Tonga, Greece and Alaska: Earthquakes 23-29 October 2014

Just one earthquake of at least magnitude 6 (≥M6.0) appeared on the United States Geological Survey’s real time earthquake map in the week of 23-29 October 2014.

The map, which records all tremors in the US and its territories and those of ≥M4.0 elsewhere, also includes 25 larger than M5.0, 118 of at least M4.0 and 259 of at least M2.5 in its total of 1,552 earthquakes worldwide.

Just one of these larger tremors (≥M5.0 in Alaska) was at a significant distance from one of the earth’s plate tectonic margins, demonstrating the relationship between these margins and larger earthquakes — although local map of smaller tremors show that low-level seismic activity can occur pretty much anywhere on the planet.

The Week’s Biggest Earthquake: M6.1, Tonga

The congested western Pacific Ocean, with its many microplates and its complex changes in the nature of the margin of the Pacific Ocean, is a regular source of larger earthquakes. This week the largest occurred at the northern end of the Tonga Trench.

Here, the subduction boundary between the Pacific and Australian plates curves sharply around before the margin becomes diffuse and its nature unclear between Tonga and Fiji.

The location of this week’s earthquake, with its epicentre on the over-riding plate, along with its depth (35 km) implies that it is a subduction-related event.

The existence of the spreading centres of the Lau Basin immediately to the south, however, introduce the possibility that it might be associated with rifting and crustal deformation.

Without further detailed information, it isn’t possibly to say for certain.

M5.2 Greece: More Than Meets the Eye

As noted above, the USGS map records only earthquakes of a certain magnitude.

This week’s M5.2 in western Greece looks isolated but a closer look at the seismicity maps produced by Greece’s Aristotle University of Thessaloniki shows that it’s just the largest in a series in the immediate area.

Tectonically, Greece is trapped in between the African and Eurasian plates, converging along the West Hellenic Trench as the Mediterranean Basin gradually closes. But as in the case of Tonga, this week’s tremor occurred just off the end of the trench at a relatively shallow depth.

In this case the implication is that the cause is crustal deformation rather than subduction — and maps of recent earthquakes, and of larger historic events, confirm that in this area earthquake activity is frequent and diffuse as the continents collide.

US Earthquakes: Alaska

With no large earthquakes in the US this week, it’s instructive to stand back and look at the overall pattern of earthquakes in Alaska. The map shows that there is a clear association between earthquakes and a narrow subduction zone along the Aleutian arc.

Further to the east, on the Alaskan continent, the zone of earthquakes departs from the subduction zone and becomes much more diffuse.

Most of these earthquakes are smaller (although large earthquakes do occur there they are more characteristic of the subduction zone itself) and shallow — the result of crustal deformation.

Earthquakes: Not Always Clear Cut

This week’s featured earthquakes in Tonga and Greece both occur just off the end of a subduction zone and both appear isolated on the USGS map. This illustrates the shortcomings of the map in a world-wide context. Comparing them with Alaska, all of whose earthquakes appear on the USGS map, indicates that medium-sized earthquakes, probably associated with crustal deformation, are a regular occurrence in such tectonic settings.