The week of 18-24 July was a sobering one, with an earthquake in China causing extensive damage and some loss of life.
The tremor, at just M5.9, was only the third largest recorded in the week, which saw two tremors of at least magnitude 6.0 (≥M6.0).
In total, 27 earthquakes of ≥M5.0 were recorded on the United States Geological Survey’s real time earthquake map: as usual the focus of seismic activity was on the Pacific Ocean, especially along the boundary between the Australian and Pacific tectonic plates but also with a concentration of smaller tremors around Japan.
The Week’s Largest Earthquake – M6.5, Cook Strait New Zealand
The largest earthquake struck on 21 July in the Cook Strait, which divides the North and South Islands of New Zealand.
Like so many other seismically active areas, New Zealand is a product of the complex interactions between the Pacific and Australian Plates: the plate boundary runs to the west of the North Island (where it forms the Hikurangi Trench) and curves eastwards through the Cook Strait before turning southwards where it becomes the Alpine Fault, along the spine of the South island.
The dominant plate motion in the Hikurangi Trench is one of subduction while the Alpine Fault is a transform boundary, characterised by lateral movement.
The July 21 tremor and its associated foreshocks and aftershocks occurred close to the transition between the two types and information from the USGS indicates that it involved reverse (thrust) faulting close to the boundary.
Deaths and Damage in China’s M5.9 Earthquake
Though not the largest quake of the week, the M5.9 earthquake which struck in Ganshu Province was the most devastating tremor, with latest reports indicating that at least 94 people lost their lives, with many more injured and homes damaged or destroyed.
The cause of the earthquake was faulting along the fold and fault belts associated with the uplift of the Himalayas and the Tibetan Plateau – a result of the collision between the continental crust of India and Eurasia.
The high death toll resulting from the earthquake reflects a number of factors other than the magnitude of the tremor. Factors such as density of population, quality and structure of buildings are influential, while in this particular case the local geological and topographical conditions conspired to cause landslides which created added devastation.
US Earthquakes: The US Virgin islands
The most earthquake-prone areas of the US may be Alaska and California, but the nation’s overseas territories are also the focus of considerable seismic activity.
The Caribbean is the location of one of only two well-developed subduction zones on the periphery of the Atlantic Ocean (the other, off the South Sandwich Islands, has recently generated several large earthquakes).
The map shows a scattering of minor earthquakes off the Lesser Antilles subduction zone (it probably excludes a number of earthquakes which occurred outwith US territories): together with the curve of the island arc, they indicate an area of subduction.
Seismic activity results from the convergence of the North and South American plates (the exact location of the boundary between them at this point is obscure) against the smaller Caribbean plate. Although the over-riding motion is convergent, the tectonic setting is complex – and extensional movement occurs locally as new crust is produced in a so-called back-arc basin.
This Week’s Quakes: Magnitude and Damage
It isn’t true to say that size doesn’t matter as far as earthquakes are concerned, but the high death toll, number of injuries and scale of damage to property resulting from this week’s Chinese tremor demonstrate once again that magnitude is not the controlling factor in the amount of devastation which an earthquake can wreak – and that a tremor which might appear minor in a geographically-different and better-developed part of the world can have fatal consequences elsewhere.
BBC news online. Rescue teams in China’s quake-hit Gansu. Accessed 24 July 2013.
USGS. Real time earthquake map. Accessed 24 July 2013.
USGS. M6.5 – 46km ESE of Blenheim, New Zealand. Accessed 24 July 2013.
Yeats, R. Active Faults of the World Cambridge University Press 2012.