The earthquake graph has its ups and downs, the results of statistical variation; but the week of 7-13 August, 2015 included a reasonably typical pattern in terms of earthquakes recorded on the United States Geological Survey’s real time earthquake map.
The USGS map, which shows all tremors in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, included a total of over 1,600 worldwide during this week.
A breakdown of the tremors shows two of ≥M6.0 (an earthquake and aftershock in the western Pacific) and 38 of at least M5.0 — the one part of the breakdown which showed a relatively high figure.
These tremors occurred largely around the margins of the planet’s tectonic plates, as is normal, but also included a pair at a great distance away from any active plate margin, in Africa — though, as we shall see below, in an area by no means seismically inactive.
The Week’s Biggest Earthquake: M6.6, Solomon Islands
Yet again, Decoded Science’s earthquake digest is back in the western Pacific for the largest earthquake — in fact, the two largest earthquakes — of the week.
World tectonic maps often simplify this margin, showing it as a straightforward boundary between the Pacific and Australian plates, but in reality, the complex nature of the margin, with its variation in nature and changes in speed and direction of movement, has created a buffer zone with several smaller plates trapped between the two larger ones.
The result of such opposing forces is a high level of earthquake activity and this week’s tectonic events occurred to the south of the zone. Scientists have produced thousands of words on the tectonics of this region (the abstract alone of a paper on the subject illustrates how many forces are at play) so there’s little option but to simplify.
The earthquakes (M6.6 and M6.4) occurred to the west of a subduction zone between the Pacific and Australian plates, but their proximity to microplates to the north means that other external stresses are at play. The earthquakes occurred very close to a subduction margin, but their location — at shallow depths in the subducting plate — suggests that deformation is the origin.
Africa Breaking Apart? M5.8, Democratic Republic of Congo
Geologists agree that Africa is splitting, along an axis from the Afar depression in the north to Malawi in the south. You can see this zone on the map — we call it the East African Rift. Here, the rising continent is breaking apart and the centre of the valley is subsiding — a process which involves the downward movement of one block of rocks relative to one another along so-called ‘normal’ faults.
There’s no detailed information on this particular tremor but a look at fault maps of the area, in scientific papers, supports the idea that both tremors occurred as the result of normal faulting along the major faults which form the western boundary of the rift.
In this area, the rate of extension is only around 0.5mm per year — which partly explains why large earthquakes are infrequent.
The western part of the rift, where this week’s two earthquakes occurred, is, however capable of generating earthquakes significantly larger than this week’s. The USGS review of seismicity on the Rift Valley identifies three tremors of ≥M7.0 this region since the beginning of the present century.
US Earthquakes: Nevada
The largest earthquake in the lower 48 this week (including the rollercoaster ride that is Oklahoma) was in Nevada, part of an ongoing swarm which has seen the immediate area experience over 1100 earthquakes of ≥M2.5 in the past year.
Such earthquake swarms are not uncommon in the western US, where the creation of the basin and range provinces (through a process of extensional tectonics) gives rise to regular minor, and occasional major, earthquakes.
This week the focus is on extensional tectonics. Africa is splitting as the continent begins to break up and the floor of the deep, narrow (in continental terms) rift valley is dropping. Subsidence in the high western US produces basin and range scenery, with alternating valleys and ranges, and again these support earth movement along narrow faults. Not all major earthquakes are at subduction zones!