M7.5 Earthquake Strikes off New Britain, 5 May 2015

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The location of the M7.8 earthquake and its foreshocks and aftershocks shows the relationship to a subduction zone.

The location of the M7.8 earthquake and its foreshocks and aftershocks shows the relationship to a subduction zone. Image by USGS.

Hot on the heels of last week’s deadly earthquake in Nepal, another of only slightly smaller magnitude (M7.5 compared to M7.8) struck just off the New Britain archipelago in the western Pacific on 5 May 2015, sparking a small tsunami.

Unlike the devastation in the Himalayas, however, this tremor, though large, was relatively harmless and at the time of writing the media were reporting only minor damage.

At M7.5, occurring at a shallow depth (42km) and with an offshore location the New Britain earthquake was well within the parameters for which a tsunami might be expected and although the initial alert from the Pacific Tsunami Warning Center noted that “hazardous tsunami waves are possible for coasts located within 300 km of the earthquake epicenter”, the eventual observed tsunami was of the order of less than a metre, according to The Guardian newspaper.

The earthquake was preceded by a series of noteworthy shocks, beginning with two of M6.7 and M6.8 (on the 30 April and 1 May respectively) and totalling over 20 in excess of M4.5 at the time of writing. It’s likely that the series of aftershocks will continue.

Tectonic Setting: The New Britain Trench

We shouldn’t be surprised to see large earthquakes in this area; a quick look at any map of tectonic plates will show that there are several slivers of crust trapped between the converging Australian and Pacific plates. A closer examination of more detailed tectonic maps, such as those produced by Robert Yeats, reveals an increasingly complicated picture.

In summary, three microplates — the North Bismarck, South Bismarck and Solomon plates — form a buffer between the two larger plates and move in different directions, and with different types of motion, relative to one another.

The earthquake occurred at the boundary between the latter two, where the Solomon plate subducts beneath the South Bismarck plate along the New Britain Trench.

The available evidence (depth and location) from the United States Geological Survey suggests that it probably occurred at or very close to the interface between the two. Such an occurrence would have involved a component of vertical movement — explaining the ensuing tsunami.

Seismic history

Large earthquakes (at least M7.0) have occurred regularly over the past century

Large earthquakes (at least M7.0) have occurred regularly over the past century. Image by USGS

In the light of the tectonic setting earthquakes — large earthquakes — in this area are nothing new. A search of the USGS earthquake archive shows around 60 of at least M7.0 along the northern margin of the Solomon plate alone in the past century.

These include an earthquake of the same magnitude in February 2015 (though this did not trigger a tsunami and is more likely to have been the result of crustal deformation). The largest earthquake on the USGS archive, an M8.1, occurred in almost exactly the same location as the most recent in 1971.

In general, though they may generate tsunamis, earthquakes in this region do not have high recorded death tolls although as the USGS earthquake summary observes, this may be “because of the remoteness of the region, though a M 8.0 earthquake in November 2000 – one of three similarly sized events over a 2-day period – did cause several deaths”.

Further to the west, an earthquake off Papua New Guinea in 1998 was followed by what Yeats describes as “an unusually large tsunami with waves as high as 15 metres” which killed an estimated 2,000 people. In this case, however, it was not the earthquake itself which generated the tsunami but a submarine landslide which resulted from it.

Western Pacific Quakes

Given the complex nature of the tectonic setting, earthquakes are an everyday feature in this part of the western Pacific. Thankfully, they rarely cause large scale devastation.

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