With just 19 earthquakes of greater than or equal to magnitude 5 (≥M5.0) and none at all exceeding M6.0, the week of 19-25 September has been another period of relatively low levels of earthquake activity.
How do we know this has been a quiet week?
Well, without having kept a detailed record of earthquakes of a particular magnitude over a long period, and given the unpredictability in terms of time and magnitude (though not, generally, location) of earthquakes, we can get an idea by looking back over USGS data for the preceding 30 days.
These show 157 events of M5.0 or greater, an average of over 5 each day – whereas the 7 days to 25 September, with their total of 19, average less than 3.
Earthquakes in the Western Pacific: The Tonga Trench
One area which hasn’t been seismically quiet is the subduction zone in the western Pacific, where the Pacific Plate collides with the Australian Plate and is forced beneath is along the line of the Tonga Trench. Over a third of the ≥M5.0 events recorded in the past week occurred at the subduction zone of the Tonga Trench.
Looking more closely at these events, we can see an illustration of how earthquake location varies with depth at subduction zones. Where a slab of crust is forced downwards, earthquakes occur at the interface between the subducting and over-riding plates. As this interface is angled downwards (the angle varies from plate boundary to plate boundary) it is logical to expect that deeper earthquakes will be recorded on the earth’s surface, increasingly far from the actual boundary.
How far does this apply? Well, in the week’s 6 events of greater than or equal to M5.0, we see a reasonably close fit. The two tremors closest to the subducting boundary were very shallow – 8 and 10km respectively. An earthquake around 100km from the boundary shows up as 104km deep. At roughly 250km behind the zone, our earthquake is at 209 km depth, and the two recorded at significant distances away (500km or more) are very deep indeed – each over 500km.
In other subduction zones the exact ratio of depth to distance might vary – but the relationship remains. And in any review of historical seismicity across a subduction zone – the so-called Wadati-Benioff Zone – the pattern of seismicity should be clear.
Canadian Earthquakes: Stable Tectonic Settings
Another point worthy of comment is the occurrence of two small earthquakes (M3.1 and M3.9) in eastern Canada. The vast majority of earthquakes are associated with plate boundaries. The eastern USA and most of Canada, however, are located far from any such boundaries and are known for their tectonic stability. Indeed, as the USGS notes, “much of the enormous region from the Rockies to the Atlantic can go years without an earthquake large enough to be felt, and several U.S. states have never reported a damaging earthquake.”
Earth tremors in such settings are generally related to faults within the rock. Some of these may be well-known, and their extent and dynamics understood; others may be unmapped, especially after long periods of inactivity or where they are deeply buried. Depth is relative, compared to subduction zone – the two Canadian earthquakes, at 10 and 18km depth, are shallow by comparison with subduction zone earthquakes but deep for the local context.
Small Earthquakes are Interesting Too!
Although the week didn’t show any earthquakes of great significance, the locations and pattern of the earthquakes which did occur allows us to investigate some specific aspects of seismology – and to demonstrate that, tectonically, there’s more to learn about quakes than just magnitude.
United States Geological Survey. Real Time Earthquake Map. Accessed September 25, 2012.
United States Geological Survey. Earthquakes in Stable Tectonic Regions. Accessed September 25, 2012.