The variation in the Earth’s seismic activity is again evident as, for a second consecutive week, the seven days from 12-18 December showed relatively little in the way of larger-magnitude earthquakes.
Of the 1397 tremors recorded on the United States Geological Survey’s real time earthquake map, just one had a magnitude of at least 6 (≥M6.0). Unlike the previous week, however, all but two of the larger tremors (≥M4.5) were confined to major plate boundaries and the two outliers were both associated with continental collision and uplift.
The Week’s Largest Earthquake: M6.2, Marianas Islands
The week’s largest tremor was an M6.2, which occurred in the tectonically-congested Western Pacific – an area which so often the location of significant tremors. This week’s quake was one of a cluster of five with magnitudes exceeding M4.5, at depths of 10-50km, which occurred west of the Marianas Trench where the Pacific plate subducts beneath the Philippine Sea plate.
Although the earthquakes themselves are unremarkable (occurring in a remote area, not of unusual magnitude, and fairly typical of a subduction zone) their tectonic setting is interesting on a number of counts, not least that subduction occurs between two plates comprising oceanic crust. The Philippine plate is, as the USGS notes, “unusual in that its borders are nearly all zones of plate convergence” and convergence at its eastern margin, where this week’s tremors occurred, is both rapid and deep. The associated Marianas Trench encompasses the deepest point on the Earth’s surface (over 11,000 metres deep).
M5.6 Quake, South Island New Zealand
This week’s M5.6 earthquake just off the South Island of New Zealand is unusual but not unprecedented. New Zealand as a whole is geologically dominated by the boundary between the Pacific and Australian plate, along which it lies. The nature of the boundary changes from subduction (off the east coast of the North Island) before changing into a transform boundary as it passes through the South Island – providing distinctive tectonic settings in both.
Although detailed information isn’t available, the location of the tremor suggests that it’s associated with the southward extension of this boundary (the Alpine Fault). Past tremors in this area have resulted from both thrust and strike slip faulting. Reports from New Zealand’s GeoNet agency suggest that it was felt across much of the South Island.
The short recorded history of New Zealand includes several significant earthquakes in both South and North Islands – including the damaging M6.3 Christchurch earthquake of 2011 and an M7.8 which occurred in the south of the country in 2009.
U.S. Earthquakes: M4.1 Forest Hills
The largest earthquake on the contiguous United States occurred east of the San Andreas Fault Zone at Lost Hills, California, and had a magnitude of M4.1.
Earthquakes are common throughout much of California but the epicentre, which was located in the southern part of the San Joaquin Valley, is not located on any known surface expression of the fault zone.
Detailed information on the fault mechanism is unavailable but the possibility exists that it may be associated with local faulting rather than movement along the main Pacific-North American plate boundary.
Earthquakes and Transform Boundaries
Most major earthquakes are associated with subduction zones. The occurrence of significant earthquakes this week in the conservative setting (tectonically speaking) of South Island New Zealand and California is a reminder that other tectonic settings are capable of generating major tremors.
GNS Science. New Zealand Earthquake Report Magnitude 6.2, Tuesday, December 17 2013 at 1:07:26 am (NZDT). (2013). Accessed December 18, 2013.
USGS. M6.2 – 195km E of Farallon de Pajaros, Northern Mariana Islands. (2013). Accessed December 18, 2013.
USGS. Real time earthquake map. (2013). Accessed December 18, 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.