Manitude 6.3 Earthquake in L’Aquila, Italy

Published 2009-04-06 by Chris

The medieval city of L’Aquila in Central Italy suffered a magnitude 6.3 earthquake this morning at 0132 GMT. Below is the shake map of this quake from the USGS. Full details can be found at the USGS.

Earthquakes in Italy are not unusual. The collision of the African tectonic plate with the European portion of the Eurasian plate cause these seismic events just as in any other tectonic collision. What IS unusual about this particular event, however, is the strength of the quake. Below is a map of all earthquakes in this area between 1973 and today from the USGS.

So since 1973, there have been 11 earthquakes in the Italy/Northern Africa region of magnitude 6 and above (0.5%), vs 2,234 earthquakes between M4 and M5.9. This is also expected, as smaller earthquakes are simply more common than large ones, but there’s a stark contrast here as all of the strongest quakes are extremely shallow (<33 Km in depth) while weaker quakes vary across the entire lithosphere/upper mantle (due to the nature of this collision). As a contrast, here’s the same segmented data from New Zealand:

Earthquakes in New Zealand >M6 (left) vs M4-M5.9 (right)

That’s 46 earthquakes of M6 or greater (1.25%), and 3660 quakes between M4 and M5.9. You’ll notice there’s a spread of depths with both quake groups in the New Zealand case, due to the subduction-related interactions of the Pacific and Australian plates (with a patch of ≥M6, 300-500 Km, purple, earthquakes at the start of the Kermadec arc, a classic case of back arc volcanism).

If you’re interested in the human side of this story, the BBC has an article about the 27+ deaths and building damage which includes video.

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