Although the overall number of earthquakes recorded on the United States Geological Survey’s real time earthquake map for the week of 12-18 September isn’t significantly different from the previous week, it was nevertheless relatively quiet at the top end of the scale with just one earthquake of at least magnitude 6 (≥M6.0) and 18 of ≥M5.0.
These larger tremors were scattered across the globe, with noteworthy events on ocean ridges in the South Atlantic and eastern Pacific and tremors in Greece and Georgia.
The Week’s Largest Earthquake: M6.1 Alaska
At just M6.1, the largest earthquake of the week was nothing out of the ordinary, particularly not for its location. Along with three more of the ten largest tremors, this quake represents an aftershock from the M7.0 which struck on 30 August.
So far there have been around 60 aftershocks of at least M4.5 and over 300 of all magnitudes. The repercussions of major shocks such as this can be expected to carry on for weeks or even months.
Continental Breakup: M5.0, Eritrea
Arguably the most interesting earthquake this week is the M5.0 which occurred in Eritrea. Geologically, this area is fascinating: A rising plume of rock sourced from deep within the earth is lifting the crust and will in time initiate continental breakup of Africa, with a new ocean forming between Ethiopia and Sudan.
The M5.0 of 18 September had its epicentre in Eritrea, north of the main hot spot.
This region is known as a triple junction, with ocean ridges along the axes of the Red Sea and Gulf of Aden already splitting Arabia from Africa and creating new ocean crust. The third arm, the East African Rift, propagates southwards to become the Great Rift Valley. With such dynamic processes in action, it’s unsurprising that this area is characterised by extensive volcanism and accompanied by earth tremors, although large earthquakes in the area are rare.
US Earthquakes: The Western States
The majority of significant earthquake activity in the States this week continued to be in Alaska. Elsewhere there was no outstanding noteworthy event, but this in itself offers an opportunity to look at the overall pattern of tremors and to get some idea of the relationship between earthquakes and topography.
The San Andreas fault zone, a major plate boundary, is clearly dominant but other, older margins are equally important.
To understand the complex geological history of California, even at a very simple level, we need to understand that the US west of the Rockies is made up of various geological blocks which over time have ‘docked’ with continental North America. The boundaries of these blocks are structurally relatively weak – as is indicated by the broadly linear pattern of seismic activity. It’s also worth noting the scattering of tremors in and around Montana-Idaho-Wyoming, which results primarily from volcanic activity associated with the Yellowstone hot spot.
Creative Processes Cause Earthquakes
The Eritrean earthquake, and its association with the Afar mantle plume, is exciting because it reminds us that earthquakes are not merely part of a destructive process – even though most major tremors are associated with subduction and melting of oceanic crust. The constructive processes by which new crust is created also involve enormous crustal forces and, as a result, are capable of generating noteworthy earthquakes.
United States Geological Survey. Real time earthquake map. Accessed 18 September 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.