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 Chris Craggs Global Crag Moderator  UKC Supporter 11 Jan 2022

We had a 6.6 magnitude earthquake off the west coast of Cyprus last night. It vibrated the apartment and rattled wardrobe doors for about 30 secs, but thankfully was pretty mild considering the headline numbers.

I was a little intrigued by the USGS graphic that shows the intensity 'contours' and wondered if anyone one of a geological bent might know why they appear to travel faster/further through the island when compared to the surrounding seabed?


 wbo2 11 Jan 2022
In reply to Chris Craggs: This is the shakemap? Function of rock velocity and distance from the epicentre.  In terms of 'shake' in the water part I'm not sure you'd observe any shaking, but you'd see a big compressive wave on any seismographs

In reply to Chris Craggs:

Think wbo2 has pretty much got it. I'm not exactly sure what the contours represent but think they are the magnitude of the surface wave arrival and this will decay exponentially with travel time from the epicenter. The surface wave travels through water at a pretty constant velocity (1500 m/s for a  Pwave) but travels through rock faster (5500 m/s P wave in granite). The variation in the contours through Cyprus actually tell you something about the rock structure below the island and is the basis of earthquake tomography.

 dunnyg 12 Jan 2022
In reply to Andy DB:

What is mapped is usually peak ground acceleration, sort of the maximum shake, thoigh the length of time the shaking occurs for may also be considered. The uniform circles under the sea are likely because the geology is assumed to be uniform. On land it is known not to be creating more complex patterns. An interesting thing to know is solid rock generally has a lower PGA than a sedimentary basin (e.g. dried up lake).

On a more practical note, I'd recommend removing any pictures above your bed, and following the "drop cover hold" if/when you get shaken again.

Also watch out for rocks being dislodged from hillsides when you are at the crag if shaking kicks off.

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