World earthquakes of M5.0 or greater in the week 19-25 September 2012: Image courtesy USGS
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.
Click to Read Page Two: Depth and Distance of Quakes
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