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Abstract:
Fault zones are major sources of hazard for many populated regions around the world.
Earthquakes still occur unanticipated, and research has started to observe fault properties
with increasing spatial and temporal resolution, having the goal of detecting signs of
stress accumulation and strength weakening that may anticipate the rupture. The common
practice is monitoring source parameters retrieved from measurements; however, model
dependence and strong uncertainty propagation hamper their usage for small and microearthquakes.
Here, we decipher the ground motion (i.e., ground shaking) variability associated
with microseismicity detected by dense seismic networks at a near-fault observatory
in Irpinia, Southern Italy, and obtain an unprecedentedly sharp picture of the fault properties
evolution both in time and space. We discuss the link between the ground-motion
intensity and the source parameters of the considered microseismicity, showing a coherent
spatial distribution of the ground-motion intensity with that of corner frequency, stress
drop, and radiation efficiency. Our analysis reveals that the ground-motion intensity
presents an annual cycle in agreement with independent geodetic displacement observations
from two Global Navigation Satellite System stations in the area. The temporal and
spatial analyses also reveal a heterogeneous behavior of adjacent fault segments in a high
seismic risk Italian area. Concerning the temporal evolution of fault properties, we highlight
that the fault segment where the 1980 Ms 6.9 Irpinia earthquake nucleated shows
changes in the event-specific signature of ground-motion signals since 2013, suggesting
changes in their frictional properties. This evidence, combined with complementary information
on the earthquake frequency–magnitude distribution, reveals differences in fault
segment response to tectonic loading, suggesting rupture scenarios of future moderate
and large earthquakes for seismic hazard assessment.
