This week (10-16 September 2015), after a few weeks of up and down activity (mostly down), the planet’s earthquake count seemed more or less back to what we might consider normal.
The United States Geological Survey’s real time earthquake map, which records tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, included a total of 1575 tremors, of which 86 were ≥M4.5, 27 were ≥M5.0 and two ≥M6.0.
The distribution of the earthquakes also fits the expected pattern, with just over 80% of those recorded as ≥M4.5 showing up in and around (largely around) the Pacific Ocean.
Again as we might expect, these included the largest tremors, which occurred on either side of the ocean, in Mexico and Indonesia.
The Week’s Largest Earthquake: M6.6, Mexico
This week’s largest earthquake occurred in the Gulf of California and had a magnitude of 6.6. On the face of it, the tectonic setting is relatively simple, and the USGS sums it up neatly: “The area west of the Gulf of California, including Mexico’s Baja California Peninsula, is moving northwestward with the Pacific plate at about 50 mm per year. Here, the Pacific and North American plates grind past each other creating strike-slip faulting, the southern extension of California’s San Andreas fault. ”
The plate boundary at this stage is ‘stepped’, with short sections of spreading centre (constructive margin) offset by transform faulting. The past half century has seen 28 earthquakes of at least M6.0 along this length of plate boundary and this week’s earthquake, which was followed by a number of aftershocks, is among the largest of them.
Seismologist Robert Yeats, adds an interesting footnote, remarking that in fact the development of this plate margin (which, he reports, ‘jumped’ from the west to the east of the Baja California peninsula some 4-5m years ago) actually initiated the activity of the San Andreas Fault.
M6.3 Earthquake in Indonesia
We’re back in the western Pacific again, where barely a week goes by without some significant earthquake activity. It only takes a quick glance at a tectonic map to show why — the whole of the region is a jumble of congested plates and microplates, major (and conflicting) subduction zones and the spaces between them complicated by the existence of minor spreading centres.
The USGS earthquake map, from which the attached map is taken, shows a simplified tectonic setting.
More detailed maps show a number of complicated and contrasting fault motions and Yeats draws attention to the (buried) Molucca Sea plate.
Covered by a thick layer of sediment, this plate doesn’t appear on maps but is subducting to both east and west. The available evidence suggests that the week’s second largest earthquake is associated with the subduction of this plate.
US Earthquakes: Nevada
Nevada has been shaking for some time: Since the beginning of the year, the area around this latest tremor has seen over 200 earthquakes of at least M2.5.
This week’s M4.7 can be viewed in two ways — either as yet another in an ongoing series, or as an unusually large tremor, the latest in the series.
It isn’t clear what’s happening in this remote corner of Nevada — but it’s well worth keeping an eye on.
Last Words: Unusual Tectonic Settings
This week we have earthquakes occurring at a plate boundary which has ‘jumped’ in relatively recent times from one place to another; in association with a tectonic plate which occurs on no surface maps; and as the latest in an cluster of earthquakes, their origin the subject of ongoing study, in Nevada. Most tectonic maps show three simple types of plate boundaries and major plates only, because these explain most of our major earthquakes; but this week is a reminder that there’s a lot more to plate tectonics than appears on the surface.