The week of 10-16 November, 2016 was dominated by one seismic event — the earthquake sequence which struck New Zealand’s South Island. The mainshock of M7.8 was the largest of the week to appear on the United States Geological Survey’s real time earthquake map, and its foreshocks and aftershocks also accounted for five of the six tremors of at least magnitude six (≥M6.0).
As often happens with so large an earthquake, the overall numbers on the USGS map, which includes tremors of all magnitudes in the US and its territories and those of ≥M4.0 elsewhere, were skewed towards the higher figures. There were 51 recorded events of ≥M5.0 in the total of just over 1500 tremors, and 32 of these belonged to the New Zealand earthquake sequence.
If the numbers were higher than usual, the distribution of the tremors was not — the vast majority of the larger earthquakes occurred at or near the main plate boundaries, most of them in the north and west Pacific.
The Week’s Biggest Earthquake: M7.8, South Island, New Zealand
The earthquake of M7.8 and its fore-and-aftershocks killed at least two people and caused extensive damage, particularly around the town of Kaikoura. New Zealand lies on a major plate boundary, between the Pacific and Australian plates. Along much of the length of the South Island the boundary is conservative, characterised by lateral movement, while in the North Island it changes to become an offshore subsection zone.
This week’s earthquake series occurred around the transition between the two. According to the USGS, the tremor was “the result of shallow oblique-reverse faulting on or near the boundary between the Pacific and Australia plates in South Island, New Zealand.”
Maps of the major faults in this transition show a series of faults branching out from the main Alpine Fault zone. Earth scientists are still updating the information on what actually happened, but the latest information form New Zealand’s GeoNet website indicates that the movement was not confined to one fault but that several of these faults ruptured during the series.
As the USGS also notes: “Because of the complexity of this plate boundary region, strain is being accommodated on many different structures of varying orientations, making it possible that more than one fault may be activated in this earthquake sequence.”
M6.2 Earthquake, Japan
Further north in the Pacific, the only other tremor to exceed M6.0 this week was an earthquake of M6.2 which occurred off the north-eastern coast of the island of Honshu. In this area, the Pacific plate subducts beneath the Okhotsk microplate along the Japan Trench; it’s an area capable of generating very large earthquakes indeed, and is notorious for the 2011 Tohoku-oki earthquake which, along with its associated tsunami, killed thousands.
This week’s M6.2 earthquake was, as might be expected from the context, the result of compressional forces, although at a depth of 45km and a distance of around 250km from the trench, it’s too far from the plate boundary to be the direct result of movement at or near the plate interface.
Instead, it’s likely to be the result of crustal deformation — especially given that the Okhotsk microplate, a southern extension of the North American plate, not only borders the Pacific plate but also the Eurasian plate, which lies just 300km or so to the east.
US Earthquakes: Oklahoma — Again
I’m reluctant to revisit Oklahoma and its anthropogenic earthquake swarm for a third consecutive week. But I did say I’d keep an eye open for more information and this week something interesting did pop up — it’s a news item from the American Geophysical Union’s blog.
It’s always been a bit of a mystery as to why induced earthquakes appear on some faults and not others, and new mapping of data has allowed scientists to produce maps which not only show which faults have been triggered by wastewater injection, but also offer the potential to identify less vulnerable areas.
The idea is that vulnerable faults might be identified and drilling confined to other areas. Whether that’s enough to halt — or even slow down — the ongoing earthquake activity remains to be seen.
Last Thoughts: Did You See the Moon?
I’m going to borrow my last words from GeoNet, which addressed a question which had, briefly, crossed my mind earlier on this week as I stared at thick cloud in an attempt to see the supermoon.
With the gravitational pull of the Moon driving tides on the surface, there’s a possible case, you might think, for a very full moon having a more significant impact and generating movement within the Earth. “Some people have raised concerns about a link with the supermoon,” observes Geonet, of this week’s M7.8 New Zealand tremor. “In large groups earthquakes exhibit slight associations with lunar cycles, but this is not reliable for forecasting.”
In other words, if there is a connection, it’s too faint to be able to identify. And in any case, the supermoon trailed the earthquake by some three days.