There have been some large and interesting earthquakes recently, but in the past week, 12-18 January 2017, Nature seems to have decided to redress the balance. (It happens.) There wasn’t a lot in the way of significant seismic activity on the United States Geological Survey’s real time earthquake map this week.
The map, which broadly speaking includes tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, showed just one tremor in excess of M6.0.
The total of 1525 included 31 of ≥M5.0, 96 of ≥M4.0 and 188 of ≥M2.5 — all very much at the lower end of the range of numbers we might expect to see in a typical week.
As usual, most of the activity was in the western Pacific, with a scattering of earthquakes along the eastern edge of that ocean, in Central and South America. And, again as usual, most of that activity was associated with the margins of the Earth’s tectonic plates — with one exception, which we’ll come to below.
The Week’s Biggest Earthquake: M6.1, Fiji
This week, in our investigation of the largest earthquake to appear on the USGS map, we’re back in that mysterious part of the western Pacific, Fiji. Here there’s a boundary between the Pacific and Australian plates, but it isn’t marked with clarity on the USGS map. It’s diffuse, probably complicated and, I venture to suggest, poorly understood.
There are one or two clues to the earthquake’s origin in the data summary from the USGS, and from (slightly) more detailed tectonic maps. Robert Yeats’s book on the world’s major faults includes, in this area, a tectonic feature where the USGS map has nothing — the Hunter Fracture Zone, which runs from south of Fiji to connect with the New Hebrides Trench (which is shown on the USGS map). It seems most likely that the earthquake is associated with some kind of movement on or near this zone.
This week’s earthquake, by the way, is one of the latest in a series of significant aftershocks following the M6.9 earthquake which occurred in the same area on the 3 January. In total there have been 16 in excess of M5.0, and there may well be more to come.
M5.5 Tremor, Madagascar
There’s one obvious outlier among the larger earthquakes this week. As I’ve said above, we expect most significant tremors to occur at or near plate margins, but this week saw an earthquake of M5.5 a very long way away from any marked plate boundary, in Madagascar.
Of course, the fact that a plate boundary isn’t developed enough to merit marking on a simplified tectonic map of the world doesn’t mean there’s no tectonic activity. Far from it — eastern Africa is subject to major rifting which may eventually split the continent. Madagascar isn’t part of that rift zone, but it may well be affected by it.
Madagascar doesn’t get a mention in my tectonics bible (Yeats’s work on active faults) so he clearly doesn’t consider it a major tectonic zone. What we know about it is very limited — the movement on the fault was lateral, and the actual earthquake shallow. That’s about it.
Earthquakes away from tectonic boundaries are often associated with very old faults. Work by A. S. Collins shows that there are many faults in the ancient rocks which make up the island — and it seems that movement on one of these is what caused this earthquake.
US Earthquakes: Way Down in Southern California
As we say in Scotland, many a mickle maks a muckle (or, a lot of little things make a big one). Not just this week, but over the past few weeks, there’s been some rumbling at the very southern edge of the San Andreas Fault Zone.
Yeats does have something to say about this. This swarm of minor earthquakes occurs between two sections of lateral movement (San Andreas and Imperial faults) which mark the boundary between the Pacific and North American plates. The two don’t connect, but are slightly offset, and the recent earthquakes occurred in the Brawley seismic zone which lies between the two.
The chances are that these small earthquakes are accommodating differential movement on the two larger faults.
Last Thoughts: No Boundaries
All three of the earthquakes featured this week occurred where there’s no definite boundary marked on tectonic maps. Madagascar is explained by reactivation of ancient faults, but the Fiji and the Brawley earthquakes pose the question of what happens when we know there must be a boundary but we can’t identify its exact route.
Okay, they’re on different scales and the distances in question are hundreds of km in one case and only tens in the other, but the question is the same. So is the answer. Not all plate boundaries are visible on the surface, certainly not as clearly as a fault line or a trench — but rocks can accommodate quite a lot of strain without breaking, or certainly without breaking in a major way.