Latest quakes on different fault to big February shake

Christchurch's most recent quakes were on a different fault to the ones which earlier devastated the city. Photo / Geoff Sloan

Yesterday's major earthquakes in Christchurch were on a different fault to one which devastated the city on February 22 but are part of the same sequence geologically dating back to September, seismologists say.

GNS Science seismologist Bill Fry said yesterday's two earthquakes -- one measuring 5.6 on the Richter scale about 1pm and a 6.3 quake about 2.20pm -- were on a fault structure 2-3km south of the Port Hills fault which ruptured for the February 22 earthquake.

He said it was likely that stresses from the first shake triggered the second one.

"Technically this makes the magnitude 5.6 a foreshock, or precursor, of the magnitude 6.3," he said.

"But overall they are still part of same aftershock sequence to the September earthquake."

However, Dr Fry said the energy released in both earthquakes was high, as it had been for most of the earthquakes since the first one on September 4 which measured 7.1.

"This contributed to the anomalously high shaking intensity of the earthquakes, as the amount of shaking is proportional to the energy released."

He said the dominant energy was horizontal, which meant it would have felt slightly different to the February earthquake, for which the dominant energy was vertical.

- NZPA