Jul 6 2011 - Taupo Earthquake: Questions and Answers

• Location: Taupo
• Date (NZ Standard Time): Tuesday, July 5 2011 at 3:36 pm
• Epicentre: 38.67°S, 175.75°E
• Focal Depth: 150 km
• Maximum Intensity: MM 5
• Richter Magnitude: 6.5
• GeoNet Summary:
  • Details
  • Maps
  • Shaking Maps

Is this earthquake unusual?

No. The last large earthquake in the area was in January of this year. It was a magnitude 6.4, with a similar depth to yesterday's. In the last 10 years, there have been about 20 deep earthquakes greater than magnitude 5.5 in the area. The largest recent earthquake occurred in May 2001. It was a M6.5, 238 km deep.

Why was this earthquake felt more widely, away from the epicentre?

So far, we've received just over 3700 felt reports for this event. These have come from as far away as Auckland and Dunedin, with no significant reports of damage. Although it was centred near Taupo, it was more widely felt on the east and south of the North Island, and throughout the South Island. This is because of where the earthquake was generated. The earthquake occurred within the subducting Pacific Plate (the east side of country), in the cooler and still relatively rigid rock of the plate. Because this rock is rigid, the seismic waves find it easier to travel back up the slab, reaching the surface along the east coast. Some energy will also move along the descending slab. The energy travelling from 150km depth, through the softer and warmer mantle, means that the seismic energy is attenuated before it reaches the surface, meaning few people above the epicentre will feel the earthquake.

Why the discrepancies in reported magnitudes?

There are several ways to calculate the magnitude of an earthquake and the result from each method depends on the physical properties of the earthquake. For example, some methods measure the amount of low frequency energy generated by the earthquake (moment magnitude) while others depend on the amount of high frequency energy (Richter magnitude). Ideally all of the methods would give the same answer but this rarely happens. However, to seismologists, the differences in the magnitude estimates can say quite a bit about the physical properties of an earthquake.

Generally, GeoNet uses Richter magnitude except for large shallow earthquakes when it becomes more appropriate to use a moment magnitude. We reported the Taupo earthquake yesterday as a Richter magnitude (M6.5). The United States Geological Survey (USGS) generally report a moment magnitude (in this case, M5.3).

Are aftershocks expected?

A distinguishing characteristic of deeper earthquakes (>30km) is the absence of observable aftershock sequences. Because the earthquake is so deep, any small aftershocks are not likely to be felt.

Will the earthquake trigger other faults to rupture?

Scientists have studied how large earthquakes trigger other large earthquakes over time frames from minutes to decades through a process known as stress triggering. When a large earthquake occurs, it releases stresses across the fault surface that ruptured, increases stress at the edges where the amount of fault slip drops to zero, and causes slight changes in the stresses within the Earth’s crust further from the fault (tens of kilometres). These more distant effects increase the stress in some places and reduce it in others, depending on the geometry of the fault rupture. Sometimes the increased stress can trigger another large earthquake, although the chances of this are low. Because the stress changes created by large earthquakes decrease very quickly with distance, major earthquakes do not trigger each other over very large distances. The earthquake waves from very large earthquakes have been shown to cause increased small earthquake activity in places such as geothermal fields, but there is no evidence that large earthquakes trigger other large earthquakes over large distances. Sometimes we notice that earthquakes around the world seem to cluster in time, but this is coincidental. Earthquake occurrence is random on a global scale.

Will the earthquake trigger volcanic activity in the area?

Yesterday's earthquake was in the subducting slab. As the subducting slab descends, it moves under the central North Island area and the Taupo Volcanic Zone (the home of many of New Zealand's active volcanoes). As the descending slab reaches depths over 180-200 km it starts to melt and gain buoyancy, allowing it to rise and ultimately feed the active volcanoes. This is a very long term process, taking hundreds to maybe even thousands of years.

"Although there is the big picture relationship between the plate boundary and volcanoes, we do not see yesterday’s earthquake will change the activity at the volcanoes." ~ Brad Scott, GNS Volcanologist

GeoNet actively monitors the Taupo Volcanic Zone.

Earthquakes and volcanoes in the subduction zone.