NZ Hot Spot: Earthquakes

New Zealand is a dynamic environment. It’s on the move. It’s a hot spot. No, I don’t mean clubs and nightlife like New York City. I mean a geothermal hotspot as in huge pockets of magma right under the surface, giant tectonic plates grinding against each other beneath your feet, and a major tsunami danger zone.

Volcanoes, earthquakes and tsunamis are all linked together and caused by the same underground forces.

The surface of the earth is made up of fifteen large tectonic plates that cover the surface of the earth like a giant jigsaw puzzle. New Zealand sits at the juncture of two of these plates, the Pacific Plate and the Indo-Australian Plate. It is as if New Zealand is straddling the boundary between these two plates with one foot on each.

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If you look closely, you can see the Indo-Australian and Pacific Plates meeting at New Zealand at the bottom left.

Basically the earth has a very hot metallic core surrounded by semi-molten rocky material (magma) and a cooler outer rocky layer called the crust.

While there are several explanations for the movement of the tectonic plates, one is that currents of magma rise and move in circular motions (thermal convection) like stuff rising and circulating in a pot of soup you’re cooking on the stove. The tectonic plates ‘float’ on this magma and convection currents drag them around.

The spots where tectonic plates meet (plate boundaries) are where geological activity commonly occurs. Where two plates meet three things can be happening depending on their movement.

The plates can be moving apart from each other, called divergence, often causing volcanic activity particularly under the ocean.

The plates can be sliding laterally across each other termed a transform fault such as the famous San Andreas Fault in California and the Alpine Fault here in New Zealand on the South Island. The Alpine Fault is the land portion where the giant Pacific and Australian Plates meet and is responsible for lifting up the Southern Alps on the South Island. This fault also has a high probability of rupturing in the next 50 years and is predicted that it will cause the largest earthquake since the European settlement of New Zealand.

Finally tectonic plates can be forced toward or into one another, termed convergence. 95% of earthquakes occur at these convergent plate boundaries. The Ring of Fire in the Pacific Ocean is where tectonic plates of the convergent variety meet.

At best when plates meet, they slowly slide past each other or one beneath the other (subduction) dissipating their energy in a gradual manner. However sometimes things get hung up and an area builds up tension which then is suddenly released causing an earthquake.

Closer view of tectonic plates meeting in New Zealand

Closer view of meeting of plates in New Zealand

What’s an earthquake like?

Well, it depends how powerful it is. Although various scales are used to measure the intensity of earthquakes, the Richter scale is probably the best known. It is important to remember that the Richter scale is logarithmic scale measuring the amplitude of the seismic waves. The difference between a Richter scale 4 and 5 means 10X the wave amplitude. But because of way wave amplitude relates to energy release, that is 32X the amount of energy. So, for example, a Richter scale 8 earthquake has 32,000 times the energy as a scale 5. Way lots more.

In most cases there is no warning. For small earthquakes a mild shaking can be felt. As the intensity increases, the shaking is more pronounced disrupting the interiors of buildings and eventually the structures themselves. For a large earthquake close to the epicenter it is said to feel like someone hit the earth with a giant sledgehammer. Following the initial shock there is a series of wavelike shaking that lasts a minute or more. Aftershocks occur after the main shock due to continuing adjustment of the rock fault that caused the initial disruption most occurring during the first hour and then diminish over the next days.

To be more scientific, two primary types of waves are produced by an earthquake. P waves are push or compressional waves which expand outward from an earthquake is the direction the waves are travelling. S waves, shear or secondary waves, are transmitted as a shearing motion at right angles to the direction of the wave travel. One author described P waves as similar to compressing a Slinky’s coils together, and S waves as making undulating waves with the Slinky.

P waves travel at 20,000 km/hr, S waves at about half that.

When an earthquake occurs, people often feel a sudden jolt which is the P waves arriving. This is followed a short time later by the ground shaking which marks the arrival of the S waves.

By having seismographs in multiple locations on the earth’s surface, one can triangulate and determine the exact location of an earthquake based on the arrival times of the P and S waves. An interesting bit of trivia is that for a long period of time it was too expensive, particularly for the scientific community, to establish accurate seismographs around the globe. However, with the Cold War and monitoring for nuclear activity and testing, an extensive series of seismic stations were created around the globe primarily by the US Department of Defense. A side effect of this was that it supplied data to geologists allowing for a better understanding of earthquakes and tectonic plate activity.

New Zealand records about 20,000 earthquakes a year! Only about 200 of these are strong enough to be felt by habitants. Here’s an image showing New Zealand’s largest earthquakes in the past.

Large earthquakes that have occurred in New Zealand

Large earthquakes that have occurred in New Zealand

One of the most recent earthquakes to cause loss of life and significant damage to property was in Christchurch, New Zealand’s second largest city, on the South Island. A previous earthquake six months earlier in Canterbury had caused some damage in Christchurch making many structures more vulnerable when a larger earthquake did occur.  Then on February 22, 2011, a magnitude 6.3 quake occurred approximately 6 miles from the city lasting only about ten seconds. 185 lives were lost and major damage to the city occurred including destroying the famous ChristChurch Cathedral. It was a devastating day in New Zealand history.

Before

Before

After

After

Along with destruction to property and loss of life related to the actual rumbling of an earthquake, fires are another risk associated with earthquakes.

Liquification is another phenomenon caused by earthquakes and lesser known by the public. Essentially the increased pressures and shaking motion of the earthquake itself cause liquefied sediment and water to force its way to the surface destroying buildings by affecting their foundations and damaging infrastructure such as gas and water pipes. Liquification was also a major cause of damage in the Christchurch earthquake.

It is inevitable that major large earthquakes will continue to strike New Zealand. The large cities most at risk include Wellington, the capital, Hastings and Napier.

Current advice from the New Zealand Earthquake Commission in case of an earthquake is termed “Drop, Cover and Hold”, to quickly drop to the ground (to avoid falling), take cover under something strong like a desk or table, and hold on until the shaking stops. Regardless of what you might think, it is also felt you are generally safer inside than outside during an earthquake.

While in Colorado, I never really worried about earthquakes, never even thought of them. Yes, there were major snowstorms, lightning, flooding and the devastating forest fires but no earthquakes. Here I have a whole new set of things to worry about.

I highly recommend you check out this site (http://www.geonet.org.nz) that shows the most recent earthquake activity for New Zealand along with the current status of the various large active volcanoes.

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2 thoughts on “NZ Hot Spot: Earthquakes

  1. And I thought New Zealand was peaceful and quiet….HA…you guys have a whole TEMPEST going on right under your feet…and ready to knock you off your little feet at any minuet. After haveing given it some thought, I think I stick with the snowstorms, forest fire, lightening …..and the biggest danger of all…. pedestrian traffic thu Manitou.

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