Japan, Greece and Alaska: Earthquakes 12-18 November 2015


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The map shows earthquakes of at least M4.5 from 12-18 November 2015

The map shows earthquakes of at least M4.5 from 12-18 November 2015. Image by USGS.

In the week of 12-18 November, the number of earthquakes recorded on the United States Geological Survey’s real time earthquake map included nothing exceptional in terms of either magnitude or number of tremors.

The map, which includes earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, showed a total of over 1250 tremors, just two of which were in excess of M6.0 with 30 larger than M5.0.

The distribution of the larger tremors was, as usual, very closely correlated to the margins of the earth’s tectonic plates. What was slightly unusual, however, is the occurrence of one of the larger events — the second largest, in fact — in one of the relatively minor subduction zones, this time in the eastern Mediterranean.

The Week’s Largest Earthquake: M6.7, Japan

The week's largest earthquake occurred south of Japan

The week’s largest earthquake occurred south of Japan. Image by USGS.

Sitting, as it does, across the coming-together of no fewer than four tectonic plates, the Japanese archipelago is no stranger to earthquakes and regularly features in this digest as the location of the largest tremor of the week. This week the activity was off the south of the island of Honshu, and comprised an earthquake of M6.7 and a cluster of aftershocks.

The tremor occurred on the Eurasian plate, to the west of its boundary with the Philippine Sea plate, with the latter subducting beneath the former to generate a chain of volcanic islands. The available information suggest that the earthquake is not, however, a subduction zone earthquake. The angle of the descending plate is such that, below the epicentre, the plate boundary is at a depth of around 200km; the depth of the earthquake series is much shallower, at 5-40km.

The reason is that the subduction zone, and the volcanic arc which it produces, create extensional movement at shallower depths as magma rises and reaches the surface. The setting implies that these shallow earthquakes are the result of normal (extensional) faulting.

M6.5 Earthquake in Greece

An earthquake in Greece killed at least two people

An earthquake in Greece killed at least two people. Image by USGS.

The eastern Mediterranean is a complex place, tectonically speaking, as Africa and Arabia close on Eurasia, crushing slivers of crust between their metaphorical jaws. Earthquakes here are not uncommon (and larger ones that this week’s are not unheard of, although this was certainly at the upper end of the scale).

South of Greece, a short subduction zone generates regular but mostly small earthquakes. This week’s tremor occurred just to the north of the western end of the zone, where the African and Eurasian plates (or slivers thereof) slide laterally past one another.

Reports in the media demonstrate that this earthquake proved deadly. At the time of writing (some 24 hours after the event) news items indicate that two people died, four were injured and much damage occurred in the nearby area.

US Earthquakes: All Quiet on Every Front

There were just four earthquakes larger than M4.5 in the US this eek, all in Alaska.

There were just four earthquakes larger than M4.5 in the US this eek, all in Alaska. Image by USGS.

There isn’t a lot to report from the US in terms of earthquakes this week. The largest offering was an M5.2 in Alaska, an aftershock from last week’s M6.2 along the Aleutian subduction zone.

Further along the arc there was an M5.0 and a couple of others of M4.7. This kind of activity is entirely normal for such an active subduction zone. Nothing to see here!

Last Thoughts: It’s Those Buildings Again

I’ve said it before; I’ve no doubt I’ll say it again. Earthquakes don’t kill people; buildings do.

This was illustrated by the news reports and eyewitness accounts of the damage from the Greek earthquake, which indicate that much of the damage occurred to older buildings and that the newer ones, built to a more rigorous code, were more likely to survive.

We can’t stop earthquakes. But we can mitigate their impacts.

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