Deep Quake

Kat -   Hi Peter, thanks for coming all the way over from the States.

Peter -   Oh yeah, just for this!

Kat -   So tell us a bit about earthquakes, what's happening when an earthquake's happening?

Peter - Well, in general what happens is that there's already a fault surface, and there's friction on the fault surface, and when that's overcome, it becomes much easier for the rocks to slide by one another.  That's really crucial, that it's hard to get it started and then once it gets started it runs away.

Kat -   Are all earthquakes the same, all we hear is 'an earthquake has happened', are they the same speed?  Where do they happen in the Earth?

Peter -   There's a huge variety of Earthquakes and maybe I'll just take the where question first, and that is; the big earthquakes are where the tectonic plates are sliding by one another, one way or another, and so the very biggest ones are in the subduction zones where oceanic plates are being slid beneath the continents.

Kat -   So where do we have these around the world?

Peter -   Well they're in the Ring of Fire, so called, or the seismic zone next to the Ring of Fire in the pacific and then in the lesser Antilles and elsewhere.

Kat -   So these are places like Japan where they are particularly susceptible...

Peter -   And then of course, that kind of plate boundary turns into what we call a straight slip fault, where plates are sliding by one another, and that's the big deal in San Francisco.

Kat -   So the sorts of earthquakes you're looking at are a particular type, called intermediate depth.  What does that mean?

Peter -   Well, we were interested in those... Well first of all I have to tell you, I'm not a seismologist, I'm a geologist, we just work on rocks, but we got interested in intermediate depth earthquakes because of things we saw in the rocks.  Intermediate depth earthquakes have been hard for seismologists to understand because they occur beneath a big pile of rock.  That could be 50 or 100 kilometres thick, and so the ordinary sliding on a fault surface is very difficult when you've got 100 km of rock sitting on top of you.  So we see the shear zones that formed at this sort of depth in the mantle of the Earth, that are now exposed by plate tectonics, and you can see that there is no fracture, but there is a zone of highly localised deformation.  Sometimes you can see that these rocks got so hot that they melted.

Kat -   Are these types of earthquakes particularly damaging?

Peter -   Yeah, they're just as bad as the shallower kind, there have been some big ones in Bolivia that were quite damaging to cities.

Kat -   We have Mark on the line now, who would like to talk about earthquakes...

Mark -   Hello there!

Kat -   What's your question?

Mark -   My question is, when geologists and seismologists talk about earthquakes, they very rarely mention the influence of tides.  Many earthquakes, including the Boxing Day earthquake which caused the tsunami in the Indian Ocean a couple of years back happen at a time of unusually high tides.  I just wandered what Peter Kelemen had to say about that.

Peter -   There is a well-known cyclicity with tides for micro-earthquakes and presumably that extends to macro-earthquakes.  There's the idea of self-localised criticality, that things get poised on the brink of failure, and then just some very very small phenomenon is all it takes to push them over the edge.  I wouldn't be at all surprised if there was a correlation in general between great earthquakes and tides.

Mark -   I think you'll find if you look at many large earthquakes and volcanic eruptions, if you check the tide patterns they often happen at periods of higher than average tides.

Peter -   Right, but that doesn't mean that if you live in San Francisco you should be worried every time the tide is higher than average.

Mark -   No, but you could be aware at that particular time rather than other times.

Peter -   Yeah, sure.

Chris -   I suppose that the point that Mark is making to us, Peter, is that the moon is exerting a gravitational effect on the Earth and if you've got big bodies of water, that's quite a big weight pushing down.  I suppose that could make a difference to the way that tectonic interaction zones do interact and so might suddenly release all the pent up energy.

Peter -   Without a doubt, there is a correlation and you can watch, we have instruments on the sea floor regularly, you can watch the mid ocean ridges kind of breathe with the tide.  The earthquakes and the hydrothermal vents, the hot water coming out, goes with the tidal cycles.

Chris -   Thanks for that Mark, that was a really good question.  Thanks for coming on the programme.

Mark -   Thanks, bye!

Chris -   It's interesting because I for one was quite shocked; I looked at the US geological survey's website today because they monitor earthquakes that are going on around the world all the time. I didn't realise that yesterday there were 22 earthquakes, today at lunchtime there had already been 12 earthquakes.  One of them was in the middle of the pacific measuring almost 6 on the Richter scale.  I was surprised at how many there were and how strong they were.

Peter -   Well actually it's kind of a matter of definition, and as our instruments get better there's going to be more and more earthquakes that people are reporting.  So the magnitude 1 and less than 1 earthquakes are extremely frequent and happen for all sorts of reasons, for example people pumping water out of the ground.

Kat -   And how is our knowledge about the causes of earthquakes, some of the work you've been doing, is it helping to predict when and where they're going to happen any better?

Peter -   One of the most important outcomes of our research is the idea that even below the so called brittle ductile transition, where it's very difficult for rocks to fracture, there can be non-steady deformation in the Earth.  Because the mantle is much, much stronger than the crust, because it's moving in a non-steady way, jerking forward and then standing still for centuries, that's going to be extremely influential in terms of causing shallower crustal earthquakes.  So if it is a factor, and we're not sure yet.  It's something you would have to know in order to predict earthquakes.