In another week of relatively low-level seismicity worldwide, the largest earthquakes recorded on the United States Geological Survey’s real-time earthquake map (which includes earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 elsewhere) between 8 -14 January 2015 were both of magnitude 5.8 (M5.8); one in the Philippines and the other in eastern Indonesia.
There were 27 tremors of at least M5 (≥M5.0), all but two of them located at or very close to the boundaries between the Earth’s tectonic plates.
The exceptions, two tremors in different parts of China, were both associated with a broader zone of convergence in the Himalayas, which is not inconsistent with the normally observed pattern. Generally, significant seismic events are linked to broad zones of continental collision or narrower ones of divergence or lateral motion.
The Week’s Biggest Earthquake: M5.8, Indonesia
Jointly the largest earthquake of the week, the M5.8 which occurred off the western coast of Papua New Guinea is the product of the complicated collision between the Pacific and Australian plates. With no detailed information available, we can only use the evidence of location and depth to consider its probable origin.
Tectonically speaking, the western Pacific is difficult.
A jumble of microplates (slivers of crust) caught between the two larger plates are subject to complex stresses caused by varying speeds and directions of movement (with different maps showing different fault zones). It’s no surprise, therefore, that earthquakes in the region are common.
The depth of the tremor (just 20km) and its location (in the main Australian plate where it overrides one of the many subducting microplates in the region) implies that it was not the result of subduction but of deformation within the overriding Australian plate.
New Oceans: Mid-Ocean Ridge Earthquakes
Oceans open and close on cycles of hundreds of millions of years. The convergence of Africa and Eurasia is the final stage of ocean closure (the Mediterranean basin is a remnant ocean) while the separation of Africa and Arabia will, in the very distant future, probably lead to a new ocean; and the Atlantic is a mature ocean, still opening.
The axis of this developing ocean is the mid-Atlantic ridge, which extends from the Arctic right down the south Atlantic. Where the Earth’s crust splits as plates move apart (in this case at around 2.5 cm per year) the inevitable occurs — earthquakes. This week’s USGS map shows this neatly, with tremors from Svalbard in the far north (the largest at M5.4), Iceland, and other tremors further south.
The Atlantic isn’t alone, and the week’s earthquake map shows further ocean ridge earthquakes in the Indian Ocean (between the African and Indian plates); south Pacific (at the junction between Pacific and Antarctic plates and also at the triple junction between the Pacific, Antarctic and Nazca plates); and in the far south west Pacific (another triple junction between the Australian, Antarctic and Pacific plates).
These minor earthquakes are often overlooked because of the dominance of the larger tremors associated with subduction or transform margins; but they illustrate that earthquakes can be generated by creative, as well as destructive, forces.
US Earthquakes: Connecticut
Not all earthquakes are associated with major plate boundaries and many occur on long-buried and often unidentified faults. As the USGS observes: “It is difficult to determine if a known fault is still active and could slip and cause an earthquake.”
This week’s M3.3 in Connecticut, far from the active plate margins, is a classic example Many millions of years ago the area was a convergent plate boundary and is criss-crossed by many deeply-buried and largely unidentified faults. This explains the (possibly unexpected) occurrence of earthquakes in this part of the US.
The largest recorded earthquake in New England had an estimated magnitude of M6.5: to quote the USGS once more: “Moderately damaging earthquakes strike somewhere in the region every few decades, and smaller earthquakes are felt roughly twice a year.”
So perhaps the tremor isn’t as unusual as it first appears.
Earthquakes and Historic Margins
The mid-ocean ridge system is part of a cycle of continental breakup and convergence. The Connecticut earthquake is a reminder that old plate margins never die and that the faults in the crust which result from them remain active even when the major stresses of continental collision and breakup are relieved.