Sometimes we feel the need to go back to our roots. I don’t know what it was this week that made me realise that it’s almost a year since I broadened the scope of my weekly digest from earthquakes only to geoscience in general — some offhand reference to seismic shifts in politics perhaps, or a Tweet about increasing earthquake activity somewhere or other.
Whatever it was, it made me a little nostalgic and so, for old times’ sake, I thought I’d go back and do what I used to do — an earthquake digest. There’s nothing particularly earth-shattering (pun intended) about this week’s roundup, but it did strike me that it’s good to keep a watch on the earth when times are quiet. And this week, they are.
Earthquakes This Week
And so, in my time-honoured tradition, we’ll start with the numbers. In the week of 2-8 November, 2018, the United States Geological Survey’s real time earthquake map recorded just over 1900 tremors of all magnitudes. Of these, 2 were of at least magnitude 6 (M6.0) with 19 of at least M5.0 and 255 of at least M2.5.
When I say that it was a quiet week, I mean it in all senses. It’s been a while since I kept a close scrutiny over the numbers of earthquakes and so there may have been changes in the criteria that I’ve missed. The number of smaller earthquakes is much smaller than I expect but that figure was always under-recorded on the map. The kay thing is that the major earthquakes are there.
The Week’s Biggest Earthquake: M6.2, Chile
The largest earthquake of the week occurred in the north of Chile, where the Nazca plate subducts beneath the South American continent at a rate of about 8cm per year. The earthquake occurred at around 200m from the plate margin at a depth of just over 100km. At this point the slab is descending at an angle of about 30 degrees which if my (admittedly rusty) algebra is correct, suggests that the earthquake occurred at or close to the plate boundary.
If this is the case we’d normally expect the mechanism of the earthquake to be compressional, but the data in the summary page indicate that in fact it was extensional. The probable explanation for this is that it was caused by deformation near the boundary, rather than by movement along the boundary itself.
It’s always worth looking at the wider context of an earthquake. While most of the earthquakes in this area (Peru and norther Chile) are indeed driven by extensional forces, a search of the UDSGS archives shows that there have been three large earthquakes in the immediate area of this week’s. All three were of similar magnitudes, all occurred at (roughly) similar depths and all were driven by extensional tectonics.
A closer look at tectonic and stress maps shows that this week’s earthquake, and the others, occurred in areas where there is extension, indicating that even within areas dominated by one type of tectonics, it isn’t unusual too find other forms of movement.
M6.0 Earthquake, Philippines
The western margins of the Pacific are a tectonic jigsaw puzzle in three dimensions, with plenty going on both on and below the surface at a variety of scales. Unlike some of the major subduction zones along continental margins (such as the Andes) the picture is anything but simple. The westward movement of the Pacific plate against Eurasia involves several other plates, from the larger Philippine Sea plate to many smaller plates, being caught up and effectively jostling for position.
This week an M6.0 occurred in the Philippines, on the island of Mindanao. Here, the position is interesting, with subduction zones to the east (Philippine and East Luzon troughs) and west (Manila, Negros, Sulu and Cotabao trenches) of the main part of the archipelago, so that the area in between in being compressed with extensive fault zones along the centre.
Unsurprisingly, there are many earthquakes in this area, and this week’s is interesting because of its depth. At almost 600km, it’s almost certainly unrelated to anything that is happening on the surface and may (possibly) be related to the Moluccan Sea plate, which is buried and doesn’t appear on tectonic maps.
US Earthquakes: A Minor Earthquake Swarm in California
The central section of the San Andreas Fault Zone which runs (roughly) from south of San Francisco to the north of Los Angeles is one dominated by creep — in other words continuous, slow, lateral movement rather than larger, less frequent shocks. For this reason, small earthquakes are so common as to be unremarkable.
Slightly larger tremors do occur, though, and are noteworthy rather than startling. This week an earthquake of M4.1 occurred close to the San Andreas fault itself, south of Hollister, and was followed by an ongoing series of aftershocks. At the time of writing there have been 60 of them, the smallest being M0.6.
Last Thoughts: Something of Everything
Scouting of the USGS map this week has been refreshing, for me at least. We’ve seen earthquakes with extensional, compressional and lateral movement. We’ve seen that in a compressional setting, it isn’t unusual to find extensional earthquakes. We’ve seen that tectonic activity at depth isn’t necessarily a reflection of the pattern on the surface. And we’ve seen that in some places strain doesn’t have to be released in a single major event.