Earthquakes made the news in the week of 28 March-3 April. A series of earthquakes in Chile, dominated by a major tremor of magnitude 8.2 (M8.2) killed remarkably few people; while a large (for its location) ‘quake of M5.1 in the outskirts of Los Angeles provided a warning of what might be to come.
The numbers reflect heightened activity. According to the United States Geological Survey’s real time earthquake map, one earthquake of at least M8.0 (≥M8.0); one of ≥M7.0; six of ≥M6.0; 54 of ≥M5.0 and 163 of ≥M4.0 shook the planet this week.
The majority of the larger ones and almost half of the total are associated with the largest tremor of the year so far, off Iquique in Chile.
The Week’s Five Largest Earthquakes – and More: Iquique, Chile
Although the tectonic setting of the Iquique earthquake of April Fool’s Day has been covered in detail on Decoded Science and elsewhere, the sheer number of aftershocks merits further investigation.
The calm of the so-called seismic gap (the USGS notes that the previous large earthquake in the region was in 1877) was shattered by an earthquake of M4.7 on 15 March, with an M6.7 the following day. This second tremor, worthy of note in itself, was followed by a sequence of (broadly) decreasing aftershocks until what now appears to be the mainshock of the sequence, the 1 April M8.2.
More aftershocks led up to an M6.5 and an M7.6 within the space of an hour on 3 April and the aftershock sequence continues: at the time of writing it included 139 tremors of a least M4.0, and that number is expected to increase.
Given the scale of the earthquake and the aftershocks, several of which are themselves significant seismic events, it’s astonishing that so few lives were lost.
A tsunami reached heights of 2m in places but jut six people have so far been reported killed, although many thousands have left their homes and the authorities have declared part of the coastal zone a disaster area.
Intervening in the Iquique sequence, 2 April saw an M6.0 in Panama. This occurred at the junction of the northern margin of the plate boundary between the Nazca and South American plates (where the Iquique earthquakes took place) and that between the Cocos and Caribbean plates.
The tremor raises the perennial question about whether a large earthquake in one location can trigger another elsewhere. Although some seismologists argue that earthquakes can shift stress from one area of a fault to another, University of San Diego’s Frank Vernon told an online Q&A session that “There is no known mechanism that can cause one large earthquake to remotely trigger another large earthquake in a different part of the world.”
With four plates in action, the nature of the boundary where the Panama tremor occurred is poorly defined and there’s little available information. It seems, however, that the most likely cause is intraplate faulting and that there is no association with the Iquique sequence.
Earthquakes in the US: California and Wyoming
The US did its best in seismic terms. The suburbs of LA received a sharp reminder of California’s vulnerability to a major tremor with an M5.1 on a thrust fault associated with the San Andreas zone; and Wyoming, not to be outdone, produced its largest tremor since 1980 with an earthquake of M4.8.
Unlike the majority of earthquakes, the one at Yellowstone is associated not with plate movement, but with a magmatic ‘hot spot’, in which expansion and contraction of a subterranean magma chamber causes rock to fracture.
There’s a trend developing in popular science. Hurricanes have been known by name for years; and more recently, the Weather Channel has taken to naming winter storms.
Probably because of their instant nature, earthquakes tend to be known by their date or their location. Major sequences such as that of Iquique perhaps deserve more. Suggestions are welcome – should we begin to name future major earthquakes and their aftershocks?