06-16-2011 08:47 PM
This is interesting. Learn something new every day.
"In some areas a geological cause can be identified. For example, in the Charlevois-
Kamouraska region, there is a large, ancient meteorite impact crater that lies across a major
geological boundary. Many earthquakes are located in and near the crater. The St. Lawrence
river covers most of the ancient crater but careful geological mapping has revealed that the
crater is a zone of weakness responsible for earthquakes."
Bit of bad news though. There were 3 M8+ earthquakes in Missouri, US in 1812. They are still recording small aftershocks nearly 200 years later.
06-18-2011 01:01 PM - edited 06-18-2011 01:05 PM
"University of Canterbury geologist Mark Quigley says even the first 7.1 Darfield quake last September has itself turned out to be a network story. A loose collection of faults unzipped together to release thousands of years of strain.
"The rupture process seems to have involved four sources," Quigley says. It started with a 6.5 event near Charing Cross, then the main 6.9 at Greendale, leading to a smaller 6.2 out at Hororata, then finally a 6.5 back at the eastern Christchurch end. "It looks like a tip-to-tip rupture as the Greendale fault has gone very quiet since."
And now the chain reaction is continuing with faults being triggered further afield.
Jarg Pettinga, head of geological science at Canterbury University, says for any particular fault, it can work either way. Either their hidden strains are going to be relieved, or the transfer of stresses will load them closer to breaking point. So there has been a race to identify the Christchurch region's other possible faults so we can be warned. Multinational teams have been probing both Canterbury and Pegasus Bay with sonar-type instruments and gravity meters which can detect subtle changes in rock density.
A fortnight ago came the first reports. A boat survey discovered a new 25km fault off Kaiapoi. But while the fault might be large enough to generate a near-7 quake, it is not thought a tsunami risk and looks to have been inactive for a million years.
Far more of a concern, however, was the finding of the "CBD faultline" that is now believed to have generated the 4.9 Boxing Day aftershock - one felt strongly, being so near the heart of town.
Pettinga says the CBD fault reaches 10km from Hagley Park to New Brighton beach and perhaps a bit out to sea. Again, it is deeply buried under sediment and so must have been dormant a long time. Also, there is no certainty about whether it is the type that would go only in isolated segments or as a long line. Yet there is the potential for a magnitude 6 jolt if it were to go with a single bang.
Quigley says the placement of the fault is bad news. However, there is reason to think its stresses have been relieved rather than heightened because, since February 22, it has gone quiet.
"We can see there is something in the subsurface that has slipped before, but has very long recurrence intervals. It switched on on Boxing Day yet has switched off again. And we can only look on that optimistically," says Quigley.
Pettinga agrees. Going quiet is not ominous as some might think, he says. It really is an encouraging seismic sign. And because the CBD fault has the look of a very old structure, there is no reason to attach any particular fears to it at the moment.
Pettinga has the responsibility for researching another of the big question-mark locations, the "gap" between the Greendale and Port Hills faults.
On the map, there is a temptation to see these two faults connecting to make a longer line. And the area around Rolleston, Lincoln and Hornby has seen a disturbing number of aftershocks that could be evidence of another concealed fault. Rolleston was hit by a 5.5 just on June 6.
Pettinga says seismic surveys have now been done in this area and so a definitive answer should be possible within a matter of weeks. However, he already believes that because the number of aftershocks has been dwindling, and because the ground has a different sub-soil orientation, there will instead prove to be just a patchwork of smaller cross-wise fractures in this gap.
"Given the patterns of activity, we're reasonably confident there isn't a single through-going fault that links the Greendale structure and the Port Hills fault," Pettinga says.
After a third surprise hit, Christchurch just wants the truth about what it needs to be worried about. And it seems like the news on two of the major concerns is going in the right direction, even if it is unlikely scientists can fully eliminate them.
But where does Monday's earthquake fit in? It was a shock that yet another fault, one unknown and which did not turn up in the recent surveys, could strike so close to the city again.
The first word was that this was indeed a new faultline some 2km to the south of the February 22 Port Hills fracture. Pettinga says it is still early in the analysis, yet there is a fair chance that Monday's quakes were instead simply further ruptures on the original Port Hill's fault - ones on a "splay" that forked off at a different angle towards the surface above.
"So they could be linked at depth in the subsurface," Pettinga explains. The different angles would explain why February 22 was felt as a fast up-and-down motion while Monday's was more a bucking side-to-side.
Pettinga says the reason why there was no warning of the fault's existence is that ground imaging equipment can only probe for faults buried under sediment layers and the Port Hills fault is part of the hard rock making up the old Lyttelton volcanic cone.
Since Monday, what scientists have been noticing is that the aftershocks have been radiating steadily around the sides of the cone, heading eastwards towards Port Levy and that side of the Banks Peninsula.
GNS Science researcher Martin Reyners, who has been tracking the progress, says even this is good news probably. Reyners says when the volcano was forming in a series of eruptions millions of years ago, it would have created a ring of small fractures around it. It now looks like these are mopping up most of the energy released by the Port Hills fault.
Certainly the activity is spreading away from Christchurch and is scattered enough not to appear to be focused on any further large faults.
Reyners adds the public should also know that scientists hold no fears for the volcano itself. "There've been a lot of calls about new hot springs appearing in the Port Hills and the water of Lyttelton harbour getting warmer." But Reyners says all the instruments show the volcano is inactive. "There's no magma bubbling up underneath because of all this."
So the evidence is that we are still in a seismically active period. Monday confirmed that. And because so much more energy was dumped into the landscape by the latest events, GNS Science has had to raise the odds to a 30 per cent chance of seeing another earthquake of between 6 and 7 in the current aftershock zone over the next year.
But on the other hand, says Pettinga, the total activity is in fact slowly subsiding as the landscape rebalances. And the patterns of activity are moving away from the city as far as we can tell."
06-30-2011 06:19 PM
Ha ha, over here in the cold and powercuts here and there, one way people have found to keep the humour is now to take verbal bets on whether we will get to 10,000 aftershocks. We are at 7,750 + aftershocks now.
08-23-2011 02:23 PM
We just had our first Earthquake since the 1800's here on the East coast of USA. The epicenter was in Virginia and we felt it in NJ. They said it was felt all along the east coast from Georgia and up into part of Canada even. My husband and I didn't feel it here in our apt. but my 2 sons felt it where they work. One son evacuated the building, but the other one didn't. First time they ever felt an earthquake. This one was a 6.0. Minor one I know, but for part of the country that never has one, it was unusual.
Success always occurs in private and failure in full view.
08-24-2011 05:26 PM
Someone knocking on your door and saying "Hi, I'm from the government and I'm here to help you"
[ducking and running before getting hit with something heavy and hard]