Earth tremor

Chat and items of interest about Crete and Greece.
bobscott
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Joined: Wed Aug 05, 2009 5:36 pm
Location: Kokkino Horio

Earth tremor

Postby bobscott » Mon Sep 27, 2021 8:20 am

Anyone feel the earth tremor in the Apokoronas area just now? 0920 ish
Yesterday today was tomorrow. Don't dilly dally!

allan
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Joined: Mon Jul 02, 2007 9:56 am
Location: Chania

Re: Earth tremor

Postby allan » Mon Sep 27, 2021 8:22 am

Felt it in Chania.

margarita
Posts: 537
Joined: Sat Jan 20, 2007 9:36 pm
Location: Kalyves area

Re: Earth tremor

Postby margarita » Mon Sep 27, 2021 8:27 am

According to this website http://www.geophysics.geol.uoa.gr/stati ... ecent.html it was SSE of Iraklio.
It must have been pretty strong there if we felt it here.
The bridge in Kalyves is not the place to be walking across during an earth tremor!

Kookla
Posts: 217
Joined: Sun Mar 27, 2016 4:24 pm

Re: Earth tremor

Postby Kookla » Mon Sep 27, 2021 8:34 am

https://www.emsc-csem.org/Earthquake/Co ... hp?lang=en This site initially said 5.6, then swiftly changed it to 6.5! At 09.17 only 2km depth 20km SSE Heraklion.
It lasted a while too - shook fixtures in our house.
Edit. Changed again to 5.9, with a smaller one a wee while later.

TweetTweet
Posts: 466
Joined: Fri Jan 13, 2012 10:35 am

Re: Earth tremor

Postby TweetTweet » Mon Sep 27, 2021 9:18 am

I use esm too - they are now showing it as 6. Initially they showed 5km deep, but now it's saying 10.

I'm near Rethymno and it felt very strong - the cats and dogs who were inside at the time vanished outside in a puff of smoke - luckily there were a few open exit points so no one got stuck in the cat flap this time :)

What a way to start the week -

https://www.youtube.com/watch?v=23zLefwiii4

TweetTweet
Posts: 466
Joined: Fri Jan 13, 2012 10:35 am

Re: Earth tremor

Postby TweetTweet » Mon Sep 27, 2021 9:21 am

Meant to say it was on land - I suppose that's why it felt so strong - they usually seem to be offshore.

Tim
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Location: Near Sitia
Contact:

Re: Earth tremor

Postby Tim » Mon Sep 27, 2021 11:11 am

One person reported dead and buildings destroyed.

https://www.dailymail.co.uk/news/articl ... 0&ito=1490

Tim

Kilkis
Posts: 13083
Joined: Sat Apr 21, 2007 3:58 pm
Location: Near Chania

Re: Earth tremor

Postby Kilkis » Mon Sep 27, 2021 11:15 am

Not very strong 20 km to the SW of Chania. Very much side-to-side movement not up and down. It certainly lasted several seconds. My seismometer detected it, i.e. a three shade ceiling light fitting with the shades at slightly different heights so they are not touching but can touch if they swing.

Warwick

TweetTweet
Posts: 466
Joined: Fri Jan 13, 2012 10:35 am

Re: Earth tremor

Postby TweetTweet » Mon Sep 27, 2021 1:00 pm

Is "side to side" versus "up and down" related to depth of activity? Is "up and down" when the ground under you feels like it's gone liquid? The Kythira quake January 8 2006 felt like that.

Kilkis
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Location: Near Chania

Re: Earth tremor

Postby Kilkis » Mon Sep 27, 2021 2:37 pm

What you experience is a mixture of exactly how the plates have moved and the geometry of where the epicentre is relative to you. I've experienced up and down motion and side to side in Chania but always at a very low level.

For example the 2004 Indian Ocean Tsunami was so big because one section of plate suddenly displaced vertically relative to the adjacent one putting a vertical pulse into the water. Sometimes one plate is sliding under another at an angle, i.e. a subduction zone, and sometimes they are sliding horizontally parallel to each other.

For buildings built with reinforced concrete pillars to resist earthquakes vertical movement is generally reckoned to be worse than lateral movement. Concrete is excellent in compression but has little strength in extension. For vertical movement you are totally relying on the rebar when the movement tries to stretch the column. For horizontal movement the vertical columns are trying to bend. That puts one side of the column in extension but the other side in compression so the strength of the concrete in compression helps resist damage.

Warwick

Kookla
Posts: 217
Joined: Sun Mar 27, 2016 4:24 pm

Re: Earth tremor

Postby Kookla » Mon Sep 27, 2021 4:39 pm

I’ve since read TweetTweet, that National and International sites vary on their grading. A couple more this afternoon, where I briefly left the office as it is old and built well before any laws on earthquake requirements were made. I’ve just had another look at the same site and it lists about 47 today in the same-ish area.
Thanks for the info Warwick, I wish they had teachers like you when I was in school. Is it true that earthquakes are more usual after or during dry weather?

Kilkis
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Location: Near Chania

Re: Earth tremor

Postby Kilkis » Mon Sep 27, 2021 5:36 pm

I've only got a fairly superficial knowledge of earthquakes, Kookla, but I am not aware that dry weather would increase earthquake activity. The United States Geological Survey, USGS, says that there isn't and I am inclined to believe them. See this FAQ. The idea seems to be associated with Aristotle's views on what causes earthquakes so your Greek neighbours may have told you this. They may have been taught this in their Greek history classes and believe it is relevant today.

Warwick

Kookla
Posts: 217
Joined: Sun Mar 27, 2016 4:24 pm

Re: Earth tremor

Postby Kookla » Mon Sep 27, 2021 8:21 pm

Ah, thanks everso! Yes, I have heard it from Greek friends. Always struck me as a bit odd as it is always (well nearly!) hot here. I thought maybe I was missing something and the plates got hot, expanded and boom, another earthquake lol.

BST
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Joined: Wed Jul 10, 2013 3:29 pm

Re: Earth tremor

Postby BST » Mon Sep 27, 2021 9:05 pm

http://www.geophysics.geol.uoa.gr/stati ... ecent.html

If you zoom right into the map on this link where the major earthquake occurred today you will see an unusually straight scarp marking the edge of the mountain range. It is a fault line. This is likely to be the fault which caused the quake today as it will probably extend a long way beneath the plateau. The Kasteli fault cuts across this area and much of the island.

As an aside, you are far more likely to feel earthquakes if your house is built on solid rock as opposed to sediment/soil..... unless the epicentre is close to you!

Kilkis
Posts: 13083
Joined: Sat Apr 21, 2007 3:58 pm
Location: Near Chania

Re: Earth tremor

Postby Kilkis » Mon Sep 27, 2021 11:36 pm

I think that the movement of the plates is driven by large scale convection currents in the underlying molten magma, Kookla. It is a bulk movement not an expansion/contraction of the rock. There is considerable friction at the boundary between two plates so the convection is trying to move them while friction is stopping the movement. Stress builds up until it overcomes the static friction force and the plates suddenly slip. All the energy built up in the stress is released in a very short time as different kinds of waves. There is a fairly simple description of the characteristics of these different waves in this short Britannica article.

Warwick


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