The timing and size of three deadly earthquakes that struck Italy in 2016 may have been pre-determined, according to new research published in Earth and Planetary Science Letters that could improve future earthquake forecasts.

A joint British-Italian team of geologists and seismologists have shown that the clustering of the three quakes might have been caused by the arrangement of a cross-cutting network of underground faults.

The findings show that although all three earthquakes occurred on the same major fault, several smaller faults prevented a single massive earthquake from occurring instead and also acted as pathways for naturally occurring fluids that triggered later earthquakes.

The cluster of three earthquakes, termed a “seismic sequence” by seismologists, each had magnitudes greater than six and killed more than 300 people in Italy’s Apennine mountains between 24 August and 30 October 2016.

Earthquake sequences

The research, led by COMET scientist Richard Walters from Durham University, comes on the second anniversary of the start of the earthquake sequence.

The researchers say the findings could have wider implications for the study of seismic hazards, enabling scientists to better understand potential earthquake sequences following a quake.

Dr Walters said: “These results address a long-standing mystery in earthquake science – why a major fault system sometimes fails in a single large earthquake that ruptures its entire length, versus failing in multiple smaller earthquakes drawn-out over months or years.

“Our results imply that even though we couldn’t have predicted when the earthquake sequence would start, once it got going, both the size and timing of the major earthquakes may have been pre-determined by the arrangement of faults at depth.

“This is all information we could hypothetically know before the event, and therefore, this could be a hugely important avenue for improving future earthquake forecasts.”

Thousands of aftershocks

Dr Walters and the team used satellite data to estimate which part of the fault failed in each earthquake, and compared this pattern with the location and timing of thousands of tiny aftershocks throughout the seismic sequence.

They found that intersections of small faults with the main fault system separated each of the three largest earthquakes, strongly suggesting these intersections stop the growth of each earthquake and prevent the faults failing in a single large event.

In addition, the scientists also found that after the first earthquake, thousands of aftershocks crept northwards along these same fault intersections at a rate of around 100 metres per day, in a manner consistent with naturally occurring water and gas being pumped along the faults by the first earthquake on 24 August, 2016.

The second earthquake, on the 26 October, occurred exactly when these fluids reached its location, therefore controlling the relative timing of failure.

Dr Walters added: “It was a big surprise that these relatively small faults were having such a huge influence over the whole sequence.

“They stop the first earthquake in its tracks, and then they channel the fluids that start the sequence up again months later. No-one’s ever seen this before.”

Seismic hazard

Co-author and COMET associate Dr Laura Gregory from the University of Leeds said it was important to understand whether or not a fault fails in a seismic sequence, and that the team’s results were only made possible by combining a varied array of different datasets.

Dr Gregory said: “A seismic sequence has vastly different implications for seismic hazard compared to a single large earthquake. If the faults in Italy in 2016 had failed together in one big event, the impact on the local population would have been much worse.

“This is the first time we’ve ever had this quality of modern data over one of these earthquake sequences, and bringing together a range of specialists was key for unpicking how the earthquakes related to one another.

“I was scrambling over the mountainside immediately after each earthquake with British and Italian colleagues, measuring the metre-high cliffs that had suddenly formed. Meanwhile, other members of our team were analysing data from seismometers stationed around the world, or were mapping the tiny bending of the ground around the faults using satellites orbiting the planet at 500 miles altitude.”

The research was partly supported by both COMET and a NERC Urgency Grant.

Satellite data has shed new light on seismic hazard in one of the world’s most deadly earthquake zones.

Published today in Nature Communications, the COMET study describes how tectonic strain builds up along Turkey’s North Anatolian Fault at a remarkably steady rate.

This means that present-day measurements can not only reflect past and future strain accumulation, but also provide vital information on events still to come.

The strain, which builds up as Turkey is squeezed between three major tectonic plates, has caused almost the entire length of the fault to rupture since 1939 in a series of major earthquakes gradually migrating east-west towards Istanbul.

Led by COMET PhD student Ekbal Hussain[1], the team used satellite images from the European Space Agency’s Envisat mission to identify tiny ground movements at earthquake locations along the fault.

Dr Hussain explained: “Because we know so much about the fault’s recent history, we could look at the strain build up at specific places knowing how much time had passed since the last earthquake.”

The 600-plus satellite images, taken between 2002 and 2010, provided insights into the equivalent of 250 years of the fault’s earthquake repeat cycle.

Remarkably, apart from the ten years immediately after an earthquake, strain rates levelled out at about 0.5 microstrain per year, equivalent to 50mm over a 100km region, regardless of where or when the last earthquake took place.

Dr Hussain added: “This means that the strain rates we measure over the short term can also reflect what’s happening in the longer term, telling us how much energy is being stored on the fault and could eventually be released in an earthquake.”

Until the satellite era, it was difficult to get a clear picture of how strain built up on the fault.  Now, satellites like Envisat, alongside the newer Sentinel-1 mission, can detect ground movements of less than a millimetre, indicating how and where strain is accumulating.

The findings suggest that some existing hazard assessment models, which presume that strain rates vary over time, need to be rethought.  This is especially true for regions where there are long gaps between earthquakes, such as the Himalayas.

Co-author and COMET Director Tim Wright said: “Discovering this consistent strain accumulation will help us to reassess how we model seismic hazards, as well as improving understanding of the earthquake cycle worldwide.”

The full paper is: Hussain et al. (2018) Constant strain accumulation rate between major earthquakes on the North Anatolian Fault, Nature Communications, doi:10.1038/s41467-018-03739-2

[1] Now Remote Sensing Scientist at BGS Keyworth.  Dr Hussain is available for comment (ekhuss@bgs.ac.uk).