Once again, the planet has experienced a relatively quiet week in seismic terms — at least according to the number of earthquakes recorded on the United States Geological Survey’s real time earthquake map.
The map, which includes tremors of all magnitudes (not all tremors) in the US and its territories and those of at least M4.5 elsewhere, isn’t comprehensive, though it does pick up on the major seismic events and is a reasonable guide to large-scale activity.
In the week of 2-8 March 2017, the earthquake map included 25 tremors of at least M6.0 and 67 of at least M4.5 in its total of almost 1300.
As usual, the majority of the larger earthquakes are concentrated around the margins of the major tectonic plates, with a particular concentration along the western margin of the Pacific Ocean and a scattering along the mid-ocean ridges.
The Week’s Biggest Earthquake: M6.3, Papua New Guinea
Here we are again in the west of the Pacific Ocean, looking at the largest earthquake of the week. The coming together of the Pacific and Australian plates has created a complex margin, with varying directions of movement and, in consequence, different tectonic styles (for want of a better word).
This tectonic mish-mash has generated a series of crustal slivers, or microplates, between the two larger plates and these lie almost as a buffer zone between the two.
Caught between such major plate movements, its hardly surprising that there’s a lot of seismic activity in this area, and this week the largest earthquake recorded on the USGS map was in Papua New Guinea. At M6.3, the quake was large but not exceptionally so, and it occurred close to the margin between two of the microplates, the South Bismarck and Solomon Sea plates, where the latter descends beneath the former along the New Britain Trench.
The available data indicate that the tremor resulted, as we might expect, from compressional movement. It occurred at a depth of 31km and at around 100km from the plate margin, implying that the tremor is associated with deformation within the over-riding plate rather than by movement along the plate interface — but it’s certainly subduction-related.
This magnitude of earthquake is by no means unusual in the region — in the past year, Papua New Guinea/New Britain has experienced three other earthquakes of M6.3 and four larger ones. The biggest of these, an M7.9 event, is an indication of the potential for destructive earthquake activity along this margin.
M5.3 Earthquake, Turkey
I always keep a particular eye out for seismic indicators in Turkey, not just because it has the capability for a mega-earthquake but because an earthquake in the proximity of Istanbul would almost certainly have very serious consequences. This week’s tremor wasn’t, fortunately, either large enough (it had a magnitude of M5.3) nor close enough to Turkey’s major population centres to do much damage — but, nevertheless, it was felt by approaching 12 million people.
Tectonically speaking, Turkey is pretty much its own province, with the Anatolian block bounded on the south by subduction zones which run offshore, and elsewhere by two broadly strike-slip fault zones, the East Anatolian and North Anatolian (the latter runs along its northern mountain ranges and down through the Bosphorus).
This week’s earthquake occurred on the East Anatolian Fault Zone, which includes components of uplift (reverse or thrust faulting) as well as lateral movement. This week’s earthquake was the result of largely lateral movement.
It’s the North Anatolian fault that’s the notorious one — and with good reason. Yeats summarises studies which indicate progression of large earthquake from east to west along its length as the strain on the fault builds westwards. The most recent of these was in 1999, at Izmit: it had a magnitude of M7.6 and killed thousands. At the western end of this fault lies the city of Istanbul, with a population of almost 15 million. The consequences of a major earthquake here don’t bear thinking about.
US Earthquakes: Alaska
So it’s back to Alaska again, where the M5.5 earthquake troubled (if that’s the word) a mere 50,000 or so residents, most of whom probably paid it no heed. At the northern edge of the Pacific, the Pacific plate disappears beneath North America with regular earthquakes and some fireworks from its accompanying volcanoes. Very few people pay it much attention.
Many of Alaska’s earthquakes result from collision and uplift, with movement along one of its major fault zones. The available data indicate that this week’s earthquake, which occurred beneath the Cook Inlet at a depth of almost 70km, was probably the result of movement close to the interface between the two plates, which, in this area, descends at a relatively flat angle.
Last Thoughts: Earthquake Potential
Major earthquakes which occur in remote places like Alaska cause relatively little damage. When they hit major centres of population, they can’t be overlooked. Seismologists have long been very concerned about the impacts of a major earthquake on a city such as Istanbul.
This week’s Turkish earthquake caused very little damage and a few injuries. An earthquake, even of that relatively minor magnitude, in or near the country’s largest city, would be a very different matter.