With just two earthquakes of at least magnitude 6 (≥M6.0) recorded on the United States Geological Survey’s real time earthquake map, it wasn’t a particularly busy week, seismologically speaking.
In the week of 2-8 October the map, which shows all tremors in the US and its territories and those of ≥M4.0 elsewhere, showed a total of 1441 tremors, of which 135 were ≥M4.0 and 30 ≥M5.0.
As usual most of the the tremors were along the margins of the Earth’s tectonic plates.
Apart from the cluster of earthquakes associated with the continuing volcanic eruption in Iceland (it produced 13 tremors of ≥M4.5) there was no significant centre of earthquake activity this week.
The Week’s Biggest Earthquake: M6.2, Southern Gulf of California
The week’s largest earthquake was an M6.2 which struck in the southern part of the Gulf of Mexico in the early hours of the 8 October, local time.
Despite its size, and although initial data from the USGS suggest that areas on both eastern and western coasts of the Gulf experienced minor shaking, at the time of writing there were no reports of any damage or injuries.
The central axis of the Gulf of Mexico is marked by the boundary between the Earth’s Pacific and North American tectonic plates. The southern extension of the San Andreas fault zone, the boundary marks an interesting phase of transition between the SAFZ, which is a notorious strike slip fault, and the constructive boundary of the East Pacific Rise (EPR).
According to seismologist Robert Yeats, the plate margin “alternates between spreading centres and right-lateral transform faults.” The location of this week’s tremor, along with its shallow depth, suggest that it may have been caused by faulting at the Tamayo Fracture zone, close to the northern end of the EPR.
M5.2, North Island New Zealand
The week saw a minor earthquake, of M5.2, strike in the North Island of New Zealand. Tectonically, this area lies to the west of the southern end of the Kermadec Trench, where the Pacific plate subducts beneath the Australian plate. Such subduction zones are commonly the source of seismic activity.
In the context of subduction zones, this week’s earthquake is small; but its location, close to several of the country’s active volcanoes, raises the question of whether is is of volcanic origin.
The available data suggest, however, that it isn’t.
Although the epicentre of the earthquake occurred along the line of volcanoes associated with this subduction process, it actually occurred at a depth of around 118km.
Given the angle of dip associated with subduction, this implies that it was a result of deformation at or near the interface between the two plates.
US Earthquakes: Now It’s Kansas
The largest tremor to occur in the southern United States this week was, just for a change, not in Oklahoma.
It might as well have been. The M4.3 (and four others >M2.5) which struck close to Harper, Kansas, is close to the state border. Earthquakes are no respecters of lines drawn on maps and the probability is that the Kansas activity (which is not new; the area has sen several smaller events in the past month) is a northwards expression of the Oklahoma swarm.
Last Words: Some Hidden Dangers
At Decoded Science we sometimes end up scratching our heads over why a particular earthquake occurs in a place where there are no mapped faults. The answer, of course, is that not all faults are visible.
Most major faults are, however, at least known, if not mapped in detail. It was a little worrying, therefore, to read reports (subject to confirmation) about a large and previously unidentified fault beneath New Zealand’s capital, Wellington – something which just indicates the limits of human knowledge about the detailed workings of our planet.