No major earthquakes occurred this week, and the United States Geological Survey’s real time earthquake map recorded just two with a magnitude of 6 or more (≥M6.0).
Both of these tremors were in the western Pacific, which again proved to be the location for much of the planet’s significant seismic activity with 12 of the 22 recorded tremors of ≥M5.0.
Smaller earthquakes largely followed the expected pattern along the planet’s tectonic margins, although this week threw up some tremors that, although small, are worthy of note in their setting.
The ongoing earthquake swarm in Oklahoma produced nine tremors of ≥M3.0 and the US also saw a further three of that magnitude in Hawaii; while perhaps the event to raise most eyebrows was the M3.9 in the UK’s Channel Islands
The Week’s Largest Earthquake: M6.5, Japan
At M6.5 the week’s largest earthquake, off Japan, can reasonably be regarded as large in world terms – the second largest earthquake to strike off the east coast of Japan in the past year (the largest was an M6.1 in October 2013).
Tectonically speaking this is a highly active area, with several plates meeting and parts of Japan being located on four of them. With a major subduction zone off the country’s west coast, frequent and significant earthquakes occur as a matter of course.
The earthquake occurred around 100km south of the epicentre of the disastrous M9.0 Tohoku-Oki earthquake of 2011, at a depth of around 10km. Earthquakes of this size are too small to generate major tsunamis although they sometimes create minor local ones; but there are no available reports of any such waves resulting from the 1 July tremor.
Although detailed information isn’t available, it’s possible that the earthquake may be an aftershock of the 2011 event. Such aftershocks may continue for years (though with decreasing frequency and magnitude) and the most recent tremor is just one of almost a hundred seismic events of ≥M6.0 to occur in the area since the 2011 megathrust ‘quake.
M6.3, the Philippines
Often overshadowed by earthquake activity nearby (experiencing just two earthquakes of ≥M8.0 in the past century compared with 13 for Japan and five in the Java-Sumatra arc) the Philippines is nevertheless highly active, as this week’s M6.3 just off the south of the archipelago demonstrates.
The Philippines are an island arc, the product of volcanism resulting from subduction of the pacific plate to the east.
The faulting associated with such subduction zones is complex and a glimpse at even a simplified fault map indicates the presence of normal and thrust faulting resulting from deformation along the length of the archipelago. The location and depth of this week’s earthquake suggest that it may have occurred at one of these faults, the Halmahera Thrust, rather than at the plate interface.
US Earthquakes: Induced Seismicity
There’s much to comment on in the United States, with ongoing aftershocks from the Arizona earthquake of 29 June and some volcanic seismicity around the island of Hawaii.
This week, as in previous weeks, it’s hard to ignore the Oklahoma earthquakes where the ongoing series of tremors seems to be increasing in magnitude. These tremors, which are now thought to be associated with wastewater injection, produced 26 seismic events of ≥M2.5 this week and continue to give cause for concern.
The maps show that very few places on the planet are immune from any kind of earthquake activity.
An unusual earthquake, however, is not necessarily a large one but one whose magnitude is unexpected in tis location – which makes a tiny earthquake in, say, the eastern US, much more worthy of comment that one of twice its magnitude in California.