This week, 24-30 October 2013, saw a relative proliferation of moderate-sized earthquakes (at least magnitude 5, ≥M5.0) recorded on the United States Geological Survey’s real-time earthquake map, with a relatively lower number of smaller and overall tremors. In total there were just over 1300 recorded earthquakes of all magnitudes in the US and its territories and ≥M4.0 elsewhere. Of these, 33 were ≥M5.0, with the usual concentration around the western Pacific.
The Week’s Biggest Earthquake: M7.1, Honshu, Japan
The largest tremor of the week, and the only one to exceed M7, was the M7.1 which struck off the west coast of Japan. This quake occurred in the same setting as the devastating tremor and subsequent tsunami of March 2011, which cost an estimated 15,000 lives and caused on-going damage at the Fukushima nuclear plant.
The most recent tremor in this area differed from the 2011 event, not just in terms of size (at just under two orders of magnitude smaller it released only a tiny fraction of the amount of energy) but in its location. Whereas the 2011 quake ruptured hundreds of kilometres along the Japan Trench, the epicentre of the 2013 tremor was in to the east of the trench and although it generated a tsunami the effects were minor and localised.
M6.2, Central Chile
Like the east coast of Japan, a major and highly active subduction zone (the Nazca plate is descending beneath the South American plate) marks the west coast of South America. This area is capable of producing some of the planet’s largest earthquakes, and is responsible for the uplift of the Andes.
This week’s M6.2 tremor occurred at a depth of 18.3km: The epicentre was onshore. Together, these facts suggest that the earthquake resulted from faulting within the over-riding plate rather than rupturing at the interface between the two.
Although the USGS Pager impact estimates suggest that there was strong to very strong shaking in the immediate area, and that there is the likelihood of significant economic loss, there was no reported damage at the time of writing.
In the wider picture, the earthquake is relatively minor as a map of seismicity in the region over the past decade shows. As recently as 2010, an earthquake of M8.8 struck on roughly the same section of the fault some hundred kilometres or so to the south.
U.S. Earthquakes: The Oklahoma Earthquake Swarm
Earthquakes – of the minor variety – are by no means unknown even in the stable continental interior of the US, where movement along ancient faults can occur through natural or human-induced processes. This week, central Oklahoma experienced two earthquakes of ≥M2.5 – bringing the total for the past 30 days to 23.
The number of small earthquakes in the region has attracted attention. “Studies show one to three magnitude 3.0 earthquakes or larger occurred yearly from 1975 to 2008, while the average grew to around 40 earthquakes per year from 2009 to mid-2013,” notes the USGS in a press release. The government agency is currently conducting monitoring to try and determine whether there is any association with human activity such as groundwater disposal.
Major and Minor Quakes
Both major and minor earthquakes made the news this week, demonstrating that we determine the newsworthiness of a seismic event not just by its scale, but by its magnitude relative to the typical size of a tremor in that setting, and by the damage which it causes.
United States Geological Survey. M7.1 – Off the east coast of Honshu, Japan. (2013). Accessed October 30, 2013.
USGS Press release. Earthquake Swarm Continues in Central Oklahoma. (2013). Accessed October 30, 2013.
USGS. Real time earthquake map. (2013). Accessed October 30, 2013
Koontz, Heidi. Press Release: Earthquake Swarm Continues in Central Oklahoma. (2013). USGS with OKGS. Accessed October 30, 2013.
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