The week of 15-21 August saw a relatively high figure – 32 – for earthquakes of at least magnitude 5 (≥5.0) although just two of these, in New Zealand and along the Southwest Indian Ridge, were greater than M6.0.
We can account for this increase in part by a cluster of earthquakes in New Zealand which included ten tremors of ≥M5.0. These apart, the pattern of earthquakes was in line with our usual tectonic expectations.
The Week’s Largest Earthquake: M6.5, New Zealand
The M6.5 which struck on the northern coast of New Zealand’s South Island, near the city of Blenheim, on 16 August was not only the largest of the week but also, along with its aftershocks, contributed significantly to the total of larger recorded earthquakes this week.
In all, 31 tremors of ≥M4.0 were recorded in the area on the USGS real time earthquake map, making a considerable contribution to the week’s total.
New Zealand is dominated, tectonically speaking, by the Pacific-Australian plate margin. To the north, the dominant motion is subduction but in the South Island the nature of the boundary changes to lateral movement.
The earthquake of 16 August and its aftershocks took place close to the transition between the two. Here, the transform Alpine Fault splits into several parallel strike-slip faults, the Marlborough Fracture Zone, and this is the location for the cluster of earthquakes.
M4.6: Central Russia
The earthquake which occurred in the central part of Russia, close to Lake Baikal, was not large (it registered just M4.6) but is noteworthy for its setting. The epicentre lies in an apparently stable continental interior hundreds of kilometres north of the main collision zone between India and Eurasia (though it’s worth noting that the impacts of this collision are very far-reaching indeed).
Lake Baikal lies in a graben, or rift, where divergent motion causes blocks of land to drop down. Though it’s impossible to be certain of the exact nature of the faulting which caused the tremor without further detailed information, it seems probable that the mechanism was normal faulting in an extensional tectonic setting.
United States Earthquakes
Alaska is regularly the location of the United States’ larger earthquakes and this week was no exception, as a cluster of earthquakes occurred on and around Little Sitkin Island in the Aleutian chain. In contrast to the swarm of tremors in New Zealand, these did not feature an obvious mainshock, with seven events of ≥M4.5, of which three were M5.0.
Such tremors are by no means uncommon in Alaska, where the northward movement of the Pacific plate against the North American plate is capable of generating damaging, large-magnitude earthquakes.
Clusters of earthquakes are by no means unusual. Often, as in the case of New Zealand’s tremor, a mainshock will be followed by a series of aftershocks which diminish in magnitude over time. In other cases, several small earthquakes will occur without a larger tremor occurring. The latter situation, which was evident in Alaska (and elsewhere) this week, is not uncommon and doesn’t necessarily presage a major earthquake event (though nor can it be said with any certainty to preclude one).
USGS. M6.5 – 29km SSE of Blenheim, New Zealand. (2013). Accessed August 21, 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.