An upsurge in seismic activity compared to the first couple of months of 2014 saw three tremors of at least magnitude 6 (≥M6.0) occurring this week, in the Pacific off California and off Papua New Guinea and in the South Atlantic to the east of the South Sandwich Islands.
These topped the list, but there were also 23 tremors of between M5.0 and M5.9 in a total of 116 which exceeded M4.0 world-wide.
The USGS interactive earthquake map shows tremors of these magnitudes worldwide while smaller ones (in the US and its territories only) brought the total for the week to 1,365.
The Week’s Largest Earthquake: M6.8, California
If it’s big in the United States, it has to be in Texas, right? Not for earthquakes. This week the largest tremor worldwide occurred just off California, 77km from Ferndale. The tremor occurred close to the Mendocino Triple Junction between the Pacific, North American and Juan de Fuca plates. The epicentre of the ‘quake was within the over-riding plate near to the point at which the nature of the tectonic margin changes from a conservative bounder to a destructive one and the dominant nature of devastating earthquakes also changes from strike-slip to subduction.
The USGS revised the preliminary magnitude of M6.9 down to M6.8 and also issued some information on the causes of the tremor. This confirms the indication of depth and location; the tremor is the result of faulting within the over-riding plate. A cluster of minor aftershocks followed, with 46 further tremors of at least M2.5 appearing on the USGS map.
M6.4 Earthquake, East of the South Sandwich Islands
The remote South Atlantic is the location of one of only two subduction zones associated with the Atlantic Ocean (the other is west of the Caribbean). Here, the Scotia microplate is caught between the Antarctic and South American plates; the volcanic arc of the South Sandwich island results from subduction of the latter.
This week’s M6.4 tremor, however, occurred not on the subduction zone but further east. The boundary here is not well studied and is likely to be either a constructive boundary or a fracture zone (where the central spreading boundary is offset).
Earthquakes in the US: Oklahoma
If Texas can’t compete with California in the earthquake stakes (Texans probably don’t mind that much) it can’t match Oklahoma either. The many minor earthquakes which have afflicted the Sooner state over the past months, and their potential association with human activities such as wastewater disposal, are already giving rise to concern (the USGS is monitoring the situation).
This week, the numbers increased slightly, with 32 small tremors in the north and centre of the state – and even creeping northwards into Kansas.
Large, Non-Tsunamigenic Offshore Earthquakes
This week the two largest earthquakes occurred offshore – but didn’t generate tsunamis. Part of the reason may be that they weren’t quite large enough (as a very broad rule, an earthquake needs to be at least M7.0 before a tsunami occurs) but it’s also likely that neither had the other key requirement – a significant amount of vertical movement. It’s this upward movement which displaces water and causes the tsunami – this is the reason most destructive tsunamis are associated with subduction zones. Although California earthquake was close to the Cascadia subduction zone, there was no significant vertical displacement.