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A complex multiple mainshocks sequence (24/08/2016, Mw 6.0; 26/10/2016, Mw 5.4 and 5.9; 30/10/2016, Mw 6.5) occurred in central Italy, causing the death of nearly 300 people and widespread destruction of entire villages. The Region-Time-Length (RTL) method is used to analyze the seismicity preceding the first Mw 6.0 Amatrice mainshock. This analysis is performed using the earthquake catalogue maintained by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) after a preprocessing, which includes the declustering of aftershocks. A well-evident quiescence that preceded the sequence was detected. The quiescence extended throughout a broad region north of the epicenter. The largest event of the sequence and the aftershocks covered most of the quiescence region, except for a small area to the west. The quiescence started from the beginning of September 2015 and lasted for approximately 1 year up to the Amatrice mainshock. [ABSTRACT FROM AUTHOR]

We perform a systematic investigation of along-strike rupture directivity of 70 earthquakes (3.0 ≤ Mw ≤ 6.1) of the 2009 L'Aquila seismic sequence by analysing azimuthal variations of broad-band seismograms recorded in the distance range 60 km < R < 230 km. We use reference spectra of events with little directivity (similar to the empirical Green's function method) to deconvolve propagation-site effects and focus on source properties. A directivity index (0 ≤ IDIR ≤ 1) calculated for each earthquake quantifies the spectral separation above the corner frequency of the target event at opposite along-strike directions. A large number (73 per cent) of events including the Mw 6.1 main shock showhigh (>0.7) IDIR values indicating predominantly unilateral rupture propagation. The preferred rupture propagation direction is generally to the southeast with no dependence on the earthquake magnitude or occurrence time. Events on two main faults (L'Aquila and Campotosto) show somewhat different behaviour. Almost all earthquakes on the L'Aquila fault have strong unilateral directivity to the southeast, whereas earthquakes on the Campotosto fault show more diverse behaviour. However, there is a predominance of unilateral ruptures (14 out of 22) also on the Campotosto fault, and the few (five) earthquakes with ruptures to the northwest are limited to the most northwestern segment of the fault. The spectral results are consistent with time-domain analysis when the latter samples adequately the frequency band above corner frequency. The preferred rupture direction may be produced at least in part by a velocity contrast across the fault. The results provide important input for estimates of seismic motion and physics of earthquake ruptures. [ABSTRACT FROM AUTHOR]

Abstract: A large earthquake preparation is often manifested in correlation of seismicity in an area whose characteristic dimension greatly exceeds a dimension of source of main shock. Zöller et al. [G. Zöller, S. Hainzl, J. Kurths, Observation of growing correlation length as an indicator for critical point behavior prior to large earthquakes, J. Geophys. Res. 106 (2001) 2167–2176] show the growth of correlation length of earthquakes prior to nine large earthquakes in California according to a power low. We argue that the algorithm of correlation length estimation proposed by Zöller et al. [G. Zöller, S. Hainzl, J. Kurths, Observation of growing correlation length as an indicator for critical point behavior prior to large earthquakes, J. Geophys. Res. 106 (2001) 2167–2176] can result in a decrease of correlation length preceding its precursory growth before large earthquakes if the area in which earthquake activity is correlated grows with time during a main shock preparation. The correlation length analysis of acoustic emission events recorded in laboratory experiments on destruction of rocks and correlation length analysis of intermediate magnitude earthquakes in the area of large earthquakes preparation on Kamchatka and in Italy confirms the theoretical argument. This effect can be considered as an additional premonitory pattern of large earthquake preparation. [Copyright &y& Elsevier]

A systematic multi-parameter and multi-platform approach to study the slow process of earthquake preparation is fundamental to gain some insights on this complex phenomenon. In particular, an important contribution is the integrated analysis between ground geophysical data and satellite data. In this paper we review some of the more recent results and suggest the next directions of this kind of research. Our intention is not to detect a particular precursor but to understand the physics underlying the various observations and to establish a reliable physical model of the preparation phase before an impending earthquake. In this way, future investigation will search for suitable fore-patterns , which the physical model of multi-layers coupling predicts and characterizes by quasi-synchronism in time and geo-consistency in space. We also present alternative explanations for some anomalies which are not actually related to earthquakes, rather to other natural or anthropic processes. [ABSTRACT FROM AUTHOR]