How to define the earthquake loading medium: an interdisciplinary approach

Since Reid formulated the elastic rebound theory in the early 20th century to describe earthquakes in brittle faulting, fault systems have been widely represented by spring–slider models, both in theoretical frameworks and laboratory experiments. From a different perspective, structural geology has long documented fault systems as geometrically complex structures, reflecting the heterogeneous physical properties of different lithologies. These systems are characterised by multiple slip surfaces and secondary fault splays and comprise large volumes of highly damaged rocks. Such damaged volumes are effectively part of the loading medium that is commonly conceptualised, in simplified models, as an elastic spring.

Over the past decades, a wealth of seismological and geodetic observations has shown that these damaged crustal volumes actively deform inelastically during the seismic cycle, rather than merely storing elastic energy. In parallel, numerical models indicate that off-fault damage can account for a significant portion of the earthquake energy budget. Together, these observations challenge the classical representation of the fault loading medium as purely elastic.

Here, we integrate observations spanning outcropping fault-zone descriptions, seismicity catalogues, and laboratory observations to explore how the earthquake loading medium could be more realistically defined and described in natural fault systems. We focus on well-studied seismogenic normal faults in Italy, namely the Gubbio and Norcia faults, where a long-standing and extensive knowledge of the involved lithologies is combined with a high-resolution fault image obtained by both high-quality outcrop exposure and enhanced seismological catalogues, and where the involved rocks have been extensively studied in the laboratory. By adopting this interdisciplinary perspective, we aim to better constrain the nature of the loading medium toward a better estimation of the forcing imbalance that is fundamental to earthquake nucleation.