I’m calling this the weekly digest, but as I wasn’t around last week I’m going to slip back and look at something I would have reported then. There can’t be many people who missed the news of the earthquake that struck in Indonesia, and although it’s slipped out of the headlines the destruction and loss it’s caused are still too recent to overlook.
There were a few other things that crept across my Twitter feed this week, too, although in all honesty it’s hard to look beyond events in Sulawesi. But I’ll try, because I believe that science should be fun as well as serious, so I’ll follow it up with the Mineral Cup roundup.
The Sulawesi Earthquake
A lot has been written about Indonesia’s M7.5 earthquake — and I mean a lot. Some of it conflicts. Some of it has been based on information which was subsequently updated, some on misinformation. With that in mind I’m going to stick to trusted sources and look at the earthquake itself, and what happened.
The death toll is still being updated and, given the nature of the damage probably will be for some time. It may never be finalised. The latest official figure I have (health warning: this comes come from Twitter and as I don’t speak Indonesian I can’t be sure of the source, though I believe it to be from Indonesia’s disaster relief agency) indicate 1,948 people confirmed dead and 10,679 injured. A further 835 people are listed as missing.
I’ve often talked about earthquake deaths and how few are the direct result of the event itself. In this case, much of the deaths came as a result of the ensuing tsunami and also from a series of devastating landslides.
Dave Petley, writing on the American Geoscience Union’s blog, notes that: “I suspect that this will be the highest loss in a landslide event since the 2013 Kedarnath debris flow. It will be the worst earthquake-induced landslide event since the 2008 Wenchuan earthquake”, though he doesn’t give numbers of deaths for either event.
Strike-Slip Earthquakes and the Palu Tsunami
Indonesia is accustomed to large earthquakes. As the Pacific plate closes in on the Philippine Sea plate and that, in turn, runs up against the Sunda plate with the Australian plate converging on them from the south, there’s a lot of strain to be accommodated. The USGS summary page notes that: “At the location of the September 28th earthquake, the Sunda plate moves south with respect to Molucca Sea plate at a velocity of about 30 mm/year”.
This implies convergent motion, but what’s perhaps most interesting about the earthquake is that the actual mechanism for faulting wasn’t convergent. It occurred on a strike-slip fault and the movement was lateral.
Strike slip earthquakes of this magnitude aren’t unusual, but what is less common is that they trigger tsunamis, which are more usually associated with submarine subduction zones. The situation is perhaps more unusual in that the faulting, which occurred over a long section of the fault, also occurred largely on land.
These two factors might initially suggest that, despite the size, an earthquake would not be expected but, as Jason Patton wrote on the Temblor blog in the immediate aftermath: “Strike-slip earthquakes can generate tsunami if there is sufficient submarine topography that gets offset during the earthquake. Also, if a strike-slip earthquake triggers a landslide, this could cause a tsunami.”
It may be, however, that this event is not unprecedented. Robert Yeats reports a series of tsunamigenic earthquakes over the last hundred years “off the west coast of Sulawesi”, many of them smaller than that of September 2018. He doesn’t give mechanisms or map the locations, but the tectonic setting implies (I put it no stronger) that they, too, may have been caused by strike-slip motion.
The debate about whether or not the warnings of the tsunami were adequate and whether, in consequence, lives could have been saved will no doubt rumble on. What the incident does show us is how little we know about earthquake behaviour — and that we should never be complacent.
Mineral Cup: The Results
The Twitter Mineral Cup came to its conclusion at the weekend with a fascinating matchup between ice (backed, as far as I could tell, by climate scientists) and garnet (cheered on by the solid Earth cohort).
It’s flippant thing, but I learned a lot during the process, even about minerals with which I thought I was familiar. To begin with, all the kerfuffle about whether ice and opal should even have been included taught me that I wasn’t, after all, 100% sure what a mineral actually was.
Garnet came out of it the winner, so here’s a bit about it. Chemically, it’s a bit of a dustbin: it can contain all sorts of things and have all sorts of forms and still be called garnet, but its principal constituent is silica. Some other forms might include iron and aluminium (in which case it’s pyrope garnet); iron and calcium (andradite garnet); or iron and magnesium (almandine garnet). It can occur (as pyrope) in some igneous rocks, but is better known for its association with metamorphism. Occasionally it will turn up in a sedimentary rock.
Most people will know the almandine version best, as a red, sometimes gem-quality, mineral, but its a mineral that comes in as many different colours as it has different chemical compositions, and it is widely used as an abrasive material in industry.
It’s common — I believe you can see it in some of the rock outcrops in Central Park, which are of metamorphic origin. And, for the record, I voted for it, purely because it’s the first metamorphic mineral I ever identified in the field.