The reverberations of September 16th’s M8.3 Chilean earthquake continued this week, although they’re dying away. Of the seismic events recorded on the United States Geological Survey’s real time earthquake map, which includes tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, 60 of the 166 ≥M4.0 were aftershocks to the main event.
The largest of these clocked a mere M6.2, more than two magnitudes smaller than the mainshock and, given that the magnitude scale is logarithmic, less than 100th of its actual size. Nevertheless, it ranked as the second largest earthquake of the week, one of three ≥M6.0 to appear on the map.
Of the total, there were 31 ≥M5.0 and four of these were in the Chilean series. If we strip these out, the overall pattern of earthquakes is pretty much as normal, with the usual concentration around the western Pacific. This week, however, there were a couple of odd outliers (one, for example, off southern Greenland and another in the Baikal Rift in central Russia.
The Week’s Biggest Earthquake: M6.6, Indonesia
So, back to normal with the largest earthquake once again occurring along the complex jumble of plate movements and crustal slivers of the western Pacific. This week’s major earthquake occurred on the Bird’s Head Peninsula at the extreme western edge of New Guinea.
The situation here is extremely complicated, with the Australian, Pacific and Philippine Sea plates coming together, with the buried Molucca Sea plate lying to the west. Tectonic maps show a dominant subduction zone to the north east of New Guinea and with thrust belts and lateral margins behind.
It’s difficult, therefore, to pick out the exact source of the tremor, but the depth and precise location of the mainshock, along with the three significant (≥M4.5) aftershocks, imply that it occurred as a result of movement along a very short subduction zone, the Monokwari Trough, rather than as a result of lateral or thrust faulting.
M5.5 Earthquake in Canada
The western seaboard of North America is fascinating for the seismologist. To the south, the active San Andreas fault zone, a conservative margin (where plates slide past one another). In the middle, the potentially dangerous sleeping giant that is the Cascadia subduction zone. And in the north, a second conservative margin along the western coast of Canada.
It was the junction between the central and northern zones which saw an earthquake this week, an M5.5 around 200km from Port Hardy. The Cascadia subduction zone is caused by the slow descent of the Juan de Fuca plate (remnant of a much larger plate) beneath North America, and the epicentre of the earthquake lay on this plate with the event itself occurring at a shallow depth of 10km.
In fact, this tremor is probably neither one thing nor the other — not the product of subduction nor of lateral movement. Because it occurred close tot the point at which the nature of the movement changes, the depth suggests that it’s most likely to be the result of deformation within the overriding plate.
US Earthquakes: Nevada
Nevada’s still shaking. This digest has been keeping an eye on the earthquake swarm in north west Nevada for some time. The state’s seismological laboratory has little or nothing to say about it (beyond the standard reports) but this week there two tremors of M4.2, which are among the largest of the ongoing sequence and suggest (no more) that it’s worth keeping a close eye out for the future.
Chance has it that this week is the centenary of the state’s largest recorded earthquake, which took place much further west on 2 October 1915 and had a magnitude of M7.1, much larger than anything in the current swarm.
It does remind us that, as the Nevada Seismological Laboratory observes, “The 1915 earthquake underscores beyond a doubt that Nevada is earthquake country and that Nevadans should be earthquake ready”.
Last Words: Earthquake Season?
They say you learn something every day.
An analysis of large, deep earthquakes undertaken by a couple of Californian seismologists, reported in the New Scientist, suggests that the ‘quakes are more likely to occur in summer than in winter. The sample size is small so there may be a random statistical variation — or it may be that there’s some seasonal cause — such as ‘solar tides’ generated by the gravitational pull of the Sun within the Earth — that triggers such earthquakes. It just goes to show that whatever we think we know about earthquakes, there’s always more to learn.