Busy day - Strait of Gibraltar and an Arctic Quake!

Large quake (M5.4) widely felt in Morocco, unusually strong but not too shallow fortunately? Interesting that there seems to have been multiple small foreshocks in a very small area - cluster only about 15km across according to the Interactive Map viewer? Trust everyone OK, no News reports to say otherwise.
It is in an area with many past quakes and will no doubt have another one at some point. In large contrast to the recent arctic quake (M5.9 / 2km depth), which sits in isolation as the only recorded quake in that area on the seismicity maps (it is also very close to the northern limit of the mapping I see). Clearly linked to the mid-Atlantic ridge system, but wonder what made it so big and so isolated - do we know much about the ridge movements that far north (as they constrained nearer the pole - compared to wider / faster spreading at lower latitudes)?
Interesting combination today and different enough to start a thread I think?!

IIRC, the spreading rates on the Gakkel ridge segment are quite low, in and of themselves. Whether that is due to them being so
far north … I’m not so sure. We’re pretty sure that in the past we had the Australian-South American ridge system spreading rapidly across the South Pole (at that time), so there’s no immediate reason to expect slow spreading in polar regions - or fast spreading in equatorial regions. It’s all about distance from the pole of rotation of the spreading.
It is hard to mathematically project the whole sphere surface onto a 2-d object - a lot of GIS-like systems I’ve met have different modes for Mercator, N-Pole or S-Pole centred. Working out the spherical trigonometry to manipulate EQ sources, paths, reflection points and surface readings is one thing, but trying to make spherical trig present comfortably, even on a Wulff Net or stereographic net, is less than intuitive. Only about 1/2 of my class ever managed it, even at a “monkey-see, monkey-do” level.

1 Like

Is this the arctic quake you were interested in? The EMSC doesn’t report much historical seismicity - which probably means I’ve forgotten how to display it. But the USGS’s data set is showing about 10 associated quakes, ridge-parallel as you’d expect from a dyke intrusion event.

1 Like

Thanks for enlightening me about spreading rates and poles of rotation, not something I was ever very familiar with tbh! Have we tried fixed reflectors and satellite monitoring to determine relative motions across plate boundaries on a global scale (I recall some local experiments on the San Andreas fault system?) It would be good to have current movement vectors displayed on the global plate boundary maps along with significant quakes as current?

There are increasing amounts of GPS (and other satellite positioning services) monitoring stations, and I certainly see plate motion (and plate motion change) discussed in geological papers on a regular basis. Just having a quick browse through EMSC and USGS earthquake websites, I don’t see specific links to those, so I suspect they’re not specifically for geological monitoring. It’s the sort of equipment that (for example) a university might maintain to provide differential GPS signals to improve monitoring of locations on building projects and the like. After an event, increasing use is being made of such information in analysis of the event.
There are also specific local networks set up around (for example) a volcano which is having a current earthquake swarm. The prime purpose of such measurements is monitoring of ground deformation (and inferred magma movement below ground), but after the event, it also provides plate motion information. But, at the time, the ground motion information is far more urgent. For example, en.vedur.is provides a lot of information about ongoing volcanic events in Iceland (eg, the current activity just outside Reykjavík, Fagradalsfjall eruption | Real time data | Icelandic Meteorological office and GPS tímaraðir ), but isn’t so hot on interpreting the data after the event because … well the general ground inflation (about 10mm over the last few days) is much more significant this week than it will be next year.