Marc Valdez Weblog: At The Eastern End Of The Port Hills Fault

Tuesday, June 14, 2011

At The Eastern End Of The Port Hills Fault

The crustal breakage continues near the seashore. Presumably the shocks will eventually start heading north, just offshore, following the locations of other small quakes since last September. That should be what it does, but what it's actually doing the last few days is arcing southeast, through Port Levy. I wonder what's going on there?:

A magnitude 5.0 tremor has rattled Christchurch this morning, as scientists say there is now a higher chance of a new large quake in the region.

The tremor, which struck at around 6.27am, was centred 20km southeast of the city at a depth of 6km.

It was followed by a 4.2 magnitude shake at 6.32am.

...Scientists analysing Monday's quakes – yesterday upgraded to magnitude 5.6 and 6.3 – now believe they occurred on another fault 2km to 3km south of the Port Hills Fault, which generated the February 22 shake.

Seismologist Bill Fry said there had been six aftershocks of magnitude 4.0 or greater on or near this fault since February.

Like most of the quakes since September 4, they had been high in energy. However, most of the energy released on Monday was horizontal, compared with vertical on February 22.

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

"The spatial size of the underground rupture area for the magnitude-6.3 quake was relatively small for the amount of energy released. This implies that the fault was very strong."

Visiting United States seismologist Kevin Furlong said Monday's major aftershocks had probably reduced the stress buildup around the eastern end of the Port Hills from the February 22 quake.

It was likely stress had now transferred further east and offshore, he said.

"So there will be aftershocks from this and they will likely mostly be on or near the fault that ruptured [on Monday], and also possibly further to the east, and also some to the north-northeast, as was the case after February."

He said the first quake had been a trigger for the second, with both showing almost identical movements.

"Although their locations relative to the February event are slightly different – more to the east – I think they reflect the same tectonics," he said.

"Whether we want to say they are the same fault or simply adjacent faults is really semantics to me. They are fault segments that are interacting with each other."

After such an incredible sequence of quakes, the problem now was knowing what "normal" was.

Earthquake scientists had "cut their teeth" on the behaviour of quakes from plate boundary faults such as the San Andreas in California and New Zealand's Alpine Fault but knew far less about small crustal quake sequences like this one.

"We know the plate boundary faults' history and behaviour that, say, every 300 years they do this or that. But with this type of event, we don't know what is normal for the Canterbury region," Furlong said.

"We assumed what we had up to September 4 was normal, but it appears it wasn't normal. We don't know what is the background condition that the Earth is now moving towards.

"Each earthquake sequence is unusual, this one both because of its character and observation.

"It's as well-recorded as any of this size has ever been. We are seeing things about it – things that we don't see in any other place.

"It's aspects of this that makes this [sequence] very important to science and why it's hard to be definite about how it's going to behave."

Geotech Consulting engineering geologist Mark Yetton said he had not seen any obvious surface rupture on the Port Hills from Monday's quakes.

Monday afternoon's biggest earthquake came close to outstripping the magnitude of the deadly February 22 quake.

GNS Science seismologists yesterday reclassified the 2.20pm aftershock as magnitude 6.338, just 0.005 of a magnitude smaller than February's 6.343 quake.