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Sunday, April 24, 2011

Port Hills fault behaving as it should

Port Hills fault behaving as it should

Last updated 05:00 23/04/2011
Christchurch's active Port Hills Fault is behaving as it should and is unlikely to be building up to create something "ominous", seismologists say.

The fault generated February 22's magnitude-6.3 earthquake and is also believed to have been responsible for the magnitude-5.1 shake on September 8, which first indicated the existence of a fault beneath the Port Hills.

Three other significant aftershocks appear likely to have occurred on the same fault – last Saturday evening's magnitude-5.3 quake, and magnitude-5.8 and 5.9 shocks on the afternoon of February 22.

Scientists have said there has been a particularly rich sequence of aftershocks around that broad area since the February event. However, despite that, they say there is no evidence yet that all of these quakes have been on the same fault. That could mean another hidden fault lies close to the Port Hills or underneath them.

GNS Science seismologist Dr Matthew Gerstenberger said if you took into account every aftershock since September 4's magnitude-7.1 quake, the sequence had been "behaving as expected".

Asked if the number of big aftershocks on or around the Port Hills Fault showed it was not behaving normally, he said: "You can't really separate out the February one."

"The 7.1 is still driving things – that is the biggest contributor to the future occurrence of earthquakes in the region."

Visiting United States seismologist Professor Kevin Furlong, of Pennsylvania State University, said it was tempting to assume all the major aftershocks were on the Port Hills Fault.

"Only after more precise relocations will we be confident of that assumption. That said, and assuming that the February 22 and the more recent magnitude-5-plus aftershocks are on the same fault, the question comes up as to whether this is behaving `badly' or strange in any way.

"In fact I think this sort of behaviour is fairly typical.

"Whether they are filling in small gaps on the fault where the slip during the magnitude-6.3 rupture varied a bit, or represent `triggered' events nearby the February rupture, the magnitude-5 events are not out of character for a fault like this.

"During the February event not all areas of the fault moved exactly the same amount, so there are likely patches of missing slip.

"I don't think there is any indication that this is leading to anything more ominous, or for which we should be worried, beyond the standard preparedness that everyone in Christchurch is dealing with.

"How this fault segment behaves over the next several months will most likely be similar to how it is currently behaving, with a decay in activity over time.

"Having aftershocks on or near the fault is expected, [and] there is no reason to think that they would be larger than what has already happened."

There was evidence the February quake had reduced stress in the Earth's crust near the Port Hills Fault, Furlong said.

However, it had instead slightly increased stress further east.

"So, similar to how faults elsewhere have behaved, we might expect more of the aftershocks to be to the east of what slipped in February.

"The good news from this is that this activity would therefore be moving away from the main population of Christchurch.

"And it is important to reiterate that the type of faulting we are having will not generate tsunami, so if the aftershocks occur offshore, they do not increase any tsunami risk."

- The Press

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