Although in numerical terms, the week of the 14-20 August 2015 was pretty much business as usual, the United States Geological Survey’s real time earthquake map, which records all tremors in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, threw up one or two slightly unusual events.
First the bare numbers. Out of a total of 1,540 tremors listed, there was just one in excess of M6.0 (in the western Pacific), with 25 ≥M5.0 and 99 measured at ≥M4.0.
As we would normally expect, most of the tremors were at plate boundaries but there was one of M5.1 in Zambia, in the East African rift; a couple at the northern edge of the India-Eurasia collision zone; and several small quakes scattered across the US.
None of these are particularly unusual, although they do depart from the pattern in most weeks, where the larger tremors are mostly restricted to subduction zones with some at mid-ocean ridges.
The Week’s Largest Earthquake: M6.6, Solomon Islands
To the keen-eyed reader it probably feels like Groundhog Day because last week’s largest earthquake was also an M6.6 in the Solomon Islands. This week, however, the location is several hundred kilometres further south-east, although along the same plate boundary.
Last week we discussed the possible influence of different direction and styles of tectonic movement, concluding that many different influences probably affected the earthquake.
This week’s tremor looks much more straightforward, occurring on a long and relatively straightforward part of the plate margin, where the Australian plate is subducting beneath the Pacific plate along the South Solomon Trench.
Medium to large earthquakes are common here, and so this week’s tremor is by no means unusual. The shallow depth (around 4km) and proximity to the surface expression of the plate margin (difficult to determine but probably of the order of a few kilometres) suggest that it is probably the result of fracturing at or near the plate interface but at very shallow depths — in other words, a straightforward subduction zone earthquake.
M5.6, Gulf of Aden
A couple of weeks ago, the digest featured two earthquakes in the East African rift valley and considered the possibility of continental breakup and the creation of an ocean. At the northern extension of the rift, this is already happening.
Typically, rifting begins at a triple junction, where the crust cracks into a Y shape. This is what has happened in Africa: the uplift of the Afar dome, in Ethiopia, has led to the creation of the East African rift as the ‘downstroke’ of the Y, while the Red Sea and Gulf of Aden form the two upper arms.
All three are extensional tectonic settings but the latter two are already incipient oceans, with oceanic crust being created along their lengths.
This week’s earthquake, at the eastern end of the Gulf of Aden, is the result of the rifting process. The rift along the Gulf is moving apart as magma rises to form new crust. Faulting and displacement occur as the two plates (in this case, Arabia and Africa) move apart; and earthquakes occur.
The past century has seen well over a hundred events of ≥M5.0 occur along its length, making this week’s interesting but by no means exceptional.
US Earthquakes: Colorado and California
The largest earthquake in the lower 48 this week was, in fact, an M4.2 in Colorado; but people are talking instead about a smaller, M4.0 in California (have a look for videos of cute puppies being woken up by it).
Why so? Well, earthquakes are a bit like writers — it’s not how big (or how good) you are, but how many people you reach. According to the USGS, probably fewer than 100,000 people will have felt the Colorado ‘quake — but California’s ‘Little ‘Un’, which occurred in a highly populated area, will have rattled several million in the San Francisco Bay Area.
Last Thoughts: Earthquake Hazard
The ‘felt’ comparisons between the Colorado and California earthquakes this week illustrate the truth about what makes earthquakes dangerous. While magnitude certainly makes a difference, a large earthquake in the middle of nowhere will affect few, if any, people while a much smaller one in a built up area will have a significant impact upon very large numbers. That’s how we define earthquake hazard — not only in terms of an actual event but of its impact on humans.