The week of 15-21 July produced some interesting, and contrasting earth tremors among those listed on the United States Geological Survey’s real time earthquake map.
The map, which shows tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, included a total of over 1,600 tremors, of which just two were in excess of M6.0 and 27 were ≥M5.0.
As is the case in most weeks, almost all of the larger tremors were associated with existing plate margins.
In addition, over half of those ‘quakes which registered ≥M5.0 occurred in the western Pacific, where the long and complicated margin between the Pacific plate in the east and the several others which march along its western edge leads to many stresses and tensions.
The Week’s Biggest Earthquake: M7.0, Santa Cruz Islands
The usual rash of western Pacific earthquakes included the largest of the week, which occurred off the Santa Cruz Islands. Although at various times the USGS and other authorities assigned the tremor a magnitude as high as M7.0 and as low as M6.9, it seems to have finally settled on the map at a respectable, but not eye-popping, M7.0.
The tremor and its series of aftershocks (at the time of writing, seven in excess of M4.5) occurred at a point where the boundary between the Australian and Pacific plates takes a near-90-degree turn, from north-south trending along the North New Hebrides Trench to pretty much east west along the South Solomon Trench.
Seven of the eight earthquake in the sequence, including the mainshock, occurred at shallow depths; just one was deeper than 10km and that was at just 44km. This implies that the cause of the sequence was deformation in the overriding plate, rather than movement along the west-to-east (or south to north) directed subduction zones.
Although at M7.0 — and certainly at the initially-recorded M7.5 — such a submarine earthquake is theoretically large enough to generate a tsunami, no such tsunami appears to have been recorded and there was certainly no wider event. This adds weight to the suggestion that no significant vertical movement took place at or near the plate boundary – the typical mechanism for tsunamigenesis.
M6.5 Earthquake, Lesser Antilles
The Atlantic, in contrast to both the Pacific and Indian Oceans, has very little in the way of active subduction, with just two relatively short subduction zones.
One, in the remote south, has been shaking away, quiet and largely unobserved (except, of course, by this digest); while the other, east of the Caribbean, is much more in the public eye.
This week, the Caribbean margin was struck by an earthquake of M6.5, over 120 miles east of Barbados.
Here the North and South American plates subduct beneath the Caribbean plate, and fault maps show that there are rows of broadly parallel, roughly north-west to south east trending, normal and thrust faults in the over-riding place.
Again, the combination of depth (10km) and location, relative to these faults and to the actual subduction zone, suggests that deformation, rather than movement at or near the plate interface, is the source for this week’s tremor. It’s worth bearing in mind that, although this earthquake is large for the area (the USGS real time map and archive shows just five larger in the past century along this part of the Caribbean margin) the largest on record in the area is, at M7.5, ten times the size (remembering that the magnitude scale is logarithmic).
US Earthquakes: Alaska
This week’s largest earthquake in the US came in at M5.7, and was located onshore around 120km from Anchorage, at a death of 71km.
It seems to have been caused by movement along the Denali fault, part of the huge fault zone which crumples eastern Alaska behind the western part of the Aleutian subduction zone.
Crustal Deformation at Subduction Zones
All three of this week’s earthquakes have one thing in common — all occurred at subduction zones and all are probably the result, not of subduction itself, but of deformation at shallow depths in the over-riding plate. All three demonstrate that subduction zones are not always simple, especially where the boundary changes direction, or nature, or both.
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