Detection of Spatial and Temporal Stress Changes During the 2016 Central Italy Seismic Sequence by Monitoring the Evolution of the Energy Index

Abstract

Investigating the strength of faults is a fundamental goal in seismology. Key open scientific questions on faults interaction and on the nucleation and preparation phase of large earthquakes are, in the final instance, all related to crustal and fault strength. However, as observed by Ben-Zion (2019), our understanding of processes occurring in the crust is inevitably hampered by the impossibility of collecting measurements directly within the seismogenic volumes.A better understanding of the in situ crustal strength cannot be achieved by studying large earthquakes only, as they are rare and provide information only on the final stages of the processes that lead to the rupture. Thus, the scientific community is mining data across wider spatiotemporal scales, both in the laboratory, by observing

On catching the preparatory phase of damaging earthquakes: an example from central Italy

“…This is certainly one of the most important limitations we must face while attempting to identify the preparatory process of large earthquakes and mitigate seismic risk. To overcome this obstacle, besides to establish denser seismic networks nearby active faults, previous works suggested to exploit microearthquakes as indicators of the mechanical state of the crust [12,44]. Indeed, a key advantage of microearthquakes is that they are too small to interact with each other and therefore contribute little to crustal deformation [45].…”

“…7b the results of this analysis for both D B and D C , where the RMSEs for the two datasets are normalized to their maximum for facilitating the comparison. Concerning D B , we observe that the ve most important features are, in descending order, the energy index (12), the fractal dimension (2), the seismic rate (5), the 3D convex Hull (i.e., volume) of the hypocenters (11), and the b-value (1). When we consider D C , we observe the most important features being the energy index (12), the b-value (1), the seismic rate (5), the volume (11), and the generalized distance (6).…”

“…We observe that only the fractal dimension (2) and the energy index (12) are excluded from this optimal set of features. Looking at the results in Fig.…”

“…Although some success on intercepting the preparatory phase of earthquakes starts to be achieved, recent analyses on two large Italian earthquakes (i.e., the Mw 6.3 L'Aquila 2009 and the Mw 6.1 Amatrice 2016), which occurred at few tens of kilometers of distance and in the same extensional environment in the Apennines in central Italy, have highlighted important differences in their initiation [11,12], with a clear preparatory phase identi ed for the former and only a long-lasting quiescence without a clear activation phase for the latter. The latter examples highlight the enigmatic nature of the preparatory phase of large earthquakes, which also in case of nearby earthquakes can result be dominated by different driving mechanisms.…”

“…3), except for the b-value and EI that show almost coincident shape. Considering that both EI and b-value have been put in relation to the stress level on the fault [12,21], the similarity of the CDF for these two features suggest that background seismicity and past seismic sequences, which did not culminate in large magnitude earthquakes, occur on small scale faults with similar properties and that both static and dynamic stresses should reach higher values over larger spatial scales to sustain the development of large ruptures.…”