The week of the 20-26 November 2014 saw the United States Geological Survey’s real time earthquake map record over 1,600 earthquakes.
The interactive – and constantly updated – map, which shows earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, showed two tremors of ≥M6.0, in Indonesia and Japan, and 27 of ≥M5.0.
The USGS Earthquake map suggests that seismic activity at Iceland’s Holuhraun lava flow may be declining, and it also records a cluster of largely minor aftershocks from last week’s M7.1 tremor in the Celebes Sea.
As usual, the concentration of activity is around the planet’s plate margins, although this week there were several away from the main plate boundaries but associated with the collision of India and Eurasia and the uplift of the Himalayas and the Tibetan plateau.
The Week’s Largest Earthquake: M6.5, Indonesia
For the second week running, the largest earthquake of the week occurred in the Celebes Sea in Indonesia. This time the epicentre was around 100km to the north east of last week’s M7.1, but it is part of the cluster of aftershocks to what now looks to have been the main event in the series of tremors which it initiated.
This week’s tremor is the largest of over 40 aftershocks of ≥M4.0; and there will have been many more which are not shown on the USGS map, indicating that the activity in this crush of plates and microplates is ongoing and will probably continue.
Though smaller than the Indonesian tremor, the second largest earthquake of the week (an M6.2 which struck on the island of Honshu, in Japan) is more newsworthy given the extensive damage it caused. The epicentre was 24km from Nagano and less than 200km from Tokyo; there was intense shaking in the immediate vicinity and weak shaking was felt as far away as Tokyo, Yokohama and Nagoya.
Tectonically, the quake is interesting in that it occurred close to what fault maps show as an inactive major fault; but in fact those same maps show so complex a web of faulting (at this point on the planet four tectonic plates are juxtaposed) that it is possible that it was caused by movement along one of these known active faults.
Though detailed information isn’t available, the most likely candidate appears to be one of several short sections of strike-slip fault align the Itoigawa-Shzuoka Tectonic Line.
Interesting though this tremor might be to the seismologist, it was probably the most damaging tremor of the week, with media images showing collapsed buildings and news reports suggesting almost 40 people injured, several of them seriously.
Given the number of people in the area and the number who felt the tremor (the USGS Pager summary suggests the number would be in excess of 20 million) the amount of damage reported seems incredibly low.
US Earthquakes: Offshore California
Though the earthquake swarms in Nevada and in Oklahoma/Kansas continued, the week’s largest tremor came further north, around 200km off the coast of Oregon, where the Juan de Fuca plate is subducting beneath North America, giving rise to the Cascadia subduction zone.
This week’s earthquake, however, was nothing to worry about. At M4.9 the tremor was small and occurred along one of the complex fracture zones (in this case the Gorda fracture zone) to the west of the plate rather than at the potentially destructive subduction zone to the east.
Earthquakes Don’t Kill People, Buildings Do
Pictures of the damage wrought by Japan’s earthquake proves the old seismologists’ truism – that earthquakes don’t kill people, buildings do.
High levels of earthquake-proof design in Japan may have saved many lives, not just as a result of this earthquake but many others in the past.