The last few weeks have been a little like living in an earthquake statistician’s version of Goldilocks and the Three Bears (not, I have to say, a particularly heavily-populated place).
If I’d been trying to find a happy medium, settling at or near the average, I’d have been frustrated. First there were too many, then there were too few. But this week turned out to be, if not just right, then normal. (Whatever normal is; but perhaps that’s philosophy, not statistics.)
So, what appeared on the United States Geological Survey’s real time earthquake map this week? The map, which, broadly speaking, includes all tremors in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, showed pretty much what we’d expect in a typical week.
There were a couple of events larger than M6.0 (both in Ecuador); 19 of ≥M5.0; 106 of ≥M4.0; and around 1720 of all magnitudes.
The Week’s Biggest Earthquake: M6.8, Ecuador
It’s not that long — about four weeks — since I was discussing an unusual number of large earthquakes. One of them, an M7.8 and a very damaging one, occurred off the coast of Ecuador in mid-April, the product of subduction of the Nazca tectonic plate beneath South America along the Ecuador Trench.
Large subduction zone earthquakes usually generate aftershocks, and this week’s largest earthquake (and another not much smaller) fit the bill exactly — same area, same mechanism, roughly the same depth, but smaller.
There have been others. “Since that mainshock, the region has experienced 5 aftershocks of M 6 or larger, including a M 6.7 earthquake which occurred 9 hours prior to this M 6.8 event on May 18,” notes the USGS.
The April earthquake caused extensive damage and significant loss of life. This week’s pair of large aftershocks, though large, are an order of magnitude smaller and caused little more than a temporary loss of power — a blessing for an area in recovery from an earlier trauma.
Earthquakes Along the Sunda Trench
The western Pacific/eastern Indian Ocean is a congested area, tectonically. The Pacific plate, subjecting from the west, and the Indo-Australian plate, from the east, provide the to jaws of a vice in which many crustal slivers are caught.
The western boundary, where subduction occurs beneath the Sunda microplate, forms a classic major subduction zone that runs from west of Burma to north of Australia — thousands of kilometres in which strain accumulates and is released, sometimes in very large earthquakes indeed.
This week the earthquakes were small, at M5.3 and M5.1, but they’re a reminder that this subduction zone is very much active. And, in case you need reminding, it was the source of one of the largest and most deadly earthquakes of this century — the M9.1 Boxing Day earthquake of 2004.
US Earthquakes: Tsunami Risk to Hawaii
Hawaii, marooned in the middle of the Pacific, is peculiarly vulnerable to tsunamis generated by major earthquakes occurring at the long subduction zones around the Pacific margin. These earthquakes are relatively regular occurrences, though not always tsunamigenic; but Hawaii can suffer when a major tsunami strikes — most notably in 1946, when a tsunami generated off Alaska killed over 150 people in Hilo.
If it’s happened once, it can happen again. This week a new study attempted to assess the chances. “There’s a 9 percent chance of Hawaii suffering a direct hit from such a mega-tsunami in the next 50 years… In other words, rare but possible.”
Last Thoughts: Distant Impacts
There’s a theme to this week’s featured events — major subduction zone earthquakes and their effects. Subduction, by definition, is submarine; and significant vertical displacement of water by a subduction zone earthquake can cause a major tsunami.
Not all megathrust earthquakes generate tsunamis and not all tsunamis are large and damaging. But they’re a reminder that the effects of an earthquake can be felt well beyond the area in which they occur.