With the United States Geological Survey’s real time map showing 42 tremors of at least magnitude 5 (≥M5.0) it appears at first glance that the week of 29 May-4 June 2014 displayed high levels of seismic activity worldwide.
A closer look at the map, which includes all tremors in the US and its territories and those of ≥M4.0 worldwide, shows that the headline figure is a little misleading.
Just five tremors were larger than M5.5 and the largest was M6.2.
The distribution of these earthquakes, if not their magnitude, was broadly as expected.
All except two (one in the Caribbean and one in Greece) were along the margins of the Pacific Ocean and the Sunda plate in Sumatra.
Distribution of the smaller tremors (249 ≥M2.5) shows one or two slightly unusual events, including minor tremors in Slovenia, the Czech Republic and in Colorado.
The Week’s Largest Earthquake: M6.2, Mexico
In a quiet week, then, even the largest earthquake is something of a damp squib. Most Mexican earthquakes are associated with the Middle America subduction zone, where the Cocos plate descends beneath Central America, but the M6.2 which struck on 31 May is different. It occurred some way offshore, at a point where the northern part of the Cocos plate narrows close to the junction with the Pacific and North American plates (a segment describes by some seismologists as the Rivera microplate).
The earthquake occurred at the western edge of the plate, which is a constructive margin, where plates move apart and new crust is created at the East Pacific Rise. This margin is periodically offset by areas of short strike-slip faults which accommodate different directions of movement.
The location of the earthquake, and its shallow depth (just 10km) suggest that it is likely to be associated with faulting at one of these fracture zones.
M5.8, Dominican Republic
If the largest earthquake of the week was nothing unusual, the M5.8 which struck near the Dominican Republic is worthy of closer inspection. Tectonically, the northern margin of the Caribbean plate is complex, with a subduction zone marking its northern boundary and a secondary subduction zone south of Puerto Rico.
Between the islands of Puerto Rico and Hispaniola are areas of rifting (sections of crust moving apart) and there are also significant segments of lateral movement.
Against this background, it’s hardly surprising that the region experiences dozens of minor tremors each week as a result of local crustal upheavals; it also occasionally experiences larger and more deadly ones (such as the 2012 Haiti earthquake, further to the west).
This week’s M5.8 appears to be the result of movement along the Yuma rift to the south of the Dominican Republic. Interestingly, the north part of this rift systems, the Mona Rift, was the location of a large (M7.3) tremor in 1918.
US Earthquakes: California
The M4.2 tremor which shook the northern suburbs of Los Angeles on 2 June occurred on one of the many faults which mark the (geologically!) recent union of California with the rest of the United States. The San Andreas Fault Zone marks the boundary between the Pacific and North American plates and LA sits upon the latter. The zone is marked by a complex system of mostly strike-slip faults and this week’s tremor occurred close to the junction between two of these minor faults, the Hollywood and Malibu Coast faults.
Earth Processes: Never So Simple
Seismologists find it convenient to simplify the Earth’s tectonic plates – both in large units and in generalisation as to the nature of the boundary. All three of this week’s featured earthquakes demonstrate that, while the simplistic approach is necessary, looking more closely shows that even a boundary broadly defined as being of one type includes great variation – and is not always what it first appears.