on the Japan Trench, east of mainland Japan, which also created a large and devastating tsunami. This event resulted in approximately 20,000 fatalities or missing people and over $210 billion in economic damage (The World Bank, 2014).

For more information: The Hikurangi Subduction Zone

Likelihood:

Scientists know the Hikurangi Subduction Zone can produce large earthquakes and tsunami, and that these events have occurred in the past. Critically, recent research indicates there is a 1 in 4 chance of a major Hikurangi Subduction Zone event, in the next 50 years.

Vulnerability:

An estimated 65% of people in Aotearoa live within 5km of the ocean (LGNZ, 2019). Specifically, a national-scale coastal exposure assessment by NIWA, found that the proportions of normally resident populations in these low-lying areas by region were (based on 2013 census):

• Te Moana-a-Toi/Bay of Plenty - 31,000 (12%)
• Tairāwhiti/Gisborne - 4,600 (11%)
• Te Matau-a-Māui/Hawke's Bay - 43,000 (29%)
• Pōneke/Wellington - 33,500 (7%)

Large populations near the coast increases vulnerability to large tsunami because issues such as traffic congestion (both foot-based and vehicle based) during tsunami evacuations becomes more prevalent and problematic, this was observed during the 2004 Indian Ocean tsunami and 2011 Tōhoku Japan tsunami (Mas et al. 2015).

The earthquake

The Toolbox has been developed using a credible magnitude 8.9 earthquake and tsunami planning scenario as a tool. This was developed by GNS Science, to help identify consequences of a large Hikurangi event.

A credible, magnitude 8.9 earthquake generates extreme (MMI 9.0-12.0) ground shaking along the east coast of Aotearoa, with MMI 7 and above shaking lasting for minutes in places.

Parts of Central Te Ika-a-Māui/North Island, also experience shaking of MMI 8.0-9.0, and most of the rest of Te Ika-a-Māui and northern Te Waipounamu/ South Island experience above MMI 7.0.

Between Mahia and East Cape the duration of intense shaking is very long, amplified by the thick sedimentary rock, with durations of severe ground shaking of more than 60 seconds.

Aftershocks occur after the main earthquake. 

(MMI - Modified Mercali Intensity Scale)

The tsunami

The offshore magnitude 8.9 earthquake initiates a large tsunami, due to displacement of the seafloor. 

Tsunami will be an ongoing hazard as multiple waves will arrive - the first wave will not necessarily be the largest.

Tsunami will represent an ongoing challenge for the initial response, reducing access to large portions of the coastline damaged by the initial earthquake. Aftershocks may also bring risk of further tsunami.

Other secondary and compounding hazards

The following list is not exhaustive but represents some secondary and compounding risks which will complicate initial response activities and reduce the mobility of responders in the regions most affected.

The Toolbox provides an overview of planning considerations relating to the following impacts, as well as recommendations for addressing these:

This list of key workstreams is intended to aid regional response planning, it is not intended to replace the Coordinated Incident Management System (CIMS) structure. The CIMS structure defines roles and responsibilities, where workstreams are intended to identify intended response outcomes, these may sit within one or across multiple CIMS functions.