Two significant earthquakes, with magnitudes of 7.2 and 6.6 (M7.2 and M6.6) struck in the New Britain area of the western Pacific on 7 July. Although the larger of the two occurred offshore, its depth (almost 380km) ensured that no major tsunami was generated and no tsunami warning was issued.
The second earthquake, which followed just an hour and 55 minutes later, was much shallower (62km) and was not only significantly smaller but also had its epicentre on land, again resulting in no tsunami being generated and no warning issued. At the time of writing there were no reports of damage or casualties.
Earthquakes: Tectonic Setting
The epicentres of the two earthquakes were around 600km apart so it’s difficult to consider the second an aftershock. Without detailed study, it’s impossible to say whether the two are in fact related or whether their occurrence so close together in time and space is pure chance: but their tectonic setting suggests the two events may be linked.
Both tremors occurred in a crowded and complex area where several microplates (in this area the North Bismarck, South Bismarck and Solomon plates) are being created and destroyed in what is effectively a buffer zone between the Australian and Pacific plates. The region is characterised by multiple crustal motions, with the type of plate motion changing repeatedly and rapidly within a very small area: convergent, divergent and transform movements all occur here.
The first, larger and deeper tremor had its epicentre on the Pacific plate, where it moves towards the Australian plate: the relative plate motion at this point is lateral, rather than vertical. The second, which had its epicentre on the island of New Britain, within the South Bismarck plate, is close to the subduction zone of the Solomon plate from the south. The depth of the first tremor, however, suggests that although the closest plate motion is a transform fault the interface between the subducting and over-riding slab may lie below and is probably the mechanism responsible.
Papua New Guinea Earthquake History
As a result of the complex and interrelated movements of the plates earthquakes are a regular occurrence and major earthquakes, of at least M7.0, are by no means uncommon. The USGS historic earthquakes archive lists 11 earthquakes of at least M6.6 and a further seven of at least M7.0 since 2000 alone and this list is not complete.
The region has experienced frequent recent activity, most notably the M8.0 and its aftershocks which occurred in February 2013 in the Santa Cruz Islands and which generated a tsunami, and tremors of M6.5, M6.6 and M7.0 which occurred in the region of the most recent earthquakes in March and April of 2013.
As the map of seismicity in the New Britain area shows, earthquakes are regular and usually related to the northward subduction of the Solomon plate beneath the South Bismarck plate and, as noted above, the depth of the first earthquake implies that it may have been a function of this movement. Whether at subduction zones or transform faults, however, the complex plate motions ensure that earthquakes will continue to occur in the Papua New Guinea region.
United States Geological Survey. Real time earthquake map. (2013). Accessed 8 July 2013.
United States Geological Survey. Historic earthquakes list. (2013). Accessed 8 July 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.