The period of seismic quiescence which has been evident over the pat several weeks continued this week, with just one earthquake of at least magnitude 6 (≥M6.0). This, an M6.2 in New Zealand, was not untypical – the western Pacific is frequently the location for the larger tremors. Overall the United State Geological Survey’s real time earthquake map showed a total of 1700 tremors (those of all magnitudes in the US and its territories and ≥M4.0 elsewhere) of which 18 were ≥M5.0, 97 ≥M4.0 and 398 ≥M2.5.
The Week’s Largest Earthquake: M6.2, North Island New Zealand
The tremor which struck the North Island of New Zealand on 20 January had its epicentre 34km from the city of Palmerston. Although there were no deaths or injuries, news media reported local damage and disruption to power and transport services and the earthquake was felt as far away as the cities of Wellington and Napier.
New Zealand, which lies on the boundary between the Pacific and Australian plates, is tectonically interesting because the North and South Islands are dominated by different tectonic processes. The nature of the boundary changes between the two, from a subduction zone in the north to a conservative boundary in the south. Monday’s earthquake was associated with the subduction zone, although information from the USGS indicates that it was not caused by movement at the boundary itself but by faulting within the over-riding plate.
Earthquakes in the North of South America
Previous weeks have seen activity along the northern margin of the Caribbean plate. This week the focus shifts southwards, as three earthquakes of 4.5 and 4.6 in Venezuela and Colombia illustrate the tectonically-active nature of the northern margin of South America.
The Caribbean plate is caught between the Nazca, North American and South American plates, and its southern margin includes an offshore subduction zone, onshore thrust zone and major strike slips faults – which between them accommodate the varying movements. The location and depth of the three earthquakes suggest that all three were generated by lateral movement along strike slip faults as both the Caribbean and Nazca plates move eastwards relative to the South America plate.
U.S. Earthquakes: Alaska
The main focus of earthquake activity in the U.S. was once again in Alaska. The Aleutian island chain is the product on one of the planet’s major subduction zones, where the Pacific plate moves north-westwards under the North American plate at a rate of around 75mm per year.
This movement is capable of generating very large, so-called ‘megathrust’ earthquake and the northern Pacific margin was the source of one of the world’s largest recorded tremors, the M9.2 which struck near Anchorage in 1964.
By comparison, this week’s earthquake was relatively minor, although it remains the largest of the 120 or so earthquakes of M2.5 or more which have occurred in the Aleutian chain over the past 30 days – and illustration of the constant activity which occurs in such highly seismically-active areas.
Earthquakes Worldwide: Constant Activity
Although there have been few major earthquakes over a period of several weeks, the planet is never still. The constant movement of the tectonic plates generates strain and releases it as earthquakes – something that is amply demonstrated by both the movements along the Caribbean and Alaskan margins.