There was so much going on, earthquake-wise, in the week of 15-21 December 2016 that it’s difficult to know where to start. The United States Geological Survey’s real time earthquake map included tremors of at least magnitude 6 (≥M6.0) in Papua New Guinea, the Solomon Islands, Brazil and East Timor: In a typical week we might expect to see two or three of this magnitude and this week there were seven.
Similarly, we might normally expect the map (which broadly includes earthquakes for all magnitudes in the US and its territories and those of at least M4.0 elsewhere) to show 20-30 tremors of ≥M5.0 in a week: This week there were 77. And a typical showing of around 100 tremors of at least M4.0 is dwarfed by this week’s 166.
There is an explanation for this, and that’s that both this week and last week saw large earthquakes with significant aftershock series, so that these two events account for an astonishing 67 out of the 139 earthquakes of at least M4.5 which are shown on the map. But there are other things to note, too — including small earthquakes in Greece, Poland and an M4.5 in Hawaii.
The Week’s Biggest Earthquake: M7.9, Papua New Guinea
The M7.9 earthquake which shook Papua New Guinea this week has the distinction not only of being the largest of the week, but also being the largest of 2016 so far — narrowly exceeding the magnitude of last week’s M7.8 in the Solomon Islands. The earthquake was followed by a series of aftershocks — 58 of at least M4.0 appear on the USGS map at the time of writing, with more likely to follow.
Tectonically speaking, this part of the globe is complicated, with the boundary between the Australian and Pacific plates buffered by a number of smaller crustal slivers, or microplates. This week’s earthquake occurred at the active subduction zone along which the Solomon Sea plate descends eastwards beneath the main Pacific plate.
At first sight the location and depth of the tremor suggest that it occurred at the interface between the two plates, but the USGS event page indicates that it is, in fact, more likely to have occurred below the main boundary and within the underlying plate.
The nature of faulting (reverse, or thrust) with its component of vertical movement, the magnitude and the offshore location are all key factors in tsunamigenesis and, in this case, a tsunami did occur. It was, however, very small — a maximum height of just 8cm — and this is probably because of the depth of the earthquake (103km). Deeper earthquakes lose energy before reaching the surface and so any damage is reduced.
M6.4 Tremor, Peru
While there was so much turbulence and turmoil on the western edges of the Pacific, there was also earthquake activity on the other side of the ocean. The subduction of the Nazca plate beneath the South American continent along the Peru-Chile trench is capable of producing some of the world’s largest earthquakes.
This week, it produced a tremor of M6.4 — though this, with its epicentre on land (and its relatively small magnitude) was not capable of producing a tsunami.
This earthquake is of particular interest because of its depth. USGS data show that it was very deep indeed — over 600km. It was also caused by extensional forces rather than the compressional ones which are more generally associated with subduction zones,
Data and maps from the USGS indicate a known zone, beneath the Peru-Brazil border, where deep earthquakes such as this occur. These earthquakes are caused not by subduction as such, but by the deformation of the descending plate deep within the asthenosphere (the upper layer of the Earth’s mantle, beneath the crust). This week’s earthquake fits this pattern exactly.
US Earthquakes: California
Last week we looked at earthquakes off the Californian coast: this week’s Californian temblor is a different beast. Indeed, the M5.0 which struck in the Coastal Ranges near The Geysers is much more what we might expect of the Golden State — a product of the San Andreas Fault Zone (SAFZ).
This earthquake is very shallow — just 1.5km — and the data suggest lateral movement, which is what we might expect in this context, where the Pacific and North American plates slide past one another. The SAFZ is not a single fault, but a series of many faults, broadly parallel. This earthquake occurred between the Maacama and Collayomi faults and, in the great scheme of things, caused no serious disturbance — the USGS estimates that light shaking was felt by just 7,000 people.
Last Thoughts: Earthquakes in a Third Dimension
The USGS earthquake map is necessarily limited, because it shows the week’s earthquakes as they appear on the surface (though it is possible to filter the earthquakes by depth). Most of the time this is fine because the majority of the earthquakes which appear are relatively shallow.
This week, however, the three earthquakes vary significantly in depths and the depth of each tells us something other about the cause of the tremor or its effect. Which is an argument, one might say, for looking at earthquake events in greater depth. (Pun intended.)