Evaluation of earthquake stress parameters and its scaling during the 2016-2017 Amatrice-Norcia-Visso sequence—Part I

The Amatrice–Norcia–Visso sequence is characterized by complex behaviour that is somewhat
atypical of main-shock–aftershock sequences, as there were multiple large main shocks that
continued for months.
In this study we focus on the Amatrice sequence (main shock 2016 August 24, Mw = 5.97)
to evaluate the apparent stress values and magnitude-dependent scaling in order to improve
our knowledge of processes that control small and large earthquakes within this active region
of Italy. Apparent stress is proportional to the ratio of radiated seismic energy and seismic
moment, and as such, these stress parameters play an important role in hazard prediction as
they have a strong effect on the observed and predicted ground shaking.
We analyse 83 events of the sequence from 2016 August 24 to October 16, within a radius
of 20 km from the main shock and with an Mw ranging between 5.97 and 2.72.
Taking advantage of the averaging nature of coda waves, we analyse coda-envelope-based
spectral ratios between neighbouring event pairs.We use equations proposed byWalter et al. to
consider stable, low-frequency and high-frequency spectral ratio levelswhich provide measures
of the corner frequency and apparent stress ratios of the events within the sequence.
The results demonstrate non-self-similar behaviour within the sequence, suggesting a change
in dynamics between the largest events and the smaller aftershocks.
The apparent stress and corner frequency estimates are compared to those obtained by
Malagnini and Munaf`o who utilized hundreds of direct S-wave spectral ratio measurements
to obtain their results. Although our analysis is based only on 83 events, our results are in
very good agreement, demonstrating once more that the use of coda waves is very stable and
provides lower variance measures than those using direct waves.
A comparison with recent Central Apennines source scaling models derived from various
seismic sequences (1997–1998 Colfiorito, 2002 San Giuliano di Puglia, 2009 L’Aquila) shows
that the Amatrice sequence source scaling in this study is well represented by the models
proposed by Pacor et al. and Malagnini and Mayeda.

This article has been accepted for publication in Geophysical Journal International ©: The Authors 2019. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved. Uploaded in accordance with the publisher's self-archiving policy.