There have been a few ups and downs (quite literally) over the past few weeks, sometimes a significant number of large earthquakes in a week, sometimes none.
Such variation is natural, especially where a very large shock is followed by significant aftershocks; but the week of 15-21 May 2015 comes much closer to what we might regard as normal in terms of numbers.
In summary, the United States Geological Survey’s real time earthquake map (which includes all tremors in the US and its territories and those of at least magnitude 4.0 elsewhere) showed:
- There were 4 earthquakes of at least M6.0.
- There were 34 earthquakes of at least M5.0.
- There were 279 earthquakes of at least M2.5.
In fact these latter numbers are rather higher than might be expected in a typical week but they are probably explained by the incidences of aftershocks from some of the larger earthquakes which the planet has recently experienced.
The Week’s Biggest Earthquake: M6.8 Tremor, Solomon Islands
Here at Decoded Science we’ve been talking a lot in recent weeks about the western margin of the Pacific and, in particular, the complex and currently pretty active stretch between Papua New Guinea and the Solomon Islands.
In the past month this stretch of the Pacific-Australian plate margin has seen five earthquakes of at least M6.5, and this week’s M6.8 is just the most recent of them.
There’s a case to made for linking some of the earlier tremors but, in fact, this week’s largest earthquake, which occurred around 184km east of the Solomon Islands, is probably too far away from the earlier ones to be anything more than just expected activity on what is, under normal circumstances, a significantly active margin.
Although the data suggest that the earthquake, close to the point at which the Australian plate subducts beneath the Pacific plate and at a depth of 12km, was probably caused by movement at or near the plate interface, there was no tsunami.
If this was indeed a subduction earthquake, then its magnitude was not quite sufficient to generate any damaging waves. Nothing abnormal, then, in a place where you might almost be more surprised the Earth was still than to feel it shaking.
M6.7 Earthquake, South Pacific — and Friends
One slightly unusual feature of this week’s earthquakes is that the second largest, at a magnitude of M6.7, is on a mid-ocean ridge, this time in the southern Pacific.
Such tremors are in fact by no means uncommon, and two or three will appear on the USGS map most weeks. They aren’t usually as large as this, but are more likely to be up to around M5.5 — an order of magnitude smaller.
For a scientist, bent on discovering the source, these tremors can be a bit of a nightmare, as they tend to occur in extremely remote places about which we know very little. It’s therefore really only possible to generalise and say that tremors like this are the result (usually) of the divergence of two plates. The plates move apart as magma upwells along a mid-ocean ridge, and earthquakes result.
The M6.7 on the Pacific-Antarctic ridge wasn’t the only such earthquake this week. There were tremors in the south Indian Ocean, the Atlantic and between Africa and Antarctica, though these were smaller and of the size we might normally expect. But there’s not much to say that’s of interest about a tremor, even a large one, that’s over 2100 km from the nearest land…
US Earthquakes: California
Next week sees the release of yet another disaster movie about the San Andreas fault, along with a series of doom-laden blogs about how the planets are lining up to unzip California and send the world careering into chaos. We should look a little more circumspectly at what’s actually going on; and the answer is, not much.
This week there was a noteworthy earthquake on the very southern end of the San Andreas fault, at the point (broadly) where the boundary shifts from transform to divergent. At M4.1 it was hardly going to shake the Golden State.
For the record, the hypothetical M9, which (I believe) is the subject of the upcoming film, would be more ten times larger than the biggest on record for California in terms of its magnitude and rather more than 22 million times larger in terms of energy released than the largest of this week.
Enjoy the movie, folks…
Quakes This Week: Movement on All Margins
The largest earthquakes occur on subduction zones with strike slip margins, such as the San Andreas, that are also capable of producing significant shock (even if this week’s wasn’t among them). This week’s featured earthquakes show that the normally relatively quiet ocean ridges can produce significant shocks, too.