Gap-filling sequence 'positive'

Posted 02 Sep 2011 by MediaStuff Popular
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The largest aftershock east of Christchurch since mid-June jolted much of the city awake yesterday and capped a week of heightened earthquake activity.

The 3.29am magnitude-4.9 shake was 7.6 kilometres deep and located about 2km northeast of Godley Head. It was marginally smaller than a magnitude-5.0 shake at 6.27am on June 15.

Scientists said yesterday's aftershock appeared to be generated by the same north-northeast/south-southwest trending fault that caused the two big quakes on June 13 and also produced Wednesday afternoon's magnitude-4.0 jolt.

United States seismologist Professor Kevin Furlong, of Pennsylvania State University, said the mechanics of yesterday's quake were similar to those shakes.

"Most of the aftershocks in the past few months, since the June event, have been in the approximate north-south trend of aftershocks that seem to be associated with the June 13 event," he said.

"So last night's event and the other one a few days ago seem to be still part of that aftershock decay."

The more "interesting" location was where Wednesday's magnitude-4.8 quake was, between Prebbleton and Rolleston, and where a magnitude-5.4 quake on June 21 was centred, between Halswell and Prebbleton.

"To me, those are at the eastern end of the subsurface [Greendale] fault and are basically slowly but surely filling in the gap between the end of the September event and the western end of the February rupture.

"One can never be definitive, but I think it is a positive sign that that area, where we have had less seismic energy released, is filling in with a bunch of magnitude-5.0 events. That doesn't preclude a slightly bigger event, but so far so good."

The February and June quakes had not "switched on" activity on other fault segments that had previously ruptured.

"So the February event turned on aftershocks along its fault but didn't change the rate of earthquakes on the Greendale Fault. Similarly, the June events `lit up' that fault segment but didn't reactivate either the Greendale or the February fault," he said.

"This seems to imply that each of those faults is basically done, so with the exception of site-specific locations that still seem to have some unresolved issues, the main faults that have ruptured seem to be relatively spent."

The Canterbury quake series since September 4 had been "an intriguing sequence".

"All this said, I can imagine that it is a bit unsettling as we approach the one-year anniversary," he said.

"It all has to end some day – hopefully sooner rather than later."

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