Of the 86 earthquakes of at least magnitude 4 (≥ M4) recorded by the USGS worldwide in the week of 16-22 January, just 16 were of magnitude ≥M5.0. The number of earthquakes occurring in any given period of time varies immensely, but by any standards the week seems to have been unusually quiet.
A Quiet Week for the Earth
To demonstrate this, we have only to glance back over previous weekly digests. These figures are not a statistically-sound database, but they do demonstrate how little earthquake activity there has been this week. Previous weeks have had as many as 42 ≥M5.0 events and the number usually exceeds 20. The lowest number of ≥M5.0 earthquakes recorded previously was just 13 – but that included a major tremor of M7.5 (week of 2-8 January 2013).
So, what of the week’s earthquakes? As is the norm, they follow the boundaries of the earth’s tectonic plates; this time without exception. (An intraplate earthquake can usually be expected somewhere in the world, but not this week!) And it’s also interesting to note that all were shallow – just 12 had a focus deeper than 100km and only one crept in at (fractionally) deeper than 200km.
M5.4 Earthquake in Iran
The northern movement of the Indian and Arabian plates against the Eurasian continent causes earthquakes along a distance of thousands of miles, from the Middle East to Indonesia, and is responsible for the uplift of the Himalayas and the Tibetan plateau. This week it was the movement of the Arabian plate which led to an M5.4 earthquake in Iran as a section of subducting crust (the Makran subduction zone) was forced north between two strike-slip (lateral) faults.
Earthquakes in the USA
Although there were no significant earthquakes in the mainland United States (the largest, apart from an M5.4 in the Aleutian Islands, was an M4.4 in Alaska) it was very much business as usual in California where numerous small earthquakes occurred along the San Andreas fault Zone. (Robert Yeats’ Active Faults of the World includes a map showing 64,000 earthquakes in the state and neighbouring Nevada over just 7 years).
Although the fault, which represents the boundary between the Pacific and North American plates, is capable of generating major seismic events, there’s also a continuous series of small-magnitude earthquakes. The USGS real-time earthquake map for California shows over 700 tremors along the zone in the past seven days, with a particular cluster at the southern end near the Salton Sea and Brawley Fault Zone.
Earthquakes: Quiet Doesn’t Mean Safe
It’s tempting to think that because earthquakes are small there’s no risk to humans. That doesn’t hold true: a relatively small earthquake can kill many more than a larger one (the M7.0 in Haiti in 2010 killed 320,000 people, over 20 times as many as the 15,000 who died in the 2011 Tohoku earthquake and tsunami – though the latter event, at M9.0, released a thousand times more energy). The week may have been quiet seismically, but it’s worth noting that an M5.4 which occurred in Indonesia caused at least one fatality.
United States Geological Survey. Real time earthquake map. (2013). Accessed 22 January 2012.
Yeats, R. Active Faults of the World. Cambridge University Press 2012.
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