This week’s geoscience roundup is more than a little eclectic. We’re on the Earth and deep within it; we’re talking about its building blocks and some of its destructive activities. Some weeks the things that catch my eye have a theme and this week there were plenty of snippets arpound, so the opportunities were significant. We very nearly ended up with an Antarctic-themed piece, but something tells me that opportunity will arise again, and because I’m in a capricious mood, we’ll go where my fancy takes me.
I’m going to start with an earthquake that struck on the island of Hokkaido, in northern Japan. At M6.6, this wasn’t the biggest of the week, though I will be talking about that one later, but it was the most damaging — at the time of writing reports indicate several dead, with the number likely to rise.
Japan lies at the junction of four tectonic plates (Pacific, North American, Eurasian and Philippine Sea) and the relative movements of these slabs of the crust create many earthquakes, some of them of very high magnitudes and with very high death tolls.
What is so striking about this particular event, however, is the intense shaking and the dramatic landslides which it generated. You may have seen some of the many images of the landslides which followed the earthquake (I can’t reproduce them here for copyright reasons, but it really is worth a search).
The USGS summary notes that: “News reports indicate that significant landsliding was triggered by the M 6.6 mainshock, burying houses in the epicentral region”. This, however, is only half of the story.
Writing on the American Geosciences Union’s landslide blog, Dave Petley gives an outline of some possible causes for this unusual — and devastating — level of instability. They include the surface geology — deep soil on mountain tops — possible liquefaction as result of the earthquake and the after-effects of heavy rain following Typhoon Jebi (saturation is a key factor in slope failure).
Whatever the reason, the disaster is a clear illustration, once again, of the fact that it isn’t only the magnitude of an earthquake that dictates the damage it causes.
Meanwhile, in the western Pacific, a very large earthquake passed almost unnoticed.
By very large, I mean almost twenty time larger that the M6.6 above (the earthquake magnitude scale is logarithmic). At M7.9, the earthquake which struck south east of Fiji is the second largest of the year, following an M8.2 which struck in the broadly same area on 19 August. Both of these are intriguing because they occurred at unusual — though not exceptional — depths, 670km and 600km respectively.
Deep earthquakes appear to have occurred at a significant distance from a plate boundary but a subducting slab of crust descends at an angle, so the deeper the ‘quake, the further away it appears from the trench and the plate margin, as the graphic shows. Both of these earthquakes, therefore, are associated with movement at the lower end of the subduction zone.
At twenty times larger, why so little damage? It’s because the energy from the earthquake must move through a significant amount of crust before it reaches the surface and, as it does so, a lot of that potentially destructive energy is lost. For this reason, among others, shaking at the surface was reported at the surface as light (USGS summary) whereas the shaking from the Hokkaido earthquake was, in some area, very strong. It’s also the reason why no tsunami was generated.
Mineral Cup — Some Losers
Moving on from the surface of the Earth to the minerals than build it, we come to the Mineral Cup on Twitter, As geologists and geophysicists engage in armchair jousts to decide their favourite, I’m learning a lot about some minerals I thought I knew well.
Leaving the winners aside, because we’ll be encountering them in future weeks, I thought I’d take a look at two of the losers — most notably the ones I voted for and why I voted for them. The tightest fight, with just two votes in it, was between quartz, which won, and muscovite.
I voted for muscovite, also known as white mica, because it’s a very beautiful mineral and one which, if you find it, you can play with. It’s a platy mineral — if you have a piece, you can slide it part with a penknife blade or even your fingernails. Unlike quartz, which is very simple in structure, it has a range of compositions. You’ll find it in granites and in minerals derived from granite.
Another loser was plagioclase, which (shockingly) went out to the Johnny-come-lately, ice. Plagioclase is, as it happens, a feldspar mineral and one which decomposes to form minerals including muscovite. It’s not immediately identifiable to the layperson in the way that (say) diamond or quartz might be, but like muscovite it’s also a key constituent of granite, and its many forms include some gorgeous variants, such as the iridescent Labradorite.
Farewell Kilauea — For Now
The eruption in the Lower East Rift Zone of Kilauea hasn’t officially been declared at an end, but there’s very little going on. Yes, there’s some activity in terms of seismicity and gas emissions but at very low levels — the latter described as “lower than at any time since late 2007”.
This being the case, it seems an appropriate time to sign off, with the caveat that, should the eruption resume, I shall be back with regular updates.