It’s still pretty quiet out there, seismologically speaking, but as I keep saying, quiet (in terms of numbers of large earthquakes) doesn’t necessarily mean dull. There were no really large earthquakes in the week of 2-8 July and the largest of the four which reached at least magnitude 6 (≥M6.0) was just M6.4.
The total of tremors recorded on the United States Geological Survey’s real time earthquake map, which shows all earthquakes on the US and its territories and those of ≥M4.0 elsewhere, was on the low side at around 1300. There was still plenty going on, however, with 21 tremors ≥M5.0 and 102 ≥M4.0.
And of course the earthquakes were the usual suspects, location-wise — mostly associated with movement at the boundaries of the Earth’s tectonic plates.
The Week’s Biggest Earthquake: M6.4, China
China has a bit of a bad reputation for earthquake fatalities. The collision between the Indian and Eurasian plate has uplifted a vast area across much of central Asia which is deeply faulted, still compressing, and highly vulnerable to earthquakes (the recent Nepal earthquake was one).
Much of this uplifted area is in China and a combination of steep slopes, traditional (or unregulated) building methods and sometimes dense population has meant that earthquakes can often produce very high death tolls.
Indeed, the USGS list of the deadliest earthquakes since 1900 includes three in China, with a combined death toll in excess of 530,000.
Further back in time, an earthquake in 1556 killed a staggering 830,000 people in Shaanxi province.
With this background, even a small earthquake can cause a lot of damage and this week’s M6.4 must have given cause for concern. The USGS pager report, which produces probable damage statistics, notes that “Overall, the population in this region resides in structures that are highly vulnerable to earthquake shaking, though some resistant structures exist” and suggested a 44% chance of up to 100 fatalities.
In the event, the latest reports (the tremor occurred on July 3) suggest far fewer deaths than might have occurred, with CNN reporting three fatalities. This time, at least, China seem to have been lucky.
M4.5 Earthquake, Iceland
About this time last year we (well, some of us) were beginning to get excited about an earthquake swarm in Iceland. That swarm eventually blossomed into the stunning volcanic eruption of Holuhraun. And although history (or in this case, geology) may not repeat itself… there’s always a chance that it might.
This week the USGS map showed a tremor of M4.5 on the Reykjanes peninsula in southwest Iceland, the largest of an ongoing seismic swarm. Experts raised the volcanic alert level at Eldey volcano to yellow. Like Holuhraun, the area overlies the rift between North America and Europe and the plates move apart with both earthquake and volcanic activity taking place on a regular basis.
That brief flurry raised the spectre of another eruption, but it appears that it is not to be. “Earthquake swarms are common on the Reykjanes ridge and the recent swarm was strong by magnitudes and number of events,” noted the Iceland Met Office, before going on to dampen any excitement.
“The region will continue to be monitored closely as increased seismic activity compared to monthly means can be expected for some more weeks following the strongest main shocks of this series.”
US Earthquakes: Oregon
Many aficionados of The Simpsons (of whom I am not one) will tell you that Matt Groening named Springfield in the TV series after Springfield, Oregon. Whether that’s true or not I can’t say; but this week, the town was the location of the largest earthquake in the Lower 48 (yes, even larger than anything in Oklahoma).
At M4.2 it wasn’t big, and at under 10km it was shallow. There’s no detailed available information but from maps it seems most likely to be the product of crustal deformation resulting from the subduction of the Juan de Fuca oceanic plate beneath the North American continent.
Rockin’ All over the World
Most large earthquakes are at subduction zones, where one plate descends beneath another. The Oregon tremor may or may not fit that bill, but it’s worth noting that the other two tremors most definitely did not. In Iceland, crust is being created rather than destroyed and in south central China the collision of continents means uplift rather than either creation or destruction of land.
Wherever you may be, if you’re near a plate boundary — or even, where those boundaries are diffuse, not very near — you may be at risk from an earthquake.