In the week of 4-10 September, the world continued to rumble, albeit rather quietly.
The United States Geological Survey’s real time earthquake map, which records all tremors in the United States and its territories, and those of at least magnitude 4 (≥M4.0) elsewhere, showed just three tremors of M6.0 or more, in Indonesia, Easter Island and Tonga.
The map does show what appears to be an unusually high number of slightly smaller tremors, with 39 of ≥M5.0 compared to the previous week’s 29 and 121 of ≥M4.0 compared to just 92.
But it’s impossible to conclude that this is anything more than natural variation, compounded by the continuing volcanic tremors in Iceland and the aftershock sequence for the Easter Island tremor.
Between them these accounted for 14 of the recorded events larger than M5.0.
The Week’s Largest Earthquake: M6.2, Indonesia
Trapped in a crowded corner of the western Pacific amid a jostling crowd of tectonic plates and microplates, Indonesia is a country vulnerable to earthquakes.
This week’s largest earth tremor was an M6.2 in the Molucca Sea.
The USGS map shows a simplified picture of the tectonic setting, with the subduction zones of the Timor Trough to the south and the Seram and Philippine troughs to the east, representing compressional motion between the Australian, Pacific and Philippine Sea plates.
Complicated enough, you might think – but a look at a more detailed fault map shows numerous other fault and thrust zones.
The epicentre of the earthquake was in the central Molucca Sea, an area termed by some seismologists the Molucca Sea plate, between two thrust faults. Here the compressional movement is lifting slabs of crust and it’s likely that the earthquake and its aftershocks were the result of this upwards movement.
Earthquakes in the Eastern Mediterranean
The eyes of European seismologists may have been very much on Iceland in recent weeks, but we shouldn’t forget that the southern margins of the Eurasian plate are also tectonically active.
This week saw three earthquakes in the eastern Mediterranean and, although they were geographically distant from one another, all are associated with the same tectonic region.
The convergence of the African and Eurasian continents is diffuse and in the eastern Mediterranean it’s expressed in the shape of a subduction zone stretching from the Ionian Sea to the Gulf of Alexandria.
One earthquake, an M4.6, occurred at the eastern end and its depth (62km) and location suggest that it may have been at or near the plate interface.
The other two tremors, though larger at M5.0 and M5.2, were shallower and further from the boundary. It seems likely that they may have been related to crustal movements caused by the rotation of the Anatolian block as it is caught between the Eurasian and African continents.
US Earthquakes: San Andreas Fault
The Napa Valley earthquake of 24 August is a reminder than there is more to California than just the San Andreas Fault. But the fault itself is moving and regular small earthquakes occur along its length.
This week there were minor tremors on or close at Ladera, Ridgemark, Greenfield and Indio, at the southern end of the zone.
Yeats calls the San Andreas “the most intensely studied fault on Earth” and this constant recording indicates how movement along the fault occurs in small increments, such as those of this week, as well as in large jumps such as that generated on the fault in San Francisco in 1906.
Although it’s tempting to look at earthquakes in isolation, they generally occur along complicated fault zones.
Even the San Andreas, which is a relatively simple fault and one which is visible on the ground, is accompanied by broad areas of parallel deformation. And the tectonic settings of the other featured tremors this week, in Indonesia and the eastern Mediterranean, illustrate just how complicated some of the interacting forces can be.