Orogen-scale uplift in the central Italian Apennines drives episodic behaviour of earthquake faults

Abstract

Many areas of the Earth’s crust deform by distributed extensional faulting and complex fault interactions are often observed. Geodetic data generally indicate a simpler picture of continuum deformation over decades but relating this behaviour to earthquake occurrence over centuries, given numerous potentially active faults, remains a global problem in hazard assessment. We address this challenge for an array of seismogenic faults in the central Italian Apennines, where crustal extension and devastating earthquakes occur in response to regional surface uplift. We constrain fault slip-rates since ~18 ka using variations in cosmogenic 36Cl measured on bedrock scarps, mapped using LiDAR and ground penetrating radar, and compare these rates to those inferred from geodesy. The 36Cl data reveal that individual faults typically accumulate meters of displacement relatively rapidly over several thousand years, separated by similar length time intervals when slip-rates are much lower, and activity shifts between faults across strike. Our rates agree with continuum deformation rates when averaged over long spatial or temporal scales (104 yr; 102 km) but over shorter timescales most of the deformation may be accommodated by <30% of the across-strike fault array. We attribute the shifts in activity to temporal variations in the mechanical work of faulting.

Fault Segmentation as Constraint to the Occurrence of the Main Shocks of the 2016 Central Italy Seismic Sequence

“…It is sufficient to consider the 1997 Umbria‐Marche sequence, where a M 5.7 event triggered 9 h later an adjacent M 6.0 event, which ruptured the same fault system farther to the north and the 1703 Norcia‐L'Aquila sequences with two approximately M 6.7 events occurred on 19 January and 2 February (Blumetti, ; Chiaraluce et al, ; Rovida et al, ). Also, paleoseismological studies based on 36 Cl concentration indicate that the Holocene activity of the Fucino fault system, located just to the south with respect to the analyzed fault systems (Lazio‐Abruzzo region), experienced periods of multiple, clustered earthquakes (Benedetti et al, ; Cowie et al, ). Furthermore, the increasing cases worldwide that document clustering of strong events suggest that earthquake synchronization can be a common behavior for seismogenic fault systems (e.g., Scholz, ; Sokos et al, ).…”

“…The waveforms were downsampled to the time step of 0.4 s. In order to strengthen the role of stations located further away from the fault, we increase their weight by a factor of 2 with respect to stations located above the fault. Synthetic Green's functions were calculated by the discrete wave number and matrix methods (Bouchon, 1981;Coutant, 1989;Kennett & Kerry, 1979) for a 1-D velocity model adopted from Gallovič and Zahradník (2012) and Ameri, Gallovic, and Pacor (2012), for the low-frequency and broadband modeling of the 2009 L'Aquila earthquake, respectively. We fix the parameter controlling the smoothing strength at a value providing a good balance between rupture complexity (minimum ghost features) and data fit and adopt this value for the three events.…”

A 667 year record of coseismic and interseismic Coulomb stress changes in central Italy reveals the role of fault interaction in controlling irregular earthquake recurrence intervals

“…Projecting the macroseismic shaking from historical earthquakes since 1349 A.D. onto a transect perpendicular to the strike of faults in the central Apennines shows that the majority of the activity during this time has occurred on faults on the northeastern side of the region (Figure b). In contrast to the asymmetric pattern of post 1349 A.D. seismicity, Holocene‐active fault scarps (Figure ) and post‐LGM strain rates [ Faure Walker et al , , ] are more evenly distributed across both sides of the central Apennines, across the crest of the regional long wavelength topographic bulge [ Cowie et al , ]. To investigate the role of elastic fault interaction in causing this skewed regional seismicity since 1349, we model the historical earthquakes and interseismic loading of faults from 1349 to 2016 A.D. to investigate how faults are interacting in the central Apennines and investigate the effects of these interactions on perturbing earthquake interevent times.…”

“…Earthquake clusters seen in high‐resolution dated stratigraphic sequences in actively extending regions suggest that fault slip rates vary over timescales of 10 2 –10 3 years [e.g., Bull et al , ; Nicol et al , ]. Evidence from 36 Cl cosmogenic exposure dating of bedrock fault scarps produced by normal faulting earthquakes show accelerated fault slip rates over periods of a few thousand years that are presumably associated with temporal clusters of earthquakes, alongside periods of quiescence (e.g., Mitchell et al []—Israel, Palumbo et al [], Schlagenhauf et al [], Benedetti et al [], Tesson et al [], and Cowie et al []—central Italy; Benedetti et al []—Greece; and Akçar et al []—Turkey). While the role of Coulomb stress changes in triggering short‐term clusters of seismicity is now widely acknowledged [ King et al , ; Stein et al , ], the role of Coulomb stress changes in causing the sort of regional changes in seismicity over 10 2 –10 3 years observed in 36 Cl cosmogenic exposure data and high‐resolution stratigraphy is less clear.…”

“…Advances in high-resolution topography and Quaternary dating tools suggest that transient fault slip rates and irregular earthquake recurrence intervals over thousand year timescales are a common feature of extensional areas [e.g., Friedrich et al, 2003;Palumbo et al, 2004;Bull et al, 2006;Nicol et al, 2010;Begg and Mouslopoulou, 2010;Akçar et al, 2012;Jewell and Bruhn, 2013;Nicol et al, 2016;Cowie et al, 2017] as well as other tectonic environments [e.g., Weldon et al, 2004;Oskin et al, 2008;Dolan et al, 2016;Gold et al, 2017]. A number of different processes have been invoked to explain this transient behavior including static elastic stress transfer [Benedetti et al, 2013], dynamic (i.e., coseismic) stress changes [Brodsky and van der Elst, 2014], temporal variations in the strength of brittle faults and ductile shear zones [Dolan et al, 2007], fluid migration through fault zones [Oskin et al, 2008], interactions with surface processes [Hetzel and Hampel, 2005], episodic release of strain stored in a crustal stress "battery" [Gold et al, 2017], and energy dissipation and minimization of work in response to flexural bending of normal fault footwalls [Cowie et al, 2017]. However, the timescales upon which these processes can affect fault slip rates are poorly constrained, and it is not clear which of these processes, if any, are the dominant mechanism controlling transient fault slip rates and irregular earthquake recurrence times.…”