OBJECTIVES AND WORKPLAN

Understanding how successive earthquakes accrue on faults to produce tectonic landforms is still poorly understood. The way deformation is accommodated throughout the crust, in response to the far-field plate tectonics force imposes at plate boundaries, strongly affects the seismic cycle and may control earthquake triggering and the spatial pattern of fault ruptures. The Apennines range, host of the 2016 seismic sequence (5 shocks Mw5-6.5 over 9 months), is a unique area where the accumulation and release of slip over multiple seismic cycles, over time scales of 1yr-1 Myr and spatial scales of 1m-100km, can be determined. We will combine frontier methodologies in geochronology, remote sensing, geodesy, geophysics, high-resolution topographical data acquisition, seismic hazard modelling, all developed and/or mastered by our teams, to quantitatively constrain how portions of the seismic cycle scale up over multiple cycles to produce the cumulative escarpments we see in the landscape.

Several recent studies have revealed episodic earthquake behaviour with alternating phases of intense seismic activity over relatively short time periods, followed by long periods of seismic quiescence. Time variability of slip accumulation has also been shown to correlate with spatial variability, as multiple faults within a system have been found to break in coeval large earthquakes over a few hundred years. Knowledge of the spatial extent of a co-seismic rupture is fundamental for deriving expected earthquake magnitude, an essential parameter for seismic design and population preparedness. Several studies have proposed systematic aspects such as structural or geometric complexities that might stop the rupture. Very few studies, however, unravel whether those barriers are persistent through multiple seismic cycles. Several approaches and models have studied the physics underlying this seismic behaviour, suggesting that there might be longer and larger spatial and time-scale control for those interactions. However, those models mostly rely on observations of the post-seismic or interseismic periods over several years and highlight the crucial need to integrate this short-term behaviour into a longer-term perspective. The conceptual breakthrough that we propose is to constrain quantitatively how the tectonic loading is released by normal fault systems over multiple cycles to ultimately produce the large (100+ m high) cumulative escarpments we see in the landscape. In particular we will address the following unresolved questions:

1. Does the segmentation, as seen in co-seismic ruptures, mimic the long-term segmentation seen in the morphology and geology ?
2. Do inherited structures control this segmentation ?
3. What is the feedback between a single fault loading-rate, and hence its slip recurrence interval, and far-field tectonic forces ?
4. What is the percentage of deformation at the surface accommodated by seismic vs. aseismic creep either on or off the main fault (i.e. on secondary faults, or volumetric deformation) ?
5. How is the regional interseismic strain accommodated at depth within the crust, and how is it seismically or aseismically released on complex and segmented fault systems ?