East Cape Earthquake: Aftershock Scenarios

GeoNet has issued an update of its aftershock forecast that was published on its website on Friday, following the magnitude 7.1 earthquake off the coast near Te Araroa.

Tuesday, 6th September 2016

GeoNet has issued an update of what it expects to happen following Friday’s magnitude 7.1 earthquake centred 125 km north-east of Te Araroa. The update now breaks the aftershock forecast into three scenarios based on GNS seismologists’ understanding of tectonics in the region, data from earthquakes since Friday morning, historical observations, and statistical models.

In issuing the scenario-based forecast, the organisation notes, “We appreciate that these scenarios may be concerning, especially scenario three.” “There are very different probabilities for each scenario; some of these are more concerning than others. We recognise that while these scenarios may increase anxiety in people living in the area of the north east corner of New Zealand, the best thing is to be prepared. Remember: If you feel a long or strong earthquake and you are on the coast, evacuate immediately.”

Scenario 1 is considered to be very likely, estimated to be accurate up to 90% in the next 30 days. In this scenario, aftershocks will continue to decrease in frequency as expected after such an event. GeoNet notes that aftershocks caused by a similar magnitude 7.1 earthquake which struck near Friday’s event on February 2nd (Waitangi Day) in 1995 followed this pattern, but aftershocks continued for more than two years. However, it is thought that there is a 50% chance that aftershocks of magnitude 6 could strike the area within the next 30 days.

Scenario 2 is considered to be unlikely with a 10% or less chance of occurring in the next 30 days. In this scenario, another earthquake of 7th magnitude might occur. “This earthquake may be onshore or offshore but close enough to cause severe shaking on land. Also there is a possibility of an earthquake either north or south of the M7.1 mainshock area e.g., in the Hikurangi Subduction Zone. Such large earthquakes have the potential to generate tsunami.”

A third scenario has been added to Friday’s projections.

Scenario 3, which is considered to be very unlikely, considers the possibility that “recent earthquake activity will trigger a significantly larger earthquake (M8 or greater).” The modelling for this scenario is complex and the uncertainty prevents GeoNet assigning a probability estimate for this occurring. Nevertheless, with Friday’s earthquake occurring near a subduction zone, where two tectonic plates collide (in this case the Pacific Plate and the Australian Plate), the possibility of a larger earthquake cannot be discounted. GeoNet notes, “This scenario is similar to what occurred in the Tohoku Earthquake in Japan in 2011. Although it is still very unlikely, the chances of this occurring have increased slightly since the M7.1 earthquake.”

Information on the Tohoku earthquake which struck off the coast of Japan in March 2011 can be found here.

GeoNet’s projections for the next 30 days include estimated numbers of aftershocks for the next week and 30 days and can be found here.

Readers need to be aware that GeoNet does not appear to consider its website postings as a matter of public record, and links provided here may simply not work. For example, I had intended to provide a link to GeoNet’s Friday projections for aftershocks, but it has been removed and replaced with this message (for as long as it lasts). How odd.

One Response to “East Cape Earthquake: Aftershock Scenarios”

  1. Lizzie from Gizzie says:

    Thankyou for the information Ken.

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