Mapping NZ’s recent earthquakes

By Carolyn Barry 23 February 2011
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Geologists have mapped the earthquakes that have shaken NZ since last year.

AT 12:51 PM LOCAL TIME on 22 February 2011, a 6.3-magnitude earthquake struck the South Island of New Zealand. It came almost six months after a 7.1-magnitude earthquake hit the Darfield region of Canterbury, about 40 km west of Christchurch.

Christchurch earthquake map

The map above, depicts the number, location and size of earthquakes since the September 2010 quake. Composed by Robert Simmon, using tectonic data from the US Geological Survey (USGS), this NASA Earth Observatory image shows the pattern of tremors along the faul lines near Christchurch.

Black circles represent earthquakes from 4 September 2010, until 21 February 2011. Red circles show the locations of the magnitude-6.3 quake and aftershocks on 22 February and the morning of 23 February. Larger circles represent stronger earthquakes. Yellow shows urban areas, including Christchurch (population 375,000).

Understanding magnitudes

Though this latest earthquake is more devastating, it’s classed as an aftershock to the 7.1-magnitude Darfield quake last year. Aftershocks can continue for about a year after the initial earthquake. The Canterbury area has endured about 5000 since September.

While the epicentre of the Darfield earthquake was about 10 km below the surface, the Christchurch quake was a mere 5 km underground and just 10 km south of the city centre. At these shallow depths, the massive energy is transferred quickly and devastatingly to the surface.

With each successive jump in the Richter scale (measured 1-10), the energy of the earthquake is tenfold. Therefore, a magnitude-7 earthquake has 10 times the ground shaking power of a magnitude-6 and 30 times more energy. The higher the magnitude, the more destructive and the fewer quakes there are.

The largest recorded earthquake occurred in Chile in 1960, whith a magnitude of 9.5.

Effects of this earthquake were felt more than 200 km away, in Dunedin and Queenstown. It also resulted in the massive collapse of a chunk of the Tasman Glacier (New Zealand’s longest glacier) into the Tasman Lake.