Hi I’m Ali Sealander with UTD GeoNews and today we are going to talk about what caused a massive
8.1 magnitude earthquake that shook the Kermadec Islands, New Zealand on the morning of March
5, 2021. This was a noteworthy event as earthquakes
of this size are a rarity. Thousands of earthquakes happen across the world every year.
On any given day, several hundred magnitude 2 or below earthquakes occur and go largely
unnoticed, even by people who live nearby. Earthquakes in the magnitude 7 range occur
about 15 times a year. But magnitude 8 or higher, these “Great Earthquakes,” are
only expected to hit maybe once a year. This magnitude 8.1 event was the largest that’s
happened in the area since a magnitude 8 earthquake struck in 1976. The largest ever recorded
earthquake in New Zealand occurred on the Wairarapa Fault in 1855 and resulted in 18
meters of horizontal land movement and pushed up the nearby Remutaka Range more than 6 meters.
And just a decade ago, a much weaker magnitude 6.3 quake struck near the city of Christchurch
and unfortunately killed 185 people. It is evident that earthquakes as strong as the
recent 8.1 Kermadec event are deadly if they happen in populated regions, so it is important
to know a little about them. The good news is that the Kermadec islands are largely uninhabited
and the resulting tsunami was minimal, less than a meter tall.
Even though this earthquake didn’t turn out to be devasting, it was still watched
quite closely. Earthquakes are taken very seriously around New Zealand. The estimate
is that there is about 150 to 200 earthquakes that happen around New Zealand every year.
So, what’s the deal? Why does this area have so much seismicity?
Well it has to do with the fact that New Zealand and the Kermadec islands sit on top of a boundary
between two massive tectonic plates, the Australian plate to the west and the Pacific plate to the east.
The Pacific plate is the biggest plate on Earth and almost everywhere around
its margin are volcanoes and earthquakes in a zone that was dubbed “the ring of fire” long
before Johnny Cash wrote his hit song.
In this region there is a deep trench where the Pacific plate
sinks beneath the Australian plate. The Pacific plate moves westward toward the Austalian
plate at a rate of about 60mm per year- this is about the same length as your little finger.
This sinking, or subducting, of the Pacific plate is not very smooth and so the convergence
of the plates builds up stress. When this becomes too much, the fault between them slips
to release the stress, creating an earthquake. The magnitude 8.1 quake occurred 20 km below
the crust, in what geoscientists call the Seismogenic Zone. The hypocenter, where the
earthquake occurred, was right on the boundary between the Australian and Pacific plate on
a westward-dipping thrust fault. A thrust fault is when the rock above a fault moves
up and over the rock below the fault. In this case, the Australian plate pushed eastward
and the Pacific plate westward, moving suddenly to produce the magnitude 8.1 earthquake.
Thankfully, this earthquake occurred in a remote area about 600 miles away from New Zealand's North Island.
and didn't turn into much more than a news story
Let’s hope future big quakes like this happen far away from where people live.
I'm Ali Sealander with UTD GeoNews and I'll see you next time!