The week of 17-23 April 2014 saw things calming down a little after a turbulent few weeks, seismically speaking — although at first glance it may not look like it.
The United States Geological Survey’s real time earthquake map recorded almost 1700 tremors of all magnitudes in the US and its territories and at least magnitude 4 (≥M4.0) elsewhere; 59 0f these were ≥M5.0.
Many of these tremors were associated with the week’s largest earthquake, an M7.5 in Papua New Guinea. Discounting these, the number of ≥M5.0 falls to 31 – still high, with large earthquakes (≥M6.0) in the western Pacific, off Mexico and in the south Pacific, but not as high as in recent weeks. And the figure included a small tremor M5.4 on Easter Sunday – near Easter Island.
The Week’s Largest Earthquake: M7.5 Papua New Guinea
The largest earthquake of the week occurred off Papua New Guinea on the 19th of April and was one of a series which began several days previously. The series included an M7.2 on 11 April and M6.6 on the 19th before what appears to have been the mainshock – an M7.5 – followed by numerous smaller tremors.
At the margin between the Pacific and Australian plates, several small crustal slivers (microplates) form a buffer zone between the two major plates. Conflicting directions, speed and style of movements combine to make this area highly earthquake-prone. The most recent information on the event from the USGS suggests that the cause of the earthquake was thrust faulting on a subducting length of the margin between the Solomon and Pacific plates. This is consistent with reports of a tsunami, though one that was small and caused no damage.
The Sumatra Arc
Further to the west, the margin between the Australian plate and the Sunda plate also saw a series of earthquakes, albeit of no great size.
The Sunda Trench, which marks the subduction zone between the two (the former descending below the latter) is one of the world’s most active such zones, capable of generating some of the planet’s largest and most devastating tremors.
Although this week’s earthquakes were minor, compared with some of the events this century (including the Boxing Day earthquake and tsunami of 2004) they are a good illustration of the fact that medium-dozed earthquakes are a common feature of subduction zone earthquakes.
U.S. Earthquakes: More on Oklahoma
The week saw a continuation of the minor earthquake swarm in Oklahoma and surrounding states. These minor tremors are associated with human activity (known as induced earthquakes) and are the subject of current attention from geologists.
Though not large, they have implications for hazard management for features such as dams and pipelines and, regardless of their origins, merit considerable ongoing research.
Last Words: Plates and Microplates
Usually, maps of the planet’s tectonic plates show only the major divisions. Many of these have been poorly defined; and the existence of microplates, which accommodate different speeds and direction of the major units, go some way to explaining the number of earthquakes which occur in areas such as the western Pacific.