It seems Þorvaldur may have got a bit overexcited last night. Here's Benedikt from the Met Office disagreeing with his assessment:
https://www.mbl.is/frettir/innlent/2023/11/13/hvad_er_sigdalur/
"A graben was discovered in Grindavík yesterday, running through the town. Þorvaldur Þórðarson, professor of volcanology at the University of Iceland, claimed that the graben was a clear sign of magma at a very shallow depth beneath the town. Benedikt Gunnar Ófeigsson, tectonics expert at the Met Office, says that this is not necessarily the case.
“This means that magma flowed down from the Sundhnúkar craters, under Grindavík and out under the sea. This causes tectonic divergence, in this case of about 1-3 metres. When something like this happens, a rift valley or graben is formed. What we are seeing is the rock breaking apart inside the rift valley and then the land sinks,” explains Benedikt.
“The best example of this is at Þingvellir, except there it is not a sign of magma movements, just of tectonic divergence,” says Benedikt.
Benedikt certainly isn’t ruling out the possibility that the magma is very close to the surface in the graben, but it doesn’t necessarily mean that.
“It doesn’t mean that the magma is very close to the surface, just that there has been significant tectonic divergence. That’s our interpretation and we have seen this many times in comparable events. We can’t rule out the possibility of magma reaching the surface, but the graben in itself doesn’t tell us anything about the magma depth. That would depend on other factors,” explains Benedikt.
Benedikt says that nevertheless, grabens like this can be very dangerous, particularly in this case since it was formed so quickly.
“When you have a graben that forms so incredibly quickly, large cracks can open up on the surface very fast. That can create a very particular kind of danger that has nothing to do with magma,” says Benedikt.
Emergency response workers are aware of this risk and take it into account as they allow Grindavík residents to return to their homes to retrieve valuables.
“Of course the other risk is there as well, the possibility of an eruption. We are monitoring the area and are in direct contact with emergency response workers,” says Benedikt.
Please if anyone sees any coverage of this sigdalur in English or is an earth scientist and knows what the proper English word is, post it here. I have never heard of a graben before and I don't know if I'm using it right :/
The word sigdalur literally means sink valley, but I'm sure that isn't the proper term.