There’s plenty of seismic activity to talk about this week, with three earthquakes of at least magnitude 6 (≥M6.0) and thirty of ≥M5.0 around the planet.
Again, the Pacific rim was the main focus of activity, with 26 of those registering ≥M5.0, including all three of the largest tremors, in this region.
The United States Geological survey’s real-time earthquake map shows a group of smaller magnitudes concentrated on the continental United States; but this is a reflection of the earthquakes selected, which include worldwide tremors only of at least M4.0 but of all magnitudes in the US and its territories.
The Week’s Largest Earthquake: M6.5 Ryukyu Islands
The largest tremor of the week occurred along the seismically-active western margin of the Pacific Ocean. The M6.5 event occurred on 3 March in the Ryukyu Islands between Taiwan and Japan. Here, the oceanic crust of the Philippine Sea plate subducts beneath the Eurasian plate, creating a chain of volcanic islands.
There’s no information available on the nature of faulting for this particular earthquake. Looking at the location (the epicentre is some way to the west of to the subduction zone in the overriding plate) and the depth (111km), however, suggests that movement at the plate interface may have been responsible.
That said, Robert Yeats remarks on the importance of normal faulting in this region and, bearing this in mind, this cannot be ruled out as a fault mechanism.
Historically, this stretch of crust isn’t known for particularly large earthquakes, with only one of M8 recorded (in 1911).
South American Earthquakes: Chile
As is the case in Japan, Chile’s earthquakes are largely driven by subduction; the country’s Pacific seaboard has seen the largest earthquake ever recorded – an M9.6 in 1960. In total, the country has seen eight tremors of ≥M8.0 since 1900.
This week’s earthquakes came nowhere near some of these historical giant in scale but the series of medium-sized ((M4.8-M5.7) tremors which struck close to the capital Valparaiso this week are a reminder of the country’s vulnerability to earth movements.
In fact the tremors, all of which had epicentres in the over-riding plate, were relatively shallow, leaving open the question of whether they were caused by movement along the plate interface or (as seems marginally more likely) were the result of faulting within the plate itself.
US Earthquakes: California
The nature of seismic activity in California is driven by lateral movement, with the Pacific and North American plates sliding past one another.
Although the plate boundary has a relatively narrow surface expression (the San Andreas Fault Zone) the deformation associated with it extends to great distances either side.
This week’s M5.0 off the California coast appears to an example of that. Its location, in the Pacific Ocean, lies at what appears to be the edge of the area of deformation.
Earthquakes are largely confined to the planet’s plate boundaries – but not exclusively so. The continents have broken apart and re-formed many times during the Earth’s history, resulting in ancient and deeply-buried faults which can occasionally be reactivated. This may be the explanation for an isolated earthquake of M4.6 in the Indian Ocean to the west of Australia, far from any active margins. Such tremors demonstrate that nowhere in the planet can be truly immune from all seismic activity.