Things quietened down a little in the aftermath of the M8.3 Chilean earthquake of 16 September.
The United States Geological Survey’s real time earthquake map, which records all tremors in the US and its territories and those of at least M4 (≥M4.0) elsewhere, showed a total of 1520 earthquakes for the week 1-7 October – and of these all these earthquakes, not one reached M6.0.
The aftershocks of the Chilean earthquake continued to contribute to the number and included the two largest tremors of the week.
The 22 tremors of ≥M4.0 show no surprises in terms of location. All of the earthquakes are located at or near major tectonic boundaries where significant earthquakes are the norm, particularly in the western Pacific (and, more specifically, the northern sector of the Tonga Trench and the boundary between the Eurasian and the Australian plate south of Indonesia.
The Week’s Biggest Earthquake: M5.9, Chile
And so it goes on. Three weeks after Chile’s M8.3 earthquake of 16 September, registered as the largest earthquake of the year so far, an earthquake in the area of the rupture once again appears as the largest of the week, with an M5.9 on 5 October.
Just as a reminder: the M8.3 mainshock, occurred as a result of subduction of the eastwards-moving Nazca plate beneath the South American continent. Despite the scale of the tremor, and an associated tsunami, only a few people were killed and injured and evacuation procedures seem certain to have saved many thousands of lives.
Large earthquakes rarely occur alone, and September’s was followed by an impressive and continuing series of aftershocks, several of which we would normally consider major events in their own right — one of M7.0 and a further dozen of ≥M6.0.
This week the Chilean aftershock sequence contributed eight of the total of 22 tremors ≥M5.0 and 38 of the 11 which registered ≥M4.0. We can expect to see more earthquakes featured along this area of coast in future weeks.
Aftershock sequences can be, and have been, statistically modelled through Omori’s law which provides “an appropriate representation of the temporal variation in aftershock activity.” The statistical process is, however, complicated.
Suffice it to say that we can expect the number of earthquakes to decline in magnitude and frequency in days to come.
M4.7 and M4.6 Earthquakes, off Madagascar
In most weeks, the USGS map shows some anomalies among the smaller earthquakes and this week the focus of attention is two intermediate-sized earthquakes with epicentres in the Mozambique Channel, between Africa and Madagascar. This area is located far from the main boundaries of the African plate, although less far (a mere 1000km or so!) from the East African rift system.
Tectonically speaking, the Mozambique channel bears some similarities to the East African Rift, though it is not part of it.
According to Annelise Miske, “Madagascar was once a part of the supercontinent Gondwana but between 160-117 Ma, it began separating — rifting southward over 1000 km away from the Africa plate. This displacement occurred along the currently inactive Davie Ridge transform fault off the west coast of Madagascar running through the middle of the Mozambique Channel”.
This fault seems likely to be the source of this week’s two earthquakes. Although Miske refers to the fault as “currently inactive,” she does go on to remark that Madagascar “is still tectonically, seismically and volcanically active.”
As an interesting footnote, Robert Yeats observes that “Seismicity in the channel between Africa and Madagascar may indicate a plate boundary,” raising the possibility that the earthquakes are the result of extensional tectonics as Madagascar continues the long process of separation from the African plate..
US Earthquakes: Offshore Oregon
The US was pretty quiet this week, with the largest tremor in the contiguous states being just M3.9 and with an epicentre some 400m off the coast of Oregon.
Unlike Madagascar, we can say with some certainty that this tremor occurred along an active fracture zone, the Blanco Fracture Zone, which marks the trailing edge of the Juan de Fuca plate, whose leading edge is the Cascadia subduction zone.
Last thoughts: Major, Minor and Unknown Plates
The Madagascar earthquakes draw our attention to the complexity of plate tectonics. For simplicity’s sake, most tectonic maps include only the major plates (and even for these, some of the boundaries are poorly understood).
Seismologists define many microplates or provinces and it’s probable that others may exist and remain undefined. As plates split and reform over a process lasting millions of years, it isn’t always clear exactly where their boundaries lie.