Complex networks and waveforms from acoustic emissions in laboratory earthquakes

Abstract

Abstract. Understanding the physics of acoustic excitations emitted during the cracking of materials is one of the longstanding challenges for material scientists and geophysicists. In this study, we report novel results of applications of functional complex networks on acoustic emission waveforms emitted during the evolution of frictional interfaces. Our results show that laboratory faults at microscopic scales undergo a sequence of generic phases, including strengthening, weakening or fast slip and slow slip, leading to healing. For the first time we develop a formulation on the dissipated energy due to acoustic emission signals in terms of shortterm and long-term features (i.e., networks' characteristics) of events. We illuminate the transition from regular to slow ruptures. We show that this transition can lead to the onset of the critical rupture class similar to the direct observations of this phenomenon in the transparent samples. Furthermore, we demonstrate the detailed submicron evolution of the interface due to the short-term evolution of the rupture tip. As another novel result, we find that the nucleation phase of most amplified events follows a nearly constant timescale, corresponding to the initial strengthening or locking of the interface. This likely indicates that a thermally activated process can play a crucial role near the moving crack tip.

Microscopic Evolution of Laboratory Volcanic Hybrid Earthquakes

“…The first intriguing result of the analysis is that we find that ultrasound excitations possess patterns of temporal evolution of network parameters that are universal among recorded events22232425. The appearance of universal patterns in any measure of excited signals shows the robustness of the collective process in the source(s) against the much faster scattering processes.…”

“…From this perspective, the system relaxes in multiple stages and time scales rather than single time scale. In general, the aforementioned phases are typical for dry events and simple friction tests conducted under dry conditions2223242526 (Fig. S2).…”

“…To evaluate reordered multiple acoustic emission (multiple time series for an occurred event), we use a previously algorithm on waveforms from our reordered acoustic emissions2225 which is the class of functional connectivity networks from multivariate time series data. The main steps of the algorithm are as follows:

The waveforms recorded at each acoustic sensor are normalized to the maximum value of the amplitude in that station.

Each time series is divided according to maximum segmentation, in a way that each segment includes only one data point.

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“…With this metric, we simply compare the amplitude of sensors in the given time-step. The employed norm in our algorithm is absolute-norm.Threshold level ( ζ ): To select a threshold level, we use an introduced method in refs 23,25 and references therein] where uses an adaptive threshold criterion and is stable for an range of deviations from ζ . The result of this algorithm is an adjacency matrix with components given by a(x i ( t ), x k ( t )) = Θ( ζ −| u i,j ( t )− u k,j ( t )|) where Θ(...) is the Heaviside function.…”

Shear ruptures of extreme dynamics in laboratory and natural conditions

“…Until recently, classic dynamic fracture theories predicted that the rupture velocity v (velocity of the rupture tip propagation) cannot exceed the velocity of the free surface or Rayleigh wave cR that is less than the shear cS and compression cP wave speed (v < cR < cS < cP). However, recent observations of shear ruptures during earthquakes and in laboratory experiments show that shear ruptures can propagate with velocities exceeding the shear wave speed cS (Heaton 1990;Olsen et al 1997;Ohnaka & Shen 1999;Rosakis et al 1999;Lei et al 2000;Rosakis 2002;Xia et al 2004;Rubinstein et al 2004;Lykotrafitis et al 2006;Griffith et al 2009;Ben-David et al 2010;Ngo et al 2012;Ghaffari et al 2014). Such dynamic ruptures and corresponding rupture velocities are called 'supershear ' or 'intersonic' (cS < v < cP).…”