Once again it was the Pacific margin which produced the largest earthquakes in the seven days from 3-9 July 2014.
Dominated by southern Mexico’s magnitude 6.9 (M6.9) tremor, which killed three people, the week also saw tremors of at least M6.0 recorded on the United States Geological Survey’s real time earthquake map — in Vanuatu, Papua New Guinea and Raoul Island, New Zealand.
All of the latter three were along the margin between the Pacific and Australian tectonic plates.
Although the distribution of earthquakes on the map was as expected, there’s an interesting footnote.
The total number (of all magnitudes) was almost 2100 compared with a usual figure (broadly speaking) of between 1400-1600. Whether this actually represents an unusually high figure of small earthquakes or (bearing in mind that he USGS map doesn’t record every tremor and very many go unreported) a change in the way they are reported remains to be seen.
The Week’s Biggest Earthquake: M6.9 Mexico
Initially reported as M7.1 but later downgraded to M6.9, the tremor which struck in southern Mexico on 7 July provides a sobering reminder of the power of earthquakes. Although not particularly large in the local context, it caused strong shaking locally and killed at least three people in Guatemala, as well as causing significant damage and power outages.
The evidence initially suggested that the event was a classic subduction zone earthquake caused by movement at the interface between the South American plate and the Cocos plate (which is descending beneath it). Updated information from the USGS, however, locates the earthquake below the plate interface. This suggests that it “is consistent with extension within the down-going Cocos slab.”
Earthquakes at the Indo-Eurasian Collision Zone
The Himalayan region, where the Indo-Australian plate is in the process of colliding with Eurasia at a rate of 36-40mm per year, is a region of frequent moderate and occasionally large earthquakes.
Unlike subduction zones, conservative margins or mid-ocean ridges, the zone of seismic activity and associated hazard in areas of continental collision is diffuse, a reflection the extent to which stresses build up in the crust as a result of convergence.
This week saw four tremors of M4.6-M5.0 in China and Pakistan, over an stretch of this margin around 500km long and 200km wide.
Although it’s unlikely that these are directly related to one another, they do give an indication of the strength and extent of the compressional forces which currently contribute to the uplift of the Himalayas.
US Earthquakes: California and Arizona
California topped the US earthquakes list this week, in the lower 48 at least, with an M4.6 in the San Bernadino Mountains. Also worthy of note is the series of aftershocks following the M5.2 tremor which struck close to the Arizona-New Mexico border on 28 June.
Both of these are related to the forces within the basin and range province of the western US and the uplift of the Rocky Mountains.
Large Earthquakes and Subduction Zones
Although most large earthquakes occur at subduction zones, areas of continental collision and extension are both capable of causing significant tremors over a wide area.
This week’s earthquakes showed evidence of both.