## Thursday, 21 February 2013

### Could we get more Earthquakes due to global warming and thermal expansion of the Earth's crust?

Thermal expansion can release quite some force as is seen in bridges and railroad tracks that are deformed due to hot weather. I am wondering whether thermal expansion of the Earth's crust due to global warming can similarly lead to seismic activity, especially at subduction zones where one crustal plate slides under another one.

I have searched the scientific literature, but was unable to find any articles on the topic, while it sounds quite straight forward to me. Did really no one have this idea before, or was it already debunked in 1960?

## Back-of-the-envelope computation

The length of the Equator is roughly 40x106 metres. And my high-school tables book gives thermal expansion coefficients for various stone types between 4 and 12x10-6 K-1. So for a one degree temperature change one would expect that the Earth's crust would expand by 160 to 480 metre. (More accurately, one would use the size of a plate not the circumference of the Earth. On the other hand, the projected temperature change is larger than one degree.) For the sake of the argument, let's say that it is about 100m. One thing I do not know is whether the crust is sufficiently brittle/solid (like a railway track or bridge that deforms by heat) or whether there are air and liquid pockets and cracks which could absorb the expansion. Furthermore, maybe the additional pressure would mainly increase the width of the crust not its length or the crust would just crumple a little more.

The next question would be how fast the Crust would expand. I would expect that whether you get additional earthquakes depends on how fast the thermal expansion is relative to tectonic plate movements already happening. Only then could the thermal expansion be a significant source of additional stress at the plate boundaries. This speed depends on how fast the heat is taken up by the surface and on how deep the brittle top layer is (does the full crust need to be warmed?). If the speed of heating of the "full" crust is in the order of 10,000 years, the contribution of expansion to tectonic plate movements would be in the order of 100m/10,000a = 0.01 m/a = 10 mm/a, which is 5 to 10% of current movements at faster plates.

The continental crust is about 10 km thick. Thus the temperature signal should penetrate the crust with a speed of around 1 meter per year. One meter is about the depth at which you no longer see an annual cycle in soil temperatures. Thus it seems to be possible, but at the margin of the possible.

An important limiting factor for the thermal conductivity is likely the fluffy humus layer, which isolates well as it contains many air pockets. The thermal conductivity of solid rock is much larger. At the end of an interglacial, just after the retreat of the ice sheets, the humus layer would still be thin, as it is just being build up. Such a situation would make the mechanism stronger. Thus, this could be an interesting period to study. Also nowadays the thickness of the humus layer is decreasing due to erosion. If the spatial variations in effective thermal conductivity are sufficiently large, it may be needed to also take lateral heat fluxes into account, for instance to model heat flows from mountains (with little or no isolating humus) to its surroundings.

## Scientific literature

I have a hard time believing no one thought of thermal expansion of the Earth's crust before. Do you know of any pointers to the literature? I did find some articles on the relationship between volcanoes and climate. It may be easier to study volcanoes as earthquakes as volcanoes leave an ash layers, which can be dated. Naturally, volcanic eruptions can cool the climate up to several years. In the other direction, there are publications that rapid warming can cause volcanic activity (Rampino et al, 1979. A recent interesting paper by Kutterolf et al. (2012) studied the influence of temperature fluctuation within a ice age on volcanoes in the Pacific “Ring of Fire” based on volcanic data from deep sea drilling cores. Their assumed mechanism is that fast melting glaciers and rising sea levels change the gravitational forces on the crust and produce regional stresses in it.

## Help wanted

Are able to compute how fast the crust would heat up or the reaction of the Earth's crust to warming? I would be very grateful for any help. Do you maybe know a colleague that may be interested in looking into this mechanism? Please forward this post.

## References

Kutterolf, S., M. Jegen, J. X. Mitrovica, T. Kwasnitschka, A. Freundt, P. J. Huybers. A detection of Milankovitch frequencies in global volcanic activity. Geology, DOI: 10.1130/G33419.1, 2012.

Rampino, M.R., S. Self, R.W. Fairbridge. Can rapid climatic change cause volcanic eruptions? Science, 206, pp. 826-829, 1979.

1. In case it helps ...

http://www.gfdl.noaa.gov/bibliography/related_files/ao8902.pdf

1. Maybe. Which aspect of that paper, Angular momentum cycle in the atmosphere-ocean-solid earth system, do you think is important for thermal expansion?

2. Sort of related, there have been a number of studies pointing to a correlation between solar and volcanic activity. Couple of papers from googling: http://onlinelibrary.wiley.com/doi/10.1029/JB094iB12p17371/abstract and http://adsabs.harvard.edu/full/2003ESASP.535..393S.

It's possible this correlation is due to variability in stratospheric aerosols affecting proxies for solar activity.

3. Many years ago as an undergrad I had a prof who specialized in this sort of calculation. He used borehole data, and some of what I recall is that there is a time lag between temperature at the surface and the change in temperature downcore; and that the amplitude of the temperature variation dropped off logarithmically with depth.

That being said, deeper cores preserved some information that could be used to calculate past temperatures.

However you don't have to go down very far before the effects of surface temperature variability are dwarfed by the heat from within the earth. So you can't look to global warming to influence the entire crust--it would be more like the top 30 m or so. Depending on your assumption of thermal coefficient of expansion, the resulting expansion would be quite small.

1. Thank you for your reply. I should really look into such bore hole measurements. I talked to someone analysing such measurements at EGU this spring. If I remember right, he saw a global warming signal down to about 100m due (and his retrieval algorithm needed data down to 600m).

The kind of calculations your professor talked about seem to be oscillating signals (the diurnal and annual cycles). In this case we are dealing with a trend.

IMHO, how far the heat penetrates into the crust is a matter of time. The penetration does not stop because the crust becomes warmer than the atmosphere above. The warmer atmosphere will make it harder for the Earth core to loose its heat and the entire depth profile will eventually (in equilibrium) heat up.

Also the colder atmosphere is able to increase the surface temperature if you add more greenhouse gasses.

4. I am not a expert on this matter, although I share your views. The moon also can cause earthquakes, this has been proven. I believe we will be seeing more mega quakes in the near future.

5. Thanks. It is just an idea. At the moment I would say an unlikely idea, but I would love to see an expert study it.

Interesting, the moon can actually trigger earthquakes via the tides. Did not know that.

Comments are welcome, but comments without arguments may be deleted. Please try to remain on topic. (See also moderation page.)

I read every comment before publishing it. Spam comments are useless.

This comment box can be stretched for more space.