A new study by IRDR PhD student Claudia Sgambato, Dr Joanna Faure Walker, Dr Zoe Mildon and Prof Gerald Roberts, has identified a novel aspect of fault interaction that links the stress loading to the geometry of the fault system, which has important implications for fault-based earthquake hazard modelling.

By analysing the earthquake sequence of the Southern Apennines (Italy) in the last 600 years, the study presents a comparison of the stress loading history of “isolated” faults and multiple faults across strike, and shows that the stress evolution is not the same for all faults, but is influenced by the way faults are arranged in the system.

In the Southern Apennines the fault system geometry is relatively simple, with most of the structures aligned along strike. This area was the location of some of the strongest earthquakes ever recorded in Italy, such as the Mw 7.1 1857 event on the Val di Diano and Val d’Agri faults, that caused ~10,000 deaths and the more recent Mw 6.8 1980 earthquake on the Irpinia fault, that caused 3,000 deaths. For these reasons, this area provides an ideal place to investigate the role of fault geometry and fault interaction in the historical earthquake sequence.

Detail of the Vallo di Diano fault scarp (photo by Claudia Sgambato).

Fault interaction during earthquakes is usually calculated through Coulomb stress transfer. Using data from historical earthquakes, the authors have calculated the coseismic stress changes on the active faults. Deformation rates measured in the field have been used to derive the annual rate of stress loading. The combined coseismic and interseismic stress components allow calculation of the stress present on the fault prior to an earthquake, or pre-stress.

The analysis of pre-stress on all the faults before each historical earthquake has shown that 94% of the earthquakes occurred on faults that were positively stressed, and that where earthquakes occurred on an isolated fault, this had the highest pre-stress of all the faults at the time of the event. This is due to the fact that when a fault is isolated, the build-up of stress is not influenced by earthquakes occurring on other faults across strike, and the stress is distributed homogeneously across their surface, compared to faults that are across strike.

Coseismic rupture of the Mw 6.8 1980 earthquake (Irpinia fault) (photo taken during fieldwork in 2019 by Claudia Sgambato).

This suggests that isolated faults have fewer areas of negative stress, which can promote the propagation of the rupture, generating earthquakes with similar magnitude. An example of this can be seen for the Irpinia fault, with the earthquakes in 1694 and 1980, which share similar magnitude and damage distribution, and a similar value of pre-stress.

This means that for studies of seismic hazard it is important to consider the pre-stress on the faults in order to understand which fault is likely to rupture, and to take into account that the fault system geometry influences the way the stress is accumulated on faults.

The article is open access and available here: Sgambato et al. (2020)

Iezzi, Roberts & Faure Walker (2020) Throw-rate variations within linkage zones during the growth of normal faults: Case studies from the Western Volcanic Zone, Iceland, J. Struct. Geol., 133, 103977

Fault throw-rates and slip-rates are a fundamental input into fault-based seismic hazard assessments (SHA) i.e. how likely are earthquakes to occur….

Francesco Iezzi, Gerald Roberts and Joanna Faure Walker studied active faults in the Western Volcanic Zone, Iceland, to determine whether changes in fault throw-rates across fault bends, as identified in previous works in central Italy, are present in other tectonic settings.

This study shows that fault throw-rate increases within fault bends in response to non-planar fault geometry are present at a range of stages of maturity of the bend and extends examples of this phenomenon to mid-ocean ridge settings. This suggests that extrapolating fault slip-rates and slip during past earthquakes from individual sites along a fault must consider the location of data collection in relation to the geometry of the fault.

Why is this so important? Because if we use individual measurements of how fast a fault is moving, we need to understand whether this measurement is representative of the fault as a whole or whether it is underestimating or overestimating the slip. If we do not do this, we will overestimate or underestimate earthquake hazard.