The week of 14-20 February 2018 has much to teach us about earthquakes. One thing this week’s research shows us is that we have much still to learn about their sheer power. Another demonstrates a way in which the planet’s processes are interlinked and shows how earthquakes can play a part in other Earth cycles — specifically, the carbon cycle.
The third article this week is perhaps slightly indulgent but it’s one of the things that caught my attention as a child and made me think about the wonders of the Earth. It bears repeating in its own right as we look back at a rare thing — the birth , life and death of a volcano.
Earthquakes and Carbon Sequestration
At first sight it may seem that there’s little connection between the cycle of plate tectonics and the carbon cycle (the recycling of carbon through the Earth, ocean and atmosphere) but in fact the two are crucially linked as earthquakes can play a role in both the sequestration (burial) of carbon and in its release — something examined in a piece of research on the 2011 Tohoku-oki earthquake in Japan.
The main mechanism for carbon burial is sedimentation, a process during which organic material is buried and removed, albeit temporarily, from the active carbon cycle. (It’s the excavation and addition of these carbon reservoirs, in the form of fossil fuels, that is a major contributor to increased atmospheric carbon dioxide).
A vast quantity of this sedimentation takes place in the deep oceans — and the deepest parts of the oceans are the trenches along which the Earth’s tectonic plates subduct. This accumulation of organic material can be enhanced by the deposition of vast quantities of material following a major earthquake.
“On longer geological time-scales,” the report notes, “the deposition, burial, and eventual subduction of organic carbon (OC) – rich marine sediments in plate subduction zones can play a key role in the earth’s long-term carbon cycle, and influence atmospheric CO2 concentrations over millions of years”.
The difficulty in evaluating the scale of this carbon removal is increased by the inaccessibility of these trenches. Based upon remote data collection and mapping of the Japan Trench (along which the earthquake occurred) the research estimates that the 2011 earthquake deposited more than 1m Tg (1 Tg = a million, million grammes) of sediment in the trench.
The researchers concluded that: “This carbon supply is comparable to high carbon fluxes described for other Earth system processes, shedding new light on the impact of large earthquakes on long-term carbon cycling in the deep-sea”.
Revelations From the 2018 Palu Earthquake
The deadliest earthquake of 2018, the Palu earthquake of September 28, was unusual in many ways and research is ongoing into its causes and its impact. As a strike-slip earthquake it would not normally have been expected to generate a tsunami, and it was a landslide the triggered it that did that damage.
This week two new pieces of research into the earthquake have been published, and both shed further light on the process — and possibly also upon the resultant landslide. Both articles are behind a paywall rather than free access, so I can’t go into any detail that isn’t in the (freely available) abstracts.
The key finding of both articles is that the rupture along the fault was extremely rapid and the rock “unzipped” at what are described as superstar speeds: “The rupture propagated at a sustained velocity of 4.1 km s–1 from its initiation to its end, despite large fault bends”.
A commentary article in the New York Times, based on interviews with some of the researchers involved, notes that this is: “9,600 miles per hour, almost 25 percent faster than is typical, and among the fastest ever recorded for rocks at shallow depths”.
Neither paper, as far as I am aware, explicitly addresses whether the speed could have contributed to the landslide and thus the tsunami, though that would be an interesting question. They do, however, raise the issue of how local geological and topographical conditions can affect the mechanics of an earthquake, and its impacts on the local population.
Happy Birthday, Parícutin
Years ago I had a book on volcanoes which recounted the extraordinary story of a Mexican farmer, quietly minding his business in his field one day (20 February 1943 to be precise) when the Earth next to him opened up and steam and fumes started emerging from the ground beside him. This fantastic-sounding tale is in fact pretty near to the truth.
The eruption was rapid with the volcanic cone growing to a height of 50 metres within the space of 24 hours. In eighteen months, lava had covered two local villages. Parícutin, as the volcano was named, continued to erupt for the best part of ten years, ending in 1952 and leaving a volcano over 400m in height.
There is, of course, more to it. The emergence of Parícutin was hardly unexpected: it’s located in an area called the Michoacán-Guanajuato volcanic field which, according to the Global Volcanism Program website: “contains over 1400 vents, including the historically active cinder cones of Parícutin and Jorullo, covering a 200 x 250 km wide area of Michoacán and Guanajuato states in west-central México”.
It’s no wonder the example attracts the attention, given that it “marks the first time scientists were able to observe the complete life cycle of a volcano, from birth to extinction” and it’s an example of how Earth processes can operate on a small scale as well as a large one.