Dear, oh dear. In the week of 9-15 June 2016, my downloading of the United States Geological Survey’s real time earthquake map came with a health alert. ‘This contains more data than your browser can display’ it warned me. But I pressed on.
In the event the map, which (broadly) includes tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, did show me all my data and the total was, indeed, unusual — over 2,800 tremors, well over 100 more than we’d normally expect.
The most likely explanation seems to be that, for some reason, more of the smaller US earthquakes were listed than is normally the case — but at the top end of the list, where it matters, the numbers of large or intermediate earthquakes was pretty much as expected.
There was nothing on the map approaching M7.0, although there were four tremors of ≥M6.0; 26 of ≥M5.0; and 101 of ≥M4.0. So, nothing unusual. And once again the larger tremors were all associated with the planet’s main tectonic margins, especially in the western Pacific.
The Week’s Biggest Earthquake: M6.2, Solomon Islands
And so, off to the western Pacific we go. This week there were actually three earthquakes of M6.2 (the largest recorded in these seven days) and two of them were in the tortured boundary between the Pacific and Australian plates. But as I haven’t mentioned the Solomon Islands in a while, this is our featured leading seismic event.
The plate boundary which separates the two plates twists and turns. In some places the Pacific descends beneath the Australian; in others the direction of subduction is reversed. Sometimes they slide past one another and sometimes they are buffered by slivers of crust trapped between them.
In this M6.2, the dominant tectonic motion is subduction of the Australian plate beneath the Pacific along the San Cristobal Trench, to the west of the Solomon Islands chain. This is the main tectonic feature shown on the map. A more detailed look, however, shows a second trench, parallel to the first, on the other side of the island chain, in which the dip is in the opposite direction.
Caught between the two, the Solomon Islands are inevitably subject to significant compressional forces. The shallow depth of the earthquake, along with the location of its epicentre (the point of the surface immediately above where it occurred) between the two trenches, indicates that it was probably caused by shallow deformation rather than by movement at or near a subducting plate surface.
M6.1 Tremor, Nicaragua
At first glance, this earthquake looks like a straightforward subduction zone earthquake. The subduction of the Cocos plate beneath Central America along the Middle America Trench is marked on the map and the earthquake epicentre lies some way into the over-riding plate.
Most subduction zones are more complicated than the inevitable simplification required by a map such as that produced by the USGS. In this case the complication arises from the relationship between the Cocos and Caribbean plates. The part of Central America, where the earthquake occurred, is the north-east corner of the Caribbean plate, squeezed by lateral movement along its northern edge and the subduction to the west.
A closer look at more detailed fault maps shows all manner of different faults at work; but the most likely appears to be movement along a long narrow depression (graben) bounded by normal faults. This is consistent both with the location of the earthquake and its depth (just 10km).
US Earthquakes: California
The largest earthquake in the US this week struck in southern California, along the San Andreas Fault Zone, though not on the San Andreas fault itself. Although the San Andreas marks the boundary between the Pacific and North American plates as they slide past one another, the zone of deformation associated with this lateral movement is broad and extends on both side of the margin.
This week the stresses and strains generated movement on the San Jacinto Fault, which runs parallel to the San Andreas, at a point some way south of Los Angeles. At M5.2 it was significant — and would have been felt (weakly) by the citizens of San Diego and even the southern suburbs of LA.
Last Words: Maps Don’t Tell Us Everything
Plate tectonics are complex and maps such as the USGS real time earthquake map — an invaluable tool — have to simplify. Tectonic settings can be complicated by different types of movement (as in the Solomon Islands, where the second subduction zone doesn’t appear); by interaction between the two (as in the Nicaraguan tremor) and by the association of complex fault zones and broader tectonic boundaries (the California tremor).
There’s often a lot more going on with an earthquake than meets the eye.