Wellington, Sept 6 NZPA
The fault that ruptured and produced the magnitude 7.1 earthquake in Canterbury on Saturday appears not to have ruptured for at least 16,000 years, scientists said today.
The earthquake produced a 22km-long surface rupture and up to 4m of horizontal displacement in alluvial terraces that were deposited about 16,000 years ago at the end of the last glaciation.
When the last ice-age ended, rivers brought large amounts of gravel from the high country and distributed it throughout Canterbury, many metres thick in some places.
“Before Saturday, there was nothing in the landscape that would have suggested there was an active fault beneath the Darfield and Rolleston areas,” manager of the Natural Hazards Platform at GNS Science Kelvin Berryman said.
Geologists had no information on when the fault last ruptured as it was unknown until last weekend.
“All we can say at this stage is that this newly revealed fault has not ruptured since the gravels were deposited about 16,000 years ago.”
Dr Berryman said it was highly likely there were other “hidden” faults around New Zealand which might be capable of producing large earthquakes in the future.
The fault had been accumulating stress for thousands of years and failed catastrophically when the stresses exceeded a certain threshold.
GNS Science geologists were now examining drill-hole data held by Environment Canterbury (Ecan) and on-land seismic recordings made by oil and gas exploration companies to learn more geological history of the area where the earthquake occurred.
One of the things they were keen to learn was if the fault extended beyond the known surface rupture.
Dr Berryman said the earthquake’s effects in terms of levels of damage and liquefaction were consistent with existing knowledge about earthquakes of this magnitude.
In the context of New Zealand’s overall seismic hazard, Saturday’s earthquake was a low probability event, he said.
“We’ve known earthquakes are possible on the Canterbury Plains, but they are infrequent.”
Seismologists believe the major earthquake risk to Christchurch still comes from known faults in North Canterbury, in the Canterbury foothills, and from the Alpine Fault which extends up the spine of the South Island.
Saturday’s earthquake produced the strongest ground-shaking ever recorded in an earthquake in New Zealand. The highest ground-shaking measurement of 1.25 times the strength of gravity was recorded at Greendale near the epicentre.
Similar recordings were made on other “strong-motion” instruments within 15km of the epicentre.
Data from these instruments, which are located throughout New Zealand, is used by engineers and is incorporated in building codes.
Dr Berryman said preliminary computer modelling had indicated there had been negligible effect on the stress regime of the Alpine Fault, 90km to the west of Christchurch.
As at 3pm seismologists at GNS Science had recorded 85 aftershocks since the main shock on Saturday.
Of these, seven have been from magnitude 5.0 to 5.4, 42 have been between magnitude 4.0 to 4.9. and 36 were between magnitude 3.5 and 3.9.
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