The United States Geological Survey’s real time earthquake map, which records earthquakes of all magnitudes in the US and its territories and some tremors of at least magnitude 4 (≥M4.0) elsewhere, shows a total of 1769 events worldwide for the week of 17-23 July.
The figure is an underestimate, but is indicative of how seismically active the planet is.
Higher-magnitude seismicity is usually concentrated along the margins of the Earth’s tectonic plates — a concept that the map shows particularly clearly this week, with no tremor ≥M5.0 and just three of ≥M4.5 (in Alaska, Bolivia and Vietnam) reported away from plate boundaries.
The greatest concentration of activity occurred in the western Pacific, where there were 18 tremors of ≥M5.0.
The Week’s Largest Earthquake: M6.9 Fiji
The largest earthquake on the map this week had its epicentre on the Lau Ridge between Fiji and Tonga. The location maps show this to be hundreds of km from the nearest plate margin — so why isn’t it included in the introduction as an intra-plate earthquake?
The USGS map shows the location of the epicentre — the point on the Earth’s surface immediately above the earthquake.
In fact the point at which it occurred was deep in the crust, at a depth of 614km. This suggests that it is probably associated with the subduction of the Pacific tectonic plate beneath the Australian plate.
Subduction takes place at an angle and earthquake epicentres along the plate interface increase in distance westwards with the depth of the actual earthquake — a concept elegantly illustrated by maps of earthquake location and depth.
M6.0 Quake in Alaska
This week an earthquake of M6.0 in Alaska draws attention to the points at which the nature of a plate margin changes. Plate tectonics recognises three types of plate boundary — constructive, destructive and conservative — and at various points there is a transition between them.
A bend in the boundary between the Pacific and North American plates in Alaska marks the transition between the Aleutian subduction zone and the transform boundary characterised by the Queen Charlotte and Fairweather faults.
This week’s M6.0 tremor occurred very close to the transition between the two margins. In this area the situation is complicated by the subduction of a sliver of crust beneath the North American content and this, combined with the transition, contributes to the fracturing of the crust into a series of onshore faults, the Chugach St Elias faults, which run broadly parallel to the coast.
The location and depth (18.5km) of the earthquake suggests that it occurred along one of these faults rather than on either the subduction zone to the west or the transform margin to the south.
US Earthquakes: Quarry Blasts
Scientists and laymen alike have paid much attention recently to human-induced earthquakes. While the origin of some is open to continuing debate, other human-induced earthquakes are clear — the USGS map goes as far as to include a separate symbol for quarry blasts which are recorded on it.
The two can be distinguished by the different seismic waves that they produce and by the analysis of their source — a complicated process but, put simply, a tectonic earthquake involves fracturing of a rock along a fault while in an earthquake the fracturing has a single source.
Quarry blasts are not large – this week’s USGS map shows five in Oregon, the largest of them with a magnitude of M2.6.
Last Thoughts: Diffuse and Complex Boundaries
The distinction between three types of boundary is useful, but simplistic. Both the Fijian and Alaskan earthquakes occurred in areas where plate boundaries change direction and nature and although the Fijian tremor was relatively straightforward, that in Alaska demonstrates the different motions which occur at the point where plate margins are in transition.