A large earthquake, of magnitude 7.6 (M7.6) struck the Solomon Islands on 13 April 2013 local time (12 April UTC).
Initial data from the United States Geological Survey indicate that the tremor struck at a depth of around 18 km around 50km south of the island of San Cristobal.
Offshore tremors of this magnitude are capable of generating damaging tsunamis and the Pacific Tsunami Warning Center issued an alert that “an earthquake of this size has the potential to generate a destructive tsunami that can strike coastlines in the region near the epicenter within minutes to hours”.
The agency later withdrew the warning when no tsunami occurred, and at the time of writing there were no reports of damage or injuries.
Tectonic Setting of the April 13 Solomon Islands Earthquake
The boundary between the Pacific and Australian tectonic plates is complex, involving several slivers of crust (microplates) and encompassing different styles of tectonism (convergent, divergent and lateral) and changes in direction of subduction along its length. The margin in general is highly active and the nature of the boundary at any point affects the type and impacts of an earthquake.
Although most maps of the area show the margin south of the Solomon Islands as a subduction zone, there are many variations and the USGS report on the tremor indicate that in fact it occurred at a point where the type of movement changes from subduction to transform motion.
The lack of vertical motion involved is a key factor in explaining why an earthquake of this magnitude didn’t cause a tsunami.
By contrast, February 2013 saw an earthquake of M8.0 strike the Solomon Islands and this did cause a tsunami – but it occurred some distance further east at a subduction zone.
Recent Earthquakes Along the Pacific-Australian Plate Margin
The western Pacific is a particularly active area seismically. The most recent tremor is the 33rd of at least M7.6 recorded since 1900 in the USGS earthquake archive for the stretch of plate margin between Papua New Guinea and Vanuatu.
More interestingly, that section of crust experienced another large earthquake (M7.1) just a day previously. This time the tremor was to the south of New Britain, where the Solomon microplate subducts beneath the Bismarck micro plate.
This was a subduction earthquake; but it also didn’t generate a tsunami, probably because the vertical displacement involved was too small.
Quakes Triggering Other Quakes?
Some scientists believe that one large earthquake can trigger another in the same area. In this case, however, it appears unlikely that the two earthquakes are linked, despite the fact that they occurred along the same part of the western Pacific within so short a space of time.
The distance between them is huge – over 1000km – and they occurred on different types of margin between different plates. This implies that the occurrence of two different tremors, on what is after all so active a margin, is coincidental.