Te Araroa and Kermadec earthquakes
Update : Sunday 7 March 2021
Council’s scientist Dr Murry Cave provides the an understanding of the Te Araroa and Kermadec earthquakes and why we’re getting so many aftershocks.
There have been over 400 quakes since Friday’s 7.1, with the biggest a 6.1 magnitude on Saturday at 1.16pm. Around 44 quakes have recorded around 4-5 but the rest have been between 1 and 3. Most of the aftershocks have been east of Te Araroa.
The location of that first big quake was originally thought to be 100km east-north-east of East Cape but has now been revised and relocated slightly further east, some 114 km off East Cape.
The quake did not occur on the Hikurangi margin but was around 24km west and is known as an intra plate earthquake.
The position of these quakes offshore of East Cape makes accurate determination of their size and precise location difficult due to the distance from the on shore seismograph network.
The magnitudes of the Kermadec quakes has been determined by USGS and GeoNet advises that it’s best to use the USGS(external link) magnitudes as the GeoNet system under reported their magnitudes.
GNS scientists will work on analysis of these events in the coming week and data will be updated. We’ll update you as soon as this information comes to hand.
There’s still some discussion about the sense of movement of the Te Araroa earthquake but the USGS have indicated that it’s largely strike slip where the movement is horizontal. This is consistent with its location, west of the Hikurangi Margin. As it did generate a small tsunami there’s likely to have been some degree of normal (vertical) movement as well as horizontal displacement.
A 6.1 earthquake associated with the Te Araroa quake was located further east, close to the main Hikurangi Margin Fault, but at this stage is still considered to be an intra plate earthquake.
Both of the Kermadec quakes were thrust faults with the 8.1 quake occurring on or very close to the plate margin, which is the northern extension of the Hikurangi Margin. Both quakes are indicative of a subduction type movement where rocks on the Pacific Plate slide underneath the rocks of East Cape.
Both the Te Araroa 7.1 and the Kermadec 8.1 quakes generated small tsunami. The Te Araroa earthquake showed as a 1m +/- oscillation on the Te Araroa tide gauge while the Kermadec quake produced a smaller 30cm-50cm oscillation at the gauge. These were expressed largely as very active shore waves surging back and forth.
At Hicks Bay, some deeper water seaweeds were deposited at the high tide mark.
The 2 tsunami occurring in rapid succession is very unusual and the Tokomaru bore was probably more obvious as a result of the wave being pushed up as it became confined in the bay. This did not occur at Te Araroa as it is more open to the ocean. Unusual wave surges were still occurring at Te Araroa at midday Saturday, highlighting that the effect of tsunami can last for more than 24 hours after the event.
Be prepared for a disaster
It's important that you're prepared for a disaster. Get ready now so that you're better prepared to cope on your own for up to 3 days or more.
As a minimum, everyone needs:
- Enough food in your home to last 2-3 days.
- Know where to get water, if your usual supply is not available.
- Alternative lighting - a torch with spare batteries or a wind up one, gas lantern or light-sticks.
- A battery operated or wind up radio and spare batteries - don't forget your car radio as a last resort.
- A first-aid kit, make sure you have essential medications.
- Blankets, survival blankets or warm waterproof clothing.
- Alternative cooking methods, BBQ or gas cooker.
- To store important family documents where you can get them easily
- An emergency plan - where to meet family and how to contact one another if separated.
No current emergency
No civil defence emergency. This does not mean that a sudden event cannot occur at any time.
Here are some suggestions on how you should be ready
When the Metservice issue a new warning the status will change to 'Weather Warning' when the event actually starts the Civil Defence status will change to 'Alert or Activated' depending on the events severity
We've been alerted to a possible situation by:
* Metservice and the event has started, or
* Institute of Geological and Nuclear Sciences, or
* Ministry of Civil Defence and Emergency Management, or
* other warning systems such as telemetry / flood warning.
Civil Defence is in a monitoring and information gathering mode.
A hazard or event of significance is imminent or is occurring and has impacted or may impact on the CDEM Group Area and requires the activation of the Group Emergency Operations Centre (GEOC).
A state of local emergency declared under section 68 or section 69 of the Civil Defence Emergency Management Act 2002. An emergency means a situation that:
* is the result of any happening, whether natural or otherwise, including, without limitation, any explosion, earthquake, eruption, tsunami, land movement, flood, storm, tornado, cyclone, serious fire, leakage or spillage of any dangerous gas or substance, technological failure, infestation, plague, epidemic, failure of or disruption to an emergency service or a lifeline utility, or actual or imminent attack or warlike act; and
* causes or may cause loss of life or injury or illness or distress or in any way endangers the safety of the public or property in New Zealand or any part of New Zealand; and
* cannot be dealt with by emergency services, or otherwise requires a significant and co-ordinated response under the Act.
State of national emergency
Declared under Section 66 of the Civil Defence Emergency Management Act 2002.