M7.3 East Cape Earthquake - Aftershock Forecasts

Published: Fri Mar 5 2021 7:40 PM
Updated: Thu Jun 24 2021 9:00 AM

While no one can scientifically predict earthquakes, we can provide forecasts of future earthquakes using computer models that are updated as an earthquake sequence continues. Forecasts based on the 5 March 2021 East Cape Earthquake are in the table below.

This forecast only considers the M7.3 East Cape earthquake. For scenarios that include the M8.1 Kermadec event, please see here.

Given that the earthquake occurred some distance offshore, we cannot detect the smaller earthquakes that we usually use in our computer models. Therefore, we base our forecast on an average of other New Zealand sequences and a global model for subduction zones.

These two models provide the spread of the average number and the probabilities in the table below. The range covers the 95% confidence bounds including both models. The forecast applies to the region around the mainshock as indicated by the box on the map below. Earthquakes can and do happen outside this region, but the box represents the most likely area for aftershocks in this sequence.

Region around the mainshock

Region around the mainshock

M5.0-5.9 M6.0-6.9 M≥7.0
Average number Range * Probability of 1 or more Average number Range * Probability of 1 or more Average number Range * Probability of 1 or more
within 365 days 5-6 1-11 >99% 0.5-0.6 0-2 40-45% 0.05-0.06 0-1 5-6%
*Calculated for 23 June 2021, 6 pm. This table shows a forecast for future earthquakes in the given time intervals from the time of issuing for the forecast area from 178.50-181.5 degrees longitude and -36.5 to -38.5 degrees latitude. 95% confidence bounds

For example, this table says that:

  • It is about as likely as not (40-45% chance) that there will be one or more M6.0-6.9 earthquakes within the next 365 days.
  • It is very unlikely (5-6% chance) that there will be one or more earthquakes of M7 or above within the next 365 days.
  • While the probability of M7 or above is small, the possible impacts of an event such as this could be severe.

Updates:

  • The earthquake sequence may have returned to normal ‘background’ levels by the end of this one year forecast window (June 2022). At that point we will assess whether an updated forecast needs to be issued. If the earthquake sequence reinvigorates in the meantime, we will issue an update sooner.
  • As time goes on, we increase the time windows for our forecasts to reflect the decay in activity. We are no longer producing the forecasts for the 7 day or 30 day window.
  • If you require the forecast window to be shorter for your decision-making (such as 7 days or 30 days), please get in touch at info@geonet.org.nz. We are also interested in hearing about what you use these earthquake forecasts for.

What you can do about this:

As described in this article about our forecasts, the best thing we can do is be prepared for earthquakes. Remember, Long or Strong, Get Gone.

Long or Strong?, Get Gone

Long or Strong?, Get Gone

Drop, Cover and Hold during the shaking. Protect yourself from the earthquake first.

As soon as the shaking stops, move immediately to the nearest high ground, out of all tsunami evacuation zones, or as far inland as possible.

NEMA (National Emergency Management Agency) have a great website with information on what to do before, during and after a tsunami. There you can also find tsunami evacuation zone maps for around New Zealand, and what supplies you need in an emergency.


Need to Talk? If you would like support or advice, have feelings of anxiety, stress, prolonged fear, hopelessness or anger, or if you just need to talk with someone. Please text or phone 1737 to speak to a trained councellor in the National Telehealth Service. Its free to call or text.


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