After two weeks with little significant earthquake activity, the Earth showed more signs of movement in the week of 4-10 July.
The USGS real time earthquake map shows a total of 199 tremors of at least magnitude 2.5 (≥M2.5), 27 of which were of ≥M5.0 (compared with 18, 18 and 19 in the previous three weeks).
As usual, the greatest focus of activity was in the western and south western Pacific, where the convergence of the Pacific plate with Australia and Eurasia creates frequent tremors.
The Week’s Largest Earthquake: M7.3, Papua New Guinea
Not all plate margins are well-defined and tectonically simple: the boundary between the Australian and Pacific plates is marked by complex changes in nature and type of plate motion and by the presence of numerous slivers of crust being created and consumed as the two large plates on either side of them converge. It’s unsurprising, therefore, that this region is regularly struck by significant tremors and this week was no exception, as it was the location of the three largest tremors.
These three tremors, of M6.0, M6.1 and M7.3, occurred in the area of the Solomon Islands/New Britain. Without detailed study it’s impossible to be clear on the exact faulting mechanism, especially given the complex faulting systems in the region, but it seems most likely that all three are associated with the northwards subduction of the Solomon microplate beneath the South Bismarck microplate to the north and the Pacific plate itself to the east.
Continental Rifting: Earthquakes in the Red Sea
While subduction, the process by which oceanic crust is consumed and destroyed, is the cause of the majority of the planet’s earth tremors, the creation of new crust is also associated with earthquakes. As hot magma rises beneath subduction zones the pressure drives the earth apart and tremors occur as overlying rock is fractured.
New ocean crust is currently forming in many areas. In the Red Sea a new ocean is slowly forming between Africa and Arabia and the spreading zone which forms its axis was the focus for an M5.5 tremor between Eritrea and Saudi Arabia.
Typically, earthquakes at spreading ridges are small because the crust is new and warm and under such conditions the energy released by an earthquake dissipates rapidly: but Yeats notes the occurrence of an M6.0 in Yemen in 1982, which may have killed over 2000 people, and other past but unmeasured tremors which have generated small tsunamis.