Seismic risks remain after Italian quakes
- 1 November 2016
- From the section Science & Environment
There remains the potential for future quakes in Italy's Apennines region, say scientists who have reviewed the latest satellite maps of the region.
The new radar imagery suggests Sunday's big tremor ruptured a segment of a fault in between sections broken by two other quakes in recent weeks.
But the Magnitude 6.6 event has still left the deeper parts of the fault system locked in place.
And this unrelieved stress now represents a risk down the line.
The researchers are keen to emphasise, however, that predicting precisely when and where a future quake might strike is not possible.
No-one was killed in Sunday's big tremor but more than 30,000 residents have been left homeless as a consequence of the damage it wrought.
The new space data comes from Europe's Sentinel-1 satellites which overfly Italy every day or so.
These platforms are able to sense ground movement by comparing before and after radar imagery acquired from orbit.
The scientists turn this information into an interferogram - a colourful, but highly technical, depiction of the displacement that occurs on a fault.
Interpreting a Sentinel interferogram
- The interferogram featured at the top of this page encompasses both Sunday's M6.6 quake and last week's M6.1 event.
- Its coloured "fringes" trace ground motion towards or away from the satellite. Each contour interval shows movement of 2.8cm.
- The peak motion recorded is about 90cm away from the satellite. This occurs to the southwest of the fault system.
- The partial nature of the interferogram reflects the track of the overflying satellite, but this will be filled by rapid, repeat viewing.
- Four observations are acquired over the region every six days, and with slightly different perspectives on each pass.
- This will allow scientists to build up a complete model of how the ground has moved and how the fault system has behaved at depth.
The new data confirms that Sunday's tremor near the town of Norcia filled a "seismic gap" created by August's M6.2 event to the south, and last week's M6.1 trembler to the north. (The three quakes cut across two neighbouring faults in the Apennine Mountains known as the Laga and the Vetorre.)
"This central section had not ruptured in the two earlier quakes but had been stressed - brought closer to failure - by those events," explained Dr Richard Walters from Durham University, UK.
Both of the previous quakes ruptured the ground to a depth of 5-6km, and although Sunday's event went slightly deeper, down to about 9km, the researchers' preliminary modelling work indicates there are even lower rock sections that could have broken but which were left untouched.
This bottom zone of the Laga-Vetorre fault system will remain a concern for the future.
"And in addition to that, also thinking about stress transfer, the southern end of the Laga fault will have been stressed by this sequence, and it was loaded also by the L'Aquila earthquake in 2009. And so while we cannot know the exact timing of future earthquakes, this seismic sequence may not yet have finished," said Dr Walters, who is affiliated to Britain's NERC Centre for Observation and Modelling of Earthquakes, Volcanoes and Tectonics (COMET).
The Sentinel system is having a transformative impact on earthquake analysis.
In the past, interferograms would take weeks to produce.
Today, the rapid data return of the two-satellite constellation, allied to new processing techniques, mean the specialist diagrams can be turned around in a matter of hours following a major quake.
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