GeologyRocksEarthquake!
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Looks like they have updated
Sat, 04/28/2007 - 13:56Looks like they have updated the article and removed someone's mess-up since I read it first time. They had a small quote from a gas company official talking about how they were trying to find out whether reported gas smells were from leaks, or natural emissions from fault lines disturbed by the quake... Isn't the gas smell added artificially during processing?
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I didn't feel it personally,
Mon, 04/30/2007 - 13:49I didn't feel it personally, but I know my Grandad did. He was outside at the timne, and he said he could feel the ground shaking. He then proceded to go ner-ner at me, cos he knows how much I like Earthquakes.
Oh well, one day I shall experience one.Â
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Magnitude is practically
Tue, 05/01/2007 - 08:49Magnitude is practically meaningless for the public. If a magnitude 8 earthquake occured at 700km we wouldn't feel it. If it occurrd in the crust - then we would! I'm not sure about the details (or where to get them) but this quake may have been shallow, hence the damage, hence the news coverage. It's the damage and misery caused that's making the news - not the fact a quake occurred. Sad really.
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Felt Deep Earthquakes
Tue, 05/01/2007 - 19:02Magnitude is practically meaningless for the public. If a magnitude 8 earthquake occured at 700km we wouldn't feel it.
Err ... I think we would. The June 9th 1994 Bolivia Deep Earthquake 647km 8.2Mw was felt over a wide area of North as well as South America
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Why the hype
Tue, 05/01/2007 - 19:10Why has this one got so much coverage then? i can remember a few, but this one seems to have been hyped up by the media.
1. People have short memories about British earthquakes and the public perception is that earthquakes are what only Jonny Foreigner gets, so this makes it news.
2. It was a slow news day
3. It was 'daarn sauf'
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I know it's not that big,
Tue, 05/01/2007 - 19:12I know it's not that big, butits still exciting.
It hasn't got that much coverage really, only the damage it caused. The manchester ones a couple of years ago got loads of more coverage.
 I remeber when we had one in Cornwall a couple of years ago,all my cds fell over, it was 4am and I thoguht a truck had driven into the side of the house.
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I really don't know what all
Tue, 05/01/2007 - 22:09I really don't know what all the fuss is about. I can remember three from well within my lifetime:
Bishops Castle - April 1990 - Magnitude 5.1
Shrewsbury - March 1996 - Magnitude 3.4
Dudley - September 2002 - Magnitude 4.7
 I felt them all, although the worst damage caused by any of them was some chimney pots toppling over, which in these parts tends to happen whenever people sneeze!
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Did you see the moment
Thu, 05/03/2007 - 10:00Did you see the moment tensor solution?
 It was a strike-slip fault mechanism which trigger the shake. Take a look on it:
http://www.earthquakes.bgs.ac.uk/earthquakes/reports/dover_straits/dover_straits_28_april_2007.htm
and to answer one of your questions about the feeling tremor look the map os isovalues of intensity (below) of this event.
But this tremor was lower (Mw = 4) than the others (Mw >5) which shake the Strait of Dover (see the map attached).Â
Javi
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I remember one of the
Fri, 05/04/2007 - 12:44I remember one of the options for projects during my geology degree was the danger to the channel tunnel from Earthquakes. I knew that historically there had been some (relatively) big earthquakes in that area but I just hadn't considered it as seismically active at all.
 The conclusion from the students who took on this project was that there could be a risk, suppose they may have been right
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Don´t think so al83. That
Fri, 05/04/2007 - 13:27Don´t think so al83.
That part of the European plate seems to me relatively stable.
But, some studies marks lithospheric folding in Brittany during the Caenozoic (close).
On the other hand, WSM shows no active stresses in that area, only just a mixture of mechanism in northern France close to the boundary with Belgium:
So I think (it´s my opinion) that it is a relatively stable area, but as part of an intraplate block, it´s possible that some processes take place because of the transmission of stresses from the Atlanthic Ridge (extension NW-SE, which press Uk in that area of the Strait of Dover) and compresión N-S and NW-SW between the continental collision Africa-Europe.
Cheers!
Javi
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Latest news about the tremor:
Fri, 05/04/2007 - 21:59Latest news about the tremor:
http://news.scotsman.com/topics.cfm?tid=907&id=669572007
more than 800 homes were damaged...a lot of them. WoW.
Ey Hypocentre, do you have the Formula to pass magnitude to intensity? I don´t remember well right now. But I think that Mw of 4 is more or less close to 6º at Richters Scale, isn´t it? So...Moderate (clearly perceptible....we know it now!).
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Magnitude and Intensity
Sat, 05/05/2007 - 12:07Latest news about the tremor:
Ey Hypocentre, do you have the Formula to pass magnitude to intensity? I don´t remember well right now. But I think that Mw of 4 is more or less close to 6º at Richters Scale, isn´t it? So...Moderate (clearly perceptible....we know it now!).
OK. (deep breath)
Earthquake magnitude and intensity are completely different things. Magnitude is a measure of the size/power of the earthquake, intensity is a measure of the damage at a particular point. Hence an earthquake will have one magnitude but a range of intensities.
(OK it might have more than one magnitude depending on how it is measured but let's not go there).
More info. here.
Obviously, the larger the magnitude the greater the maximum intensity but this will depend on other factors like the depth of the 'quake and the nature of the near surface rocks so there is no direct correlation.
A note on scales. The 'Richter Scale' is a measure of magnitude. It is calibrated for Californian rocks and a Wood-Anderson seismograph. It is no longer used (despite what the press would have you believe!). The modern magnitude scale is Mw or moment-magnitude. The seismic moment (a measure of the forces at the hypocentre) is calculated from the inversion of seismic waveforms and the result is then converted to an equivalent magnitude.
Intensity in Europe is measured on the European Macroseismic Scale (not the Mercalli Scale) as this is calibrated for European buildings and ground conditions.
See here. for more details
The Folkstone Mw 4.2 has a maximum intensity of 8 EMS
see picture.
The strike-slip mechanism is common for UK earthquakes - the result of compression from SE from the Alps and NW from Mid-Atlantic ridge.
The Kent earthquake comes as no real surprise to me. This area is one of the most active areas for large earthquakes in the UK, albeit with long return times. The most significant events were on 21st May 1382 and 6th April 1580, both estimated at Local Magnitude 5.8. The latter caused much damage in the southeast of England and is one of Britain's most fatal earthquakes with two apprentices being killed by falling masonry in a church in Newgate.
See my blog.
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Sometimes you make me seem
Sat, 05/26/2007 - 14:51Sometimes you make me seem stupid Hypo!!.
I appreciate your aptitude and willingness, but I´m disgusted. (i´m talking about your deep breath......)
The point is that I know what you have posted and what I was talking about.
 But.... there is a relationship between magnitude and intensity for estimating the size of an earthquake. They are different....but they have some common features:
There is an empirical relationship as I´m going to show you:
Imax = 1.5 Ms- 1.8log 10 h +1.7
 But the there is another one (Mezcua and Pego, 2002) with the Moment magnitude (Mw):
Mw= 0.96 + 0.6 Imax
So, the earthquake of Kent with Mw=4.3 is related with an Imax= 5.5
On the other hand, as previously I have told...the pushing of the Atlantic Ridge from the Northwest and the African plate to the South makes Britain seismically active. But it is not a specially high activity because....if you make measures of velocity vectors (I´m talking about GPS measures) you could see that que Euler pole that moves Europe and Africa from a fixed America plate show a low velocity because is far away from the plate boundary, inversely, Spain (2-5 mm/y) but overcoat Argel (7mm/y)  and the E of the Mediterranean area (>10 mm/y) are much more active zones. So, if the velocity of pushing is low, the elastic energy accumulated on fault planes and surfaces is low too.
 Cheers!
Javi
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Javi
Sat, 05/26/2007 - 22:43Javi
I hope you didn't take offence at my post as none was intended.
There are some magnitude / maximum intensity relationships published as you say, but as you can see don't work well as there are just too many variables - predicted maximum intensity 5.5, actual maximum intensity 8 - not very good.
On your second point I think it depends what you mean by active - I think that an area with two historic magnitude ~6 events is active.
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Hi,(forgive my insistence
Sun, 06/03/2007 - 10:14Hi,(forgive my insistence Hypo).
But It still seems to me very high that value of 8 EMS. So I was seeking for some papers related to seismicity at Britain because that value seemed to me very exagerated.
 Everyone knows that Intensity is a "subjective" parameter on the earthquake´s stimations. But always could be use a reliable formula to achieve a mathematic value of it.
I have found what´s the Imax Formula that uses the BGS related to EMS scale which obtain the best results for British earthquakes  (Musson 2004).
Where ML is the Local Magnitude of Richter and R the hypocentral distance. So if we calculate R as an approximation (because the path of the seismic ray is not a straight line) by the Pythagora´s Theorem:10 Km------------epycentral distance2 Km--------------depthH= (C2+c2)1/2 = (102+22)1/2 = 10.19 KmThen if we introduce ML=4.3 and work with the Formula above:Imax= 3.31 + (1.28 x 4.3) - 1.22 (ln 10.19) = 5.9That is the Formula that uses the BGS to calculate the Imax. This kind of Intensity formulas are correlated with historical seismicity of events in each country can uses one different related with their own events. Through some scales they correlated some variables of magnitud, intensity, distance and so on to calibrate hazard models against hystorical earthquakes.Sorry for the brick Cheers,Javi
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What happened with my text?
Sun, 06/03/2007 - 10:37What happened with my text? it puts all sentences together....
I forgot to say that I think (it´s my opinion) that the BGS slip itself up with that value of 8 EMS.Â
Anyway...I came across this paper about the seismicity at the British Islands and elsewhere around it.
 http://www.hse.gov.uk/research/otopdf/2002/oto02005.pdf
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Javi, I think the point is
Sun, 06/03/2007 - 13:06Javi,
I think the point is that 8 EMS is an observation of the maximum damage i.e. Furniture may be overturned. Many ordinary buildings suffer damage: chimneys fall; large cracks appear in walls and a few buildings may partially collapse.
... and is more than EMS 7 - small cracks and partial collapse of chimneys.
So if you get large cracks and lose whole chimneys, it is, by definition EMS 8.
Calculating Imax from magnitude is not, IMHO, accurate. Things to consider that will influence the result ... magnitude, distance to source, source depth, source mechanism orientation, source directivity, attenuation of transmission path, thickness of near surface sediments, rheological properties of near surface materials (has been shown can amplify acceleration by a factor of at least 2) and condition and nature of buildings involved and their foundations.
Calculation of Imax from magnitude can only give a ball-park estimate of the true Imax as there are too many parameters not accounted for - it is the equation that it is too simple - not the BGS' observations being inaccurate.
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Hi Hypo,Well, the BGS has
Fri, 06/22/2007 - 11:22Hi Hypo,
Well, the BGS has fitted the DC of the focal mechanism during these days. It´s therefore a bit different than the former one.
 The point is that they put an isosismal map with EMS =6 and reduce the maximum intensity as well. More information:
 http://www.earthquakes.bgs.ac.uk/earthquakes/reports/folkestone/folkestone_28_april_2007.htm
 I have been studying the focal mechanism to achive the strie´s plane of the fault using some geologic software related to Slide Model. And the result seems to be the plane with strike 327, oriented with a general compresion or horizontal maximum shortening of NW-SE in the area of Dover Strait.
So finally, owing to the orientation of the stress tensor and the type of focal mechanism I would say that it ´s a normal oblique-slip fault and obiously left-handed.
I´m going to try to show you some of the plots I have done with some geologic software.
Cheers,
Javi
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Hi Javi, Thanks for the
Sun, 06/24/2007 - 11:53Hi Javi,
Thanks for the update. Reading between the lines it looks like they downgraded from EMS 8 to EMS 7 because some of the damage was due to the falling masonry rather than the earthquake itself.
From the isoseismal map skew to the NW it looks like source directivity effects may contribute to the high Imax
I'm unsurprised by the focal mechanism. Due to the NW-SE compressional stress field over most of the UK most British earthquakes tend to be predominantly sinistral strike-slip on approximately N-S faults.
On the subject of British earthquakes I've just read an article about a possible event in SE England at the time of Boudica/Romans - might blog about it latter if I get time
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Nice
Thu, 07/12/2007 - 18:01Nice paper:
http://www.ingentaconnect.com/content/els/00401951/2002/00000357/00000001/art00365
If anyone wants to read it, you don´t hesitate e-mail me to ask for the paper, and I´ll send you.
cheers,
Javi
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Earthquake!
Submitted by Jon on Sat, 04/28/2007 - 09:12.http://news.bbc.co.uk/1/hi/uk/6602677.stm
Did anyone feel this earthquake in Kent?
Geologists are gneiss!!