After a quiet period seismically, normal service (more or less) resumed in the week of 22-28July 2016, with the United States Geological Survey’s real time earthquake map showing (more or less) the expected number and distribution of earthquakes.
The map, which includes earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, included a total of just under 1500 tremors, of which three were at least M6.0, 31 at least M5.0, and 95 at least M4.0.
The Week’s Biggest Earthquake: M6.4, Papua New Guinea
This week, as in so many other weeks, the largest earthquake was to be found among the jumbled traffic jam of crust in the Western Pacific. Here, the coming together of the Pacific and Australian plates creates a tectonic mishmash, with reversal of direction and nature of motion and the consequent creation of microplates trapped between the two major plates. It’s unsurprising that this digest is a regular visitor here.
The earthquake, an M6.4, occurred along the northern margin of the South Bismarck microplate, which is bounded on the south by the Australian plate and to the north by the (poorly-defined) North Bismarck microplate and the Pacific plate. The southern margin is defined by a subduction zone; the northern margin is a conservative boundary, where the plate slide past one another.
If there’s anything slightly unusual about this week’s tremor, it’s that it was in the north rather than the south, since the subduction margin is the location for most of the plate’s earthquakes. But, as this week’s tremor shows, subduction zones don’t have a monopoly on significant earthquakes.
Iceland, Earthquakes and Volcanoes
They don’t show up on the map, but a series of small earthquakes certainly showed up in the media this week. “When will she blow?” wondered the Iceland Volcano Monitor after two (very minor) earthquakes in what the website describes as “Iceland’s vicious Katla volcano.”
This kind of speculation happens pretty much every time there’s minor activity in Iceland (and that’s often) — back in June everyone was getting het up over Katla’s neighbour, Hekla. Hekla is also a bit on the vicious side when disturbed, and has also been rumbling this week .
Small earthquakes are characteristic of volcanic regions and while earthquake activity may be a sign of magma rising it can also indicate the opposite. Earthquakes are just one of the signs which the authorities monitor in predicting volcanic eruptions
US Earthquakes: California
Far larger than anything in Iceland this week was the M4.7 in northern California. In itself it wasn’t especially newsworthy, with the USGS suggesting that only around 150,000 people may have felt it. The tremor occurred at the northern end of the San Andreas Fault Zone, just before the point where it becomes the major Cascadia subduction zone and is joined by the Mendocino Fracture zone.
This junction between three plates (the Pacific, North American and Juan de Fuca) creates complex stresses within the crust so it’s hardly surprising that earthquakes here are regular and unremarkable.
Last Words: Hold the Front Page
Iceland wasn’t the only place to generate a few shock headlines this week. “Tsunami fears for Australia as 6.1 magnitude earthquake strikes …” warned the Daily Express, while, not to be outdone, the Sun reported that “Massive 6.1 magnitude earthquake rocks southern Australia as experts issue tsunami warning”.
In fairness, neither of these newspapers is known for either their restraint or their scientific reporting, but this one did make me smile. It takes a few seconds to fact check. The earthquake, on an ocean ridge, wasn’t the type to generate tsunamis. It was thousands of km from Australia. And it was way below the magnitude at which a tsunami is expected.
It must have been a quiet news week.
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