After a somewhat random selection of articles on the digest last week, this week’s roundup (2-8 August, 2018) does have some sort of unifying theme — that of plate tectonics. Earthquakes and volcanoes are both linked to the movement of the planet’s tectonic plates — and the theory of plat tectonics itself has some interesting aspects.
And, of course, we have the weekly update on what’s happening at Kilauea.
The Lombok Earthquake: More Devastation
Last week I wrote about an M6.4 earthquake on the Indonesian island of Lombok, which killed (at the time of writing) 15 people and caused extensive damage. As is sometimes the way, we know know that that particular earthquake was not the mainshock in a sequence, but merely the largest foreshock.
The 28 July event was to be followed by a larger event, of M6.9, which killed (at the time of writing) close to a hundred people with the death toll likely to rise, and caused considerable damage and many injuries.
As I said last week, this area of the planet is complicated, with different directions and types of plate movement, but the overriding context is the collision of the Indo-Australian and Eurasian continents, with a range of smaller plates trapped between them. In this case, the earthquake was caused by collision between the Indo-Australian plate with the Sunda plate.
More specifically, the USGS earthquake summary attributes it to movement: “on or near The Flores Back Arc Thrust. Preliminary focal mechanisms for the earthquake indicates slip occurred on either a shallow, south-dipping thrust fault, or on a steep, north dipping reverse fault”.
One interesting aspect of this fatal sequence of earthquakes is a discussion of what we consider a mainshock. The M6.4 earthquake may well have been reported as such last week given that it was the first in the sequence and followed by smaller events — but the occurrence of the larger tremor also changed this.
The mainshock is, by definition, the largest earthquake in a sequence and we cannot know for certain what that is until the sequence is concluded. If a larger earthquake occurs in the same area within a short period of time, it will become the mainshock and all smaller earthquakes preceding it will be redefined as foreshocks.
Volcano Watch: A New Resource
I’ve long been a fan of the Smithsonian Institution/National Museum of Natural History’s Global Volcanism Program website. Although it isn’t the best place to keep up with breaking news, it’s an authoritative source of information on volcanoes around the world and throughout their known histories, full of weekly updates, links to useful documents and even downloadable data that allows you to manipulate spreadsheets to identify patterns of activity over time and by region.
This week I’ve noticed that a new feature has popped up. Whether or not it’s a deliberate response to the questions that have accompanied the eruptions of Kilauea, Fuego and others — often relating to whether we should be worried about a perceived increase of volcanic activity, or whether it’s been in the pipeline for a while, I don’t know. But I do know that it’s a useful, and welcome, addition to current resources.
So what is it? It’s a panel that appears on the home page and gives information on current eruptions. As well as flagging up a weekly report in volcanic activity, it gives an easy and at-a-glance indication of how many volcanoes are currently erupting and lists new activity for the week. And for those who want a comparison, it includes data for 2018 to date, 2017 and averages for 2008-2017.
Those who are concerned about a perceived upsurge in volcanic activity this year should be reassured. The average number of volcanoes erupting in a year over the 2008-17 period was 79, 36 of them new. At not quite two-thirds of the way through 2018 the figures are 19 and 58 — in other words, nothing particularly unusual.
Plate Tectonics and Life
The Earth is the only planet with confirmed life, and the only planet with confirmed plate tectonics. The connection between these two is not necessarily random: in brief (very brief), one of the processes of volcanism is degassing (allowing gases such as carbon dioxide to escape from within the planet) and this feeds into the recycling and sedimentation processes which sequester carbon dioxide and maintain the atmosphere at levels suitable for life.
The volcanoes, like earthquakes, are mainly found along the boundaries of tectonic plates, and this might seem to support the theory that, without plate tectonics, the balance of the atmosphere couldn’t be maintained. But new research from Penn State University questions this assumption.
We know from examples on Earth (Kilauea being just one) that volcanism exists without the need for plate tectonics. The researchers modelled the life cycle of a planet based on different sizes and chemical compositions and concluded that degassing can take place at a sufficient level to maintain habitable conditions.
Whisper it — the eruption at Kilauea is slowing down. According to the USGS: “Activity and lava output from fissure 8 remains low. The morning overflight crew observed a small active lava lake within the fissure 8 cone, a weak gas plume, and a drained upper lava channel. The surface of the lava lake was about 5-10 m below the spillway entrance. There were a diminishing number of small active ooze outs near the coast on the Kapoho Bay and Ahalanui lobes and the laze plume was greatly diminished. Active lava remains close to the Pohoiki boat ramp but has not advanced significantly toward it.”
Furthermore, gas measurements have shown lower levels than previously and deformation at the summit crater has also slowed.
What this means is by no means clear. Volcanic activity increases and decreases during the course of an effusive eruption such as this, and a pause in activity is not necessarily an indication that the eruption is coming to an end.