Quake: New Zealand and the Hikurangi Trough

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By now you’re likely aware of the devastating earthquake that struck near Christchurch, New Zealand Tuesday morning (local time).

Here’s the latest: The strong and very shallow quake was located at 43.59S and 172.71E, or about three miles north northwest of Christchurch. The depth of the focus was estimated to be only about 3 miles deep. Strong seismic aftershocks between 4.5 and 5.5 on the Richter Scale will likely occur over the next two to three weeks as the initial break in the Earth’s crust slowly reaches equilibrium. Buildings already weakened by the initial strong quake could collapse with those additional strong aftershocks.

Subduction. As the name implies, a subduction zone is where two of Earth's rubbing plates converge and one slides under the other. Image: Wikipedia

The nearest major fault is the Hikurangi Trough (a subduction zone) where the oceanic crust is subducting into the mantle underneath New Zealand which is on the Australian Plate. It’s more than likely a fault zone associated with this subduction zone is what caused this quake.

The Hikurangi Trough is a subduction zone. Image: Wikipedia

This region has been experiencing a series of quakes since early September. On September 4th, a 7.1 magnitude quake struck South Island near Darfield (about 45 kilometers west of Christchurch). Fortunately, this significantly stronger quake struck further away from Christchurch and deeper than Tuesday’s quake. In addition, it struck very early in the morning. Damage was much less and there were no deaths attributed to the quake.

Quakes with a relatively shallow depth such as Tuesday’s allow a large part of the energy generated by the quake to be translated directly to the surface in the form of strong seismic waves. Surface damage with a shallow quake tends to be much more significant.

For geological purposes these are the most import criteria in judging the destruction capability of  an earthquake:

  • Magnitude. The stronger the magnitude, the higher the risk potential.
  • Depth. The shallower the epicenter, the higher the risk potential.
  • Soil Consistency. If the soil is composed of sediments (loose sand or clay) rather than solid igneous or metamorphic bedrock, the greater the shaking risk and destruction potential. Loose sand or clay causing increased damage may sound counter-intuitive, but it allows the ground to shake like a bowl of Jell-O. as opposed to a single (or series) or sharp jolts.

What’s considered “shallow” when considering the depth of the epicenter? Typically anything within 20 miles of the surface is considered shallow. Between  five and 10 miles is considered very shallow, while extremely shallow is reserved for those epicenters within five miles of the surface. Deeper quakes can reach well into the mantle, with depths greater than 50 miles.

Epicenters of shallow quakes originate in the Earth's crust. Deeper quakes can come from the mantle. Image: Wikipedia

Perhaps adding to the misery, yesterday’s sunshine across most of New Zealand is now being replaced with rain as a cold front moves from the Indian Ocean and Tasman Sea across New Zealand, from south to north.

To donate, contact the N.Z. Red Cross at RedCross.org. Donors are urged to donate cash rather than food or supplies. As always, be aware of scam donation schemes which always pop-up within hours of such a crisis.

Showers passing by Christchurch should enter the Friday forecast, but the heaviest rain should remain across southern areas of South Island. Image: ImpactWeather Gmaps.

Showers passing by Christchurch should enter the Friday forecast, but the heaviest rain should remain across southern areas of South Island. Image: ImpactWeather Gmaps.

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