After the excitement of last week’s unusual (and classic) earthquake doublet, things have quietened down a little.
In the week of 26 November-2 December, the United States Geological Survey’s real time earthquake map included nothing larger than a tremor of magnitude 6.7 (M6.7) — large enough, but just an aftershock to last week’s M7.6 twins.
The map, which shows tremors of all magnitudes in the US and its territories and those of at least M4.0 elsewhere, recorded a total of over 1350 events, just two of which were ≥M6.0, with 25 of ≥M5.0.
As usual, these were concentrated around the boundaries of the planet’s main tectonic plates.
The Week’s Largest Earthquake: M6.7, Brazil
This week’s largest earthquake had its epicentre (the point on the Earth’s surface directly above the point at which it occurred) in Brazil whereas last week’s doublet earthquakes occurred in Peru. Don’t be fooled by political geography, though — deep beneath the surface these boundaries mean nothing and this week’s M6.7 is an aftershock to last week’s.
I mentioned depth above, and discussed it last week. This week’s aftershock is one of a series (including the two mainshocks) of ten measuring at least M4.0 recorded on the map. All of them occurred at a depth of 595-650 km. (For perspective, that’s roughly the distance between Boston and Washington DC.)
The depth is what’s interesting about this whole earthquake series. The USGS notes that “deep earthquakes in this region occur at depths of 500 to 650 km and are concentrated into two zones: one that runs beneath the Peru-Brazil border” (this most recent series) “and another that extends from central Bolivia to central Argentina.”
It goes on to say that: “these earthquakes generally do not exhibit large magnitudes” — which is what makes this series unusual.
M5.4 Quake in Iran
Meanwhile, on the other side of the world, the Arabian plate, driven by divergence along the Red Sea, is splitting apart from Africa and moving towards Eurasia.
Continental collision raises mountains and that’s happening in this case, with the uplift of the Zagros Mountains. And mountain-building causes earthquakes.
This week saw a tremor of M5.4 in western Iran. Tectonically speaking this occurred on the Arabian side of the plate boundary but in fact it’s part of a broad and diffuse mountain building zone, comprising a series of roughly parallel, northwest-southeast trending thrust faults (and occasional cross-cutting strike-slip faults).
In such settings earthquakes are common, so this week’s event is surprising in neither location nor magnitude.
US Earthquakes: Glacier Peak
Something may or may not be stirring under Glacier Peak in Washington State. This week there was a series of small earthquakes which have aroused the interest of both seismologists and volcanologists. Geophysicist Seth Moran, quoted on the My Northwest website, noted that these are unusual because they are in an area where tremors of this magnitude are unusual.
Glacier Peak isn’t an active volcano, but the Smithsonian Institution’s Global Volcanism Program database records its last eruption s being around the year 1700 (though with a margin of error of 100 years either way!).
The volcano has no record of significant explosivity but volcanic seismicity is often the result of magma movement — though of course normal faulting is an alternative option.
Last Thoughts: Different Types of Earthquake
So this week’s digest features three very different types of earthquake which illustrate the diversity of our planet. Deep deformation resulting from subduction caused the Brazilian earthquake.
Reverse faulting as continents collide is the probable source of the tremor in Iran. And it’s possible (though by no means certain) that magma movement may have caused minor seismicity at Glacier Peak.
There’s never a dull day in seismology!