The Sassy Scientist – Climate Change Causality

Tom Doehne has been watching the news reports on the effects of climate change on human civilization. Dumbfounded that the main stream has not peered beyond the variable intensity of weather patterns and the mere notion of rising sea levels, he pondered:

Will the increase in ocean level trigger more slippage along subduction zones?

Dear Tom,

In terms of geophysical research angles this really is a doozy. Human-induced increase in seismicity (rates) have been documented proficiently all around the world (Foulger et al. 2018). The two foremost categories are: 1) underground material extraction or injection, and mining and 2) additional loading of the crust. This first point includes the exploration of petrochemical resources, water extraction, under- and aboveground mining for ore, coal and salt, and fluid injection into reservoir rock as documented clearly in Europe, the USA and Canada. Man is not just great at extracting or introducing substances to and from the Earth’s crust, thereby modifying the stress state and loading pre-stressed faults. We’re also great at loading the crust from above. Hydromechanical structures (some may call these dams) stop the downward flow of water into artificial reservoirs (some may call these lakes). Look and behold, seismicity increases as the water level rises, and as seasonal variations in water level take place (e.g., Simpson et al., 1988). A long story short: a direct link between human actions and seismicity is unambiguous. So, can we put another mark on the not-so-squeaky-clean-wall of human-induced (increase in) seismicity through human-induced (acceleration of) climate change?

To return to your question, the main notion is whether a couple of meters of (mean) ocean sea level rise will actually induce additional seismicity. What’s the common denominator in the aforementioned human-induced seismicity: a perturbation of the stress field in the vicinity of faults that apparently were critically loaded. Or unloaded; seismicity at the (“quiet intraplate”) New Madrid seismic zone has been related to the removal of crustal material (i.e. unloading) above buried fault segments (Calais et al. 2010). Whereas loading at dams results from the addition of several tens of meters of material over a relatively long period of time (e.g., Simpson et al., 1988), we can also link the triggering of subduction zone earthquakes to weather (some may call this a short time period). Typhoons to be exact (Lui et al. 2009). Although it has to be stated that these were slow earthquakes. Not the inherently destructive ‘fast’ ones. Nevertheless, a relatively small-magnitude pressure change was enough to nucleate seismic events, and thus trigger more slippage. So, with the projected sea level rise of less than half a meter over the next two decades, I would not necessarily be too concerned about a massive increase in seismicity. Maybe you’ll see some increase in slow slip (in the data, you’ll never feel this), but this may also be related to an increase in extreme weather conditions. In case you’re still worried about living in the ring of fire, and an increase in subduction zone slippage remains a major concern, I’ve got one last piece of advice for you. Move.

Yours truly,

The Sassy Scientist

PS: This post was written to explore the depths of human-induced seismicity. Earthquake nucleation (in subduction zones) remains a poorly understood phenomenon and stress perturbations due to sea level variations a fairly absent research angle, despite all of our hard work. Who knows, maybe an amoeba turd landing on the seabed is enough to set things off. Perhaps your turd has drifted down into the subduction interface and is blocking the next nucleation. And to be really out there; perchance there are simply underground gnomes bringing down their hatchets and pushing those crustal blocks asunder.

References:
Calais, E., Freed, A.M., Van Arsdale, R. and Stein, S. (2010), Triggering of New Madrid seismicity by late-Pleistocene erosion. Nature, 466, doi:10.1038/nature09258.
Foulger, G.R., Wilson, M., Gluyas, J., Julian, B.R. and Davies, R. (2018). Global review of human-induced earthquakes. Earth-Science Reviews, 178, 438-514.
Liu, C., Linde, A.T. and Selwyn Sacks, I. (2009), Slow earthquakes triggered by typhoons. Nature, 459, doi:10.1038/nature08042
Simpson, D.W., Leith, W.S. and Scholz, C.H. (1988), Two types of reservoir-induced seismicity. Bulletin of the Seismological Society of America, 78, 6, 2025-2046.